U.S. patent application number 15/086764 was filed with the patent office on 2016-07-21 for low phase noise voltage controlled oscillators.
This patent application is currently assigned to Broadcom Corporation. The applicant listed for this patent is Broadcom Corporation. Invention is credited to Ahmadreza Rofougaran, Maryam Rofougaran, Farid SHIRINFAR, Tirdad Sowlati.
Application Number | 20160211801 15/086764 |
Document ID | / |
Family ID | 50232683 |
Filed Date | 2016-07-21 |
United States Patent
Application |
20160211801 |
Kind Code |
A1 |
SHIRINFAR; Farid ; et
al. |
July 21, 2016 |
Low Phase Noise Voltage Controlled Oscillators
Abstract
A voltage controlled oscillator (VCO) with low phase noise and a
sharp output spectrum is desirable. The present disclosure provides
embodiments of LC tank VCOs that generate output signals with less
phase noise compared with conventional LC tank VCOs, while at the
same time limiting additional cost, size, and/or power. The
embodiments of the present disclosure can be used, for example, in
wired or wireless communication systems that require low-phase
noise oscillator signals for performing up-conversion and/or
down-conversion.
Inventors: |
SHIRINFAR; Farid; (Los
Angeles, CA) ; Sowlati; Tirdad; (Irvine, CA) ;
Rofougaran; Maryam; (Rancho Palos Verdes, CA) ;
Rofougaran; Ahmadreza; (Newport Coast, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Broadcom Corporation |
Irvine |
CA |
US |
|
|
Assignee: |
Broadcom Corporation
Irvine
CA
|
Family ID: |
50232683 |
Appl. No.: |
15/086764 |
Filed: |
March 31, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14561799 |
Dec 5, 2014 |
9312807 |
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15086764 |
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13610016 |
Sep 11, 2012 |
8933757 |
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14561799 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03B 5/1228 20130101;
H03B 5/1293 20130101; H03B 27/00 20130101; H03B 2200/0074 20130101;
H03B 1/04 20130101; H03B 2200/0052 20130101; H03B 5/1262 20130101;
H03B 5/1296 20130101; H03B 5/1852 20130101; H03B 2200/0048
20130101; H03B 5/1243 20130101; H03B 5/1215 20130101 |
International
Class: |
H03B 1/04 20060101
H03B001/04; H03B 5/12 20060101 H03B005/12 |
Claims
1. A multi-injection voltage controlled oscillator (VCO) for
providing a low phase noise oscillating signal at an output, the
multi-injection VCO comprising: a first inductor-capacitor tank VCO
configured to generate a first oscillating signal; a second
inductor-capacitor tank VCO configured to generate a second
oscillating signal; and an output coupler configured to
electrically couple the first oscillating signal to the output
using a first transmission line and to electrically couple the
second oscillating signal to the output using a second transmission
line.
2. The multi-injection VCO of claim 1, wherein the first
transmission line is used to form, at least in part, a resonator of
the first inductor-capacitor tank VCO, and the second transmission
line is used to form, at least in part, a resonator of the second
inductor-capacitor tank VCO.
3. The multi-injection VCO of claim 1, wherein the output coupler
is configured to provide both the first oscillating signal and the
second oscillating signal at the output substantially in-phase with
each other.
4. The multi-injection VCO of claim 1, wherein the output coupler
is configured to couple the first inductor-capacitor tank VCO to
the second inductor-capacitor tank VCO in series or in
parallel.
5. The multi-injection VCO of claim 1, wherein the first
transmission line has a length that is greater than 1/10 of a
wavelength of the first oscillating signal.
6. The multi-injection VCO of claim 1, wherein the first
transmission line comprises a ladder line, a stripline, a
dielectric slab, or a waveguide.
7. The multi-injection VCO of claim 1, wherein the output coupler
is configured to electrically couple the first oscillating signal
and the second oscillating signal to the output in parallel using,
the first transmission line and the second transmission line.
8. A multi-band, multi-injection voltage controlled oscillator
(VCO), for providing a low phase noise oscillating signal at an
output, the multi-band VCO comprising: a first cluster of
inductor-capacitor tank VCOs configured to generate a first
oscillating signal at a first frequency by electrically coupling
output oscillating signals generated by a first number of the
inductor-capacitor tank VCOs in the first cluster to the output
using a first set of transmission lines; and a second cluster of
inductor-capacitor tank VCOs configured to generate a second
oscillating signal at a second frequency different from the first
frequency by electrically coupling output oscillating signals
generated by a second number of the inductor-capacitor tank VCOs in
the second cluster to the output using a second set of transmission
lines.
