U.S. patent application number 14/879285 was filed with the patent office on 2017-04-13 for i/q modulator and demodulator with wide instantaneous bandwidth and high local-oscillator-port-to-radio-frequency-port isolation.
The applicant listed for this patent is NATIONAL INSTRUMENTS CORPORATION. Invention is credited to Justin R. Magers.
Application Number | 20170104617 14/879285 |
Document ID | / |
Family ID | 58461820 |
Filed Date | 2017-04-13 |
United States Patent
Application |
20170104617 |
Kind Code |
A1 |
Magers; Justin R. |
April 13, 2017 |
I/Q Modulator and Demodulator with Wide Instantaneous Bandwidth and
High Local-Oscillator-Port-to-Radio-Frequency-Port Isolation
Abstract
An improved quadrature modulator/demodulator (IQMD) may use
two-phase quadrature local oscillator (LO) signal generation for
generating 0.degree. and 90.degree. LO signals, and an anti-phase
combiner/divider (at 0.degree. and 180.degree.) on the RF (radio
frequency) port. The IQMD may include mixers (which may be
double-balanced passive mixers) that function as downconverters
when a signal is incident at their radio frequency (RF) ports, and
function as upconverters when signals are incident on their
intermediate frequency (IF) ports. Accordingly, the IQMD may
function as an I/Q modulator by connecting digital-to-analog
converters (DAC) to the differential I and Q ports, and/or it may
also function as an I/Q demodulator by connecting analog-to-digital
converters (ADC) to the differential I and Q ports.
Inventors: |
Magers; Justin R.; (Santa
Rosa, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NATIONAL INSTRUMENTS CORPORATION |
Austin |
TX |
US |
|
|
Family ID: |
58461820 |
Appl. No.: |
14/879285 |
Filed: |
October 9, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03D 7/166 20130101;
H04B 1/48 20130101; H04L 27/2273 20130101; H03C 3/40 20130101; H04L
27/364 20130101; H04B 1/403 20130101; H04L 27/206 20130101 |
International
Class: |
H04L 27/20 20060101
H04L027/20; H04B 1/403 20060101 H04B001/403; H04B 1/48 20060101
H04B001/48; H04L 27/227 20060101 H04L027/227 |
Claims
1. A modulator/demodulator comprising: an RF port; a first circuit
comprising a first port coupled to the RF port, and further
comprising a second port and a third port, wherein the first
circuit is configured to: generate a pair of out-of-phase input
signals at the second port and third port, responsive to the first
port being excited by a first signal received at the RF port; and
generate a summed output signal at the first port, responsive to
the second port and the third port being excited by a pair of
out-of-phase output signals; a first mixer coupled to the second
port, and configured to: downconvert a first out-of-phase input
signal of the pair of out-of-phase input signals to a first pair of
incoming baseband signals; and upconvert a first pair of outgoing
baseband signals to a first out-of-phase output signal of the pair
of out-of-phase output signals; and a second mixer coupled to the
third port, and configured to: downconvert a second out-of-phase
input signal of the pair of out-of-phase input signals to a second
pair of incoming baseband signals; and upconvert a second pair of
outgoing baseband signals to a second out-of-phase output signal of
the pair of out-of-phase output signals.
2. The modulator/demodulator of claim 1, further comprising a local
oscillator (LO) port configured to receive an LO signal; wherein
the first mixer and the second mixer are configured to perform
upconversion and downconversion based on the LO signal.
3. The modulator/demodulator of claim 2, further comprising: a
zero-phase power splitter coupled to the LO port and configured to
receive the LO signal and derive a first LO signal and a second LO
signal from the LO signal, wherein the first LO signal and the
second LO signal are in phase with respect to each other; wherein
the first mixer is configured to perform upconversion and
downconversion based on the first LO signal; and wherein the second
mixer is configured to perform upconversion and downconversion
based on the second LO signal.
4. The modulator/demodulator of claim 2, further comprising: a
phase shifter coupled to the LO port and configured to receive the
LO signal an generate an in-phase LO signal and a quadrature LO
signal based on the LO signal; wherein the first mixer and the
second mixer are each configured to perform upconversion and
downconversion based on the in-phase LO signal and the quadrature
LO signal.
5. The modulator/demodulator of claim 4, further comprising: a
first zero-phase power splitter configured to receive the in-phase
LO signal and derive a first in-phase LO signal and a second
in-phase LO signal from the in-phase LO signal; and a second
zero-phase power splitter configured to receive the quadrature LO
signal and derive a first quadrature LO signal and a second
quadrature LO signal from the quadrature LO signal; wherein the
first mixer is configured to perform upconversion and
downconversion using the first in-phase LO signal and the first
quadrature LO signal; and wherein the second mixer is configured to
perform upconversion and downconversion using the second in-phase
LO signal and the second quadrature LO signal.
6. The modulator/demodulator of claim 1, wherein the first circuit
is one of: a hybrid circuit further comprising a fourth port, and
further configured to generate a second summed output signal at the
first port, responsive to the second port and the third port being
excited by a pair of in-phase output signals; or a balun circuit,
wherein the first port of the first circuit is an unbalanced port
of the balun circuit, and the second and third ports of the first
circuit are balanced ports of the balun circuit.
7. The modulator/demodulator of claim 1, wherein the first pair of
incoming baseband signals comprises a first incoming in-phase
baseband signal and a first incoming quadrature baseband signal,
and wherein the first pair of outgoing baseband signals comprises a
first outgoing in-phase baseband signal and a first outgoing
quadrature baseband signal; wherein the second pair of incoming
baseband signals comprises a second incoming in-phase baseband
signal and a second incoming quadrature baseband signal, and
wherein the second pair of outgoing baseband signals comprises a
second outgoing in-phase baseband signal and a second outgoing
quadrature baseband signal; wherein the first incoming in-phase
baseband signal and the second incoming in-phase baseband signal
form an incoming pair of differential in-phase baseband signals;
wherein the first incoming quadrature baseband signal and the
second incoming quadrature baseband signal form an incoming pair of
differential quadrature baseband signals; wherein the first
outgoing in-phase baseband signal and the second outgoing in-phase
baseband signal form an outgoing pair of differential in-phase
baseband signals; and wherein the first outgoing quadrature
baseband signal and the second outgoing quadrature baseband signal
form an outgoing pair of differential quadrature baseband
signals.
8. The modulator/demodulator of claim 1, wherein each mixer of the
first mixer and the second mixer comprises: a first mixer circuit
configured to: receive an in-phase baseband signal when
upconverting; and provide an in-phase baseband signal when
downconverting; and a second mixer circuit configured to: receive a
quadrature baseband signal when upconverting; and provide a
quadrature baseband signal when downconverting.
9. The modulator/demodulator of claim 8, wherein the first mixer
circuit and the second mixer circuit are each double-balanced
passive mixer circuits.
10. The modulator/demodulator of claim 8, wherein each mixer of the
first mixer and the second mixer further comprises: a phase shifter
circuit configured to: generate an in-phase oscillator signal and a
quadrature oscillator signal based on a received local oscillator
signal; provide the in-phase oscillator signal to the first mixer
circuit, wherein the first mixer circuit is configured to perform
modulation and demodulation using the in-phase oscillator signal;
and provide the quadrature oscillator signal to the second mixer
circuit; wherein the second mixer circuit is configured to perform
modulation and demodulation using the in-phase oscillator
signal.
