FM demodulator system with threshold extension

A FM demodulator system which provides threshold extension by frequency selective phase modulation of the input FM signal in a band pass filter to reduce phase error between the input and output of the filter. The output of the filter is applied to an FM discriminator which provides a base band signal. This base band signal is applied to a frequency selective phase compensation network to derive a signal which phase modulates the band pass filter. The filter rejects uncorrelated thermal noise so as to effect threshold extension in the demodulator system.

DESCRIPTION 
The present invention relates to signal threshold extension FM 
demodulation, and particularly to an FM demodulator system with threshold 
extension capability. 
The invention is especially suitable for use in receivers used in those 
satellite communication systems where FM television signals are 
transmitted. 
Threshold extension of FM signals has been obtained using various 
techniques, such as FM feedback with tracking filters tuned so that their 
center frequency tracks the instantaneous frequency of the FM signal. Such 
feedback techniques generally involve the use of regenerative limiters and 
voltage controlled oscillators. The latter technique is discussed in 
reference data for Radio Engineers, Fifth Edition, pages 21-12 and 21-13. 
Phase locked loop demodulators are also used in FM threshold extension. A 
survey of such techniques and a system using a regenerative limiter for 
threshold extension will be found in U.S. Pat. No. 4,101,837 issued to 
Clayton and Livaditis on July 18, 1978. Circuits for improving FM 
threshold extension, of the type discussed above, are implemented using 
oscillating circuits, the performance of which may vary unpredictably 
because of instabilities which develop over time and with temperature 
variations. While such circuits have performed satisfactorily in many 
applications, they are more complex and critical in design than is 
desired. 
It is an object of the present invention to provide an improved FM 
demodulator system with threshold extension which eliminates oscillating 
and regenerative circuits which can be unstable in frequency and amplitude 
response due to aging and environmental effects, and also to provide a 
system which can be implemented by circuitry less complex than that used 
in FM receivers which involve the use of oscillating and regenerative 
circuits. 
It is another object of the present invention to provide an improved FM 
demodulator system which extends the FM threshold by rejecting noise, such 
as thermal noise, which is uncorrelated with the FM signal. 
It is a further object of the present invention to provide an improved 
threshold extending FM demodulator system which allows the use of a filter 
of lower band width than that occupied by the effective spectrum of the FM 
signal and therefore has a smaller band width to noise than predetection 
filters conventionally used in FM receivers so that noise outside the band 
width can be reduced and threshold extension results. 
It is a still further object of the present invention to provide an 
improved FM demodulator system which enables the use of a filter of 
narrower band width than that of the FM spectrum of interest for threshold 
extension by providing phase modulation in the filter, causing the phase 
of the output signal from the filter to track the phase of the FM input 
signal thereby precluding signal distortion and consequent degradation of 
the threshold by such a narrower band width filter. 
Briefly described, a system for demodulating FM signals and providing 
threshold extension in accordance with the invention makes use of means 
for band pass filtering input FM signals to be demodulated and providing 
output signals. The band pass filtering means includes means for 
modulating the phase of the input FM signals in accordance with a steering 
signal. Means are provided including an FM discriminator for demodulating 
the output FM signals from the filter to provide demodulated signals. 
These demodulated signals are video base band signals when the input FM 
signal is a TV signal of higher frequency, for example from the 
intermediate frequency stages of the receiver incorporating the 
demodulating system. Means are provided in the demodulating system which 
are responsive to the output base band signal for providing a signal which 
tracks the phase of the input FM signal and applying this phase tracking 
signal to the filter as the steering signal. The steering signal providing 
means is implemented by a frequency sensitive phase compensating circuit. 
The band pass filter may have a band width less than the band width of the 
input FM signal. The phase modulating means is desirably a parametric 
phase modulating circuit in the band pass filter utilizing variable 
capacitor (varactor) diodes.