9. The multi-band, multi-injection VCO of claim 8, wherein a first
transmission line of the first set of transmission lines is used to
form a resonator of a first inductor-capacitor tank VCO of the
first cluster of inductor-capacitor tank VCOs.
10. The multi-band, multi-injection VCO of claim 9, wherein the
first transmission line has a length that is greater than 1/10 of a
wavelength of the first oscillating signal.
11. The multi-band, multi-injection VCO of claim 9, wherein the
first transmission line comprises a ladder line, a stripline, a
dielectric slab, or a waveguide.
12. The multi-band, multi-injection VCO of claim 8, further
comprising: a controller configured to control the first cluster of
inductor-capacitor tank VCOs and the second cluster of
inductor-capacitor tank VCOs such that only one of the first
cluster inductor-capacitor tank VCOs and the second cluster of
inductor-capacitor tank VCOs is on.
13. The multi-band, multi-injection VCO of claim 8, wherein the
first number of the inductor-capacitor tank VCOs that are on in the
first cluster is adjustable.
14. The multi-band, multi-injection VCO of claim 8, wherein the
first number of the inductor-capacitor tank VCOs that are on in the
first cluster is adjustable based on a power requirement.
15. The multi-band, multi-injection VCO of claim 8, wherein the
first number of the inductor-capacitor tank VCOs that are on in the
first cluster is adjustable based on a phase noise requirement.
16. The multi-band, multi-injection VCO of claim 8, wherein the
first number of the inductor-capacitor tank VCOs in the first
cluster is adjustable based on a signal-to-noise ratio of a
signal.
17. .A multi-injection voltage controlled oscillator (VCO) for
providing a low phase noise oscillating signal at an output, the
multi-injection VCO comprising: a first VCO configured to generate
a first oscillating signal using a first tank circuit comprising a
parallel combination of a first inductor and a first capacitor; a
second VCO configured to generate a second oscillating signal using
a second tank circuit comprising a parallel combination of a second
inductor and a second capacitor; and an output coupler configured
to electrically couple the first oscillating signal to the output
using a first transmission line and to electrically couple the
second oscillating signal to the output using a second transmission
line, wherein the first transmission line is used to form, at least
in part, the first inductor, and the second transmission line is
used to form, at least in part, the second inductor.
18. The multi-injection VCO of claim 17, wherein the output coupler
is configured to provide both the first oscillating signal and the
second oscillating signal at the output substantially in-phase with
each other.
19. The multi-injection VCO of claim 17, wherein the output coupler
is configured to couple the first inductor-capacitor tank VCO to
the second inductor-capacitor tank VCO in series or in
parallel.
20. The multi-injection VCO of claim 17, wherein the first
transmission line has a length that is greater than 1/10 of a
wavelength of the first oscillating signal.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is a continuation of U.S. patent
application Ser. No. 14/561,799, filed Dec. 5, 2014, which is a
continuation of U.S. patent application Ser. No. 13/610,016, filed
Sep. 11, 2012, both of which is incorporated herein by reference in
its entirety.
FIELD OF THE INVENTION
[0002] This application relates generally to oscillators and more
specifically, to voltage controlled oscillators (VCOs).
BACKGROUND
[0003] An ideal voltage controlled oscillator (VCO) generates a
single-tone output signal at a frequency determined as a linear
function of an input control voltage. For such an ideal VCO, the
spectrum of the output signal assumes the shape of an impulse. In
practice, however, the output signal generated by a VCO includes
random fluctuations referred to as phase noise. Phase noise is seen
in the spectrum of the output signal as "skirting" around the
impulse. In communication systems that use a VCO output signal to
up-convert or down-convert a signal, this phase noise can corrupt
the resulting frequency-translated signal.
[0004] For example, in a received signal, a desired channel
centered at a frequency .omega..sub.0 can be spaced very close to a
strong, undesired channel centered at a frequency
.omega..sub.0-.DELTA.f. To down-convert the desired channel to
baseband, the VCO can be tuned to provide an output signal with a
frequency equal to the center frequency .omega..sub.0 of the
desired channel, and the two signals can be mixed. In the ideal
case, the VCO output signal consists of a single tone, with no
phase noise, at a frequency .omega..sub.0, and only the desired
channel is down-converted to baseband. In practice, however, the
VCO output signal includes phase noise around the single tone at
.omega..sub.0. This phase noise further mixes with the received
signal, which may result in the strong, undesired channel being
down-converted to baseband if the bandwidth of the phase noise is
larger than the distance separating the two channels (i.e., larger
than .DELTA.f).
[0005] Assuming the bandwidth of the phase noise is larger than the
distance separating the two channels, the undesired channel will
interfere with the desired channel at baseband in the
down-converted signal, reducing the sensitivity of the receiver.
