Abstract:
A signal receiving system for receiving messages from each of several unequal amplitude FM carriers occupying the same portion of the frequency band.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to frequency modulation (FM) radio systems, more particularly, systems for receiving all desired broadcast messages from multiple FM carriers of the unequal amplitude. In addition, an inherent feature of the present invention is that all suppressed signals, both carrier and modulation, are recoverable to substantially the same extent. 
     In a conventional frequency modulation (FM) radio receiver, the demodulator circuit suppresses weaker FM signals. The receiver is said to be captured by the strongest signal and all presently known FM demodulators exhibit this &#34;capture effect.&#34; Capture effect, a weak-signal-suppression characteristic, is a well-known property of frequency modulation theory. 
     The prior art includes work by E. J. Baghdady as described in &#34;Signal Cancellation Techniques for Capturing the Weaker of Two Co-Channel FM Signals&#34;, Electro-Magnetic Wave Propagation, 1960, pages 183-207, Academic Press. Two signal cancellation techniques were discussed, namely, (1) dynamic trapping and (2) feed-forward. 
     Dynamic trapping attempts to reduce the amplitude of the stronger signal by tuning an electrical bandpass filter. The filter reduces the signal strength of the stronger signal so that it becomes the weaker signal. A conventional demodulator is then used to recover the message from its dominant input, which was originally the weaker desired signal. The theoretical performance of the dynamic trapping technique is uncertain, since it appears to require that the concepts of instantaneous frequency and Fourier transform frequency be equated. Therefore, Baghdady relies on experimental results. 
     The feed-forward technique uses signal suppression provided by narrow band limiters. Such limiters are also difficult to analyze mathematically and again, Baghdady relies on experimental results for supporting this technique. By the very principles of their operation, neither dynamic trapping nor feed-forward function as intended when the instantaneous frequency of the applied FM signals are equal or approximately equal. 
     The prior art also includes U.S. Pat. Nos. 3,226,646 -Ludwig, 3,753,123 - Carpenter, et. al, and 4,739,518 -Bickley, et. al. All of these references attempt to recover information from a weaker signal in the presence of one or more stronger signals by signal cancellation and related filtering techniques. 
     In Ludwig, a cancellation bridge is used for cancellation of interfering signals and employs a strong signal tracking filter for isolating the interference. The tracking filter incorporates a high signal capture-type demodulator, which provides an instantaneous analog signal for controlling coincidence of the instantaneous center frequency of an electronically tunable filter with the frequency of the undesired stronger signal. Only one output, namely the demodulated weaker signal, is obtained. Again, the concepts of instantaneous frequency and Fourier transform frequency are equated to explain the operation. 
     Carpenter, et. al., describes a system for subtracting unwanted signals from input signals to provide error signals at the output. This reference also teaches use of a phase-locked loop (PLL) in signal extraction means for producing an estimate signal, which includes any incidental amplitude variations of the carrier signal, and which is vectorially subtracted from the input signal. More than one estimate signal may be derived for vector subtraction and, since each extractor remains locked on its own signal, the effect of removing one or more input signals by such subtraction has little or no effect on the remaining signals. Cancellation only of undesired signals is provided. 
     Finally, in Bickley et. al., the capture effect of a limiter is used to detect a desired signal received with an interfering signal at nearly the same frequency as the desired signal but at significantly greater amplitude. In the described system, a gain-controlled amplifier provides a constant amplitude signal having the amplitude of the interfering signal portion equal to the amplitude of the interfering signal portion produced by the limiter. The constant amplitude and limiter signals combine through a subtraction operation to effectively cancel the interfering signal while causing only small attenuation of the desired signal. Again, only suppression of undesired signals is described. 
     None of these references apparently appreciate other advantages and uses of capture effect of FM demodulators. In addition, while Carpenter, et. al, uses a phase-locked loop (PLL) in extracting signals from input signals, none of the references appreciate other advantages and uses of phase-locked loop technology for discriminating more than one signal component of the input signal. 
