WO1998017000A1

WO1998017000A1 - Tunable filter with q-multiplication

Info

Publication number: WO1998017000A1
Application number: PCT/US1997/018432
Authority: WO
Inventors: Ellsworth R. Moss
Original assignee: Adc Telecommunications, Inc.
Priority date: 1996-10-15
Filing date: 1997-10-15
Publication date: 1998-04-23
Also published as: AU4982397A

Abstract

A Q-multiplier circuit for a filter that is tunable over a wide range of frequencies. Broadband splitter/combiners and broadband linear amplifiers used in the Q-multiplier circuit need little or no compensation. In one embodiment, a tunable bandpass filter having a frequency tuning range of 30 to 108 MHz is used in the Q-multiplier circuit and achieves a ten-to-one increase in Q, thus narrowing the bandpass width. In other embodiments, lowpass, highpass, or notch filters are used in the Q-multiplier circuit. Q-multiplication is accomplished at frequencies as determined by the tunable filter section without needing to change characteristics of the first splitter/combiner, the second splitter/combiner or the amplifier. In one embodiment, a frequency-hopping filter is Q-multiplied and used within the transmitter, the receiver, or both, of a spread-spectrum radio system. In one embodiment, a tunable phase-compensation network is used in the feedback path and is programmably tuned to compensate for inaccuracies in other parts of the circuit. In one such embodiment, a digitally tuned filter, a digitally tuned phase compensation network, and a compensation look-up table are used together to achieve Q-multiplication and frequency accuracy across a frequency band of approximately three octaves.

Description

TUNABLE FILTER WITH Q-MULTIPLICATION

Field of the Invention This invention relates to the field of electronic filter circuits and Q- multipliers, and more specifically to a method and apparatus of Q-multiplication in electronically tunable filter circuits.

Background of the Invention Frequency-hopping filters can be used in spread-spectrum military battlefield radios operating, for example, across the frequency range of 30 MHz to 108 MHz. One system in which frequency hopping can be used is the United States Army's Single Channel Ground and Airborne Radio System (SINCGARS). Typical communications on the SLNCGARS system involves many data packets each having a short duration. Another use for frequency-hopping filters is in base-station and cordless handset systems operative to receive and transmit cordless radiotelephone signals, for example, in a frequency band of between 902 MHz and 928 MHz.

Yet another use for frequency-hopping filters is in cellular radiotelephone systems. U.S. Patent No. 4,479,226, issued to Prabhu et al, discloses a frequency- hopping single side band mobile radio system. The transmitter for this system functions to modulate the input signal by "hopping" it to a different carrier frequency every few seconds while the receiver to the system employs the identical carrier sequence as used by the transmitter to demodulate the transmitter carrier-frequency-hopped SSB signal thereby recovering the original single side band signal.

U.S. Patent No. 5,303,394, issued to Hrncirik, describes a Feedback Stabilized O-Multiplier Filter Circuit. Referring to Fig. 1 of Hrncirik, a prior-art Q-multiplier circuit 15 includes variable-gain amplifier 16, bandpass filter 18, signal combiner 20 and signal splitter 22. The feedback signal path 8 is intended to provide positive feedback or reinforcement of the applied signal and includes a phase shifter 24. Figures 3 and 4 describe circuits which are functionally similar. There is no mention of using this circuit in a broadband, spread- spectrum, or frequency-hopping applications, nor of providing lowpass, highpass, or notch filters.

U.S. Patent No. 5,491,604, issued to Nguyen et al, describes Q Controlled Microresonators and Tunable Electronic Filters Using Such

Resonators. This amplifier provides negative feedback to adjust overall system quality factors. Nguyen et al.

U.S. Patent No. 5,386,198, issued to Ripstrand et al., describes a Linear Amplifier Control circuit to reduce system distortion. This is a feed-forward power-compensation system. This reference provides background on methods for controlling the Q-factor in multistage devices using linear amplifiers.

The name "Q Multiplier" is given to a circuit that increases the effective "Q" of a filter. The method by which the Q improvement takes place is as follows. A bandpass filter composed of ordinary inductors and capacitors when connected together as a bandpass filter will have a "loaded Q." The natural Q of the filter is determined mainly by the quality of the individual components, i.e., low-loss inductors with high unloaded Q's may be constructed of large copper or silver wire to reduce the resistive part or component of the coil structure. Capacitors of high Q will be constructed of low-loss materials. For example, an air dielectric, a ceramic dielectric or a mica dielectric, combined with using silver-plated plates for the capacitors anodes.