The effect of phase noise is similar for up-conversion. Therefore,
for this reason and others, a VCO with low phase noise and a sharp
output spectrum is desirable.
BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
[0006] The accompanying drawings, which are incorporated herein and
form a part of the specification, illustrate the embodiments of the
present disclosure and, together with the description, further
serve to explain the principles of the embodiments and to enable a
person skilled in the pertinent art to make and use the
embodiments.
[0007] FIG. 1 illustrates a generic inductor-capacitor (LC) tank
VCO in accordance with embodiments of the present disclosure.
[0008] FIG. 2 illustrates a specific implementation of an LC tank
VCO in accordance with embodiments of the present disclosure.
[0009] FIG. 3 illustrates a multi-injection LC tank VCO with
parallel coupled VCOs in accordance with embodiments of the present
disclosure.
[0010] FIG. 4 illustrates a multi-injection LC tank VCO with series
coupled VCOs in accordance with embodiments of the present
disclosure.
[0011] FIG. 5 illustrates a layout of a transformer for use in a
multi-injection LC tank VCO in accordance with embodiments of the
present disclosure.
[0012] FIG. 6 illustrates another multi-injection LC tank VCO with
parallel coupled VCOs in accordance with embodiments of the present
disclosure.
[0013] FIG. 7 illustrates a compact, multi-band VCO in accordance
with embodiments of the present disclosure.
[0014] FIG. 8 illustrates a layout of a three winding transformer
for use in a compact, multi-band VCO in accordance with embodiments
of the present disclosure.
[0015] FIG. 9 illustrates a multi-band, multi-injection VCO in
accordance with embodiments of the present disclosure.
[0016] The embodiments of the present disclosure will be described
with reference to the accompanying drawings. The drawing in which
an element first appears is typically indicated by the leftmost
digit(s) in the corresponding reference number.
DETAILED DESCRIPTION
[0017] In the following description, numerous specific details are
set forth in order to provide a thorough understanding of the
embodiments of the present disclosure. However, it will be apparent
to those skilled in the art that the embodiments, including
structures, systems, and methods, may be practiced without these
specific details. The description and representation herein are the
common means used by those experienced or skilled in the art to
most effectively convey the substance of their work to others
skilled in the art. In other instances, well-known methods,
procedures, components, and circuitry have not been described in
detail to avoid unnecessarily obscuring aspects of the
invention.
[0018] References in the specification to "one embodiment," "an
embodiment," "an example embodiment" etc., indicate that the
embodiment described may include a particular feature, structure,
or characteristic, but every embodiment may not necessarily include
the particular feature, structure, or characteristic. Moreover,
such phrases are not necessarily referring to the same embodiment.
Further, when a particular feature, structure, or characteristic is
described in connection with an embodiment, it is submitted that it
is within the knowledge of one skilled in the art to affect such
feature. structure, or characteristic in connection with other
embodiments whether or not explicitly described.
1. Overview
[0019] A VCO with low phase noise and a sharp output spectrum is
desirable.
[0020] The present disclosure provides embodiments of
inductor-capacitor (LC) tank VCOs that generate output signals with
less phase noise compared with conventional LC tank VCOs, while at
the same time limiting additional cost, size, and/or power. The
embodiments of the present disclosure can be used, for example, in
wired or wireless communication systems that require low-phase
noise oscillator signals for performing up-conversion and/or
down-conversion. Before describing the specific embodiments of the
present disclosure, a general overview of LC tank VCOs and their
implementations is provided.
[0021] Referring now to FIG. 1, a generic LC tank VCO 100 is
illustrated. The generic LC tank VCO 100 generates an oscillating
output signal using a tank circuit 110 and an active circuit 120.
The tank circuit 110 includes an inductor 130 placed in parallel
with a capacitor 140. The parallel combination of inductor 130 and
capacitor 140 resonates at a frequency given approximately by:
.omega..sub.res=1 LC (1)
From equation (1), it becomes apparent that the resonant frequency
of tank circuit 110 can be tuned by adjusting either the inductance
(L) of inductor 130 or the capacitance (C) of capacitor 140. In
practice, the capacitance of capacitor 140 is typically made
adjustable via an input control signal. For example, capacitor 140
can be implemented as a programmable bank of capacitors and/or a
varactor that can be controlled by an input control signal to vary
its capacitance. The energy resonating within the tank circuit 110
is used to form the oscillating output signal of LC tank VCO
100.
