JPS6255321B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a demodulator.

Conventionally, an FM demodulator is comprised of a discriminator that converts a phase change in an input signal into an amplitude change corresponding to this change, and a bandpass filter that extracts a demodulated signal from the output of the discriminator. Therefore, when an unwanted signal (hereinafter referred to as interference wave) that is stronger than the signal to be demodulated (hereinafter referred to as desired wave) occurs within the demodulation band of the discriminator, the demodulation output of the desired wave decreases or disappears, and the desired signal It was difficult to detect.

That is, the modulation frequency of the desired wave is P, and the carrier frequency is
_D , the modulation frequency of the interference wave is Q, and the carrier frequency is _UD . When a strong interference wave is superimposed on the desired wave, the sine waveform of frequency P is suppressed at the output of the discriminator, and the sine waveform of frequency Q is instead suppressed. wave signal and
A cross-modulation group containing a complex modulation signal component whose amplitude and phase change at frequencies P and Q is generated.

Therefore, if the output of the discriminator is simply passed through a bandpass filter, when there is interference between the desired wave and the interference wave, the same amount of suppression effect as that up to the discriminator will occur in the output.

The present invention provides a demodulation device that makes it possible to reproduce a demodulated signal in the presence of interference waves by using a cross-modulation group with a desired wave that is generated in the presence of interference waves.

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

1 is an input terminal, which receives the desired FM wave and unwanted interference waves at the same time. These received signals are made constant in amplitude change by an amplitude limiter 2, and then a phase change is detected by a discriminator 3.

Here, the desired FM wave S _D is S _D = Acos {ω _c t + ∫Ω (t) dt} ... (1) A: amplitude, ω _c : angular frequency, ∫Ω (t): frequency deviation Also, interference When the wave S _UD is expressed as S _UD = Bcosωt (2) where B is the amplitude and ω is the angular frequency, the received signal t is the sum of the desired wave S _D and the interference wave S _UD t = S _D + S _UD = Acos { ω _c t+∫Ω(t)dt}+Bcosωt (3). Therefore, the discriminator 3 output U(t)
is given by the time differential of the phase term of the received signal _t , so if A/B≪1, U(t)=ω−(A/B)×{Δω−Ω(t)}cos{Δωt−∫Ω( t) dt} ...(4) However, Δω=ω _c −ω.

As is clear from this equation (4), the discriminator output U
(t) includes a term of the modulation component Ω(t) of the FM modulated wave. When this discriminator output is applied to the next stage AM detector 4 and its amplitude change is detected, an AM detection output of {Δω−Ω(t)} (5) is obtained. Since Δω in equation (5) is a DC component, the modulation component Ω(t) of the FM modulated wave can be found from the AC component.

Note that equation (5) is for the case where the interference wave is unmodulated, but
When the frequency modulated wave of the interference wave is denoted by Ω'(t), the interference wave _SUD is expressed as _SUD =Bcos{ωt+∫Ω'(t)dt}...(6). However, ∫Ω'(t)dt is the frequency shift.

In this case, the output U(t) of the discriminator 3 is determined as the time differential of the phase term of the sum component of the desired wave and the interfering wave, and is as follows.

U(t)≒ω+Ω′(t)−A/B{Δω−Ω(t)−Ω′(t)}cos{Δωt −∫Ω(t)dt +∫Ω′(t)dt} ……( 7) When the output of this discriminator 3 is subjected to AM detection, the AM detector output becomes {Δω−Ω(t)−Ω′(t)} ……(8).

As shown in equation (8), the desired wave component Ω(t) does not disappear and remains in the AM detector output.

Therefore, if there is a difference in frequency between the modulation signal Ω(t) of the FM modulated wave and the modulation signal Ω'(t) of the interference wave, the desired wave is modulated by passing the AM detection output to the bandpass filter 5. The signal Ω(t) can be detected.

According to the present invention, which is configured to limit the amplitude of the received modulated wave as described above, detect the phase change, and perform AM detection, even when the desired wave is received buried in interference waves, the desired wave can be detected. Modulated wave components can be detected.

[Brief explanation of the drawing]

The drawing is a circuit configuration diagram showing an embodiment of the present invention. 1...Input terminal, 2...Amplitude limiter, 3...Discriminator, 4...AM detector, 5...Band wave detector, 6
...Output terminal.

Claims (1)

[Claims] 1. An amplitude limiter that keeps the amplitude change of the received signal containing the desired modulated wave constant, a discriminator that detects the phase change of the signal output from the amplitude limiter, and a discriminator that detects the amplitude change of the output of the discriminator. , and an AM detector that outputs a signal in which a desired modulated wave component remains.