U.S. patent application number 09/430922 was filed with the patent office on 2001-11-22 for method and apparatus for filtering radio frequency signals.
Invention is credited to WALTMAN, STEVEN D..
Application Number | 20010043116 09/430922 |
Document ID | / |
Family ID | 23709655 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010043116 |
Kind Code |
A1 |
WALTMAN, STEVEN D. |
November 22, 2001 |
METHOD AND APPARATUS FOR FILTERING RADIO FREQUENCY SIGNALS
Abstract
A variable frequency active filter circuit for tuning in a
specific bandpass frequency is disclosed. The active filter
incorporates a controlled oscillator circuit for tuning into a
specific frequency range. A frequency input signal controls the
active filter bandpass frequency by tuning the oscillator circuit
to a desired frequency. The controlled oscillator circuit further
includes a gain input. The gain input is set to a value just below
where the oscillator circuit would oscillate.
Inventors: |
WALTMAN, STEVEN D.;
(BOULDER, CO) |
Correspondence
Address: |
JAMES Y. GO
BLAKELY, SOKOLOFF, TAYLOR & ZAFMAN LLP
12400 WILSHIRE BOULEVARD
SEVENTH FLOOR
LOS ANGELES
CA
90025-1026
US
|
Family ID: |
23709655 |
Appl. No.: |
09/430922 |
Filed: |
November 1, 1999 |
Current U.S.
Class: |
327/559 |
Current CPC
Class: |
H03B 2201/0208 20130101;
H03B 5/1847 20130101; H03J 3/08 20130101 |
Class at
Publication: |
327/559 |
International
Class: |
H03B 001/00 |
Claims
We claim:
1. A method of filtering an input signal, said method comprising:
setting a frequency input on a controlled oscillator circuit to a
desired frequency; coupling an input signal to said controlled
oscillator circuit; setting a gain input on said controlled
oscillator circuit to a value just below where said oscillator
circuit would oscillate; coupling an output signal to said
controlled oscillator circuit.
2. The method as claimed in claim 1, said method further
comprising: determining a gain value by feeding a reference
oscillator signal through said input signal.
3. The method as claimed in claim 1 wherein said coupling is
performed using coupled microstrips.
4. The method as claimed in claim 1 wherein said coupling is
performed using a pair of coupled stripline transmission lines.
5. The method as claimed in claim 1 wherein said controlled
oscillator circuit comprises a voltage controlled oscillator.
6. The method as claimed in claim 1 wherein said voltage controlled
oscillator comprises a resonator and a negative resistance
circuit.
7. An active filter circuit for filtering an input signal, said
circuit comprising: an oscillator circuit, said oscillator circuit
coupled to said input signal, said oscillator circuit having a
frequency input and a gain input, said gain input set to a gain
value just below where said oscillator circuit would oscillate; and
an output signal, said output signal coupled to said oscillator
circuit.
8. The apparatus as claimed in claim 7 further comprising: an
oscillation detection circuit.
9. The apparatus as claimed in claim 7 wherein said oscillator is
coupled to said input signal using microstrips.
10. The apparatus as claimed in claim 7 wherein said oscillator is
coupled to said input signal using a pair of coupled stripline
transmission lines.
11. The apparatus as claimed in claim 7 wherein said oscillator
circuit comprises a voltage controlled oscillator.
12. The method as claimed in claim 1 wherein said voltage
oscillator comprises a resonator and a negative resistance circuit.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of radio
frequency electronics. In particular the present invention
discloses an active filter for filtering signals over a relatively
large bandwidth range.
BACKGROUND OF THE INVENTION
[0002] Computer and digital communication networks have
traditionally been constructed using wired network technologies.
However, the expense and difficulty of installing a wired network
has sped the growth of a wireless digital communication industry.
Cellular telephone networks, satellite communication networks, and
wireless computer networks all now use digital wireless
communication technologies.
[0003] The military and large corporations have used digital
wireless communication systems for many years now. However, the
consumer market for digital wireless communications is still
relatively young. To penetrate the consumer market, digital
wireless communication systems must be simple, reliable, and most
importantly inexpensive. Therefore, it would be desirable to
improve the designs of wireless communication circuitry such that
the wireless communication circuitry can be used in the consumer
market.
