JPH0846534A - Noise elimination system for fm signal - Google Patents

Abstract

PURPOSE:To obtain the noise elimination system of an FM signal in which a noise of the FM signal is accurately eliminated by detecting the direction of rotation of the noise. CONSTITUTION:An input FM signal is distributed and frequency-converted by mixers 11,12 and local carrier oscillators 13,14, and a difference frequency component is obtained by BPFs 15,16. Then output signals of the BPFs 15,16 are given to noise detection circuits 17,18. The phase component of noise detected by the noise detection circuits 17,18 and the input signal are given to mixers 19,20, in which a phase fluctuation component due to noise is cancelled. Furthermore, an undesired frequency component is eliminated from the output of the mixers 19,20 by BPFs 21,22, then a vibration fluctuation component is eliminated by limiters 23,24 and FM-demodulated respectively by FM detectors 25,26 respectively. Moreover, an inversion circuit 27 is provided on an output of the FM detector 26 to match the phase of the demodulation signal. Then an output of the FM detector 26 and an output of an inversion circuit 27 are given to a switch 28, a switch 26 is switched by a control signal from a switch control circuit 50 and a signal from which phase fluctuation due to noise is selected and the FM signal whose noise is eliminated is extracted from a terminal 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise elimination system for FM signals.

[0002]

2. Description of the Related Art FIG. 4 is a vector diagram showing the relationship between FM signals and noise components. In FIG. 4, the FM signal S has an angular frequency ωs
The noise signal is rotating at the tip of the FM signal S. Here, assuming that the noise component when the phase difference with the FM signal S is φ1 is N1, the noise component when the phase difference is φ2 is N2, and φ1 = φ2 (absolute value), the noise phase amount is the same. There will be noise components in opposite directions. Japanese Patent Laid-Open No. 4-1502 as a noise removal method for FM signals
There is a method disclosed in No. 31 that detects an instantaneous phase component of noise from an amplitude variation of an FM signal caused by noise.

[0003]

However, in the noise elimination method for FM signals disclosed in Japanese Patent Laid-Open No. 4-150231, the actual rotation direction cannot be determined because the absolute phase of the noise is detected (that is, as shown in FIG. In 4, it is impossible to determine whether the noise vector is N1 or N2.)
Therefore, when noise that rotates in the opposite direction is detected when canceling the noise component detected from the input signal, the phase fluctuation may increase, and the noise removal of the FM signal may be inaccurate. There was a point.

The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to provide an FM signal noise elimination system capable of accurately eliminating the noise of an FM signal by detecting the rotation direction of noise. And

[0005]

The noise in the band of the FM signal exists randomly at frequencies above and below the FM carrier (upper and lower sidebands). The noise of the upper sideband and the noise of the lower sideband have different rotation directions (leading phase and lagging phase) even if the noise phase amount is the same. The present invention uses FM by noise
In the method of detecting the phase component of noise from the fluctuation of the signal amplitude,
The noise rotation direction detecting means is added to remove the noise in the FM band.

Specifically, in order to achieve the above-mentioned object, the noise elimination method of the FM signal of the first invention is the first and second methods.
First and second frequency conversion means for extracting the difference frequency component from the FM signal branched to the path, and first and second noise for detecting the phase component of the noise with respect to the carrier signal from the respective difference frequency component. Detection means, first and second multiplication means for multiplying the outputs of the first and second noise detection means and the FM signal, respectively, and first demodulation for FM demodulation of the outputs of the first and second multiplication means, respectively. And a second demodulating means, an inverting means for inverting the output of the second demodulating means, and an FM demodulated signal in which noise is removed from either the output of the first demodulating means or the output of the inverting means based on the control signal. And switch control means for outputting a control signal.

According to a second aspect of the present invention, in the noise elimination system of the first aspect, the switch control means performs FM modulation on the output from the switch means, and the inverted output of the output of the switch means is FM. Second modulating means for modulating, and third and third signals for multiplying respective signal components obtained by removing amplitude fluctuation components from outputs from the first and second multiplying means by outputs of the first and second modulating means. 4 multiplication means and output level comparison means for comparing the output levels of the third and fourth multiplication means, detecting which output level is smaller, and outputting a control signal. .

[0008]

According to the first aspect of the present invention, unnecessary frequency components are removed by the first and second multiplication means, and amplitude fluctuation components are removed by the first and second demodulation means to perform FM demodulation. And
An FM in which noise is removed from either the output of the first demodulating means or the output of the inverting means by a switch based on a control signal.
Select the demodulated signal.

In the second aspect of the invention, the switch control means compresses the FM modulation component from the modulation means by the first and second multiplication means to extract the noise component, and the output level comparison means makes the first control signal.
And the output levels of the outputs (noise components) of the second multiplying means are compared with each other, which output level is smaller, and the control signal is output.

[0010]

1 is a block diagram showing the configuration of an embodiment of an FM signal noise elimination system of the present invention. In FIG. 1, an FM signal is input from an input terminal 1, the input FM signal is branched into two paths, one of which is frequency-converted by a mixer (MIX) 11 and a local carrier oscillator (OSC) 13, and the other one is a mixer ( MIX) 12 and local carrier oscillator (O
(SC) 14 frequency conversion. Here, the frequency of the FM signal is fc + fm (fc is the carrier frequency, fm is the instantaneous frequency), and the oscillation frequency of the local carrier oscillator 13 is f.
1. The oscillation frequency of the local carrier oscillator 14 is set to f2 (however, f2> (fc + fm)> f1).