[0022] At the resonant frequency defined by .omega..sub.res, the
impedance of inductor 130 and capacitor 140 are ideally equal and
opposite, yielding a theoretically infinite impedance. However,
practical inductors and capacitors, such as inductor 130 and
capacitor 140, have an associated series resistance. Tank circuit
110 models this series resistance of inductor 130 and capacitor 140
(as well as other sources of resistances in VCO 100) as a finite,
parallel resistance Rtank 150.
[0023] In order for tank circuit 110 to oscillate, the losses from
Rtank 150 need to be overcome. Therefore, active circuit 120 is
typical used to offset (i.e., cancel) the losses incurred by Rtank
150. Active circuit 120 includes a negative, active resistance,
-Ractive 160, that satisfies the following equation in order for
circuit 110 to oscillate:
1 R active .gtoreq. 1 R tank ( 2 ) ##EQU00001##
[0024] Referring now to FIG. 2, an exemplary implementation of a
differential LC tank VCO 200 is illustrated. As shown in FIG. 2.
differential LC tank VCO 200 includes a tank circuit 210 with an
inductor 220 and a capacitor 230, an active circuit 240 that
provides a negative resistance to offset resistive loses of
differential LC tank VCO 200, and a tail current source 250. A
differential oscillating output signal Vout is provided as output
across tank circuit 210.
[0025] Active circuit 240 is formed by a differential pair of
transistors M1 and M2 that have been cross coupled in a positive
feedback configuration. Specifically, transistors M1 and M2 are
n-type metal-oxide semiconductor field effect transistors
(MOSFETs), where the gate of each transistor has been connected to
the other transistors drain. The source of each transistor is
connected to a supply voltage VSS through tail current source 250.
Supply voltage VSS is either at ground or a negative potential
relative to a supply voltage VDD coupled to a center tap of
inductor 220. Tail current source 250 is generally used to bias
transistors M1 and M2.
[0026] Transistors M1 and M2 essentially form a common-source
amplifier with a complex, tuned load that includes tank circuit
210. Through the cross coupled pair, active circuit 240 is
configured to provide a negative resistance to offset the losses
incurred by the positive, finite resistance of the complex, tuned
load (and other sources of resistances in differential LC tank VCO
200). The negative resistance is substantially equal to -2/g.sub.m,
where g.sub.m represents the transconductance of transistors M1 and
M2.
[0027] The exemplary implementation of differential LC tank VCO 200
is used to describe the various embodiments of the present
disclosure below. However, it should be noted that the various
embodiments of the present disclosure are not limited to using this
specific differential LC tank VCO implementation. For example,
other differential LC tank VCO implementations that include
different transistor types to implement the cross coupled pair of
active circuit 240, such as p-type MOSFETs, or a different active
circuit implementation all together can be used. A complementary
metal oxide semiconductor (CMOS) implementation of active circuit
240 is also possible, with two cross coupled pair of transistors: a
first cross coupled pair with n-type MOSFETs and a second cross
coupled pair with p-type MOSFETs. In general, any reasonable
differential LC tank VCO implementation can be used with the
various embodiments of the present disclosure as will be
appreciated by one of ordinary skill in the art.
[0028] 2. Multi-Injection VCOs
[0029] FIG. 3 illustrates a multi-injection VCO 300 in accordance
with embodiments of the present disclosure. Multi-injection VCO 300
includes two LC tank VCOs 302 and 304, and a parallel VCO output
coupler 306. Multi-injection VCO 300 is configured to provide a low
phase noise oscillating output signal at a differential output Vout
by combining the oscillating output signals of LC tank VCOs 302 and
304 and, potentially, the oscillating output signals of other LC
tank VCOs (not shown). LC tank VCOs 302 and 304 are configured and
controlled to provide oscillating output signals in phase with one
another (not considering their respective phase noises) and at the
same frequency.
[0030] In general, it can be shown that a signal containing random
noise can have its signal-to-noise ratio (SNR) improved by
combining multiple copies of the signal generated in substantially
similar manners. The basic concept is that, if the noise is random
and the signal is constant, the signal will add with each copy
combined (assuming the signal from each copy is correlated) but the
noise will, at least in part, cancel due to its random nature
(assuming the noise from each copy is uncorrelated). The phase
noise inherent in the oscillating output signals produced by LC
tank VCOs 302 and 304 is random. Therefore, by combining the
oscillating output signals of LC tank VCOs 302 and 304, the SNR of
the combined oscillating output signal can be improved relative to
any one of the individual oscillating output signals provided by LC
tank VCOs 302 and 304.
