JPS6252961B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device that removes the influence of interference on frequency modulated waves and performs normal demodulation.

Conventional FM receivers equipped with an amplitude limiting circuit (hereinafter referred to as a limiter) and a frequency demodulation circuit (hereinafter referred to as a discreet) receive interference due to a signal stronger than the desired FM wave. The disadvantage was that it was not possible to demodulate weak desired signals.

In order to eliminate these drawbacks, the present invention connects a limiter and a disc after the conventional FM demodulation circuit, and will be explained in detail below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which 1 is a first limiter, 2 is a first disk drive, 3 is a second limiter, and 4 is a first limiter.
is the second disk. The first limiter 1 and the first disk drive 2 have the same circuit configuration as the conventional one, and when there is no interference or the interference wave is weaker than the desired FM wave, the desired FM wave is demodulated and output 1.
Output to the terminal. On the other hand, when the interference wave becomes stronger than the desired FM wave, an interference wave between the desired FM wave and the interference wave is output to the output 1 terminal. The second limiter 3 removes the amplitude variation of this interference wave, and the second disk 4 demodulates the FM component of the interference wave, thereby making it possible to extract the desired FM wave signal.

Let us consider the case where the desired FM wave is expressed as Acos {ωct+∫Ω(t)dt} and is subjected to interference by an unmodulated interference wave Bcosωt.

At the input to the FM receiver, these are superimposed and become (t)=Acos{ω _c t+∫Ω(t)dt} +Bcosωt (1). The difference in angular frequency between the interfering wave and the desired wave is △ω=
If we set ω−ω _c , equation (1) becomes It can be rewritten as however It is. When this is demodulated using the circuit shown in Figure 1, we get
The amplitude variation represented by the square root of equation (2) is made constant by the first limiter 1, and only the phase variation is extracted by the first disk 2. Assuming an ideal demodulator that produces an output that is completely independent of amplitude and proportional to instantaneous angular frequency (the proportionality constant is 1), and calculating its output u(t), we get: becomes. The interference wave is larger than the desired signal. In other words, assuming that B/A≫1, equation (4) becomes further It can be approximated as In other words, equation (5) shows that when the interference wave is stronger than the desired wave, the desired signal Ω(t) cannot be directly extracted by the first disc 2. It can be seen that the second term in equation (5) represents the interference wave caused by the interference wave and the desired wave, and is amplitude-modulated and frequency-modulated by the desired wave signal component Ω(t). Therefore, after FM demodulation, the desired signal Ω(t) can be extracted by further performing AM detection or FM demodulation on the interference wave. On the other hand, when the interference wave is an AM wave, the output signal corresponding to equation (5) is the interference wave Bg(t)cosωt, where g(t)=1+
If m<1 in mV(t), equation (4) is Assuming that Bg(t)/A≫1, equation (4') becomes It can be approximated as From equation (6), if the interference wave is an AM wave
The amplitude variation of the interference wave represented by the second term of equation (5) becomes a signal that is further varied by the interference wave amplitude component g(t). Therefore, the desired signal Ω(t) cannot be extracted only by AM detection of the interference wave shown by equation (6). On the other hand, in the case of the FM demodulation method, as shown in FIG. 1, by installing the second limiter 3 in front of the second disk 4, the amplitude fluctuation of the interference wave shown by equation (6) can be removed. Desired wave signal Ω(t)
can be demodulated by the second disc 4. Therefore, by switching output 1 and output 2 shown in FIG. 1 depending on the presence or absence of interference waves, normal FM demodulation can be performed even if interference occurs.

Note that the first limiter 1 and first disk 2 shown in FIG. A low-frequency disc that can demodulate FM waves with a carrier angular frequency of △ω (for example, a pulse counting method that measures and integrates the zero crossing or interval of a signal with pulses, etc.)
The limiter level of the second limiter 3 is (5)
It is necessary to make adjustments to limit the AM signal component A/B{△ω-Ω(t)} or A/Bg(t){△ω-Ω(t)} in the second term of equation or (6). There is. Also
It goes without saying that FM waves also include FSK waves.

As explained above, normal demodulation can be performed by removing the suppression effect caused by strong interference waves that are unique to frequency modulation communication, so it has the advantage of being very effective in receiving FSK waves in the shortwave band where there is a lot of interference.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the invention. 1...1st limiter, 2...1st limiter,
3...Second limiter, 4...Second limiter.

Claims (1)

[Claims] 1 In a receiver used for frequency modulation communication,
An interference removal device for frequency modulation communication, characterized in that an amplitude limiting circuit and a second demodulating circuit are connected after the demodulating circuit of the receiver.