JPS6237563B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM signal demodulation method, and improves the signal-to-noise ratio by substantially realizing a tracking filter in the FM signal demodulation circuit by a fixed frequency narrow bandwidth filter. The purpose of this research is to provide an FM signal demodulation system that can perform the following steps.

As a conventional FM signal demodulation method, for example, the first
The system used is as shown in the block system in the figure. In the same figure, the input FM signal 7 is on the one hand fed to the phase shifter 1 and after being phase shifted by 90 degrees,
In the amplitude limiting circuit 2, amplitude fluctuations of the signal are removed, and the signal is supplied to the phase detection circuit 3. On the other hand, the input FM signal 7 is supplied to a narrow bandwidth tracking filter 5, and after the amplitude fluctuations of the signal are removed by a subsequent amplitude limiting circuit 6, it is supplied to the phase detection circuit 3. The phase detection circuit 3 supplied with these signals supplies its output signal to the loop filter 4, and the FM demodulated output signal 8 containing only the necessary frequency components obtained via the loop filter 4 is converted into a narrow bandwidth trass. It is also used as a control signal supplied to the king filter 5.

Since the narrow bandwidth tracking filter 5 passes only the signal in the required instantaneous spectrum width band of the input FM signal 7, it completely removes noise outside the passband of the narrow bandwidth tracking filter 5. be able to. On the other hand, the signal passing through the phase shifter 1 and the amplitude limiting circuit 2 is supplied to the phase detection circuit 3 as it is without noise removal. That is, among the two signals supplied to the phase detection circuit 3, only the signal from the amplitude limiting circuit 6 has noise removed, so the noise removed in the sum of the two signals from the amplitude limiting circuits 2 and 6 is as follows. Narrow bandwidth tracking filter 5
outside the passband, it becomes half (6dB). Note that noise is not removed within the passband of the narrowband tracking filter 5.

For example, if noise exists in the input FM signal 7 in the range shown by diagonal lines in FIG. 3A, the noise distribution of the output signal from the amplitude limiting circuit 2 will remain as shown in FIG. 3A; The noise distribution of the output signal from the limiting circuit 6 is shown by diagonal lines in FIG. 3C. Therefore, the noise distribution in the phase detection circuit 3 is as shown in Figure 3B, and the noise improvement level is 6 dB.
It will be about. Therefore, there was a drawback that the degree of noise improvement could not be sufficiently improved.

The present invention eliminates the above-mentioned drawbacks,
An embodiment thereof will be explained with reference to FIG. 2 and the following figures.

FIG. 2 is a schematic block diagram showing an embodiment of the FM signal demodulation method according to the present invention.

Generally, the frequency spectrum of a frequency modulated wave (FM signal) is as shown in Figure 4A, and a signal (carrier wave) C having a narrower instantaneous spectrum distribution within a predetermined band corresponds to the amplitude of the modulated signal. and move. Therefore, in FIG. 2, there is a local oscillation circuit 11 that generates a single frequency spectrum outside the band of the FM signal (indicated by "SF" in FIG. 4A), an input FM signal 22, and an output signal of the local oscillation circuit 11. As shown in FIG. 4B, the analog multiplier 13 supplied and the bandpass filter 14 supplied with the output signal of the analog multiplier 13 allow the center frequency of the signal with the same bandwidth to be adjusted to the local oscillation circuit 11. A frequency modulated wave is generated that is shifted (frequency shifted) to a frequency that is smaller (or may be larger) by the local oscillation frequency of , and the signal thus obtained is supplied to the analog multiplier 15 . In other words, the input FM signal 22
is frequency-converted by a local oscillation circuit 11, an analog multiplier 13, and a bandpass filter 14.

In addition, the input signal is adjusted so that the signal passing through the narrow bandwidth tracking filter 21 has the same group delay time as the signal passing through the bandpass filter 14.
The group delay time of the FM signal 22 is corrected by a group delay time correction circuit 20. The output signal of the narrow bandwidth tracking filter 21 obtained in this way is
The output signal of the bandpass filter 14 is supplied to an analog multiplier 15 which is also supplied with the output signal.

2 supplied to the analog multiplier 15 as described above.
The frequency difference between the two signals is equal to the local oscillation frequency, and the output signal spectral distribution of the narrow bandwidth bandpass filter 16, which passes only this frequency difference, is as shown by diagonal lines in FIG. 4C. The bandwidth of the narrow bandwidth bandpass filter 16 is set to be narrower than or equal to the bandwidth of the narrow bandwidth tracking filter 21. The output signal of the narrow bandwidth bandpass filter 16 thus obtained is supplied to a phase detection circuit 18 via a subsequent amplitude limiting circuit 17.

On the other hand, the output signal of the local oscillator circuit 11 is supplied to the phase detection circuit 18 after its phase difference with the signal from the amplitude limiting circuit 17 is set to 90 degrees by the phase correction circuit 12. The output signal has a spectral distribution as shown in FIG. 4D.