SUMMARY OF THE INVENTION
[0004] A variable frequency active filter circuit for tuning in a
specific bandpass frequency is disclosed. The active filter
incorporates a controlled oscillator circuit for tuning into a
specific frequency range. A frequency input signal controls the
active filter bandpass frequency by tuning the oscillator circuit
to a desired frequency. The controlled oscillator circuit further
includes a gain input. The gain input is set to a value just below
where the oscillator circuit would oscillate.
[0005] Other objects, features, and advantages of present invention
will be apparent from the company drawings and from the following
detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The objects, features, and advantages of the present
invention will be apparent to one skilled in the art, in view of
the following detailed description in which:
[0007] FIG. 1 illustrates a satellite data distribution system with
an uplink transmitter, a communications satellite, and a number of
receiver systems.
[0008] FIG. 2 illustrates a block diagram of a typical satellite
receiver system that receives, demodulates, and decodes digital
satellite signals.
[0009] FIG. 3 illustrates a bandwidth diagram of twenty-four
transponders with a bandwidth of thirty-six megahertz each on a
communication satellite where half of the transponders in the
horizontal polarization and half of the transponders in the
vertical polarization.
[0010] FIG. 4 illustrates a block diagram of a receiver circuit
that uses a bandpass filter.
[0011] FIG. 5 illustrates a block diagram of a receiver circuit
that uses a variable active filter circuit.
[0012] FIG. 6 illustrates a block diagram of one embodiment of a
variable active filter circuit.
[0013] FIG. 7 illustrates a simple block diagram of an oscillator
circuit.
[0014] FIG. 8 illustrates one embodiment of an Ultra-high Frequency
(UHF) voltage controlled oscillator circuit.
[0015] FIG. 9 illustrates one embodiment of an active filter
circuit that uses a voltage controlled oscillator circuit.
[0016] FIG. 10 illustrates a block diagram of a receiver with two
voltage controlled oscillator based active filter circuits such
that one active filter is used to calibrate the gain signal to be
applied to the other active filter circuit.
[0017] FIG. 11 illustrates an embodiment of a tunable bandpass
filter circuit that may or may not filter an RF signal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0018] A method and apparatus for implementing an active filter
circuit for receiving radio frequency signals across a wide
frequency range is disclosed. In the following description, for
purposes of explanation, specific nomenclature is set forth to
provide a thorough understanding of the present invention. However,
it will be apparent to one skilled in the art that these specific
details are not required in order to practice the present
invention. For example, the present invention has been described
with reference to filtering digital satellite signals. However, the
same techniques can easily be applied to other types of radio
frequency signals.
Satellite Communications
[0019] FIG. 1 illustrates a conceptual diagram of a typical
satellite communication system 100. In the typical satellite
communication system 100 of FIG. 1, an uplink earth station 110
modulates and transmits an uplink signal 115 to a communication
satellite 120. The communication satellite 120 transmits the
received signal back down to a plurality of receivers 151, 153,
155, and 157 with a downlink signal 125. In a Ku-band satellite
system, the downlink signal 125 is in the frequency range of 10.7
to 15.0 GHz. The receivers 151, 153, 155, and 157 demodulate the
downlink signal 125 to extract the encoded information.
[0020] FIG. 2 illustrates a block diagram of a typical satellite
receiver system 200. In the satellite receiver system 200 of FIG.
2, a dish antenna 210 receives a downlink signal transmitted by a
communications satellite. The dish antenna usually includes a
low-noise block down converter (LNB) 220 that translates the high
frequency satellite carrier signal to a lower intermediate
frequency (IF) that will be used for signal processing. In a
typical consumer satellite receiver system, the downlink signal is
down-converted to an L-band signal in the range of 950 to 2150 MHz.
The down-conversion allows the signal to be carried on coaxial
cables 231 and 233 to a receiver unit 240. Instead of using two
coaxial cables 231 and 233, a single coaxial cable may be used with
DC voltage used to switch between polarizations.
[0021] A receiver circuit 250 in the receiver unit 250 amplifies
the signal and tunes into a specific frequency using bandpass
filters and a tuner circuit. The receiver circuit 250 outputs an
In-phase (I) data signal and a Quadrature (Q) data signal. The I
and Q data signals are demodulated by a demodulator circuit 260.
Many satellite communication systems, such as direct broadcast
satellite television systems, use Quadrature phase shift keying
(QPSK) modulation. Quadrature phase shift keying (QPSK) is used in
satellite communication because of its power efficiency and its
robustness against phase noise. The demodulator circuit 260 outputs
an encoded data stream. The encoded data stream usually comprises a
forward error correction encoded (FEC) data stream. A decoder
circuit 270 decodes the encoded data stream to generate a decoded
data stream.
