JPS6310606B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM demodulator that can demodulate a desired signal even when there is interference.

In general, FM demodulators demodulate the frequency modulated signal and also detect the amplitude modulated component of the input signal, so if the input signal is amplitude modulated by noise, noise will increase in the demodulated output. An amplitude limiter is added before the frequency discriminator to remove unnecessary amplitude modulation components. Another effect of introducing an amplitude limiter is the effect of eliminating interference. That is, when the ratio of the signal level of the desired wave to be received to the signal level of the unwanted interference wave (hereinafter referred to as D/U) is greater than 1, interference disappears. However, this advantage may also be a disadvantage in some cases. That is,
When an interference unnecessary wave whose signal level is higher than that of the desired wave appears, the desired wave disappears from the normal demodulated output. This is not desirable as a communication means, and there has been a desire for a demodulator that can demodulate even a small amount of the desired wave even if D/U is less than 1.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks and to provide an FM demodulator that is capable of demodulating a signal containing a desired wave even in the case of interference where D/U is less than 1.

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of one embodiment of the present invention. In FIG. 1, an input signal supplied to an input terminal 1 is amplitude limited by an amplitude controller 2, and its output is subjected to frequency discrimination by a frequency discriminator 3 and FM demodulated. When the input signal is only one FM wave, an FM demodulated signal exists at the output of the frequency discriminator 3 and can be extracted at the output terminal 5 of the low-pass filter 4. When an interference unnecessary wave is simultaneously present in addition to the desired wave as an input signal, the output E _DSC of the frequency discriminator 3 is as follows.

E _DSC = A/D ² +U ² +2DUcos {(ω _D −ω _U )t+θ _D −θ _U
}……(1) However, A=D ² (ω _D +θ′ _D )+U ² (ω _U +θ′ _U )+DU(ω _D +
ω _U +θ′ _D +θ′ _U ) ×cos {(ω _D −ω _U )t+θ _D −θ _U } Here, D: Amplitude of desired wave U; Amplitude of unnecessary wave ω _D ; Carrier angular frequency of desired wave ω _U ; Carrier angular frequency of unwanted waves θ′ _D ; Frequency modulation component of desired waves θ′ _U ; Frequency modulation component of unwanted waves θ _D ; Phase term of desired waves θ _U ; Phase term of unwanted waves D/U>1 In this case, the denominator and numerator on the right side of equation (1) are
Dividing by D ² , equation (1) becomes E _DSC = (ω _D +θ′ _D )+(U/D) ² (ω _U +θ′ _U )+
U/D (ω _D +ω _U +θ′ _D +θ′ _U )cosZ/1+2U/D
cosZ+(U/D) ² (2) However, Z={(ω _D −ω _U )t+θ _D −θ _U }. The first term of the numerator on the right side of equation (2) (ω _D +θ′ _D )
By making it independent as the first term on the right-hand side and rearranging the equation, E _DSC = (ω _D +θ′ _D )−U/D(ω _D −ω _U +θ′ _D −θ
' _U )・U/D+cosZ/1+2・U/DcosZ+(U/D
) ² ......(3).

Here, the expansion ^formula∞ 〓 〓 〓 ⁼⁰ x〓cos {(γ+1)θ}=cosθ−X/1−2Xcosθ
+X ² ......(4) (however, |X|<1), set X=-U/D (however, U/D<1), and then multiply both sides by -U/D to obtain the following equation. obtain. ^∞ 〓 〓 ^r=0 (-1)〓 ⁺¹・(U/D)〓 ⁺¹ cos {(γ+1)θ}
=-U/D・cosθ+U/D/1+2・U/Dcosθ+(
U/D) ² ………(5) In this equation (5), if we set θ=Z and γ+1=n, ^∞ 〓 〓 ⁿ⁼¹ (-1) ⁿ・(U/D) ⁿ cos(nZ )=-U/D・cosZ
+U/D/1+2・U/DcosZ+(U/D) ² ………(6)
becomes. Therefore, if we rewrite equation (3) using equation (6), E _DSC = (ω _D +θ′ _D ) + (ω _D −ω _U +θ′ _D −θ′ _U )・
^∞ 〓 ⁿ⁼¹ (-1) ⁿ (U/D) ⁿ cos (nZ) ......(7) However, Z={(ω _D −ω _U )t+θ _D −θ _U ) is derived. In addition, when D/U<1, dividing the denominator and numerator on the right side of equation (1) by U ² , equation (1) becomes E _DSC = (U/D) ² (ω _D +θ′ _D ) + (ω _U + θ′ _U )+
U/D (ω _D +ω _U +θ′ _D +θ′ _U )cosZ/1+2U/D
cosZ(U/D) ² (8) However, Z={(ω _D −ω _U )t+θ _D −θ _U }. The second term of the numerator on the right side of equation (8) (ω _U +θ′ _U )
If we separate the equation as the first term on the right-hand side and rearrange the equation in the same way as in the case of D/U>1 above using the expansion formula of equation (4), we get E _DSC = (ω _U + θ′ _U ) −(ω _D −ω _U −θ′ _D −θ′ _U )・
^∞ 〓 ⁿ⁼¹ (-1) ⁿ (D/U) ⁿ cos (nZ) ......(9) However, Z={(ω _D −ω _U )t+θ _D −θ _U } is derived.