The amplitude characteristics and phase characteristics of the narrow bandwidth tracking filter 21 are shown in FIGS. 9A and 9B, respectively. The amplitude and phase characteristics of the narrow bandwidth tracking filter 21 are as shown in FIG.
It changes depending on the center frequency f ₀ of the FM signal. Note that in FIG. 9, a line I connecting three points indicates that the demodulation point differs depending on the phase.

As a result, the narrow bandwidth tracking filter 2
The noise at one out-of-band frequency is removed, and only the noise at that in-band frequency remains unremoved, as shown by diagonal lines in FIG. 3C.

Next, noise improvement of the in-band frequency of the narrow bandwidth tracking filter 21 will be explained.

First, it is assumed that the signal and noise at the time of no modulation are expressed by the following equations (1) and (2), respectively.

E _S sin (ω _S t + ψ _S ) ...(1) E _N cos (ω _N t + ψ _N ) ... (2) In the above formulas (1) and (2), E _S and E _N are the amplitudes of each signal,
ω _S and ω _N represent the angular frequency of each signal, and ψ _S and ψ _N represent the phase of each signal.

From the above equations (1) and (2), one signal supplied to the analog multiplier 15 is E _S sin (ω _S t + ψ _S ) + E _N cos (ω _N t + ψ _N ) ... (3), and the other signal is The signal becomes E _S sin {(ω _S −ω ₀ )t+ψ _S −ψ ₀ }+E _N cos{(ω _N −ω ₀ )t+ψ _N −ψ ₀ } (4). In the above equation (4), ω ₀ and ψ ₀ represent the angular frequency and phase of the output local oscillation signal of the local oscillation circuit 11, respectively.

_From these _equations , ^the output _{signal P} of the _{analog multiplier 15 is expressed as} +ψ _S ) cos {(ω _N −ω ₀ )t+ψ _N −ψ ₀ }+E _N E _S cos (ω _N t+ψ _N ) sin {(ω _S −ω ₀ )t +ψ _S −ψ ₀ }+E _N ² cos ( ω _N t+ψ _N )cos {(ω _N −ω ₀ )t+ψ _N −ψ ₀ } (5). Moreover, _{the difference} component Q of ^the output signal _P expressed by _the above _equation ( ₅ ⁾ _is : _{( 6 ) _ _ _ _ _ _ _ _ _ _}

Fig. 6A is a vector composite diagram of signal S and noise N, Fig. 6B is a spectral distribution diagram of signal S and noise N, and Fig. 7A is a vector of difference component Q expressed by equation (6) above. The composite diagram and Figure B are its spectral distribution diagrams. In FIG. 7, as is clear from equation (6) above, the noise components N ₁ and N ₂ have a frequency of |ω _S −ω _N | with respect to the angular frequency ω ₀ , and a phase of |ψ They exist simultaneously with the same amplitude with a difference of _N −ψ _S |. This means that, for example, the composite signal shown in Figure 5C, which is a composite signal of the signals and noise shown in Figures A and B, respectively, becomes an amplitude modulated wave with the signal as a carrier wave and the noise as a modulation signal. It shows. Therefore, when such a composite signal passes through the amplitude limiting circuit 17, it becomes a single frequency with no noise in the local oscillation frequency, and by supplying the output signal of the phase detection circuit 18 to the subsequent low-pass filter 19, the DC component is removed. An output signal containing only the signal is obtained from this low-pass filter 19. The output spectrum of the low-pass filter 19 is shown in FIG. 4E.

As mentioned above, the narrow bandwidth tracking filter 2
The noise of the in-band frequency of 1 is an analog multiplier 15,
The narrow bandwidth bandpass filter 16 and the amplitude limiting circuit 17 eliminate the signal almost completely. However, in reality, the input FM signal is passed through the low-pass filter 1.
The narrow band tracking filter 21 controlled by the output signal of 9 performs phase modulation corresponding to the instantaneous frequency of the input FM signal, and the signal thus obtained is finally reproduced by the phase detection circuit 18. be done.

Therefore, since the noise itself is also phase modulated, the seventh
A phase shift occurs in both sidebands of the amplitude modulated wave shown in Figures A and B, and the degree of noise reduction changes depending on the deviation from the frequency when no modulation is performed, as shown by "F" in Figure 8. do.

The composite signal of FIG. 5C is phase modulated by the narrow bandwidth tracking filter 21, and
At the same time, the amplitude is modulated by noise, and since an error due to "F" in FIG. 8 occurs, the output signal of the narrow bandwidth bandpass filter 16 is phase modulated by the correction error due to the phase shift, and the output signal of the narrow bandwidth bandpass filter 16 is as shown in FIG. The waveform will be as shown in D. The signal thus obtained is subjected to phase modulation in the amplitude limiting circuit 17 and phase modulation due to the error component of noise, resulting in a waveform as shown in FIG. 5E. Therefore, the noise level becomes as shown by diagonal lines in FIG. 3D. Here, for example, if the ratio of the bandwidth of the FM signal to the bandwidth of the instantaneous spectrum of the narrow bandwidth tracking filter 21 is 10:1, as shown in FIG. While the improvement is about -dB, as shown by the dashed-dotted line a, the degree of improvement is about 30 dB when there is no modulation, as shown by the dashed-dotted line a, and about 20 to 30 dB when modulated, as shown by the dashed-dotted lines c, b, and a.