[0031] Parallel VCO output coupler 306 is used to combine the
individual oscillating output signals provided by LC tank VCOs 302
and 304. Specifically, parallel VCO output coupler 306 includes two
transformers: a first transformer 308 configured to magnetically
couple the oscillating output signal generated by LC tank VCO 302
to differential output Vout, and a second transformer 310
configured to magnetically couple the oscillating output signal
generated by VCO 304 to differential output Vout. Because the
oscillating output signals of LC tank VCOs 302 and 304 are coupled
to the same output, they are combined. As is shown in FIG. 3,
parallel VCO output coupler 306 specifically combines the
oscillating output signals of LC tank VCOs 302 and 304 in
parallel.
[0032] In an embodiment, and as shown in FIG. 3, one of the
windings of transformer 308 is used not only to magnetically couple
the oscillating output signal generated by LC tank VCO 302 to
differential output Vout, but is further used to form, at least in
part, the LC tank resonator of LC tank VCO 302. More specifically,
one of the windings of transformer 308 is further used to form the
inductor of the LC tank resonator of LC tank VCO 302. In this way,
area can be conserved. Similarly, in an embodiment, one of the
windings of transformer 310 can be further used to form, at least
in part, the LC tank resonator of LC tank VCO 304 to conserve
area.
[0033] In order to substantially synchronize the phases of the
oscillating output signals generated by LC tank VCOs 302 and 304,
before they are combined by parallel VCO output coupler 306, LC
tank VCOs 302 and 304 can be adjusted based on a common oscillating
reference signal. For example, LC tank VCOs 302 and 304 can be
implemented within respective phase-locked loops (PLLs), and the
phase of their respective oscillating output signals (or some
frequency divided version of their respective oscillating output
signals) can be compared to the phase of the same oscillating
reference signal (e.g., the same oscillating reference signal
produced by a crystal oscillator) using the PLLs. Any difference in
phase between the oscillating output signals of VCOs 302 and 304
(or some frequency divided version of the oscillating output
signals of VCOs 302 and 304) from the oscillating reference signal
can be used to adjust their respective phases. The phases can be
adjusted by varying the capacitance of the capacitors and/or
varactors in the tank circuits of LC tank VCOs 302 and 304.
[0034] In another embodiment, as opposed to using an individual PLL
for each LC tank VCO 302 and 304 to adjust their respective
oscillating output signals based a common reference signal, a
single PLL can be used. The single PLL can be used to compare the
phase of the combined output oscillating signal provided at
differential output Vout (or some frequency divided version of the
combined oscillating output signal provided at differential output
Vout) to a reference signal and, based on this comparison, adjust
the respective phases of each LC tank VCO 302 and 304. Again, the
phases can be adjusted by varying the capacitance of the capacitors
and/or varactors in the tank circuits of LC tank VCOs 302 and
304.
[0035] After the oscillating output signals of LC tank VCOs 302 and
304 are made to be in phase with each other, parallel VCO output
coupler 306 can be configured to maintain this phase relationship.
For example, parallel VCO output coupler 306 can be constructed to
provide equal routing delays from the respective points where the
oscillating output signals generated by LC tank VCOs 302 and 304
are sensed as inputs (i.e., at transformers 308 and 310), to the
point where they are sensed as a combined output (i.e., at
differential output Vout). Equal routing delays can be achieved by
making the routing lengths from the respective points where the
oscillating output signals are sensed as inputs, to the point where
they are sensed as a combined output, equal. This assumes similar
routing materials (e.g. materials with similar per unit resistances
and capacitances) and routing layouts (e.g., same number of turns)
are used.
[0036] It should be noted that multi-injection VCOs, such as
multi-injection VCO 300, has further benefits other than phase
noise improvement. For example, the distribution of VCOs in
multi-injection VCOs can allow a required power to be divided among
the VCOs, thereby placing less stress on the devices implementing
them (e.g., transistors) and resulting in a longer lifetime. In
addition, the distribution of VCOs in multi-injection VCOs can
improve reliability. For example, if one or more of the distributed
VCOs stops functioning or stops functioning within required design
parameters, the multi-injection VCO can still be used to provide an
oscillating output signal.
[0037] Referring now to FIG. 4, another multi-injection VCO 400 is
illustrated in accordance with embodiments of the present
disclosure. Multi-injection VCO 400 includes two LC tank VCOs 402
and 404, and a series VCO output coupler 406. Similar to
multi-injection VCO 300 in FIG. 3, multi-injection VCO 400 is
configured to provide a low phase noise oscillating output signal
at a differential output Vout by combining the oscillating output
signals of LC tank VCOs 402 and 404 and, potentially, the
oscillating output signals of other LC tank VCOs (not shown).