A plurality of coils Ll-Ln and capacitors Cl-Cn can be combined to form a multi-section filter. When these elements are connected to form a filter as shown in prior-art Figure la, they form a bandpass filter. The filter of Figure la is a three-section filter in which the sections are coupled to one another capacitively, and coupled to the input and output inductively. Coils and capacitors can be alternatively arranged in many other forms for other particular filter characteristics, including lowpass, highpass and notch filters. Generally, Figure lb illustrates a typical "shape" characteristic of such a bandpass filter as in Figure la, wherein the "bandwidth" of the filter can be defined as the difference between the frequencies at which the response drops 3 decibels (the 3dB points).

This "bandwidth" characteristic can be narrowed or broadened, depending on several factors, such as the unloaded Q's of each element, the combined loaded Q of the circuit combination, inter-coupling between sections, the number of sections, and shielding. At higher frequencies, typically above 100 MHz, cavities using microwave topologies can provide higher Q's than "lumped elements" (coils and capacitors). After all bandpass filter characteristics are met, such as bandpass frequencies and band-reject characteristics, the filter may exhibit a high bandpass loss, anywhere from a few tenths of a dB to several dB. By trying to make the filter less lossy, cost and size would increase, sometimes dramatically, due to use of silver and other low-loss materials.

Active filters can be used to synthesize filter characteristics, and can be digitally tuned. However, such filters tend to have rather low Q factors, and can become quite expensive when a narrow bandwidth is desired.

Thus there is a need in the prior art to address these and other problems and limitations of standard approaches to filters. Many systems require low-cost filters having narrower bandwidths and lower losses. There is also a need for frequency-agile filters.

Summary of the Invention The present invention provides a circuit and a method for a tunable Q- multiplier circuit having a signal input and a signal output. The Q-multiplier includes a first and a second splitter/combiner, each having a first terminal, a second terminal and a third terminal. The first terminal of the first splitter/combiner is coupled to the Q-multiplier's signal input. The second terminal of the first splitter/combiner is coupled to an input of a tunable filter section that is tunable to a range of frequencies. An output of the tunable filter section is coupled to the third terminal of the second splitter/combiner. The first terminal of the second splitter/combiner is coupled to the Q-multiplier's signal output. An amplifier coupled either between the first splitter/combiner and the tunable filter section, or between the tunable filter section and the second splitter/combiner. A phase-slope network is coupled between the third terminal of the second splitter/combiner and the third terminal of the first splitter/combiner. Q-multiplication is accomplished at frequencies as determined by the tunable filter section without needing to change characteristics of the first splitter/combiner, the second splitter/combiner or the amplifier.

In one embodiment, the tunable filter section responds to an electronic signal to adjust a characteristic frequency of the filter. In one such embodiment, the electronic signal provides a digital value representative of the characteristic frequency to be tuned.

In one embodiment, the tunable filter section includes a bandpass filter. In another embodiment, the tunable filter section includes a notch filter. In yet another embodiment, the tunable filter section includes a highpass filter. In still another embodiment, the tunable filter section includes a lowpass filter. In one embodiment, the Q-multiplier includes a filter controller coupled to the tunable filter section, the filter controller including a look-up table PROM that converts an input frequency value to a filter-control value.

In another embodiment, the Q-multiplier includes a filter controller coupled to the phase-slope network, the filter controller including a look-up table PROM that converts an input frequency value to a phase-control value. In one such embodiment, the filter controller coupled to the phase-slope network further includes a digital-to-analog converter having an analog output, and the phase- slope network includes a varactor whose capacitance is controlled by the analog output. In one embodiment, the tunable filter section responds to an electronic signal to adjust a characteristic frequency across a range of approximately 30 to approximately 108 megahertz, and wherein the splitter, amplifier, and combiner provide broadband responses covering at least the range of approximately 30 to approximately 108 megahertz. In one embodiment, the first splitter/combiner provides two-way combining, the second splitter/combiner provides two-way splitting, and the slope network provides a phase compensation.

Brief Description of the Drawings FIG. 1 A is a schematic of a prior-art bandpass filter 10.