As mentioned above, the FM signal demodulation method according to the present invention includes a frequency conversion means for converting the frequency of an input FM signal using the output signal of a local oscillation circuit, and a narrow frequency converter that is supplied with the input FM signal and removes noise outside the instantaneous spectrum width of the input FM signal. A bandwidth tracking filter, an analog multiplication circuit that is supplied with the output signal of the frequency conversion means and the narrow bandwidth tracking filter, and a narrowband tracking filter that extracts the frequency difference component between the two input signals of the analog multiplication circuit from the output signal of the analog multiplication circuit. a bandwidth bandpass filter; and an amplitude limiting circuit that substantially eliminates noise within the instantaneous spectral width of the narrow bandwidth tracking filter by removing amplitude modulation components in the output signal of the narrow bandwidth bandpass filter; a phase correction circuit that corrects the phase difference between the output signal of the local oscillation circuit and the output signal of the amplitude limiting circuit to 90 degrees; a phase detection circuit that is supplied with the output signals of the amplitude limiting circuit and the phase correction circuit; The structure consists of a low-pass filter that extracts the low-frequency components of the output signal and supplies them as a control signal to a narrow-bandwidth tracking filter, as well as outputting them as an FM demodulation signal. Noise components outside the band of the width tracking filter are removed by this narrow bandwidth tracking filter, and noise components within the band are removed by the analog multiplication circuit, narrow bandwidth bandpass filter, and amplitude limiting circuit.
This method is characterized in that noise in the FM signal is effectively removed as a whole and the signal-to-noise ratio is effectively improved.

[Brief explanation of the drawing]

Figure 1 is a schematic block system diagram showing an example of a conventional FM signal demodulation system, and Figure 2 is an FM signal demodulation system according to the present invention.
A schematic block system diagram showing an example of a signal demodulation method, FIGS. 3A to 3D are diagrams showing noise levels, respectively, FIGS. 4A to E are frequency spectrum diagrams of frequency modulated waves, and FIGS. 5A to E. 6A and 6B are vector composite diagrams of the signal and noise and their spectral distribution diagrams, respectively. FIGS. 7A and B are vector composite diagrams of the difference component Q of the output signal P and their spectral distribution diagrams, respectively. FIG. 8 is a diagram showing the degree of noise improvement, and FIGS. 9A and 9B are diagrams showing the amplitude characteristics and phase characteristics of the narrow bandwidth tracking filter during modulation, respectively. 1... Phase shifter, 2, 6... Amplitude limiting circuit, 3...
...Phase detection circuit, 4...Loop filter, 5...
Narrow bandwidth tracking filter, 7...Input FM
Signal, 8...FM demodulation output signal, 11...Local oscillation circuit, 12...Phase correction circuit, 13, 15...
Analog multiplier, 14...Band pass filter,
16... Narrow bandwidth bandpass filter, 17...
Amplitude limiting circuit, 18... Phase detection circuit, 19...
Low pass filter, 20...Group delay time correction circuit, 21...Narrow bandwidth tracking filter, 2
2...Input FM signal, 23...FM demodulation output signal.

Claims (1)

[Claims] 1. Frequency conversion means for frequency converting an input FM signal using an output signal of a local oscillation circuit;
a narrow bandwidth tracking filter that is supplied with the signal and removes noise outside its instantaneous spectral width; an analog multiplication circuit that is supplied with the frequency conversion means and the output signal of the narrow bandwidth tracking filter;
A narrow bandwidth bandpass filter extracts a frequency difference component between two input signals of the analog multiplication circuit from the output signal of the analog multiplication circuit, and an amplitude modulation component in the output signal of the narrow bandwidth bandpass filter is removed. an amplitude limiting circuit that substantially removes noise within the instantaneous spectral width of the narrow bandwidth tracking filter; and a phase that corrects to 90 degrees a phase difference between the output signal of the local oscillation circuit and the output signal of the amplitude limiting circuit. a correction circuit; a phase detection circuit that is supplied with the output signals of the amplitude limiting circuit and the phase correction circuit; and extracting a low frequency component of the output signal of the phase detection circuit and sending it as a control signal to the narrow bandwidth tracking filter. An FM signal demodulation method characterized by comprising a low-pass filter that supplies the signal and outputs it as an FM demodulated signal. 2. The FM signal demodulation method according to claim 1, wherein the narrow bandwidth bandpass filter has a bandwidth narrower than or equal to that of the narrow bandwidth tracking filter.