[0022] Communication satellites carry a number of different
transponder units that receive and retransmit a number of different
frequency ranges. Many communication satellites contain twenty-four
(24) different transponders that each broadcast 36 Mhz wide
frequency "channels". FIG. 3 illustrates a diagram that graphically
illustrates the frequency channels in a twenty-four transponder
satellite embodiment. As illustrated in FIG. 3, a first set of
twelve channels use horizontal polarization and a second set of
channels use a vertical polarization. In the diagram of FIG. 3,
each channel is 36 Mhz wide. To prevent interference between the
transponders, the channels using the same polarity are separated
with a four MHz guard band. Note that many different satellite
transponder embodiments exist.
[0023] To tune into a specific frequency channel, most RF receiver
systems (such as satellite receiver systems) use a fixed bandpass
filter that eliminates signals outside of the desired frequency
range. For example, FIG. 4 illustrates a block diagram of the
receiver system that uses a fixed bandpass filter 420 to tune into
a specific transponder frequency. By eliminating the signals
outside of the desired frequency range, the receiver will be able
to better demodulate the desired signal. In receiver systems with a
fixed bandpass filter, the receiver may only access a few
transponders within the limited bandpass frequency range of the
bandpass filter.
A Tunable Bandpass Filter
[0024] To be able to tune into all the transponders on a satellite
with sufficient signal quality at any transponder frequency range,
the present invention introduces a variable bandpass filter that
allows the tuner to tune into any transponder. FIG. 5 illustrates a
block diagram of a receiver with the variable bandpass filter 510.
In the tuner circuit of FIG. 5, the variable bandpass filter 510
provides a high quality input signal for any of the transponders on
a communications satellite since the variable bandpass filter 510
filters out signals from the transponders not being tuned.
[0025] FIG. 6 illustrates a block diagram of one embodiment of a
variable bandpass tuner 610. The variable bandpass tuner 610
receives the radio frequency input signal 605 from the satellite
antenna and outputs a filtered radio frequency signal 695. The main
feature of bandpass filter 610 is the active filter circuit 620.
The active filter circuit 620 is used to filter out signals that
are not within a specified pass band and provide some gain for
signals within the pass band. If the entire satellite transponder
spectrum is to be examined, the active filter circuit 620 may be by
passed using the full bandwidth bypass 630 under the control of a
wideband/narrowband control signal 635.
[0026] The active filter circuit 620 is controlled with two input
signals. The first input signal is a frequency control input signal
640. The frequency control input signal 640 is used to select the
desired transponder frequency.
[0027] The second control signal of the active filter circuit 620
is a gain control input signal 650. The gain control input signal
650 determines a gain setting for the active filter circuit 620.
Due to the new active filter design of active filter circuit 620,
the new active filter design of the active filter circuit 620 must
be calibrated to set the gain value of the gain control input
signal 650.
[0028] To calibrate the active filter circuit 620 to set the gain
value, a calibration control signal 660 is activated in order to
have multiplexor 680 direct an oscillator signal 670 into the
active filter circuit 620. Referring back to FIG. 5, the oscillator
signal 670 is from the oscillator 550 used by the tuner circuit
530. While the oscillator signal 670 passes through the active
filter circuit 620, a calibration system adjusts the gain control
input 650 to a proper setting as set forth in the following
section.
New Active Filter Design
[0029] The present invention introduces a new type of variable
active filter circuit design. In the new active filter design of
the present invention, an oscillator circuit is used to provide
gain such that there is minimal signal loss at the desired
frequency. However, the oscillator circuit is not allowed to
oscillate since oscillations would distort the received input
signal.
[0030] FIG. 7 illustrates a block diagram of a standard negative
resistance oscillator. Referring to the diagram of FIG. 7, a
resonator 710 that experiences some loss is coupled to a "negative"
resistance unit 720 that provides gain. The resonator 710 provides
an oscillating signal. The negative resistance unit 720 amplifies
the oscillating signal by a set gain. The resultant oscillating
signal is passed to an output with a matched coupling.
[0031] FIG. 8 illustrates a typical circuit embodiment of the
negative resistance oscillator of FIG. 7. The oscillator circuit of
FIG. 8 is a typical Ultra-High Frequency (UHF) voltage controlled
oscillator (VCO) circuit 800. Referring to FIG. 8, a transmission
line resonator 810 is coupled to a negative resistance circuit 840
that provides gain. The output signal from the oscillator circuit
800 is from an output capacitor C.sub.out. Note that the oscillator
circuit 800 of FIG. 8 includes a tuning voltage V.sub.t that is
used to select the oscillation frequency of the voltage-controlled
oscillator (VCO) circuit 800.