However, unlike multi-injection VCO 300, which combines the
oscillating output signals of LC tank VCOs 302 and 304 in parallel,
multi-injection VCO 400 combines the oscillating output signals of
LC tank VCOs 402 and 404 in series. This is shown by the
configuration of series VCO output coupler 406.
[0038] Referring now to FIG. 5, an exemplary integrated circuit
layout of a transformer 500 that can be used to couple the
oscillating output signal of an LC tank VCO to the oscillating
output signals of other LC tank VCOs is illustrated in accordance
with embodiments of the present disclosure. For example,
transformer 500 can be used to implement transformer 308 or 310 in
FIG. 3 or transformer 408 or 410 in FIG. 4.
[0039] As shown in FIG. 5, transformer 500 specifically includes
two spiral inductors that form its windings: a first spiral
inductor 502 with two ends 506 and 508 and a center tap 510, and a
second spiral inductor 512 with two ends 514 and 516. Although the
two spiral inductors 502 and 512 are shown in FIG. 5 as being laid
out flat next to each other for ease of illustration, in actual
implementation the two spiral inductors 502 and 512 are stacked,
with one of the spiral inductors on top of the other. In an
integrated circuit, the two spiral inductors can be stacked by
using different metal layers. For example, spiral inductor 502 can
use a second metal layer in the integrated circuit, and spiral
inductor 512 can use a third metal layer that is positioned above
the second metal layer on the substrate of the integrated
circuit.
[0040] In the embodiment of FIG. 5, spiral inductor 502 includes
two loops that are predominately in one metal layer of an
integrated circuit. A small portion 504 of spiral inductor 502 is
implemented in another metal layer to prevent the two loops from
being coupled together at an undesired point. It should be noted
that, in other embodiments of transformer 500, spiral inductor 502
can be implemented with more than two loops.
[0041] When transformer 500 is used to implement transformer 308 or
310 in FIG. 3 or transformer 408 or 410 in FIG. 4, spiral inductor
502 can be used to form the inductor of an LC tank VCO, and spiral
inductor 512 can be used to magnetically couple the signal flowing
through spiral inductor 502 to an output.
[0042] Referring now to FIG. 6, a multi-injection VCO 600 is
illustrated in accordance with embodiments of the present
disclosure. Multi-injection VCO 600 includes two LC tank VCOs 602
and 604, and a parallel VCO output coupler 606. Similar to
multi-injection VCO 300 in FIG. 3 and multi-injection VCO 400 in
FIG. 4, multi-injection VCO 600 is configured to provide a low
phase noise oscillating output signal at a differential output Vout
by combining the oscillating output signals of LC tank VCOs 602 and
604 and, potentially, the oscillating output signals of other LC
tank VCOs (not shown). However, unlike multi-injection VCOs 300 and
400, which combine the oscillating, output signals of LC tank VCOs
using transformers, multi-injection VCO 600 combines the
oscillating output signals of LC tank VCOs 602 and 604 in parallel
using transmission lines 608 and 610.
[0043] In general, a transmission line is a conductor that carries
signals with frequency components having wavelengths comparable to
or less than the length of the conductor. For example, conductors
with lengths greater than 1/10 of the wavelengths of the signals
they carry can be considered transmission lines. Thus, in at least
one embodiment, transmission lines 608 and 610 are constructed to
have lengths greater than 1/10 of the wavelengths of the
oscillating output signals generated by LC tank VCOs 602 and 604.
However, other lengths are possible (e.g., greater than or equal to
a half wave length or quarter wave length). Transmission lines 608
and 610 can be constructed using a ladder line, stripline,
dielectric slab, or waveguide, for example.
[0044] Transmission lines 608 and 610 have a characteristic
impedance as seen by LC tank VCOs 602 and 604 that is dependent on
at least: (1) the length of the transmission lines (as measured
from the point where they are respectively coupled to LC tank VCOs
602 and 604, to the output at Vout); (2) the load impedance coupled
at the output Vout; and (3) the wavelength of the oscillating
output signals generated by VCOs 602 and 604 as carried by
transmission lines 608 and 610. In the embodiment of FIG. 6, one or
more of these values are set or adjusted such that the
characteristic impedance of transmission lines 608 and 610, as seen
by LC tank VCOs 602 and 604, provides a desired inductance for the
LC tank resonators of LC tank VCOs 602 and 604. In this way, area
can be conserved because the transmission line conductors, used to
couple the oscillating output signals generated by VCOs 602 and
604, are further used to form, at least in part, the LC tank
resonators of LC tank VCOs 602 and 604.