11. A transceiver comprising: a first modulator/demodulator
circuit; and a second modulator/demodulator circuit; wherein each
modulator/demodulator circuit of the first modulator/demodulator
circuit and the second modulator/demodulator circuit comprises: an
RF port; a first circuit comprising a first port coupled to the RF
port, and further comprising a second port and a third port,
wherein the first circuit is configured to: generate a pair of
out-of-phase input signals at the second port and third port,
responsive to the first port being excited by a first signal
received at the RF port; and generate a summed output signal at the
first port, responsive to the second port and the third port being
excited by a pair of out-of-phase output signals; a first mixer
coupled to the second port, and configured to: downconvert a first
out-of-phase input signal of the pair of out-of-phase input signals
to a first pair of incoming baseband signals; and upconvert a first
pair of outgoing baseband signals to a first out-of-phase output
signal of the pair of out-of-phase output signals; and a second
mixer coupled to the third port, and configured to: downconvert a
second out-of-phase input signal of the pair of out-of-phase input
signals to a second pair of incoming baseband signals; and
upconvert a second pair of outgoing baseband signals to a second
out-of-phase output signal of the pair of out-of-phase output
signals.
12. The transceiver of claim 11 wherein the first
modulator/demodulator circuit is configured to perform modulation
and the second modulator/demodulator circuit is configured to
perform demodulation.
13. The transceiver of claim 12, wherein each modulator/demodulator
circuit further comprises a respective local oscillator (LO) port
configured to receive a respective LO signal; wherein the first
mixer and the second mixer of the first modulator/demodulator
circuit are configured to perform upconversion based on the
respective LO signal received by the first modulator/demodulator
circuit; and wherein the first mixer and the second mixer of the
second modulator/demodulator circuit are configured to perform
downconversion based on the respective LO signal received by the
second modulator/demodulator circuit.
14. The transceiver of claim 13, wherein each modulator/demodulator
circuit further comprises: a respective zero-phase power splitter
coupled to the respective LO port and configured to receive the
respective LO signal and derive a respective first LO signal and a
respective second LO signal from the respective LO signal, wherein
the respective first LO signal and the respective second LO signal
are in phase with respect to each other; wherein the first mixer of
the first modulator/demodulator circuit is configured to perform
upconversion based on the respective first LO signal derived from
the respective LO signal received by the first
modulator/demodulator circuit; wherein the second mixer of first
modulator/demodulator circuit is configured to perform upconversion
based on the respective second LO signal derived from the
respective LO signal received by the first modulator/demodulator
circuit; wherein the first mixer of the second
modulator/demodulator circuit is configured to perform
downconversion based on the respective first LO signal derived from
the respective LO signal received by the second
modulator/demodulator circuit; and wherein the second mixer of the
second modulator/demodulator circuit is configured to perform
downconversion based on the respective second LO signal derived
from the respective LO signal received by the second
modulator/demodulator circuit.
15. The transceiver of claim 13, wherein each modulator/demodulator
circuit further comprises a respective phase shifter coupled to the
respective LO port and configured to receive the respective LO
signal and generate a respective in-phase LO signal and a
respective quadrature LO signal based on the respective LO signal;
wherein the first mixer and the second mixer of the first
modulator/demodulator circuit are each configured to perform
upconversion based on the respective in-phase LO signal and the
respective quadrature LO signal generated from the respective LO
signal received by the first modulator/demodulator circuit; and
wherein the first mixer and the second mixer of the second
modulator/demodulator circuit are each configured to perform
downconversion based on the respective in-phase LO signal and the
respective quadrature LO signal generated from the respective LO
signal received by the second modulator/demodulator circuit.
16. The transceiver of claim 12, further comprising: a plurality of
digital-to-analog converters coupled to the first
modulator/demodulator circuit and configured to provide unmodulated
analog baseband signals to the first modulator/demodulator circuit;
and a plurality of analog-to-digital converters coupled to the
second modulator/demodulator circuit and configured to receive
demodulated analog baseband signals from the second
modulator/demodulator circuit.
17. The transceiver of claim 11, wherein the first circuit is one
of: a hybrid circuit further comprising a fourth port, and further
configured to generate a second summed output signal at the first
port, responsive to the second port and the third port being
excited by a pair of in-phase output signals; or a balun circuit,
wherein the first port of the first circuit is an unbalanced port
of the balun circuit, and the second and third ports of the first
circuit are balanced ports of the balun circuit.
18. The transceiver of claim 11, wherein the first pair of incoming
baseband signals comprises a first incoming in-phase baseband
signal and a first incoming quadrature baseband signal, and wherein
the first pair of outgoing baseband signals comprises a first
outgoing in-phase baseband signal and a first outgoing quadrature
baseband signal; wherein the second pair of incoming baseband
signals comprises a second incoming in-phase baseband signal and a
second incoming quadrature baseband signal, and wherein the second
pair of outgoing baseband signals comprises a second outgoing
in-phase baseband signal and a second outgoing quadrature baseband
signal; wherein the first incoming in-phase baseband signal and the
second incoming in-phase baseband signal form an incoming pair of
differential in-phase baseband signals; wherein the first incoming
quadrature baseband signal and the second incoming quadrature
baseband signal form an incoming pair of differential quadrature
baseband signals; wherein the first outgoing in-phase baseband
signal and the second outgoing in-phase baseband signal form an
outgoing pair of differential in-phase baseband signals; and
wherein the first outgoing quadrature baseband signal and the
second outgoing quadrature baseband signal form an outgoing pair of
differential quadrature baseband signals.
19. The transceiver of claim 11, wherein each mixer of the first
mixer and the second mixer comprises: a first mixer circuit
configured to: receive an in-phase baseband signal when
upconverting; and provide an in-phase baseband signal when
downconverting; and a second mixer circuit configured to: receive a
quadrature baseband signal when upconverting; and provide
quadrature baseband signal when downconverting.
20. The transceiver of claim 11, further comprising: a first
plurality of modulator/demodulator circuits; and a second plurality
of modulator/demodulator circuits; wherein the first
modulator/demodulator circuit and the first plurality of
modulator/demodulator circuits are configured to perform modulation
for multiple-input-multiple-out wireless signal transmissions; and
wherein the second modulator/demodulator circuit and the second
plurality of modulator/demodulator circuits are configured to
perform demodulation for the multiple-input-multiple-out wireless
signal transmissions.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the fields of wireless
communication and instrumentation, and more particularly to the
design of an I/Q modulator and demodulator.
DESCRIPTION OF THE RELATED ART
[0002] Wireless communication systems are rapidly growing in usage.
In recent years, wireless devices such as smart phones and tablet
computers have become increasingly sophisticated. In addition to
supporting telephone calls, many mobile devices now provide access
to the internet, email, text messaging, and navigation using the
global positioning system (GPS), and are capable of operating
sophisticated applications that utilize these functionalities.
