Abstract An FM demodulation system having means for noise reduction. The system has a signal path which includes a limiter, an FM demodulation circuit and a gate circuit, and a control circuit which includes a differentiator, two mono-stable-multivibrators, an adder and a gate signal generator. The input of this control circuit is coupled to the output of the limiter and the output of the control circuit is coupled to the control terminal of the gate circuit so as to apply a gate signal to the gate circuit. The gate circuit in the signal path operates in such a way that a demodulated signal is passed to the output terminal of the system when the gate signal is not present and the demodulated signal is prevented from passing to the output terminal when the gate signal is present. The control circuit generates the gate signal when a dropout in the FM signal occurs in the following manner. The signal from the limiter is differentiated by the differentiator and is formed into a narrow pulse train having positive and negative polarities. The positive spikes and the negative spikes of this pulse train trigger the respective ones of two mono-stable-multivibrators, and the outputs from these multivibrators are added by the adder. The output pulse-width of these multivibrator is controlled so that a marker signal appears at the output of the adder when carrier dropout in the FM signal occurs. The gate signal generator generates the gate signal from the marker signal and the resulting gate signal is fed to the control terminal of the gate circuit. a result, a noise component does not appear at the output terminal.
An FM demodulation system for accomplishing noise reduction comprising: a limiter which receives an input frequency modulated signal to be demodulated and eleminates an amplitude variation in the input frequency modulated signal; a demodulation means coupled to said limiter for demodulating said input frequency modulated signal; a differentiator connected to the output of said limiter, said differentiator differentiating the output of said limiter and forming a narrow pulse train having positive and negative spikes; a first mono-stable-multivibrator which is coupled to the output of said differentiator and is triggered by the positive spikes of the pulse train from said differentiator; a second mono-stable multivibrator which is coupled to the output of said differentiator and is triggered by the negative spikes of the pulse train from said differentiator, the output pulses of said first and second mono-stable-multivibrators having a pulse-width with a length of time slightly shorter than the interval of the zero crossing points of said input frequency modulated signal where the frequency deviation of said input frequency modulated signal becomes a maximum in a negative direction; an adder coupled to said mono-stablemultivibrators for adding said output pulses of said monostable-multivibrators; a gate signal generator coupled to said adder for generating a gate signal according to the added output from said adder; and a gate circuit which is coupled to the output of said demodulation means and which has a control terminal coupled to said gate signal generator to receive said gate signal from said gate signal generator, said gate circuit passing the signal from said demodulating means when said gate signal is not present at said control terminal and preventing said signal from passing when said gate signal is present at said control terminal, whereby a demodulated signal in which the noise component is eliminated appears as output of said system.
Claim 2
Dependent Claim
302 words
An FM demodulation system for accomplishing noise reduction comprising: a limiter which receives an input frequency modulated signal to be demodulated and eleminates an amplitude variation in the input frequency modulated signal; a demodulation means coupled to said limiter for demodulating said input frequency modulated signal; a differentiator connected to the output of said limiter, said differentiator differentiating the output of said limiter and forming a narrow pulse train having positive and negative spikes; a first mono-stable-multivibrator which is coupled to the output of said differentiator and is triggered by the positive spikes of the pulse train from said differentiator; a second mono-stable multivibrator which is coupled to the output of said differentiator and is triggered by the negative spikes of the pulse train from said differentiator, the output pulses of said first and second mono-stable-multivibrators having a pulse-width with a length of time slightly shorter than the interval of the zero crossing points of said input frequency modulated signal where the frequency deviation of said input frequency modulated signal becomes a maximum in a negative direction; an adder coupled to said mono-stablemultivibrators for adding said output pulses of said monostable-multivibrators; a gate signal generator coupled to said adder for generating a gate signal according to the added output from said adder; and a gate circuit which is coupled to the output of said demodulation means and which has a control terminal coupled to said gate signal generator to receive said gate signal from said gate signal generator, said gate circuit passing the signal from said demodulating means when said gate signal is not present at said control terminal and preventing said signal from passing when said gate signal is present at said control terminal, whereby a demodulated signal in which the noise component is eliminated appears as output of said system.
Claim 3
Dependent Claim
22 words
An FM demodulation system as claimed in claim 1, wherein said demodulation means is connected directly to the output of said limiter.
Claim 4
Dependent Claim
22 words
An FM demodulation system as claimed in claim 1, wherein said demodulation means is directly connected to the output of said adder.
Claim 5
Dependent Claim
29 words
An FM demodulation system as claimed in claim 1, wherein said demodulation means is a low-pass filter having the input thereof connected directly to the output of said adder.