[0032] The variable active filter design of the present invention
uses an oscillator circuit such as the oscillator circuit 800 in
FIG. 8 to provide gain to the variable active filter circuit. FIG.
9 illustrates one possible embodiment of an active filter circuit
constructed according to the teachings of the present
invention.
[0033] In the active filter circuit of FIG. 9, a radio frequency
input signal 905 is coupled to the resonator 910 of the oscillator
type circuit. In one embodiment, the radio frequency input signal
905 and the resonator 910 are coupled using coupled microstrips or
stripline transmission lines. The oscillator type circuit is
controlled with a tuning voltage V.sub.t 960 that selects a desired
filter frequency and an oscillator gain control V.sub.g 970 that
selects a gain value. The output of the active filter circuit is
obtained by coupling an output coupling 990 to the resonator 910 of
the oscillator type circuit. The output signal 995 is a signal
filtered at the desired tuning frequency as selected by the tuning
voltage V.sub.t 960.
[0034] As set forth in the beginning of this section, the
oscillator type circuit in the active filter should not be allowed
to oscillate. To enforce this requirement, the present invention is
calibrated to set the oscillator gain control V.sub.g 970 to a
value just below where the oscillator would oscillate. Referring
back to FIG. 6, a calibration is initiated by activating the
calibration control signal 660. The activated calibration control
signal 660 will cause multiplexor 680 to deliver the reference
oscillator signal 670 through the active filter circuit 620. An
oscillation detection circuit may be used to detect the gain
control 650 setting where the active filter circuit 620 begins to
oscillate. Then the oscillation detection circuit will reduce the
gain control 650 setting to a voltage value just below the
oscillation point.
[0035] When designing tunable active filters using negative
resistance elements (such as the negative resistance element 720 of
FIG. 7) with a large tuning range (i.e. e.g. 2:1), it can be
challenging to avoid spurious oscillations due to resonance's
caused by biasing elements. This design problem has already been
solved in the case of broadly tunable voltage controlled
oscillators (VCOs) for a variety of gain topologies. It is a much
simpler problem to redesign VCOs to have barely insufficient gain
to oscillate than to design tunable negative resistance elements
without spurious gain. Since the active filter of the present
invention relies VCOs instead of negative resistance elements, the
present invention yields designs with unprecedented tunability.
Specifically, the active filter design of the present invention has
been used to create an active filter with a 2:1 frequency
range.
An Alternate Receiver with Tunable Bandpass Filter Design
[0036] In the receiver system embodiment of FIG. 5 that includes an
active bandpass filter 520, the receiver system may be periodically
taken off-line to calibrate the variable bandpass filter 520. In an
alternate embodiment disclosed in FIG. 10, the receiver circuit
uses two active filter circuits such that one active filter circuit
may be used to continually calibrate a gain signal for the other
variable bandpass filter.
[0037] In the embodiment of FIG. 10, a first active filter circuit
is used within a tunable bandpass filter 1020 to filter the signals
received by the tuner 1030. FIG. 11 illustrates an embodiment of
the tunable bandpass filter circuit 1020. The active bandpass
filter circuit 1125 in the tunable bandpass filter circuit 1020 can
be constructed according to the teachings in FIG. 9.
[0038] Referring back to FIG. 10, a second active filter circuit,
active filter clone circuit 1090, is coupled to receive a reference
signal from oscillator 1050. The active filter clone circuit 1090
is built identical to the first active filter circuit in tunable
bandpass filter 1020.
[0039] An oscillation detection and control circuit 1095 monitors
the output of the active filter clone circuit 1090. The oscillation
detection and control circuit 1095 analyzes the output of the
active filter clone circuit 1090 and generates a gain input into
active filter clone circuit 1090 that causes the oscillator in the
active filter clone circuit 1090 to begin oscillating. The
oscillation detection and control circuit 1095 uses a gain input
slightly
[0040] The foregoing has described an active filter circuit for
filtering signals across a wide frequency range is disclosed. It is
contemplated that changes and modifications may be made by one of
ordinary skill in the art, to the materials and arrangements of
elements of the present invention without departing from the scope
of the invention.
* * * * *