[0003] Digital wireless communications are typically implemented
through the use of modulators and demodulators, which provide a
necessary RF (radio frequency) interface for systems such as
cordless phones, wireless networks, and wireless peripheral devices
for computers, in addition to test and control systems that may use
a wireless interface to couple certain system elements. One
commonly used modulation method that lends itself well to digital
processes is "quadrature modulation", which employs two carriers
out of phase by 90.degree. and modulated by separate signals. This
modulation technique is also referred to as "I/Q Modulation", where
"I" refers to the "in-phase" component of the waveform, and "Q"
refers to the quadrature component, or 90.degree.-out-of-phase
component of the waveform. In its various forms, I/Q modulation
provides an efficient way to transfer information, and also works
well with digital formats. An I/Q modulator may be used for
amplitude modulation (AM), frequency modulation (FM) and phase
modulation (PM). There are also many digital encoding standards
that allow for the transmission of vast amounts of data over
wireless RF interfaces in shorter periods of time.
[0004] The development of next generation wireless systems calls
for increased data transfer capabilities. One example of a next
generation standard is 5G (5th generation mobile networks or 5th
generation wireless systems) which represents the next major phase
of mobile telecommunications standards beyond the current
4G/IMT-Advanced standards. One essential goal for 5G systems is to
deliver increased capacity for cellular access, with a rough goal
of approximately 10 Gbps (10 gigabits per second) peak data rates
and 100 Mbps (100 megabits per second) at cell edges. Possible
schemes of achieving increased capacity include high rank MIMO
(multiple input multiple output) and beamforming techniques, higher
order modulation schemes, and increased instantaneous bandwidth
(IBW), or some combination of each. Increased IBW is also
beneficial for other systems in which wireless data transmission
may be a less costly solution than installation of fiber optic
systems. Such systems include cellular front-haul (the connection
from remote radio heads to base station controllers), cellular
backhaul (the connection from the mobile network to the wired
network), and point-to point radio links. Large IBW also enables
high capacity short range radio links in the 60 GHz range due to
atmospheric oxygen absorption. The applications for these links are
wireless LAN (local area network; e.g. IEEE 802.1 lad) and wireless
transmission of high-definition video, as well as cellular
backhaul, cellular access, Wi-Fi offloading, connector free
platform extension (wireless docking), device-to-device
collaboration, multi-media kiosks, and wireless PCIe extension.
[0005] I/Q modulators and demodulators are key components of modern
digital radio systems because they offer potentially higher levels
of integration due to decreased reliance on high-Q filters as
required by super-heterodyne systems. Additionally, I/Q modulation
and demodulation is advantageous for systems with high IBW because
twice the IBW can be achieved for the same sample rate as compared
with super-heterodyne systems that employ intermediate frequency
(IF) sampling. Unfortunately, most I/Q modulators and demodulators
come with their own set of impairments such as DC offsets, I/Q
mismatch, even-order distortion, flicker noise, and LO (local
oscillator) leakage (from the LO port to the RF port) that can
reduce the dynamic range of digital radio systems that employ them.
LO leakage reflecting back into modulator/demodulator result in DC
offsets via self-mixing. These static DC offsets can be corrected
using DSP (digital signal processing) techniques at the expense of
decreased dynamic range.
[0006] In systems using orthogonal frequency division multiplexing
(OFDM), the sub-carriers near DC are often left unused such that it
is more tolerant of DC offsets. Unlike OFDM single carrier
modulation is susceptible to DC offsets. However, single-carrier
modulation is preferred in high frequency wide IBW systems over
OFDM due to lower peak-to-average power ratio which decreases the
linearity requirement on power amplifiers leading to reduced power
consumption and higher efficiency. Power amplifier linearity and
efficiency are especially important at millimeter wave frequencies
where the available spectrum exists to support communications links
with large IBW, but low cost CMOS power amplifiers have difficulty
providing the required transmit power. In addition to static DC
offsets, LO leakage that radiates off the receiver antenna and
reflects off an external obstruction which is moving relative to
the receiver can cause time-varying DC offsets. Finally, LO leakage
can be amplified by the transmitter's power amplifier and make it
difficult for receiver to demodulate. Thus, radio standards such as
802.1 lad specify that the power at the carrier frequency must be
significantly less than total signal power.
[0007] Most present day solutions employ multiple mixers and LO
distribution networks with proper phase relationships in order to
decrease LO leakage and the DC offsets resulting from self-mixing.
In some systems, four mixers are employed and are connected using
four-phase quadrature LO generation (0.degree., 90.degree.,
180.degree., and 270.degree.) and an in-phase RF divider/combiner.
Some of these systems feature a single node that ties the RF ports
of the four mixers together, which is suitable for integrated
circuits. Some systems use a suitable in-phase combining network
which is more appropriate for printed circuit boards where longer
lengths between mixers are required. In both cases, LO signals
which leak to the RF ports of the mixers due to the non-infinite
LO-to-RF isolation are summed at the RF port of the modulator or
demodulator. Specifically, the 0.degree. and 180.degree. LO leakage
signals cancel each other, and the 90.degree. and 270.degree. LO
leakage signals cancel each other.
[0008] Other corresponding issues related to the prior art will
become apparent to one skilled in the art after comparing such
prior art with the present invention as described herein.
SUMMARY OF THE INVENTION
[0009] Various embodiments of an improved I/Q modulator/demodulator
(IQMD) feature a modulator and demodulator topology which utilizes
a two-phase quadrature LO (local oscillator) signal generation
process for providing 0.degree. LO signals and 90.degree. LO
signals, and further feature an anti-phase combiner/divider
(0.degree. and 180.degree.) on the RF (radio frequency) port. The
IQMD may include mixers (in some embodiments, double-balanced
passive mixers) that function as downconverters when a signal is
incident on their RF ports or they may function as upconverters
when signals are incident on their IF (intermediate frequency)
ports. Thus, the IQMD may equally function as an I/Q modulator,
when digital-to-analog converters (DAC) are connected to the
differential I and Q ports, and as an I/Q demodulator when
analog-to-digital converters (ADC) are coupled to the differential
I and Q ports.
[0010] The IF port of the mixer (hereinafter also simply referenced
as "mixer") may be most often DC-coupled. Therefore, embodiments of
the IQMD may accommodate modulated signals with baseband frequency
content at 0 Hz. Another feature of the mixer may be a wide
bandwidth on its IF port, thus allowing the IQMD to handle signals
with very wide instantaneous bandwidth. As opposed to single-ended
I/Q modulators and demodulators, the differential baseband ports
are convenient for connecting to DACs and ADCs, as they usually
have differential analog outputs and inputs, respectively. The use
of differential signaling provides substantial benefits such as
cancellation of even-order intermodulation distortion, immunity to
far-field radiation, and common-mode rejection of interfering
signals. The LO and RF ports of the IQMD topology may remain
single-ended, making it convenient to interface with typical
high-frequency circuits.