     SUMMARY OF THE INVENTION 
     In the present invention, capture effect associated with frequency demodulators is used in cooperation with phase-locked loops in a new demodulator to provide improved demodulation of all of several FM carriers including weaker signals in the presence of dominant carriers. A frequency demodulator converts the instantaneous frequency of the applied signal to a voltage. When the sum of two or more signals is present at the input to the demodulator of the present invention, the output voltage m o  (t) is proportional to the instantaneous frequency of the dominant portion of the input signal. Thus, the dominant signal is said to capture the demodulator. 
     When m o  (t) is used to frequency modulate another sinusoid, then a replica of the original dominant signal is created in the receiver. The replica signal is now isolated from the other weaker receiver input signals and can be subtracted from the composite input to effectively suppress the dominant signal. After subtraction, weaker carriers of the input signal remain. The remaining signal then can be successively demodulated in the same way virtually as many times as desired for demodulation of as many component carriers of the input signal as desired. 
     For effective cancellation using the present invention, the carrier frequency and the extent of modulation of the replica signal must be substantially the same as the dominant signal. In practice, it is not possible to achieve this condition using a conventional signal source and frequency modulator. However, by using a voltage-controlled oscillator (VCO) in a PLL, near-exact replication of the dominant signal is possible. Thus, when the phase-locked loop is operated as a frequency demodulator, the output of the VCO is the replica of the dominant input signal to be cancelled. 
     Recovery of weaker signal information is inherent in the design of a signal receiving system constructed according to the principles of the present invention. A plurality of output signals representing the messages contained in the modulation of a succession of dominant input signals, derived from remaining components of the original input signal in descending order of dominance, is obtainable from each phase-locked loop demodulator comprising the signal receiving system of the present invention. Consequently, this invention makes possible multiple reuse of FM bands, that is, several FM carriers having unrelated messages can coexist in the same frequency band with all or selected messages being recoverable by application of this invention. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of a signal receiving system constructed according to the principles of the present invention. 
     FIG. 2 is a block diagram of the variable delay employed in the system of FIG. 1. 
     FIG. 3 is a block diagram of the variable-gain difference amplifier employed in the system of FIG. 1. 
     FIG. 4 is a block diagram of the PLL employed in the system of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, signal receiving system 10 comprises a plurality of PLL demodulators, each of which includes a mixer, lowpass filter (LPF) and VCO. The system further includes pluralities of variable delays and variable-gain difference amplifiers coupled, respectively, to each of the phase-locked loop demodulators as shown and further described elsewhere in this specification. 
     With the continuing reference to FIG. 1, input signal V i  (t) is assumed to include many FM carrier signals of various strengths and will be described in more detail elsewhere in this specification. The receiving system of the present invention may be connected to the front end, i.e. radio frequency (RF) antenna, RF amplifier, mixer and intermediate frequency (IF) amplifier, of any conventional superheterodyne radio receiver. In such systems, the mixer downconverts received RF energy into a received IF signal. Typically, the received IF signal preserves the signal-to-interference ratio of the received RF energy, and the frequencies of the desired as well as interfering signals. Thus, frequency components of the RF energy are preserved in the IF signals. 
     Phase-locked loop 11 (also referred to as PLL 1 ) comprises mixer 102, LPF 103 and VCO 104. One input of mixer 102 is coupled to the input signal V i  (t). The output of mixer 102 is coupled to the input of LPF 103. The output of LPF 103 is coupled to the input of VCO 104 and produces message information m 1  (t) demodulated from the most dominant FM signal. The output of VCO 104 is applied to another input of mixer 102 and to the input of difference amplifier 106. 
     PLL 2 , PLL 3 . . . PLL n  are essentially the same circuits as that just described for PLL 1 . 
     The input signal is also applied to the input of variable delay 105. The outputs of delay 105 and VCO 104 are applied to the inputs of variable-gain difference amplifier 106. The output of amplifier 106 comprises input signal V i  (t) with the most dominant carrier suppressed. Demodulation of the most dominant carrier of the output signal from amplifier 106 is provided by PLL 2 . It should be noted that the most dominant carrier of the output signal from amplifier 106 is, typically, the second most dominant carrier of input signal V i  (t). 