FIG. IB is a graph showing the shape of the frequency response of prior- art bandpass filter 10. FIG. 2A is a schematic of a Q-multiplied tunable filter 100 according to the present invention. FIG. 2B is a schematic of another Q-multiplied tunable filter 100 according to the present invention. FIG. 2C is a schematic of yet another Q-multiplied tunable filter 100 according to the present invention. FIG. 3 A is a schematic of a portion of a transceiver including bandpass and notch filters in the transmit and receive paths.

FIG. 3B is a schematic of a portion of a transmitter including transmit- frequency bandpass and receive-frequency notch filters in the transmit path. FIG. 3C is a schematic of a portion of a receiver including receive- frequency bandpass and transmit-frequency notch filters in the receive path. FIG. 4A is a graph showing the frequency-response of one embodiment of the present invention showing the upward shift in filter center frequency at approximately 30 MHz due to phase shift in the feedback network.

FIG. 4B is a graph showing the frequency-response of one embodiment of the present invention showing the downward shift in filter center frequency at approximately 60 MHz due to phase shift. FIG. 4C is a graph showing the frequency-response of one embodiment of the present invention showing the downward shift in filter center frequency at approximately 85 MHz due to phase shift. FIG. 4D is a graph showing the frequency-response of one embodiment of the present invention showing notch-type response at the edges of the bandpass response. FIG. 4E is a graph showing the frequency-response of one embodiment of the present invention showing a radio spectrum, Q-multiplied filter response, and the combination of the two. Description of Preferred Embodiments In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

Figure 2a is a block diagram showing a Q-multiplied tunable filter 100 according to the present invention. An input signal 109 applied to the input of the Q-multiplied tunable filter 100 is coupled to broadband splitter/combiner 110 which combines the input signal 109 with positive feedback 141 from phase- slope ("phase-adjustment") circuit 140. In one embodiment, broadband splitter/combiner 110 is a quadrature signal splitter/combiner. Signal 121 from the broadband splitter/combiner 110 is applied filter/amplifier 120. In the embodiment shown, filter/amplifier 120 includes tunable filter 122 and low- noise, linear, high-intercept class "A" amplifier 124. The amplifier output signal 125 is then split in two by broadband splitter/combiner 130; with one-half going to the circuit output 131, and the other one-half goes to the phase-slope feedback circuit 140. This feedback signal 141 is applied to the input splitter/combiner 110 in phase with the input signal 109. The magnitude of this signal is adjusted in magnitude and phase by the phase-slope circuit 140. In one embodiment, phase-slope network 140 includes a capacitor of approximately 35 picofarads in parallel with a resistor of approximately 50 ohms (this RC pair in the signal path), with shunt resistors of approximately 150 ohms to ground; the capacitor providing increased feedback at higher frequencies at a rate of approximately 6 dB per octave. This results in controlled feedback which increases the gain, overcoming the losses of the filter, and, in direct proportion to the magnitude of the positive feedback, narrows the bandpass, or in effect, increasing the loaded "Q" of the filter. Ten-to-one (10:1) improvements in loaded effective Q have been demonstrated.

In one embodiment, filter/amplifier 120 is a VHF (very-high frequency) integrated filter and power amplifier (IFPA), such as available from the Xetron subsidiary of Westinghouse Electric Company. Such a filter amplifier 120 can be used in a SINCGARS-type spread-spectrum radio system that uses frequency hopping. Without improvement, such a filter-amplifier is unsuitable for certain applications due to its relatively broad bandwidth. However, when used in a Q- multiplied tunable filter 100 such as Figure 2a, the bandwidth is narrowed suitably, and acceptable frequency-agile hopping for a transmitter/receiver system is achieved. Figure 2B is a schematic of another Q-multiplied tunable filter 100 according to the present invention. In Figure 2B, frequency-control circuit 150 converts frequency-control signal 149 into a suitable filter-control signal 127. In the embodiment shown circuit 150 does not provide a phase-control signal 159, since phase-slope circuit 140 is not adjustable. For some filters, the phase shift needed for proper frequency control is more than that provided by phase-slope circuit 140 at some frequencies, and less than that provided by phase-slope circuit 140 at some other frequencies. This results in the center frequency of the Q-multiplier output being shifted from the center frequency that would otherwise be provided by filter 122 (and shifted upwards in frequency for some specified frequencies, and downwards for others), thus producing inaccurate frequency responses. For example, see Figures 4A and 4B, described below. Thus, in one embodiment, frequency control 150 includes a programmable read-only memory (PROM) that converts a digitally specified frequency-control signal 149 into a different suitable filter-control signal 127, such that the center frequency of the Q-multiplier output matches that specified by frequency-control signal 149. Figure 2C is a schematic of yet another Q-multiplied tunable filter 100 according to the present invention. In Figure 2C, one embodiment of combiner 110 is shown, having a tapped coil 117 including windings 112 and 113 on a suitable core, resistor 111, capacitor 114, a tapped coil 118 including windings 112 and 113 on a suitable core; wherein the tap between windings 112 and 113 is connected to capacitor 114 and the tap between windings 115 and 116. Splitter 130 is constructed similarly, but connected in the Q-multiplier circuit in a complimentary fashion relative to combiner 110, as shown. In one embodiment, the combiner 110 and splitter 130 of Figure 2C are substituted into the circuit shown in Figure 2B for the corresponding combiner 110 and splitter 130 of Figure 2B. In Figure 2C, frequency-control circuit 150 converts frequency- control signal 149 into a suitable filter-control signal 127. In the embodiment shown in Figure 2C, circuit 150 does provide a phase-control signal 159, since phase-slope circuit 140 is adjustable. In this embodiment shown in Figure 2C, frequency control 150 receives a serial frequency-specifying word on signal 149, then uses a serial-to-parallel (S/P) converter 151 to providing a 13-bit-wide parallel output word corresponding to the inputted serial frequency-specifying word from signal 149. In one embodiment, frequency PROM 152 converts the 13-bit-wide parallel output word from S/P converter 151 corresponding to the inputted serial frequency-specifying word from signal 149 into a suitable filter- control signal 127, and phase PROM 153 converts the 13 -bit- wide parallel output word from S/P converter 151 into a digital value that is converted into a suitable analog phase-control signal 127 by D/A converter 154, such that the center frequency of the Q-multiplier output matches that specified by frequency- control signal 149. In one such embodiment, frequency PROM 152 is omitted, and the 13 -bit-wide parallel output word from S/P converter 151 is used directly by tunable filter 122 (in this embodiment, the center-frequency adjustment needed is provided by appropriately adjusting the phase shift through phase PROM 153, D/A converter 154, and phase-slope circuit 140). In another such embodiment, phase PROM 153 is omitted, and selected bits (e.g., the high-order 8 bits) from the 13 -bit- wide parallel output word from S/P converter 151 are coupled to D/A converter 154 to generate the analog phase-control signal 159 (in this embodiment, the center-frequency adjustment needed is provided by appropriately adjusting the specified frequency output by frequency PROM 152 to tunable filter 122). In yet another embodiment, a D/A converter is inserted between frequency PROM 152 and tunable filter 122, in order to convert the digital output of frequency PROM 152 into a suitable analog input signal as required by a tunable filter 122 requiring an analog control signal.

Figure 3 A is a schematic of a portion of a radio transceiver including Q- multiplied bandpass and notch filters in the transmit and receive paths. In the exemplary embodiment shown, transmit signal 310 is narrowed by Q-multiplied bandpass filter 310 to a selected center transmit-frequency as specified by transmit frequency-hopping control 330 that provides control signal 331. The resultant signal is then passed through Q-multiplied notch filter 320 which further removes frequency components at a selected center receive-frequency as specified by receive frequency-hopping control 340 that provides control signal 341, thus reducing signal frequency components that could otherwise interfere with the received signal. The outputted transmit signal is then coupled to antenna 350. In a complementary manner, a received signal from antenna 350 is narrowed by Q-multiplied bandpass filter 310' to a selected center receive- frequency as specified by receive frequency-hopping control 340 that provides control signal 341. The resultant received signal is then passed through Q- multiplied notch filter 320' which further removes frequency components at a selected center transmit frequency as specified by transmit frequency-hopping control 330 that provides control signal 331, thus reducing signal frequency components that could otherwise interfere with the received signal.

Figure 3B is a schematic of a portion of a transmitter including transmit- frequency bandpass and receive-frequency notch filters in the transmit path. Such a transmitter is identical in function to the corresponding transmit section (310 and 320) as described for Figure 3 A. Figure 3C is a schematic of a portion of a receiver including receive- frequency bandpass and transmit-frequency notch filters in the receive path. Such a receiver is identical in function to the corresponding receive section (310' and 320') as described for Figure 3A.