[0011] In addition to the mixers, various embodiments of an IQMD
circuit may include several other functional blocks. In one set of
embodiments, the IQMD may include a first circuit (which may be a
180.degree. hybrid circuit 4-port circuit) that, when excited by an
RF signal at a first port (also referred to as a "delta" port), may
generate signals at a third port (also referred to as a "+" port)
and a fourth port (also referred to as a "-" port). The generated
signals may be power attenuated by a specified amount (e.g. 3 dB),
and may be out of phase (e.g. by 180.degree.) with respect to each
other. If the first circuit is instead excited by a signal at a
second port (also referred to as a "sum" port), the generated
signals at the third ("+") port and fourth ("-") port may each be
attenuated by a specified amount (e.g. 3 dB), and may be in phase,
or 0.degree. degrees out of phase. If the first circuit is excited
at the third ("+") and fourth ("-") ports with two signals that
have the same phase and equal magnitude, then the signals may
appear at the second ("sum") port because they are added, and no
signal may appear at the first ("delta") port because they are
subtracted. Likewise, if the first circuit is excited at the third
("+") and fourth ("-") ports with two signals that are 180.degree.
out of phase and have the same magnitude, then the signal may be
added at the first ("delta") port and no signal may appear at the
second ("sum") port.
[0012] At the LO port input, the IQMD may include a 0.degree. power
divider. When excited at the LO port, the power divider may output
two signals which are each attenuated by a specified amount (e.g. 3
dB) and have the same phase. It should be noted that the power
divider circuit may also function as a power combiner for signals
that are in-phase and are of equal magnitude. A respective second
circuit, which may be a 90.degree. hybrid circuit, may be connected
to the oscillator port of each mixer. The second circuit may be a
3-port circuit which, when excited by a signal at its input port,
may generate two respective output signals that are 90.degree.
out-of-phase. In some embodiments, the second circuit may be 4-port
hybrid circuit that also includes an isolation port terminated in a
resistive load.
[0013] The IQMD circuit may operate as either a modulator or
demodulator. When combined with analog-to-digital converters (ADCs)
on the respective differential "I" and "Q" ports (i.e. on the
differential pair of I ports and the differential pair of Q ports),
the IQMD may function and operate as an I/Q demodulator. In this
mode of operation, a signal may be provided to the RF port and may
be split into two signals of opposite polarity on the third and
fourth ports of the first circuit. The split signals may be further
split, in phase, and each phase-split signal may be provided into
the input of a respective corresponding one of the mixers. The LO
signal may be provided to an in-phase power divider and the
power-divided LO signals may be provided to respective second
circuits that drive each mixer with two sets of signals that are
separated by 0 or 90 degrees. When the RF and LO signals combine in
the mixers, the respective baseband signals at the I port and
.about.I (i.e. "not I") port have the same magnitude while being
180.degree. out of phase with respect to each other, that is, the
amplitude of the signal at the I port is commensurate with the
amplitude of the signal at the .about.I port, while there is a
180.degree. phase difference between the signal at the I port and
the signal at the .about.I port. The respective baseband signals at
the Q port and .about.Q (i.e. "not Q") port also have the same
magnitude and are 180.degree. out of phase with respect to each
other, that is, the amplitude of the signal at the Q port is
commensurate with the amplitude of the signal at the .about.Q port,
while there is a 180.degree. phase difference between the signal at
the Q port and the signal at the .about.Q port. Furthermore, there
may be a phase difference of 90.degree. between I and Q (and
.about.I and .about.Q), respectively. With these relative phase
shifts and appropriate digital signal processing, digitally
modulated signals may be demodulated.
[0014] When the IQMD circuit is operated as an I/Q modulator, the
incident baseband signals may be provided on the differential "I"
and "Q" ports and the RF port may be used as the output port. The
operation of the circuit is similar as described above with respect
to operating the IQMD as a demodulator, except that the signal is
flowing in the opposite direction. It should be noted that the
mixers may not have infinite isolation between their respective LO
ports and RF ports, which may result in the LO signal leaking into
the RF path. In the case of an I/Q modulator, the LO signal may
fall at the center of the instantaneous RF bandwidth. If the LO
signal is too large, it may prevent the corresponding I/Q
demodulator at the other end of the wireless link from demodulating
the signal. In various embodiments of the IQMD circuit, the
respective LO signals that leak from the respective LO ports of the
mixers associated with the pair of differential in-phase ports to
the third and fourth ports of the first circuit, respectively, both
have the same relative phase. Thus, as these two signals enter the
first circuit, they are dissipated into the load connected at the
second port (or "sum" port) of the first circuit instead of
emanating out of the first port (or "delta" port). The same is true
for signals which leak from the respective LO ports of the mixers
(associated with the pair of differential quadrature ports) to the
third and fourth ports of the first circuit, respectively. Hence,
the LO-to-RF isolation of the I/Q modulator may approximately equal
to, or be within a specified percentage of the sum (in dB) of the
LO-RF isolation of each mixer and the isolation of the first
circuit.
[0015] Other aspects of the present invention will become apparent
with reference to the drawings and detailed description of the
drawings that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] A better understanding of the present invention can be
obtained when the following detailed description of the preferred
embodiment is considered in conjunction with the following
drawings, in which:
[0017] FIG. 1 shows a circuit diagram of first exemplary I/Q
modulator/demodulator, according to some embodiments;
[0018] FIG. 2 shows a circuit diagram of a second exemplary I/Q
modulator/demodulator, according to some embodiments;
[0019] FIG. 3 shows a partial circuit diagram of an exemplary I/Q
modulator/demodulator that uses a balun, according to some
embodiments;
[0020] FIG. 4 shows a partial circuit diagram of an exemplary
demodulator configuration of the baseband ports for an I/Q
modulator/demodulator, according to some embodiments;
[0021] FIG. 5 shows a circuit diagram of an exemplary I/Q
modulator/demodulator circuit, according to some embodiments;
and
[0022] FIG. 6 shows a partial circuit diagram of an exemplary
transceiver that uses multiple instances of I/Q
modulators/demodulators, according to some embodiments.
[0023] While the invention is susceptible to various modifications
and alternative forms, specific embodiments thereof are shown by
way of example in the drawings and are herein described in detail.
It should be understood, however, that the drawings and detailed
description thereto are not intended to limit the invention to the
particular form disclosed, but on the contrary, the intention is to
cover all modifications, equivalents and alternatives falling
within the spirit and scope of the present invention as defined by
the appended claims.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0024] Embodiments of an I/Q modulator/demodulator (IQMD) circuit
described herein may be used in a variety of systems and devices
that use I/Q modulation/demodulation. Such devices and systems
include systems designed to perform test and/or measurement
functions, to control and/or model instrumentation or industrial
automation hardware, or to model and simulate functions, e.g.,
modeling or simulating a device or product being developed or
tested, etc. Embodiments of the IQMD circuit may also be included
in various radio frequency (RF) devices such as wireless
communication devices (e.g. cellular phones, tablets, wearable
devices such as smart watch and smart glasses, etc.). More
specifically, embodiments of the disclosed IQMD may be used in
various instances where modulation/demodulation, more specifically
I/Q modulation/demodulation of signals is performed. However, it is
noted that various embodiments may be used for a variety of
applications, and such applications are not intended to be limited
to those enumerated above. In other words, applications discussed
in the present description are exemplary only, and various
embodiments of I/Q modulator/demodulator circuits with differential
baseband may be used in any of various types of systems.
[0025] FIG. 1 shows a circuit diagram of an exemplary I/Q
modulator/demodulator (IQMD) 300 according to some embodiments.