3. Compact Multi-Band VCO
[0045] Described below is another approach for reducing VCO phase
noise, while keeping area requirements low, when oscillating output
signals with frequencies over a large range are desired. For
example, today's wireless devices often support multiple wireless
standards that operate over multiple frequency bands. A cellular
phone or handset may be configured to communicate with both second
and third generation wireless communications systems using multiple
standards (e.g., EDGE/GSM/WCDMA) and frequency bands (e.g., the
900, 1800, and 2100 MHz bands). A wireless local area network
device is another common wireless device that may be configured to
support multiple standards (e.g., IEEE's 802.11a/b/g/ac/ad
standards) and frequency bands.
[0046] These multi-band wireless devices include a radio frequency
(RF) front-end to up-convert a signal for transmission over a
wireless link and to down-convert a signal received over a wireless
link. The RF front-end typically includes a VCO to generate a local
oscillator (LO) signal to perform this up-conversion and
down-conversion. In wireless devices, the VCO is typically
implemented using an LC tank circuit that includes an inductor and
one or more capacitors as described above. The frequency of the LO
signal generated by the VCO can be controlled by varying the
capacitance of the LC tank. A VCO can generally achieve good
performance over a small tuning range. However, for a large tuning
range the resistive impedance of the LC tank circuit can vary
considerably, which results in a relatively large and undesirable
variation in the VCO phase noise over the different, supported
frequency bands.
[0047] To combat phase noise, a wireless device that supports
multiple bands that are spaced far apart can employ multiple VCOs.
Each VCO can then be designed to achieve good phase noise
performance for a specific frequency band. However, the use of
multiple VCOs for multiple frequency bands increases cost and area
in an integrated circuit (IC) implementation.
[0048] FIG. 7 illustrates one potential solution to these
drawbacks. Specifically. FIG. 7 illustrates a multi-band VCO 700 in
accordance with embodiments of the present disclosure. Multi-band
VCO 700 includes two LC tank VCOs 702 and 704 and a three winding
transformer 706. LC tank VCO 702 is configured to generate an
oscillating output signal with a frequency that can be tuned over
(or that is centered within) a first one of two different frequency
bands, and LC tank VCO 704 is configured to generate an oscillating
output signal with a frequency that can be tuned over (or that is
centered within) a second one of the two different frequency bands.
Because LC tank VCOs 702 and 704 each respectively provide an
oscillating output signal with a frequency over (or centered
within) one frequency band, each can be designed to achieve good
phase noise performance over its specific frequency band.
[0049] Three winding transformer 706 provides for a compact
implementation of multi-band VCO 700 and is configured to couple
the oscillating output signals generated by LC tank VCOs 702 and
704 to a differential output Vout. Three winding transformer
specifically includes a first winding 708 electrically coupled to
the oscillating output signal generated by LC tank VCO 702, a
second winding 710 electrically coupled to the oscillating output
signal generated by LC tank VCO 704, and a third winding
electrically coupled to the differential output Vout. Three winding
transformer 706 is configured to magnetically couple the
oscillating output signal generated by LC tank VCO 702 to the
output Vout using first winding 708 and third winding 712, and to
magnetically couple the oscillating output signal generated by LC
tank VCO 704 to the output Vout using second winding 710 and third
winding 712.
[0050] By using three winding transformer 706 to couple the
oscillating output signals generated by LC tank VCOs 702 and 704 to
differential output Vout, as opposed to using two separate
transformers, multi-band VCO 700 can be more compactly implemented.
However, because third winding 712 is shared between the two LC
tank VCOs 702 and 704 a controller (not shown) can be used to turn
one of the to LC tank VCOs 702 and 704 off while the other one is
operating. For example, a switch S1 and a switch S2 can be
respectively included in LC tank VCOs 702 and 704 to turn one of
them off while the other is on, as directed by a controller.
[0051] In an embodiment, first winding 708 of three winding
transformer 706 is used not only to magnetically couple the
oscillating output signal generated by LC tank VCO 702 to
differential output Vout, but is further used to form, at least in
part, the LC tank resonator of LC tank VCO 702. More specifically,
first winding 708 is further used to form the inductor of the LC
tank resonator of LC tank VCO 702. In this way, further area can be
conserved. Similarly, in an embodiment, second winding 710 of three
winding transformer 706 can be further used to form, at least in
part, the LC tank resonator of LC tank VCO 704 to conserve
area.
[0052] Referring now to FIG. 8, an exemplary layout of a three
winding transformer 800 that can be used to couple the oscillating
output signals of two LC tank VCOs to a common output is
illustrated in accordance with embodiments of the present
disclosure. For example, transformer 800 can be used to implement
three winding transformer 706 in FIG. 7.