Figure 4A is a graph showing the frequency-response of one embodiment of the present invention showing the upward shift in filter center frequency at approximately 30 MHz due to phase shift in the feedback network. Response graph 410A shows the response of one embodiment of tunable filter 122 (such as described above for Figure 2A) alone. Response graph 420A shows the response of one embodiment of Q-multiplied filter 100, with the same specified frequency control 149 as used for response graph 410A, and having a non-adjustable phase- adjustment circuit 140 such as described above for Figure 2B. Note that the center frequency shifted upward from response graph 410A to response graph 410B, apparently due to the amount of phase shift provided by phase-adjustment circuit 140 relative to other components in Q-multiplier 100. Note also, that the Q-multiplied signal output level (420A) is increased approximately 8 dBs from the signal level of the filter alone (410A).

Figure 4B is a graph showing the frequency-response of the same embodiment of the present invention as measured in Figure 4A, but showing the downward shift in filter center frequency at approximately 60 MHz due to phase shift. Figure 4C is a graph showing the frequency-response the same embodiment of the present invention as measured in Figure 4 A, but showing the downward shift in filter center frequency at approximately 85 MHz due to phase shift.

In one embodiment, the center-frequency shift at each desired frequency is measured, and the phase PROM 153 is programmed using empirically derived look-up values to compensate for the measured frequency shift, in order to keep the center frequency 420 of the Q-multiplied circuit 100 the same as the center frequency 410 of the filter alone, and thus maintain frequency accuracy to the specified frequency value at signal 149.

In another embodiment, the center frequency at each desired frequency is measured (either before or after performing the phase measurements just described), and the frequency PROM 152 is programmed using empirically derived look-up values to compensate for the measured frequency inaccuracies, in order to maintain frequency accuracy to the specified frequency value at signal 149.

Figure 4D is a graph showing the frequency-response of one embodiment of the present invention showing notch-type response at the edges of the bandpass response having a center frequency at approximately 31.42 MHz.

Figure 4E is a graph showing the frequency-response of one embodiment of the present invention showing a radio spectrum 430E, Q-multiplied filter response 420E, and the combination 440E of the two (i.e., passing radio spectrum 430E though Q-multiplied filter circuit 100 of Figure 2C). Conclusion

A relatively low-cost filter (e.g., filter 122) is enhanced (e.g., by a reduction in its losses and a significant improvement in loaded Q) with a Q- multiplier (i.e., Q-multiplier 100) to improve the noise-figure gain and bandpass characteristics with a low-cost active circuit addition to the filter.

In one embodiment, a digitally tuned filter uses this same system as long as the bandwidth of the splitter/combiner and amplifier cover the digitally tuned filter's tuning range and the phase and amplitude of the feedback signals are maintained. In particular, a frequency-hopping filter advantageously uses the Q- multiplication circuit

It is understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

What is claimed is:

1. A tunable Q-multiplier circuit having a signal input and a signal output comprising: a splitter having a first terminal coupled to the Q-multiplier's signal input; a second terminal; and a third terminal; a tunable filter section that is tunable to a range of frequencies and having an input coupled to the second terminal of the splitter, and having an output; a cornbiner having a first terminal coupled to the output of the filter section, a second terminal coupled to the Q-multiplier's signal output, and a third terminal; an amplifier coupled either between the splitter and the tunable filter section, or between the tunable filter section and the combiner; a phase-slope network coupled between the third terminal of the combiner and the third terminal of the splitter; whereby Q-multiplication is accomplished at frequencies as determined by the tunable filter section without needing to change characteristics of the splitter, combiner or amplifier.

2. The tunable Q-multiplier according the claim 1 , wherein the tunable filter section responds to an electronic signal to adjust a characteristic frequency.

3. The tunable Q-multiplier according the claim 2, wherein the electronic signal is a digital value representative of the characteristic frequency.

4. The tunable Q-multiplier according the claim 2, wherein the tunable filter section includes a notch filter having a notch frequency centered at the characteristic frequency.

5. The tunable Q-multiplier according the claim 1, further comprising a filter controller coupled to the tunable filter section, the filter controller including a look-up table memory that converts an input frequency value to a filter-control value.

6. The tunable Q-multiplier according the claim 1, further comprising a filter controller coupled to the phase-slope network, the filter controller including a look-up table memory that converts an input frequency value to a phase-control value.