IQMD 300 may feature a modulator and demodulator topology that
utilizes two-phase quadrature LO generation, using a local
oscillator (LO) input (19) and circuits 306 and 312 for generating
0.degree. and 90.degree. phase LO signals. The IQMD 300 may further
include an anti-phase combiner/divider circuit 302 for combining
out-of-phase signals present on ports 3 and 4 onto port 1, and
terminating in-phase signals at port 3 and 4 onto the load
connected to port 2. The IQMD 300 may also include mixers 304, 308,
310 and 314 that may function as downconverters when a signal is
provided at their respective RF ports (5-8), and may also function
as upconverters when signals are provided at their IF (intermediate
frequency) ports (15-18). The IQMD may thereby function as an I/Q
modulator by coupling digital-to-analog converters (DAC) to the
differential I and Q ports (15-18), or it may function as an I/Q
demodulator by coupling analog-to-digital converters (ADC) to the
differential I and Q ports (15-18). The differential I ports (15,
17) are also referred to herein as differential in-phase ports, and
differential Q ports (16, 18) are also referred to herein as
differential quadrature ports.
[0026] The respective IF ports (15-18) of mixers 304, 308, 310 and
314 may be DC-coupled or they may be AC coupled, and various
embodiments of the IQMD 300 may accommodate modulation formats with
no information centered at 0 Hz. Furthermore, the respective IF
ports (15-18) of mixers 304, 308, 310 and 314 may be wide bandwidth
ports, allowing the IQMD 300 to handle signals with very wide
instantaneous bandwidth. In contrast to single-ended I/Q modulators
and demodulators, differential baseband ports may be used to
connect to DACs and ADCs, as DACs usually have differential analog
outputs and ADCs usually have differential analog inputs. The use
of differential signaling may facilitate cancellation of even-order
intermodulation distortion, immunity to far-field radiation, and
common-mode rejection of interfering signals. The LO port (19) and
RF port (1) of IQMD 300 may remain single-ended, allowing IQMD 300
to conveniently interface with high-frequency circuits that are
most often single-ended.
[0027] In addition to the mixers 304, 308, 310 and 314, embodiments
of IQMD circuit 300 may include several other functional blocks as
shown in FIG. 1. For example, IQMD 300 may include a first circuit
302 (in some embodiments, a 180 degree or 180.degree. hybrid
circuit), which may feature four ports. When excited by an RF
signal at a first port (also referred to as a "delta" port, and
indicated by the numeral `1`), first circuit may output signals at
a third port (also referred to as a "+" port, and indicated by the
numeral `3`) and a fourth port (also referred to as a "-" port, and
indicated by numeral `4`). The output signals at ports 3 and 4 may
be power attenuated by a specified amount (e.g. 3 dB), and may be
out of phase with respect to each other, e.g. by 180.degree.. If
the first circuit 302 is instead excited by a signal at a second
port (also referred to as a "sum" port, and indicated by numeral
`2`), the output signals at the third ("+") port and fourth ("-")
port may each be attenuated by a specified amount (e.g. 3 dB), and
may be in phase, or 0.degree. degrees out of phase. It should be
noted, however, that as shown in FIG. 1 (and also FIG. 2), the port
2 is terminated, and is therefore not configured to be functionally
excited by a signal as opposed to port 1. If the first circuit is
excited at the "+" (3) and "-" (4) ports with two signals of the
same phase and of equal magnitude, then the sum of those two
signals may therefore appear at the "sum" (2) port, and no signal
may appear at the "delta" (1) port, resulting from the difference
of the two signals trending to zero. Similarly, if the first
circuit 302 is excited at the "+" (3) port and the "-" (4) port
with two signals, respectively, that are 180.degree. out of phase
with respect to each other and have the same magnitude, then the
two signals may be added at the "delta" (1) port and no signal may
appear at the "sum" (2) port.
[0028] The IQMD 300 may also include a 0.degree. power divider 316
at the LO port input (19). When excited at the LO port (19), the
power divider may output two signals at ports 13 and 14. The
signals output by power divider 316 may each be attenuated by a
specified amount (e.g. 3 dB) and may both have (or they both may be
of) the same phase. It should be noted that the power divider
circuit 316 may also function as a power combiner for signals that
are in-phase and are of equal magnitude, though circuit 316, as
shown in the figures is not used in that manner. Respective second
circuits 306 and 312 (in some embodiments, 90.degree. hybrid
circuits also having an isolation port--not shown--terminated in a
resistive load) may be connected to corresponding oscillator ports
of each mixer circuit 304, 308, 310 and 314 as shown. Each second
circuit (306 and 312) may have three ports. It should be noted,
however, that the second circuits (306 and 312) may be 90.degree.
hybrid circuits that may have a fourth port, specifically an
isolation port, which may be terminated with a resistive load for
the purposes of operation disclosed herein. When excited by a
signal at its respective input port (input port 13 for second
circuit 306, and input port 14 for second circuit 312), each second
circuit (306 and 312) may generate two respective output signals
(output signals 9 and 10 for second circuit 306, and 112 and output
signals 12 for second circuit 312) that are 90.degree. out-of-phase
with respect to each other. The IQMD circuit 300 may operate as
either a modulator or demodulator.
Operation of Various Embodiments of IQMD Circuits
[0029] Referring to at least FIGS. 1 and 2, the operation of
various embodiments of an IQMD circuit may be described as follows.
A modulator/demodulator (e.g. IQMD 300 or 350) may include an RF
port (e.g. RF port), and may further include a first circuit (e.g.
circuit 302) having a first port (e.g. port 1) coupled to the RF
port, and further having a second port (e.g. port 3) and a third
port (e.g. port 4). The first circuit may generate a pair of
out-of-phase input signals at the second port and third port,
responsive to the first port being excited by a first signal
received at the RF port, and may further generate a summed output
signal at the first port, responsive to the second port and the
third port being excited by a pair of out-of-phase output signals.
The modulator/demodulator may also include a first mixer (e.g. a
mixer that includes mixer circuits 304 and 306) coupled to the
second port. The first mixer may downconvert a first out-of-phase
input signal of the pair of out-of-phase input signals to a first
pair of incoming baseband signals (e.g. baseband signals received
at ports 15 and 16), and upconvert a first pair of outgoing
baseband signals (e.g. baseband signals provided at ports 15 and
16) to a first out-of-phase output signal of the pair of
out-of-phase output signals. The modulator/demodulator may further
include a second mixer (e.g. a mixer that includes mixer circuits
310 and 314) coupled to the third port. The second mixer may
downconvert a second out-of-phase input signal of the pair of
out-of-phase input signals to a second pair of incoming baseband
signals (e.g. baseband signals received at ports 17 and 18), and
upconvert a second pair of outgoing baseband signals (e.g. baseband
signals provided at ports 17 and 18) to a second out-of-phase
output signal of the pair of out-of-phase output signals.
[0030] The modulator/demodulator may also include a local
oscillator (LO) port (e.g. port 19) to receive an LO signal, and
the first mixer and the second mixer may perform upconversion
and/or downconversion based on the LO signal. The
modulator/demodulator may also include a zero-phase power splitter
(e.g. power splitter 316) coupled to the LO port to receive the LO
signal and derive a first LO signal (e.g. LO signal provided at
port 13) and a second LO signal (e.g. LO signal provided at port
14) from the LO signal, where the first LO signal and the second LO
signal are in phase with respect to each other. The first mixer may
perform upconversion and/or downconversion based on the first LO
signal, and the second mixer may perform upconversion and/or
downconversion based on the second LO signal. In some embodiments,
the modulator/demodulator may include a phase shifter (e.g. phase
shifter 362) coupled to the LO port to receive the LO signal and
generate an in-phase LO signal (e.g. the LO signal provided at port
20) and a quadrature LO signal (e.g. the LO provided at port 21)
based on the LO signal. The first mixer and the second mixer may
each perform upconversion and/or downconversion based on the
in-phase LO signal and the quadrature LO signal.
[0031] In further embodiments, the modulator/demodulator may also
include a first zero-phase power splitter (e.g. power splitter 356)
and a second zero-phase power splitter (e.g. power splitter 364).
The first zero-phase power splitter may receive the in-phase LO
signal and may derive a first in-phase LO signal and a second
in-phase LO signal from the in-phase LO signal. The second
zero-phase power splitter may receive the quadrature LO signal and
derive a first quadrature LO signal and a second quadrature LO
signal from the quadrature LO signal. The first mixer may then
perform upconversion and downconversion using the first in-phase LO
signal and the first quadrature LO signal, and the second mixer may
perform upconversion and downconversion using the second in-phase
LO signal and the second quadrature LO signal.
[0032] In some embodiments, the first circuit may be a hybrid
circuit that further includes a fourth port (e.g. port 2), and may
generate a second summed output signal at the first port,
responsive to the second port and the third port being excited by a
pair of in-phase output signals. It should be noted that in the
various embodiments explicitly disclosed herein, port 2 is shown as
being terminated with a resistor to ground. In some embodiments the
first circuit may be a balun circuit (e.g. balun circuit 402), with
the first port of the first circuit being an unbalanced port of the
balun circuit, and the second and third ports of the first circuit
being balanced ports of the balun circuit.
[0033] In some embodiments, the first pair of incoming baseband
signals may include a first incoming in-phase baseband signal and a
first incoming quadrature baseband signal, and the first pair of
outgoing baseband signals may include a first outgoing in-phase
baseband signal and a first outgoing quadrature baseband signal.
Furthermore, the second pair of incoming baseband signals may
include a second incoming in-phase baseband signal and a second
incoming quadrature baseband signal, and the second pair of
outgoing baseband signals may include a second outgoing in-phase
baseband signal and a second outgoing quadrature baseband signal.
The first incoming in-phase baseband signal and the second incoming
in-phase baseband signal may form an incoming pair of differential
in-phase baseband signals (e.g. signals I and .about.I), and the
first incoming quadrature baseband signal and the second incoming
quadrature baseband signal may form an incoming pair of
differential quadrature baseband signals (e.g. Q and .about.Q). In
addition, the first outgoing in-phase baseband signal and the
second outgoing in-phase baseband signal may form an outgoing pair
of differential in-phase baseband signals (e.g. I and .about.I),
and the first outgoing quadrature baseband signal and the second
outgoing quadrature baseband signal may form an outgoing pair of
differential quadrature baseband signals (e.g. Q and .about.Q).
[0034] In some embodiments, each mixer may include a first mixer
circuit (e.g. mixer circuit 304 or mixer circuit 310) which may
receive an in-phase baseband signal when upconverting, and provide
an in-phase baseband signal when downconverting, and may also
include a second mixer circuit (e.g. mixer circuit 308 or mixer
circuit 314) which may receive a quadrature baseband signal when
upconverting, and provide a quadrature baseband signal when
downconverting. In some embodiments, the first mixer circuit and
the second mixer circuit may each be double-balanced passive mixer
circuits. Furthermore, in one set of embodiments, each mixer may
include a phase shifter circuit (e.g. phase shifter circuit 306 or
phase shifter circuit 312) which may generate an in-phase
oscillator signal and a quadrature oscillator signal based on a
received local oscillator signal, provide the in-phase oscillator
signal to the first mixer circuit and provide the quadrature
oscillator signal to the second mixer circuit. The first mixer
circuit may then perform modulation and demodulation using the
in-phase oscillator signal, while the second mixer circuit may
perform modulation and demodulation using the in-phase oscillator
signal.
Demodulator Operation
[0035] Referring again to FIG. 1 (and/or FIG. 2), when combined
with analog-to-digital converters (ADCs) on the respective pair of
differential "I" ports (15, 17) and respective pair of differential
"Q" ports (16, 18), IQMD 300 may function and operate as an I/Q
demodulator. FIG. 4 shows a partial circuit diagram of an exemplary
demodulator configuration of the baseband ports (I, .about.I, Q,
and .about.Q) for IQMD 300, according to some embodiments. As seen
in FIG. 4, ADC 512 is coupled to the pair of differential in-phase
ports (15, 17), while ADC 514 is coupled to the pair of
differential quadrature ports (16, 18) to provide a demodulated
digital baseband signal at the output of summing circuit 518.
[0036] Referring again to FIG. 1, a signal may be provided to the
RF port (1) and may be split into two signals of opposite polarity
on nodes 3 and 4. The split signals may be further split, in phase,
and each phase-split signal (at ports 3 and 4, respectively) may be
provided into the input of a respective corresponding one of the
mixers 304, 306, 310, 314. The LO signal (at node 19) may be
provided to the in-phase power divider 316, and the power-divided
LO signals may drive each mixer (304, 306, 310, 314) with two sets
of signals that are separated by 0.degree. or 90.degree.. When the
RF and LO signals combine in the mixers 304, 306, 310, 314, the
respective baseband signals at the pair of differential in-phase
ports (15, 17) have the same magnitude and are 180.degree. out of
phase with respect to each other, that is, the amplitude of the
signal at the I port (15) is commensurate with the amplitude of the
signal at the .about.I port (17), while there is a 180.degree.
phase difference between the two signals. The respective baseband
signals at the pair of differential quadrature ports (16, 18) also
have the same magnitude and are 180.degree. out of phase with
respect to each other, that is, the amplitude of the signal at the
Q port is commensurate with the amplitude of the signal at the
.about.Q port, while there is a 180.degree. phase difference
between the two signals. Furthermore, there is a phase difference
of 90.degree. between I and Q (and .about.I and .about.Q),
respectively.
Modulator Operation
[0037] When the IQMD 300 circuit is operated as an I/Q modulator,
the incident baseband signals may be provided on the pair of
differential "I" ports (15, 17) and on the pair of differential "Q"
ports (16, 18). In this case the RF port (1) may be used as the
output port. The operation of the circuit is similar to the
demodulator operation described above, with the difference that the
signal is flowing in the opposite direction, that is, from the two
pairs of differential signal ports (15-18) to the RF port (1). It
should be noted that mixers 304, 306, 312, 314 may lack infinite
isolation between their respective LO ports (9-12) and RF ports
(5-8), which may result in the LO signal leaking into the RF path.
In the case of an I/Q modulator, the LO signal may fall at the
center of the instantaneous RF bandwidth. If the LO signal is too
large, it may prevent a corresponding I/Q demodulator at the other
end of the wireless link from demodulating the signal.