The first term on the right side of equations (7) and (9) above is the DC component and the FM component of the desired wave or unnecessary wave, and the second and subsequent terms are the fundamental wave and harmonics of the beat signal caused by interference between the desired wave and unnecessary wave. shows.

When D/U is greater than 1, the desired wave can be demodulated by extracting the FM component θ' _D of the desired wave in the first term of equation (7) using the low-pass filter 4.
However, when D/U is less than 1, (9)
Since the FM component θ' _D of the desired wave disappears from the first term of the equation, the low-pass filter 4 cannot demodulate the desired wave. On the other hand, the beat signal from the second term onwards in equation (9) includes the phase terms θ _D and θ _U of the desired wave and the unwanted wave, and is also subjected to amplitude demodulation by the FM component θ′ _D of the desired wave. I understand that. Therefore, D/U is 1
Even if the desired wave FM component is no longer included in the low frequency component of the frequency discriminator 3 output, the desired wave FM component is still included, although the modulation component of the unwanted wave is mixed in by demodulating the beat reception. The signal can be extracted.

In actual equipment, it is not possible to transmit all the high frequency components of the beat signal shown in equations (7) or (9) above, and therefore the beat signal is deformed by harmonic interference and becomes the desired signal. Maintenance of FM components is no longer guaranteed. In order to prevent this, it is most likely to use only the fundamental wave of the beat signal. After selecting the frequency region containing the beat signal from the output of the frequency discriminator 3 by a high-pass filter 6, only the fundamental wave component of the beat signal is changed in frequency by a band filter 7 whose center frequency can be arbitrarily set. Extract. Although changing the frequency has many advantages for subsequent signal processing, it does not concern the essence of the present invention. The center frequency of the band filter 7 is set by the following method. That is, the frequency of the beat signal that is the output of the high-pass filter 6 is counted by the counter 8. In the second term on the right side of equation (9), (D/U) ⁿ < 1, so among the beat signal components, the fundamental wave (n = 1 component) has the largest amplitude, and the amplitude of the harmonics is more sufficient. small. Therefore, the beat signal output from the high-pass filter 6 is influenced by harmonics in such a way that its waveform has a sharp peak, but its period coincides with the period of the fundamental wave of the beat signal. Therefore, the count result fb of the counter 8 gives the frequency of the fundamental wave of the beat signal, and the output of the counter 8 controls the center frequency of the band filter 7. band filter 7
An example of the configuration is shown in FIG. The counting result fb of the counter 8 is processed by a circuit not shown, and the fixed center frequency fo of the band filter 71 in FIG.
It is converted into +fb form and applied to the control terminal 72. In response to this control signal, the oscillation frequency of the frequency synthesizer 73 is set to fo+fb. On the other hand, the beat signal applied from the high-pass filter 6 to the input terminal 74 is interfered with by the output signal of the frequency synthesizer 73 at the frequency mixer 75 and converted into a frequency fo.
Only this component passes through the bandpass filter 71 and is obtained at the output terminal 76. That is, the center frequency of the band filter 7 shown in FIG. 1 is automatically set in cooperation with the frequency counter 8 so as to extract the fundamental wave component of the beat signal. The fundamental wave component of the beat signal thus obtained as the output of the band filter 7 is detected by the envelope detector 9 in FIG.
Desired wave FM by detecting the amplitude modulation component of the beat signal
A signal containing the component θ′ _D can be extracted by the output terminal 11 of the low-pass filter 10.

