JPH01297906A - Fm signal demodulating device - Google Patents

Abstract

PURPOSE:To remove an amplitude variation component effectively and to enable excellent FM demodulation by providing a frequency shifting means in front of the input of a limiter circuit and increasing the carrier frequency of an input FM wave. CONSTITUTION:The input FM wave is multiplied by a sine wave through a balanced modulator 5, whose amplitude-modulated output is passed through a BPF 6 which passes only an upper side-band wave, so that an FM wave which is shifted in carrier frequency is outputted. The output is passed through the band-pass limiter 3 composed of a hard limiter 1 and a BPF 2 and an FM demodulator 4, so that a demodulated signal is outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an FM signal demodulation device used for demodulating frequency modulated signals.

[Conventional technology]

When demodulating a frequency modulated wave (hereinafter abbreviated as FM wave), F
A bandpass limiter is generally used to suppress amplitude fluctuations of the M wave before demodulation. FIG. 4 is a block diagram when a bandpass limiter is used, in which a hard limiter 1 and a bandpass filter 2 constitute a bandpass limiter 3. The operation of the bandpass limiter will be explained below.

Hard limiter 1 has input/output characteristics shown in FIG. Therefore, when the input signal ■mu=COS θ is applied to the hard limiter, its output is the sixth
The waveform will be as shown in the figure. If the waveform shown in FIG. 6 is expressed as a Fourier series with respect to θ, the following is obtained. FM waves are generally VFM(tl −A cosθ(11θ(t)=
It can be expressed as ωc t + kt core m (α) dα. Here ω. is the carrier frequency, m(α)
is the original signal being modulated, A and are constants.

However, the amplitude of the FM wave may be modulated when passing through the transmission system. At this time, the amplitude of the FM wave is no longer constant, and V yn(t) = A (t) cosθ(
1). During demodulation, it is desired that this amplitude fluctuation component A (t) remains constant.

Therefore, in order to eliminate this amplitude fluctuation, vl (1) is added to the above-mentioned bandpass limiter 3. When the FM wave having the amplitude modulation component is applied to the bandpass limiter 3,
The output of the hard limiter 1 is as shown in the following equation.

V (1 (θ) = vo ((1) Ct + kt f2 mccx”) dα>-... ] The above formula is 3 times, 5 times 161 for the fundamental wave component of the first term.
．． It is the sum of waves with frequency components.

Therefore, the first term cos (ωct +k, !J
If the frequency components of other terms do not overlap with the frequency components of I, , m(α)dα), it is possible to extract only the first term using the bandpass filter 2. At this time,
The output Vto(t) of the bandpass filter 2, that is, the bandpass limiter 3 is, and this VLo(t) is FM
If the signal is input to the demodulator 4, demodulation can be performed without being affected by amplitude fluctuation components.

[Problem to be solved by the invention]

Since the conventional FM signal demodulator is configured as described above, in low-carrier FM of a VTR, etc., the spectra of the first term and other terms of the output Vo (θ) of the hard limiter 1 become close, and the As shown in Figure 7, since overlap occurs, even if it is passed through the band-pass filter 4, only the first term cannot be extracted, and frequency components other than the first term that fall within the band of the band-pass filter 4 become noise components. as F
It will be input to the M demodulator 4.

This invention was made to solve the above-mentioned problems, and even when the spectrum of the sideband of the FM wave is relatively large relative to the carrier frequency, such as low carrier FM of a VTR, It is an object of the present invention to provide an FM signal demodulation device that can effectively remove amplitude fluctuation components and perform good FM demodulation without causing spectrum overlap as described above.

[Means to solve the problem]

The FM signal demodulator according to the present invention has a frequency shifting means for shifting the frequency of an input FM signal inserted before a limiter circuit that keeps the amplitude of the input wave constant.

[Effect]

In this invention, the frequency of the FM wave is shifted in advance and then input to the limiter, thereby preventing the spectrum of unnecessary signal components from entering the band of the necessary signal at the output of the hard limiter.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
In the figure, 1 is a hard limiter, 2 is a bandpass filter, 4 is an FM demodulator, 5 is a balanced modulator for frequency shifting the input FM wave, and 6 is a bandpass filter for extracting the frequency shifted FM wave. be. Here, 1
and 2 constitute a bandpass limiter 3. Further, 5 and 6 have the same configuration as that for generating a single-sideband amplitude modulated wave (SSB), and constitute frequency shifting means.

The input FM wave is multiplied by a sine wave in a balanced modulator 5. The frequency of this sine wave is ω1, and the input FM wave is VrI4(t)
−A(t) cos (ale t + ,, f−m
(α) dα), the output Vv14 of the balanced modulator 5 (
t) is V vs(t) = 2 cosω t
・A(t) cos (t + (”-m(α)dα
) = A(t) cos ((a+.+(El, )
t+JI,,,m(α)dα)+A(t) cos (
((ds (r)c)t-5-m(α)dcr). Here, the amplitude of the sine wave was set to 2.

The above equation shows that the input FM wave is amplitude-modulated by carrier wave 2 cos ω, t, where the first term corresponds to the upper side band wave and the second term corresponds to the lower side band wave. Therefore, the balanced modulator 5
The output of the FM wave V 5r(tl −A (tlcos ((ω. +
ω1)t+LCoffl(α)dα) is extracted. Figure 3 explains this process in the form of a frequency spectrum, and the spectrum of the input FM wave is
3 (b), the output of the balanced modulator 5 is
) is obtained. Therefore, the frequency ω1+ω. By passing the output of the balanced modulator 5 through a bandpass filter 6 with a passband centered at , a frequency-shifted FM wave shown in FIG. 3 (C1) is obtained. Therefore, even if the amplitude is kept constant by the hard limiter 1, the spectrum of higher-order frequency components will not overlap with the frequency components shown in FIG. 3 (C1).

Note that any type of FM demodulator 4 may be used here, but if a pulse count type FM demodulator is used, the bandpass filter 2 is not required, so the method shown in FIG. It can be configured as follows. Further, in the above embodiment, the balanced modulator 5 and the bandpass filter 6 are used as frequency shifting means, but any other frequency shifting means, that is, means for generating a single-side band amplitude modulated wave, may be used. In addition, although the above embodiment uses a portion of the output of the balanced modulator 5 that corresponds to the upper sideband, the same effect can be obtained even if only the lower sideband is extracted.

〔Effect of the invention〕

As described above, according to the present invention, a frequency shift means is provided before the conventional limiter circuit input to increase the carrier frequency of the input FM wave, so that it is effective even for low carrier FM in a VTR. Amplitude limitation can be performed and a good demodulated output can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an FM signal demodulator according to one embodiment of the present invention, and FIG. 2 is a block diagram of an FM signal demodulator according to another embodiment of the present invention.
A block diagram of the signal demodulator, FIG. 3 is a spectrum diagram showing an example of the operation of the frequency shift means of the above embodiment, FIG. 5 is a characteristic diagram showing the input/output characteristics of the hard limiter, and FIG. A waveform diagram showing the output when a wave is input, and FIG. 7 is a spectrum diagram for explaining the conventional problems. 1 is hard limiter, 2 is band pass filter, 4 is F
M demodulator, 5 is a balanced modulator, and 6 is a bandpass filter. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] In an FM signal demodulation device that demodulates an FM signal, an input F
An FM signal demodulation device comprising: a limiter circuit used to remove an amplitude modulation component from an M wave; and a frequency shift means inserted before the limiter circuit to shift the frequency spectrum of an input FM wave.