[0038] It is worth noting that when used as an I/Q modulator,
embodiments of IQMD circuit 300 (and 350) may operate to also
reject common-mode signals that are present on the baseband
differential pairs (I/.about.I and Q/.about.Q). For example, if a
common-mode signal is present on the differential pair Q/.about.Q
and that common-mode signal is upconverted, then it may appear at
the RF ports of each I/Q mixer with the same amplitude and phase
and may thus be combined and dissipated in the 50.OMEGA.
termination on the sum port (i.e. the resistor coupled at port 2)
of the first circuit 302.
Additional Considerations
[0039] It should further be noted that in various embodiments of
IQMD circuit 300, the respective LO signal that may leak from node
9 (via RF port 5) at mixer 304 to node 3 of circuit 302, and the
respective LO signal that may leak from node 11 (via RF port 7) at
mixer 310 to node 4 of circuit 310, both have the same relative
phase. Thus, as these two signals appear at the first circuit (at
ports 3 and 4), they may be dissipated into the load connected at
terminal (or node) 2 of the first circuit 302 instead of emanating
out of RF port 1. The same is true for the respective LO signal
that may leak from node 10 at mixer 308 (via RF port 6) to node 3
of circuit 302, and the respective LO signal that may leak from
node 12 (via RF port 8) at mixer 314 to node 4 of circuit 302.
Hence, the LO-to-RF isolation of the IQMD circuit 300 may
approximately equal to, or be within a specified percentage of the
sum (in dB) of the LO-to-RF isolation of each mixer 304, 306, 312,
314 and the isolation of the first circuit 302.
Alternate Embodiments
[0040] FIG. 2 shows a circuit diagram of an alternate
implementation 350 of an I/Q modulator/demodulator, according to
some embodiments. The port numbers in FIG. 2 correspond to the port
numbers in FIG. 1 for each port number that is included in both
figures. As shown in FIG. 2, as opposed to including two phase
shifter circuits (such as circuits 306 and 312 in FIG. 1), circuit
350 includes a single phase shifter circuit 362, with its input
connected to LO port 19 to receive the local oscillator signal. The
circuit 362 may be excited by the LO signal, and responsive to the
LO signal, generate two respective output signals that are
90.degree. out-of-phase with respect to each other. In other words,
phase shifter circuit 362 may generate an in-phase LO signal at
port 20, and a quadrature LO signal at port 21. Thus, the in-phase
oscillator signal may be provided to a first 0.degree. power
divider 356 at port 20, and the quadrature oscillator signal may be
provided to a second 0.degree. power divider 364 at port 21. When
excited at their respective input ports (20, 21), the power
dividers may output respective pairs of signals at ports 9 and 11,
and 10 and 12. Each respective pair of signals output by the power
dividers (356 and 364) includes two respective signals that may be
attenuated by a specified amount (e.g. 3 dB) and may be in phase
with respect to each other. For example, the pair of signals output
by power divider 356 at ports 9 and 11 may be in phase with respect
to each other, and/or they may each be attenuated by a specified
amount. The same holds for power divider 364.
[0041] In this manner, rather than splitting the LO signal into two
LO signals, and using two respective phase shifter circuits for
phase shifting each of the two LO signals to generate a respective
in-phase LO signal and a respective quadrature LO signal for each
pair of I/Q signals (as performed in circuit 300), in embodiments
exemplified in FIG. 2, a single phase shifter circuit 362 is used
to generate an in-phase LO signal and a quadrature LO signal, which
are then each divided/split to provide the appropriate respective
in-phase LO signals and respective quadrature LO signals for the
respective mixers. For example, the in-phase LO signal provided by
circuit 362 may be divided/split by divider circuit 356 to provide
in-phase LO signals to mixers 304 and 310 corresponding to the pair
of differential in-phase baseband signals. Similarly, the
quadrature LO signal provided by circuit 362 at port 21 may be
divided/split by divider circuit 364 to provide quadrature LO
signals to mixers 308 and 314 corresponding to the pair of
differential quadrature baseband signals.
[0042] Thus, in some embodiments the LO signal may first be
divided/split first into a first LO signal and a second LO signal,
and respective pairs of in-phase LO signals and quadrature LO
signals may be generated from the first LO signal and the second LO
signal, respectively, to provide the appropriate LO signals to the
respective mixers. In other embodiments, an in-phase LO signal and
a quadrature LO signal may first be generated from the LO signal,
and the in-phase LO signal and quadrature LO signal may each be
divided/split into respective first and second in-phase LO signals
and first and second quadrature LO signals to be provided to the
appropriate respective mixers.
Exemplary IQMD Circuit According to One Set of Embodiments
[0043] FIG. 5 shows the circuit diagram of an exemplary
implementation of the proposed I/Q demodulator according to some
embodiments. The RF node (1) is shown on the left, the LO (carrier)
input (19) is shown on the right, and the pairs of differential
in-phase and quadrature baseband signals are shown at ports 15-18,
respectively. Again, the port numbers in FIG. 5 correspond to the
port numbers in FIG. 1 for each port number that is included in
both figures. As previously mentioned, circuit 600 may operate as a
modulator by exciting the baseband inputs and making the RF port
(1) the output, or it may operate as a demodulator when the RF port
(1) is used as an input and the baseband ports (15-18) are used as
outputs. The circuit 600 includes four sub-circuits. The first
sub-circuit is a 180.degree. hybrid 502, the second sub-circuit is
a first I/Q mixer 604 (including phase splitter circuit 306 and
mixer circuits 304 and 308), the third cub-circuit is a second I/Q
mixer 606 (including phase splitter circuit 312 and mixer circuits
310 and 314), and the fourth sub-circuit is an in-phase power
divider 516. In the exemplary circuit shown in FIG. 5, the hybrid
circuit 502 is implemented as a microstrip three-section rat-race
180.degree. hybrid circuit. The sum port (2) of the hybrid circuit
is terminated with R1, which may be practically formed by the
parallel combination of two surface mount thin-film resistors (e.g.
two 0201 100.OMEGA. surface mount thin-film resistors). The hybrid
circuit 502 may be fabricated using microstrips (traces/lines) A-G,
with a specified substrate height and dielectric constant (e.g.
substrate height of 10 mil and a relative dielectric constant of
3.66). The in-phase power divider 516 may be implemented as
two-section Wilkinson combiner. The I/Q mixers may be surface mount
GaAs MMIC double-balanced mixers operating over a specified
frequency range (e.g. a frequency range of 8.5 GHz to 13.5 GHz. In
some embodiments, the hybrid circuit 502 may be shrunk using
fractal techniques.
Design Advantages
[0044] Various embodiments of the IQMD design disclosed herein
provide several advantages in addition to the ability to function
as both a modulator and/or a demodulator. High-speed analog
baseband circuits are commonly constructed using differential
methodologies. Differential circuits offer advantages such as
immunity to common-mode noise and spurs, and even-order rejection.
Additionally, high-speed ADCs and DACs most often have differential
input and output interfaces. Simultaneously, RF and microwave
circuits predominantly use single-ended 50.OMEGA. interfaces.
Embodiments of the disclosed IQMD circuit conveniently accommodate
both. In addition to providing a single-ended interface on the RF
port, the 180 degree hybrid circuit significantly improves the
LO-to-RF isolation of the modulator/demodulator and decreases
self-mixing.