FIG. 3 is a block diagram of another embodiment of the present invention. The parts up to extracting the fundamental wave component of the beat signal are the same as the embodiment shown in FIG. 1, and the same parts are given the same reference numerals. The explanation will be omitted. The configuration in Figure 3 focuses on the fact that desired wave information exists as the phase term θ' _D of the beat signal in the second term on the right side of equation (9), and the desired wave information included in the beat signal fundamental wave in the form of FM. This is to extract the FM component. That is, the output of the bandpass filter 7 is amplitude limited by the amplitude limiter 12 shown in FIG. 3, and the frequency discriminator 13 demodulates the frequency modulated signal of the fundamental wave of the beat signal from the output. The low-pass filter 14 is the frequency discriminator 13
Only the audio component is extracted from the demodulated output, and the audio component is taken out from the output terminal 15.

Although the circuit shown in FIG. 3 has the disadvantage that it is somewhat more complex than the circuit shown in FIG. 1, all changes in the beat signal level due to D/U changes are absorbed by the amplitude limiter 12, resulting in a demodulated output with a stable level. is obtained from the output terminal 15 of the low-pass filter 14.

In addition, in FIGS. 1 and 3, the band filter 7 has a function of extracting only the fundamental wave of the beat signal of the desired wave and the unnecessary wave from the output of the high-pass filter 6,
Since the signal bandwidth of the bandpass filter 7 can be limited, the S/N ratio is improved.

In addition, as mentioned above, two-wave interference is thought to occur most frequently, but even if there is three or more waves of interference,
If there is only one unnecessary wave with a higher level than the desired wave, the level of the beat signal due to the interference between the maximum unnecessary wave and the desired wave is the maximum, so the same effect can be obtained. Can deal with interference.

As explained above, according to the present invention, by extracting the fundamental wave components of the beat signals of the desired wave and the unnecessary wave, and further extracting the amplitude modulation signal or the frequency modulation signal of the fundamental wave components, the D/U is lower than 1. Even when there is small interference, it is possible to demodulate a signal containing the frequency modulation component of the desired wave. Furthermore, as described above, since interference in most cases can be dealt with, the conditions under which communication is possible are greatly expanded.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of one embodiment of the present invention, FIG. 2 is a specific block diagram of the bandpass filter 7 of FIG. 1, and FIG. 3 is a block diagram of another embodiment of the present invention. . 1...Input terminal, 2...Amplitude limiter, 3...Frequency discriminator, 4...Low pass filter, 5...Output terminal, 6...High pass filter, 7...Band filter,
8... Counter, 9... Envelope detector, 10...
Low pass filter, 11...Output terminal, 12...Amplitude limiter, 13...Frequency discriminator, 14...Low pass filter, 15...Output terminal, 71...Band filter, 72...Control terminal, 73 ... Frequency synthesizer, 74 ... Input terminal, 75 ... Frequency mixer, 76 ... Output terminal.

Claims (1)

[Scope of Claims] 1. An amplitude limiter that limits the amplitude of a received signal including unnecessary waves and frequency-modulated desired waves; and a frequency discriminator that is supplied with the output signal of this amplitude limiter and extracts a frequency-modulated signal. , a first filter to which the output signal of the frequency discriminator is supplied and extracts beat signal components of the desired wave and the unnecessary wave;
a counter that counts the frequency of the output signal of the first filter; and the center frequency is controlled such that the output signal of the first filter is supplied and the frequency signal of the fundamental wave of the beat signal component is extracted by the output signal of the counter. and output means that is supplied with the output signal of the second filter, extracts the amplitude modulation signal of the fundamental wave, and outputs a signal containing the frequency modulation component of the desired wave. FM demodulator. 2. Claim 1, characterized in that the output means is comprised of a detector that performs envelope detection of the output signal of the second filter, and a low-pass filter connected to this detector. FM demodulator. 3. An amplitude limiter that limits the amplitude of a received signal including unnecessary waves and frequency-modulated desired waves; a frequency discriminator that is supplied with the output signal of this amplitude limiter and extracts a frequency-modulated signal; a first filter to which an output signal is supplied and extracts beat signal components of the desired wave and the unnecessary wave;
a counter that counts the frequency of the output signal of the first filter; and the center frequency is controlled such that the output signal of the first filter is supplied and the frequency signal of the fundamental wave of the beat signal component is extracted by the output signal of the counter. and output means that is supplied with the output signal of the second filter, extracts the frequency modulation signal of the fundamental wave, and outputs a signal containing the frequency modulation component of the desired wave. FM demodulator. 4. The output means includes an amplitude limiter that limits the amplitude of the output signal of the second filter, a frequency discriminator that extracts a frequency modulation signal from the output of the amplitude limiter, and a low-pass filter connected to the frequency discriminator. The FM demodulator according to claim 3, characterized in that it is comprised of: