US4087756A
Expired / Lapsed
US
⏱️ 0.0 years remaining
FM Feedback demodulator having threshold extension circuit with two-pole crystal filters
Assignee: Harris Corp
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Filing: 1977-06-14
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Granted: 1978-05-02
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Expires: 1997-06-14
Abstract
Abstract Demodulation of FM signals employing an FM feedback loop of the present invention includes a first mixer circuit for mixing an IF input signal with a voltage controlled oscillator signal to down-convert the IF input signal to a predescribed center frequency IF signal. This signal is coupled through a tuned amplifier having an automatic gain control loop and is then filtered in a predetection filter and amplified. The predetection filter is composed of a two-pole crystal bandpass filter having a very narrow bandwidth. The filtered signal is then coupled to a frequency discriminator circuit comprised of a frequency detection second mixer and filter circuit, the frequency detection filter circuit being composed of a second two-pole crystal bandpass filter, the bandwidth of which is an order of magnitude larger than that of the predetection filter. The output of the frequency discriminator is coupled through a loop compensation filter to the voltage controlled oscillator so that changes in the output frequency of the voltage controlled oscillator reduce the deviation of the FM signal in the intermediate frequency and frequency discriminator stages. The use of two-pole crystal bandpass filters improves the frequency stability of the system and significantly reduces post detection noise levels, as compared with conventional limiter/discriminator techniques. The two-pole crystal bandpass filters have a Butterworth configuration. In addition to the FM feedback loop, a squelch circuit is also coupled to the output of the frequency discriminator.
Inventors
- 👤 Walter McDowell Rogers, Jr.
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-10-25 11:49
CPC Classifications
H; H03; H03D; H03D3/00; H03D3/001; H03D3/003; H03D3/004
Claims Analysis
Total Claims: 10
Claim 1
Independent Claim
230 words
In a frequency modulation feedback demodulator having: a first mixer circuit for receiving an IF input FM signal and a local oscillator signal and converting said IF input FM signal to a first frequency-converted IF FM signal, an IF amplifier stage coupled to said first mixer circuit for amplifying and selectively filtering said first frequency-converted IF FM signal within a first prescribed narrow bandwidth, an FM discriminator stage, coupled to said IF amplifier stage, for selectively filtering the output of said IF amplifier stage within a second prescribed narrow bandwidth wider than said first prescribed narrow bandwidth, and mixing the selectively filtered output of said IF amplifier stage with the output of said IF amplifier stage to obtain an FM detection discriminator output signal, a controlled oscillator having an output coupled to said first mixer circuit for supplying said local oscillator signal to said first mixer circuit, and a feedback connection coupling the output of said FM discriminator stage to said controlled oscillator for causing the frequency of the local oscillator signal to change to reduce the deviation of the FM signal through said IF amplifier and FM discriminator stages, the improvement wherein said IF amplifier stage includes a predetection filter comprising a two-pole bandpass filter having said first prescribed bandwidth, said FM discriminator stage includes a frequency discriminator filter comprising a two-pole bandpass filter having said second prescribed bandwidth.
Claim 2
Dependent Claim
230 words
In a frequency modulation feedback demodulator having: a first mixer circuit for receiving an IF input FM signal and a local oscillator signal and converting said IF input FM signal to a first frequency-converted IF FM signal, an IF amplifier stage coupled to said first mixer circuit for amplifying and selectively filtering said first frequency-converted IF FM signal within a first prescribed narrow bandwidth, an FM discriminator stage, coupled to said IF amplifier stage, for selectively filtering the output of said IF amplifier stage within a second prescribed narrow bandwidth wider than said first prescribed narrow bandwidth, and mixing the selectively filtered output of said IF amplifier stage with the output of said IF amplifier stage to obtain an FM detection discriminator output signal, a controlled oscillator having an output coupled to said first mixer circuit for supplying said local oscillator signal to said first mixer circuit, and a feedback connection coupling the output of said FM discriminator stage to said controlled oscillator for causing the frequency of the local oscillator signal to change to reduce the deviation of the FM signal through said IF amplifier and FM discriminator stages, the improvement wherein said IF amplifier stage includes a predetection filter comprising a two-pole bandpass filter having said first prescribed bandwidth, said FM discriminator stage includes a frequency discriminator filter comprising a two-pole bandpass filter having said second prescribed bandwidth.
Claim 3
Dependent Claim
18 words
The improvement according to claim 1, wherein each of said two-pole bandpass filters comprises a two-pole crystal filter.
Claim 4
Dependent Claim
20 words
The improvement according to claim 2, wherein said IF input FM signal is a single channel per carrier FM signal.
Claim 5
Independent Claim
191 words
A method of demodulating a single channel per carrier FM signal comprising the steps of: (a) combining said single channel per carrier FM signal at a prescribed intermediate frequency in a first mixer circuit with a local oscillator signal so as to convert said FM signal to a first frequency converted IF FM signal, (b) selectively filtering and amplifying said first frequency-converted IF FM signal within a first prescribed narrow bandwidth, (c) further selectively filtering the signal obtained from step (b) within a second prescribed narrow bandwidth greater than said first prescribed narrow bandwidth while controlling the phase of said obtained signal in accordance with the frequency thereof, (d) combining the signals obtained from steps (b) and (c) in a second mixer circuit to provide an FM discriminator output signal, and (e) controlling said local oscillator signal in accordance with the level of said FM discriminator output signal, and wherein each of steps (b) and (c) includes the selective filtering of said first frequency-converted IF FM signal and the signal obtained from step (b), respectively, in a respective two-pole crystal bandpass filter having said first and second prescribed narrow bandwidths, respectively.