7. The tunable Q-multiplier according the claim 6, wherein the filter controller coupled to the phase-slope network further includes a digital-to-analog converter having an analog output, and the phase-slope network includes a varactor whose capacitance is controlled by the analog output.

8. The tunable Q-multiplier according the claim 1, wherein the tunable filter section responds to an electronic signal to adjust a characteristic frequency across a range of approximately 30 megahertz to approximately 108 megahertz, and wherein the splitter, amplifier, and combiner provide broadband responses covering at least the range of approximately 30 megahertz to approximately 108 megahertz.

9. The tunable Q-multiplier according the claim 1, wherein the splitter provides two-way splitting, the combiner provides two-way combining, and the phase-slope network provides a phase compensation.

10. The tunable Q-multiplier according the claim 1, further comprising a radio-wave transmitting antenna coupled to the signal output of the Q-multiplier circuit.

11. The tunable Q-multiplier according the claim 1 , further comprising a radio-wave receiving antenna coupled to the signal input of the Q-multiplier circuit.

12. A method for increasing the Q of a broad-range tunable filter comprising the steps of: splitting an input signal between a first path signal and a second path signal; tunably filtering and amplifying the first path signal to produce a filtered and amplified signal; adjusting the second path signal to produce a phase-adjusted signal; and combining the filtered and amplified signal with the phase-adjusted signal to produce an output signal.

13. The method according the claim 12, wherein the step of tunably filtering and amplifying includes the step of electronically adjusting a characteristic frequency.

14. The method according the claim 12, wherein the step of tunably filtering and amplifying includes the step notch filtering, highpass filtering, or lowpass filtering.

15. The method according the claim 12, further comprising the step of using an input frequency value to electronically look up a filter-control value.

16. The method according the claim 12, further comprising the step of using an input frequency value to electronically look up a phase-control value.

17. The method according the claim 16, further comprising the step of using the phase-control value to generate an analog value used to control a phase of the phase-adjusted signal.

18. The method according the claim 12, wherein the step of tunably filtering and amplifying includes the step of responding to an electronic signal to adjust a characteristic frequency across a range of approximately 30 megahertz to approximately 108 megahertz.

19. The method according the claim 12, wherein the step of splitting provides two-way splitting, the step of combining provides two-way combining, and the step of adjusting provides a phase compensation.

20. The method according to claim 12 further comprising the step of: transmitting a radio wave signal corresponding to the output signal.

21. The method according to claim 12 further comprising the steps of: receiving a radio- wave signal and generating the input signal as corresponding to the received radio-wave signal.

22. A radio communications system comprising: a radio; one or more tunable Q-multiplier circuits in each of one or more signal paths of the radio, each Q-multiplier circuit having a signal input and a signal output and comprising: a splitter having a first terminal coupled to the Q- multiplier's signal input; a second terminal; and a third terminal; a tunable filter section that is tunable to a range of frequencies and having an input coupled to the second terminal of the splitter, and having an output; a combiner having a first terminal coupled to the output of the filter section, a second terminal coupled to the Q-multiplier's signal output, and a third terminal; an amplifier coupled either between the splitter and the tunable filter section, or between the tunable filter section and the combiner; a phase-slope network coupled between the third terminal of the combiner and the third terminal of the splitter; whereby Q-multiplication is accomplished at frequencies as determined by the tunable filter section without needing to change characteristics of the splitter, combiner or amplifier.

23. The radio communications system according the claim 22, wherein the radio includes both a receiver having one of the tunable Q-multiplier circuits in one of its signal paths, and a transmitter having one of the tunable Q-multiplier circuits in one of its signal paths.

24. The radio communications system according the claim 23, wherein the radio receiver and transmitter each operate on spread-spectrum radio signals.

25. The radio communications system according the claim 23, wherein the tunable filter section responds to an electronic signal to adjust a characteristic frequency, and the electronic signal includes a digital value representative of the characteristic frequency.

26. The radio communications system according the claim 22, further comprising a filter controller coupled to the tunable filter section, the filter controller including a look-up table memory that converts an input frequency value to a filter-control value.

27. The radio communications system according the claim 22, further comprising a filter controller coupled to the phase-slope network, the filter controller including a look-up table memory that converts an input frequency value to a phase-control value.

28. The radio communications system according the claim 27, wherein the filter controller coupled to the phase-slope network further includes a digital-to- analog converter having an analog output, and the phase-slope network includes a varactor whose capacitance is controlled by the analog output.