Referring to FIG. 1 there is shown a system for demodulating FM signals, 
such as are obtained from the IF (intermediate frequency) circuits of a 
satellite receiver which receives TV signals from a satellite. The FM 
modulated signal passes through a band pass (band limiting) filter 10. 
This filter includes phase modulating means. The output signal from the 
filter 10 is applied to an envelope limiter 12. The output of the limiter 
12 contains the FM carrier modulated by both the desired signal plus 
thermal noise. An FM discriminator circuit 14 detects both the FM 
modulation as well as the phase noise present at the discriminator input. 
The output of the discriminator is the demodulated video base band signal. 
This output is applied to a frequency selective phase compensating circuit 
16. This circuit provides a phase compensated output as a phase steering 
signal to the phase modulating circuits of the band pass filter 10. 
The filter 10 reduces the noise presented to the limiter 12 thereby 
providing FM threshold extension. The filter 10 is of lesser band width 
than the effective FM spectrum. The effective FM spectrum is meant to 
include the bandwidth occupied by the modulated FM carrier. The pass band 
of the filter 10 is lower than this spectrum and therefore lower than the 
band width of predetection filters which are conventionally used in FM 
receivers. 
The filter 10 taken alone would cause severe signal distortion as well as 
threshold penalty, since it filters signal spectral content as well as 
unwanted thermal noise. The frequency sensitive phase compensating circuit 
insures that the FM spectrum will be accurately transmitted while the 
uncorrelated thermal noise is rejected. The steering signal from the phase 
compensating circuit 16 parametrically phase modulates the band pass 
filter so that the output of the filter phase tracks the input FM signal. 
This tracking is achieved using the output signal from the FM 
discriminator circuit 14. The phase of this signal is compensated in 
accordance with the frequency of the output signal from the discriminator 
14 to provide a closed loop response which achieves thermal noise 
rejection without introducing signal distortion in the base band output 
signal from the discriminator circuit 14. 
The phase modulated band pass filter 10 is shown in FIG. 2. It includes a 
double mesh filter circuit wherein the capacitance in the branches is 
provided primarily by varactor diodes 19 and 20. The diodes are biased to 
their operating point from a source of operating voltage indicated at +V. 
The voltage is adjustable using a potentiometer 22. The response of this 
filter is shown in FIG. 3. 
The limiter and discriminator 12 and 14 may be provided by an integrated 
circuit 24. The base band output signal from this circuit 24 is applied to 
the frequency sensitive phase compensating circuit 16 via a coupling 
resistor 26. The response of the circuit 16 versus frequency is 
illustrated in FIG. 4. The output of the circuit 16 is coupled through a 
capacitor 28, an inductor 30 and a resistor 32 to the node of the filter 
10 to which the varactors 19 and 20 are connected. 
The phase modulating effects of the steering signal on the output of the 
filter 10 as well as the characteristics of the filter 10 cause phase 
modulation which generates the required FM side bands filtered by the band 
pass filter characteristic. No oscillating circuitry is used so the design 
is stable and reliable with aging and temperature. The system is 
insensitive to variations in the level of the input FM signal since noise 
band width is not a function of input level. The following lists an 
exemplary complement of components for use in the system and their nominal 
values: 
Diode 19--1.94 pF 
Diode 20--1.94 pF 
Inductor L1--6 nH (nano-Henry) 
Inductor L2--6 nH (nano-Henry) 
Inductor L3--46 nH (nano-Henry) 
Inductor L4--46 nH (nano-Henry) 
Inductor L5--100 .mu.H (micro-Henry) 
Inductor L6--0.47 .mu.H (micro-Henry) 
Inductor L7--0.22 .mu.H (micro-Henry) 
Capacitor C1--68 pF 
Capacitor C2--1600 pF 
Capacitor C3--2700 pF 
Capacitor C4--1600 pF 
Resistor 26--75 ohms 
Resistor 32--56 ohms 
Inductor 30--0.27 .mu.H (micro-Henry) 
From the foregoing description it will be apparent that there has been 
provided an improved system for demodulating FM signals and providing 
threshold extension. Variations and modifications in the herein described 
system, within the scope of the invention, will undoubtedly become 
apparent to those skilled in the art. Accordingly, the foregoing 
description should be taken as being illustrative and not in a limiting 
sense.