Claim 6
Independent Claim
191 words
A method of demodulating a single channel per carrier FM signal comprising the steps of: (a) combining said single channel per carrier FM signal at a prescribed intermediate frequency in a first mixer circuit with a local oscillator signal so as to convert said FM signal to a first frequency converted IF FM signal, (b) selectively filtering and amplifying said first frequency-converted IF FM signal within a first prescribed narrow bandwidth, (c) further selectively filtering the signal obtained from step (b) within a second prescribed narrow bandwidth greater than said first prescribed narrow bandwidth while controlling the phase of said obtained signal in accordance with the frequency thereof, (d) combining the signals obtained from steps (b) and (c) in a second mixer circuit to provide an FM discriminator output signal, and (e) controlling said local oscillator signal in accordance with the level of said FM discriminator output signal, and wherein each of steps (b) and (c) includes the selective filtering of said first frequency-converted IF FM signal and the signal obtained from step (b), respectively, in a respective two-pole crystal bandpass filter having said first and second prescribed narrow bandwidths, respectively.
Claim 7
Dependent Claim
284 words
A frequency modulation feedback demodulator for demodulating single channel per carrier FM input signals comprising: a first mixer circuit having a first input port to which an intermediate frequency FM input signal is applied, a second input port to which a local oscillator signal is supplied, and an output port from which a first frequency-inverted IF FM signal is derived; a tuned IF amplifier circuit connected to the output port of said first mixer circuit and having an automatic gain control loop for controlling the output amplitude thereof; an IF amplifier circuit coupled to the output of said tuned IF amplifier; a predetection filter comprised of a two-pole crystal bandpass filter having a first prescribed narrow bandwidth coupled to the output of said IF amplifier circuit; an amplifier driver circuit connected to the output of said predetection filter; a frequency/discriminator network including a series-connected attenuator/frequency detection filter circuit, including a two-pole crystal filter having a second prescribed narrow bandwidth greater than said first prescribed narrow bandwidth, coupled to the output of said amplifier driver circuit, and a second mixer circuit having a first input port connected to the output of said amplifier driver circuit, a second input port coupled to said two-pole crystal filter having said second prescribed bandwidth, and an output port; a loop compensation filter connected to the output port of said mixer circuit; a voltage controlled oscillator having an input coupled to said loop compensation filter and an output connected to the said second input port of said first mixer circuit for supplying said local oscillation signal thereto; and an output amplifier circuit coupled to the output of said loop compensation filter from which a baseband demodulated FM signal is derived.
Claim 8
Dependent Claim
284 words
A frequency modulation feedback demodulator for demodulating single channel per carrier FM input signals comprising: a first mixer circuit having a first input port to which an intermediate frequency FM input signal is applied, a second input port to which a local oscillator signal is supplied, and an output port from which a first frequency-inverted IF FM signal is derived; a tuned IF amplifier circuit connected to the output port of said first mixer circuit and having an automatic gain control loop for controlling the output amplitude thereof; an IF amplifier circuit coupled to the output of said tuned IF amplifier; a predetection filter comprised of a two-pole crystal bandpass filter having a first prescribed narrow bandwidth coupled to the output of said IF amplifier circuit; an amplifier driver circuit connected to the output of said predetection filter; a frequency/discriminator network including a series-connected attenuator/frequency detection filter circuit, including a two-pole crystal filter having a second prescribed narrow bandwidth greater than said first prescribed narrow bandwidth, coupled to the output of said amplifier driver circuit, and a second mixer circuit having a first input port connected to the output of said amplifier driver circuit, a second input port coupled to said two-pole crystal filter having said second prescribed bandwidth, and an output port; a loop compensation filter connected to the output port of said mixer circuit; a voltage controlled oscillator having an input coupled to said loop compensation filter and an output connected to the said second input port of said first mixer circuit for supplying said local oscillation signal thereto; and an output amplifier circuit coupled to the output of said loop compensation filter from which a baseband demodulated FM signal is derived.
Claim 9
Dependent Claim
93 words
A frequency modulation feedback demodulator according to claim 5, further comprising a squelch circuit connected to the output of said loop compensation filter, said squelch circuit including a highpass filter receiving the output of said loop compensation filter, a fullwave rectifier connected to the output of said highpass filter, a smoothing filter connected to the output of said fullwave rectifier, and a differential amplifier comparator circuit for comparing the output voltage level of said smoothing filter with a reference voltage level, the squelch output signal being derived from said differential amplifier comparator circuit.
Claim 10
Dependent Claim
63 words
A frequency modulation feedback demodulator according to claim 6, wherein said automatic gain control loop comprises an IF amplifier connected to the output of said tuned IF amplifier circuit, an envelope detector connected to the output of said IF amplifier, and an integrator circuit connected between the output of said envelope detector and an AGC control terminal of said tuned IF amplifier circuit.
References Cited (Backward Citations)
USPTO Annuity & Maintenance Windows
| Annuity Stage | Window Opens | Payment Deadline | Surcharge Period Until | Estimated Fee | Status |
|---|---|---|---|---|---|
| 3.5 Years (Year 4) | 1981-05-01 | 1981-10-31 | 1982-05-02 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 1985-05-01 | 1985-10-31 | 1986-05-02 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 1989-05-01 | 1989-10-31 | 1990-05-02 | $7700.00 | Window Closed / Lapsed |