[0053] As shown in FIG. 8. transformer 800 specifically includes
three spiral inductors that form its windings: a first spiral
inductor 802 with two ends 806 and 808, a second spiral inductor
804 with two ends 810 and 812, and a third spiral inductor 818 with
two ends 820 and 822. In the embodiment of FIG. 8, spiral inductors
802 and 804 are implemented predominately in the same metal layer
of integrated circuit. Two small portions 814 and 816 of spiral
inductors 802 and 804 are implemented in another metal layer to
prevent the two spiral inductors from being electrically coupled
together.
[0054] Although spiral inductor 818 is shown as being laid out next
to spiral inductors 802 and 804 for ease of illustration, in actual
implementation spiral inductor 818 is stacked on top of or below
spiral inductors 802 and 804 using a different metal layer. For
example, spiral inductors 802 and 804 can use a second metal layer
in an integrated circuit, and spiral inductor 818 can use a third
metal layer that is positioned above the second metal layer on the
substrate of the integrated circuit.
[0055] When transformer 800 is used to implement three winding
transformer 706 in FIG. 8, spiral inductor 802 can be used to form
the first winding 708 and the inductor of LC tank. VCO 702, spiral
inductor 804 can be used to form the second winding 710 and the
inductor of LC tank VCO 704, and spiral inductor 806 can be used to
form the third winding.
[0056] Referring now to FIG. 9, a multi-band, multi-injection VCO
900 in accordance with embodiments of the present disclosure is
illustrated. Multi-band, multi-injection VCO 900 includes four
clusters 902, 904, 906, and 908 of LC tank VCOs, and each cluster
902-908 includes four LC tank VCOs. Cluster 902 includes LC tank
VCOs 1-1 through 1-4, cluster 904 includes LC tank VCOs 2-1 through
2-4, cluster 906 includes LC tank VCOs 3-1 through 3-4, and cluster
908 includes LC tank VCOs 4-1 through 4-4. It should be noted that,
in other embodiments, more or less clusters can be used, and each
cluster can include more or less LC tank VCOs.
[0057] Each of the four clusters 902-908 is configured to generate
an oscillating output signal with a frequency that can be tuned
over (or that is centered within) a different frequency band.
Because clusters 902-908 each respectively provide an oscillating
output signal with a frequency over (or centered within) one
frequency band, their respective LC tank VCOs can be designed to
achieve good phase noise over their respective frequency bands.
[0058] In an embodiment, the oscillating output signals of the LC
tank VCOs of one or more of clusters 902-908 are combined in
parallel or series in a manner consistent with that disclosed above
in FIG. 3 and FIG. 4, respectively. For example, the oscillating
output signals of LC tank VCOs 1-1 through 1-4 in cluster 902 can
be combined in series using a first set of transformers in a manner
consistent with that disclosed above in FIG. 3. By combining the
oscillating output signals of LC tank VCOs 1-1 through 1-4, phase
noise can be reduced.
[0059] In another embodiment, a controller (not shown) is
configured to turn off all but one of the clusters in multi-band,
multi-injection VCO 900. This prevents the clusters from
interfering with each other due to the fact that their outputs are
all coupled to the same differential output Vout as shown in FIG.
9.
[0060] In yet another embodiment, a controller (not shown) is
configured to turn on/off an adjustable number of LC tank VCOs
within one or more of clusters 902-908. For example, assuming
cluster 902 is currently in operation and providing an oscillating
output signal at differential output Vout, the number of LC tank
VCOs 1-1 through 1-4 currently being used by cluster 902 can be
controlled by the controller based any number of different
criteria. The different criteria can include, for example, a power
requirement, a phase noise requirement, a SNR or modulation order
of a signal to be down-converted using the oscillating output
signal generated by cluster 902, or the presence of nearby blockers
to a desired channel in a signal to be down-converted. For example,
if low power is required., less of the LC tank VCOs 1-1 through 1-4
can be turned on to generate the oscillating output signal provided
at differential output Vout. If low phase noise is required, more
of the LC tank VCOs 1-1 through 1-4 can be turned on to generate
the oscillating output signal provided at differential output
Vout.
[0061] It should be noted that, rather than supporting separate
frequency bands, the LC tank VCOs noted above in regard to FIGS.
7-9 can support separate frequency channels, or can be said to
support separate frequency channels. Frequency channels are
generally understood to be narrower in terms of bandwidth than
frequency bands.
4. Conclusion
[0062] The present disclosure has been described above with the aid
of functional building blocks illustrating the implementation of
specified functions and relationships thereof The boundaries of
these functional building blocks have been arbitrarily defined
herein for the convenience of the description. Alternate boundaries
can be defined so long as the specified functions and relationships
thereof are appropriately performed.
* * * * *