     Since PLL 3  is substantially the same as PLL 1  and PLL 2 , demodulation of the third most dominant carrier of input signal V i  (t) is provided by PLL 3 . The input of PLL 3  is coupled to the output of variable-gain amplifier 116. One input of variable-gain amplifier 116 is coupled to the output of the VCO forming a part of PLL 2  (not shown). The other input of difference amplifier 116 is coupled to the output of variable-gain amplifier 106 via delay 115. Again, the output signal from variable-gain amplifier 116 is the input signal V i  (t) with the first two most-dominant carriers suppressed. 
     To the extent that the strength of the individual carriers of input signal V i  (t) permit, any number of individual demodulations of the carriers of V i  (t) can be obtained as each such carrier becomes dominant in later stages of the receiving system of the present invention. Thus, the input to phase-locked loop 14 (PLL n ) is V i  (t) demodulated n times, where n is the number of demodulated carriers desired, assuming appropriate amplification of weaker and weaker signals can be added to later stages of receiving system 10 if required. Thus, recovery of the information contained in the modulation of all carriers of V i  (t), in descending order of dominance, is obtained. 
     Variable delay 105 may also be merely a variable-phase shift circuit for appropriately adjusting the phase of the output from the previous stage of receiving system 10 for coherence with the output of VCO 104. Delay 105 may use operational amplifier circuit techniques in order to alter the signal phase in a precise and predictable manner. Thus, variable delay 105 may include four (4) LM-318 operational amplifier stages, where each stage has the possibility of continuously variable phase change from 0° to 90°, as shown in FIG. 2. 
     Variable-gain difference amplifier 106 may be any circuit suitable for combining a signal having the same amplitude and frequency as one component of another signal. Such circuits are well known and may include an LM-318 operational amplifier configured as shown in FIG. 3. 
     Phase-locked loops, PLL 1 . . . PLL n , are conventional, each of which consisting of such wellknown components as a mixer, a lowpass filter and a reference voltage-controlled oscillator for producing a reference or replica signal. Typically, all of the components of a PLL are integrated as a single semiconductor product, such as part number 562, manufactured by Signetics, Inc., which may be used in the present invention as shown in FIG. 4. 
     When the VCO follows the frequency change of V i  (t), the VCO output is the frequency of the dominant component of V i  (t) because of capture effect. 
     Input signal V i  (t) is given by the following relation: ##EQU1## 
     Modulation components, m 1 . . . m n , are arbitrary. Carrier frequencies, f 1 , f 2 . . . f n  where ##EQU2## are typically in the same band. If all other components or V i  (t), namely j 2  (t), do not exceed in amplitude the carrier signal s 1  (t), then the output x 1  (t) of PLL 1  is equal to m 1  (t) because of the capture effect of the first demodulator. Since message m 1  (t) is recovered from the dominant carrier of V i  (t), and since VCO 104 is a frequency modulator itself being modulated by m 1  (t), then y 1  (t) of FIG. 1 is a replica of the dominant carrier of V 1  (t). 
     Referring again to FIG. 1, the delays and variable-gain summers are adjusted to minimize the level of previously dominant signals present at the input of the PLL of interest. Owing to the capture effect of subsequent FM demodulator stages, complete suppression of the undesired signal components is unnecessary, i.e., it is not necessary that z k  (t) equals j k  (t), where z k  (t) is a signal in which s k+1  (t) is the dominant component. Rather, it is only necessary that the level of s k  (t) merely exceed the level of all other signal components of j k  (t) where k is any integer in the range 1, 2, . . . n, in order to recover information from any component of V i  (t). 
     While the present invention has been particularly shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that changes in form and detail may be made without departing from the spirit of the invention or exceeding the scope of the appended claims. In particular, for example, this invention may be used for phase modulated carriers as well as for FM carriers.