29. The radio communications system according the claim 22, further comprising: a filter controller; wherein the filter controller is coupled to the tunable filter section, the filter controller including a look-up table memory that converts an input frequency value to a filter-control value; and wherein the filter controller is coupled to the phase-slope network, the filter controller including a look-up table memory that converts an input frequency value to a phase-control value and a digital-to-analog converter having an analog output, and a phase of the phase-slope network is controlled by the analog output.

30. The radio communications system according the claim 22, wherein the tunable filter section responds to an electronic signal to adjust a characteristic frequency across a range of approximately 30 megahertz to approximately 108 megahertz, and wherein the splitter, amplifier, and combiner provide broadband responses covering at least the range of approximately 30 megahertz to approximately 108 megahertz.

31. A radio communications system comprising: a radio; and a tunable Q-multiplier circuit in a transmitter signal path of the radio, and having a signal input and a signal output, the Q-multiplier circuit comprising: a splitter having a first terminal coupled to the Q- multiplier's signal input; a second terminal; and a third terminal, wherein the splitter provides two-way splitting; a tunable filter section that is tunable to a range of frequencies and controlled by an output of a look-up table memory that converts an input frequency value to a filter-control value, and having an analog signal input coupled to the second terminal of the splitter, and having an output; a combiner having a first terminal coupled to the output of the filter section, a second terminal coupled to the Q-multiplier's signal output, and a third terminal, wherein the combiner provides two-way combining; an amplifier coupled either between the splitter and the tunable filter section, or between the tunable filter section and the combiner; and a phase-slope network coupled between the third terminal of the combiner and the third terminal of the splitter, wherein the phase-slope network controlled by an output of a look-up table memory that converts an input frequency value to a phase-control value wherein the phase-slope network provides a phase compensation.

32. The radio communications system according the claim 31 , further comprising a radio-wave transmitting antenna coupled to the signal output of the Q-multiplier circuit in the transmitter signal path of the radio.

33. The radio communications system according the claim 31 , further comprising: a tunable Q-multiplier circuit in a receiver signal path of the radio, and having a signal input and a signal output, the Q-multiplier circuit comprising: a splitter having a first terminal coupled to the Q- multiplier's signal input; a second terminal; and a third terminal, wherein the splitter provides two-way splitting; a tunable filter section that is tunable to a range of frequencies and controlled by an output of a look-up table memory that converts an input frequency value to a filter-control value, and having an analog signal input coupled to the second terminal of the splitter, and having an output; a combiner having a first terminal coupled to the output of the filter section, a second terminal coupled to the Q-multiplier's signal output, and a third terminal, wherein the combiner provides two-way combining; an amplifier coupled either between the splitter and the tunable filter section, or between the tunable filter section and the combiner; and a phase-slope network coupled between the third terminal of the combiner and the third terminal of the splitter, wherein the phase-slope network controlled by an output of a look-up table memory that converts an input frequency value to a phase-control value wherein the phase-slope network provides a phase compensation.

34. The radio communications system according the claim 33, further comprising a radio-wave receiving antenna coupled to the signal input of the Q- multiplier circuit in the receiver signal path of the radio.

35. A radio communications system comprising: a radio; and a tunable Q-multiplier circuit in a receiver signal path of the radio, and having a signal input and a signal output, the Q-multiplier circuit comprising: a splitter having a first terminal coupled to the Q- multiplier's signal input; a second terminal; and a third terminal, wherein the splitter provides two-way splitting; a tunable filter section that is tunable to a range of frequencies and controlled by an output of a look-up table memory that converts an input frequency value to a filter-control value, and having an analog signal input coupled to the second terminal of the splitter, and having an output; a combiner having a first terminal coupled to the output of the filter section, a second terminal coupled to the Q-multiplier's signal output, and a third terminal, wherein the combiner provides two-way combining; an amplifier coupled either between the splitter and the tunable filter section, or between the tunable filter section and the combiner; and a phase-slope network coupled between the third terminal of the combiner and the third terminal of the splitter, wherein the phase-slope network controlled by an output of a look-up table memory that converts an input frequency value to a phase-control value wherein the phase-slope network provides a phase compensation.

36. The radio communications system according the claim 35, further comprising a radio-wave receiving antenna coupled to the signal input of the Q- multiplier circuit.