[0045] Referring again to the exemplary implementation shown in
FIG. 5, the LO signal may be applied to both I/Q mixers 604 and 606
with the same phase via the Wilkinson power divider 516. The LO
signals may leak from the LO ports to the RF ports of the I/Q
mixers, as indicated by dashed arrows K, L, M and N from the I/Q
mixers 604 and 606, respectively, towards hybrid circuit 502. The
amplitudes of the LO leakage signals (K, L, M, and N) may be
reduced by the LO-to-RF isolation of each respective I/Q mixer 604
and 606. The phase and amplitude of signals K and M will be
equivalent, as will the phase of signals L and N. Thus, the K and M
signals appear in-phase on ports 3 and 4 of the hybrid circuit 502,
and may be combined at port 2 and dissipated in R1 instead of
appearing at the RF port (1). Similarly, signals L and N may also
be dissipated in R1. Therefore, the overall LO-to-RF isolation of
IQMD 600 may be significantly improved.
[0046] The LO-RF isolation improvement may also be achieved by
replacing the first circuit 302 (or hybrid circuit 502) with a
balun, as shown in FIG. 3. Balun 402 is different from hybrid
circuit 502, as a balun has an even-mode reflection coefficient of
.+-.1 while a 180 degree hybrid circuit may typically have a
reflection coefficient of zero for even-mode excitation. Referring
back to FIG. 5, signals K and M, as well as signals L and N may be
considered even-mode signals which may be reflected back into the
I/Q mixers 604 and 606. It should be noted that the reflected LO
signals may re-mix in the I/Q mixers 606 and 606 with the LO signal
(a process referred to as self-mixing), and may create voltages at
DC and 2f.sub.LO (i.e. two times the LO frequency) on the baseband
ports 15-18.
[0047] In some embodiments, IQMD may be designed for a specified
tuning range. For example, in some embodiments, the IQMD circuit
may have a tuning range of approximately 8.5 to 13.5 GHz. The
tuning range may be limited by the bandwidth of the circuit 502,
the baluns in the I/Q mixers, and the LO power divider. In order to
extend the tuning range, these three components may be extended.
For example, in some embodiments, commercially available broadband
I/Q mixers with RF/LO ranges exceeding three octaves may be used.
In some embodiments, decade bandwidth 180.degree. hybrid circuits
may be obtained using asymmetric stepped or tapered backward-wave
directional couplers. For example, in order to realize practical
coupling profiles, two 8.34 dB couplers may be coupled in tandem.
In order to maintain equal even and odd mode phase velocity,
homogeneous stripline may be used. The couplers may be implemented
in inhomogeneous dielectric (such as microstrip) using wiggly or
serpentine techniques for equalizing the even and odd mode phase
velocities. In one set of embodiments, the LO power divider 316 may
be extended in frequency by using a multi-section stepped or
tapered Wilkinson power divider. In other embodiments, a second 180
degree hybrid may be used, and the sum port (3) may be driven with
the LO, while the delta port (1) is terminated. In addition, a
3-resistor power divider may also be used, which may benefit from a
smaller size but may dissipate more LO power (relative to reactive
power division) and may provide less isolation.
Benefits
[0048] I/Q modulators and demodulators with wide instantaneous
bandwidth are key circuit elements that enable digital radio
systems with data rates above 1 Gbps (1 gigabits per second) in the
millimeter-wave spectrum. Various embodiments of the IQMD topology
disclosed herein features high LO-to-RF isolation, low DC offsets,
and convenient interfaces for LO, RF, and baseband circuitry. In
various embodiments, the IQMD may be realized using commercial
off-the-shelf technology, and may be widely used in numerous
applications, exemplified by but not limited to millimeter-wave
digital radio systems.
Exemplary Transceiver Incorporating IQMD Circuits
[0049] FIG. 6 shows a partial circuit diagram of an exemplary
transceiver that includes I/Q modulators/demodulators, according to
some embodiments. Instances of the IQMD circuit, such as those
shown in FIGS. 1, 2, and 5, for example, and may be incorporated
into a transceiver system capable of delivering a high encoded data
rate at a specified carrier frequency, for example a data rate of
10.2 Gbps at a carrier frequency of 74 GHz. The transceiver circuit
in FIG. 6 may include a transmitter sub-circuit 620 and a receiver
sub-circuit 630. In transmitter sub-circuit 620, the baseband data
may be modulated up to a specified first frequency (e.g. 12 GHz)
using I/Q modulators 652 and 654, both of which may be instances of
the IQMD detailed, for example, in FIG. 1, and operated as
modulators (as previously detailed above). A super-heterodyne stage
662 may be used to upconvert the signal to a specified second
frequency (e.g. up to 74 GHz). Similarly, in the receiver
sub-circuit 630, the modulated signal may be downconverted from the
specified second frequency (e.g. from 74 GHz) to an IF frequency
(e.g. 12 GHz) through conversion stage 664, and then converted to
baseband using I/Q demodulators 656 and 658, both of which may be
instances of the IQMD detailed, for example, in FIG. 1, and
operated as demodulators (as previously detailed above).
[0050] As shown in FIG. 6, there are bandpass filters coupled to
the respective RF and LO ports of the I/Q modulators 652 and 654
and I/Q demodulators 656 and 658. One reason for filtering the LO
signal is to eliminate high harmonic levels which have been shown
to imbalance I/Q modulators. One reason for filtering the RF signal
is to protect the I/Q demodulators (656 and 658) from the
higher-order images centered at integer multiples of the carrier
frequency, and to prevent the I/Q modulators (652 and 654) from
transmitting harmonics of the modulated signal. Another reason for
the bandpass filters is to prevent unwanted signals (spurs) and
noise at baseband frequencies from entering the respective RF ports
of the I/Q demodulators (656 and 658), travelling through the mixer
to the IF port, and entering the baseband. This may be especially
true if the excess noise ratio at the respective RF inputs of the
I/Q demodulators (656 and 658) is high at baseband frequencies
(e.g. due to high gain and/or high noise affecting the signals
prior to entering the I/Q demodulators), or if the RF-to-IF
isolation of the I/Q demodulators is low.
[0051] As shown in FIG. 6, the transceiver circuit may use two data
streams simultaneously at the same carrier frequency by utilizing
both horizontal and vertical antenna polarization and 2.times.2
MIMO (multiple-input-multiple-output) processing. In some
embodiments, the specified encoded data rate (e.g. 10.2 Gbps) may
be achieved using a single-carrier 16-QAM (quadrature amplitude
modulation) with RRC filtering (.alpha.=0.3), 7/8 encoding, and a
symbol rate of 1536 MBd. The DACs shown in FIG. 6 may implemented
as 14-bit DACs while the ADCs shown in FIG. 6 may be implemented as
12-bit ADCs, and they may each be clocked at a sample rate of 3072
MHz (2.times.oversampling). The DC-offsets and I/Q mismatch may be
improved using single-point corrections. It should be noted that
the numeric values provided above as well as those provided in FIG.
6 are for illustration purposes only, and various embodiments may
use different appropriate values as required for any given
implementation.
[0052] Although the embodiments above have been described in
considerable detail, numerous variations and modifications will
become apparent to those skilled in the art once the above
disclosure is fully appreciated. It is intended that the following
claims be interpreted to embrace all such variations and
modifications.
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