The sum and difference frequency components are obtained from the mixer 11 and the mixer 12, respectively.
(Bandpass filters) 15 and 16 extract the difference frequency component. Therefore, in the output of BPF15,

## EQU1 ## (fc + fm) -f1 = (fc-f1) + fm (1), and the carrier frequency is converted from fc to (fc-f1). On the other hand, in the output of BPF16,

## EQU00002 ## f2- (fc + fm) = (f2-fc) -fm (2), and the frequency shift fm is inverted with respect to the output of the BPF 15. Here, if the frequencies of f1 and f2 are set so that (fc-f1) = (f2-fc), the carrier frequency can be made the same in the output of each BPF.

The reversal of the frequency shift is an operation of exchanging upper and lower side waves with respect to the carrier frequency. Noise is uniformly distributed in the FM band, and by this operation, the frequency above and below the carrier frequency of the noise component is switched. As a result, the rotation of the vector diagram shown in FIG. 4 is reversed in both the FM component and the noise component.

The noise detection circuit 1 outputs the output signal of the BPF 15.
7 and the output signal of the BPF 16 is input to the noise detection circuit 1
Enter in 8. A terminal 2 provided between the BPF 15 and the noise detection circuit 17 and a terminal 3 provided between the BPF 16 and the noise detection circuit 18 are input terminals of the noise detection circuits 17 and 18, respectively, and will be described later with reference to FIG. Corresponds to the input / output terminals (block diagram of noise detection circuit).

At the input terminals of the noise detecting circuits 17 and 18, the phase component φ of noise with respect to the carrier signal is detected as shown in FIG. 5, but the two noise detecting circuits 17 and 18 are detected by the inversion operation of the frequency shift described above. At 18, the phase components (± φ) are in opposite directions. Noise detection circuit 17, 1
The phase components of these noises detected in 8 and the input signal are input to mixers (MIX) 19 and 20 to cancel the phase fluctuation components due to noises. However, in this case, since the polarity of the phase fluctuation is unknown because the phase fluctuation component is detected from the vibration of the FM signal, one of the mixers 19 and 20 cancels the noise phase fluctuation, but the other one doubles the phase fluctuation amount. There will be an inconvenience that

That is, even if the mixer 19 can cancel the phase fluctuation of the noise, if the mixer 20 detects the noise rotating in the opposite direction when canceling the noise component detected from the input signal, the phase fluctuation amount is reduced. However, even if the mixer 20 can cancel the noise phase fluctuation, noise that rotates in the opposite direction when canceling the noise component detected from the input signal by the mixer 19 can be increased. When detected, the phase fluctuation amount may increase (double).

Therefore, from the outputs of the mixers 19 and 20, B
The PFs 21 and 22 remove unnecessary frequency components, the limiters (LIM) 23 and 24 remove amplitude fluctuation components, and the FM detectors (FM DET) 25 and 26 perform FM demodulation, respectively. Further, since the phase of each demodulated signal is inverted due to the inversion operation of the frequency shift at the input stage, F
An inverting circuit 27 is provided at the output of the M detector 26 to match the phase of the demodulated signal.

Next, the output of the FM detector 26 and the output of the inverting circuit 27 are input to the switch 28, the switch 26 is switched by the control signal from the output terminal 6 of the switch control circuit 50, and the phase fluctuation is removed by noise. One of the signals is selected, and the FM demodulated signal from which noise is removed is taken out from the terminal 7. In this embodiment, the mixer 11, the local carrier oscillator 13, and the BPF 15 are the first conversion means,
Mixer 12, local carrier oscillator 14, and BP
F16 is the second conversion means, limiter 23 and FM detector 25 are the first demodulation means, limiter 24 and FM.
The detector 26 constitutes a second demodulation means.

FIG. 2 is a block diagram showing a configuration example of the switch control circuit 50 for controlling the switch 28.
2, the FM detectors 25 and 26 and the inverting circuit 27 are the same as those shown in FIG. 1, and the terminals 8 and 9 are the limiter 2 of FIG.
Outputs from 3 and 24 are shown. With the switch control circuit 50,
The FM demodulation signal from which the phase fluctuation due to the noise from the terminal 7 is removed is input to the VCO (voltage controlled oscillator) 52, 53 and F
M-modulate. Here, the inverting circuit 51 provided in the preceding stage of the VCO 53 is for matching the phase with the signal whose frequency deviation has been inverted.

The outputs of these VCOs 52, 53 and the terminals 8,
The signals from 9 (the outputs from the limiters 23 and 24 in FIG. 1) are input to the mixers 54 and 55, respectively, and the FM modulation components are compressed to leave only the noise components. Therefore,
The outputs of the mixers 54 and 55 are passed through LPFs (low pass filters) 56 and 57, respectively, and the noise level is converted into a DC level. Here, since the noise has been removed from the output of either one of the LPFs, the output becomes 0 (zero),
Since the noise component remains in the output of the other LPF, the DC component corresponding to the noise component is output.

The outputs of these LPFs are input to the voltage comparator 58 to detect which DC level is smaller, and the output of the voltage comparator 58 controls the switch 28 to perform noise removal for FM demodulation. Select an output. Note that FIG.
In the embodiment, the inverting circuit 51 and the voltage controlled oscillator 52 are the first
Of the LPF 56, 57 and the voltage comparator 58.
Constitutes an output level comparing means.

FIG. 3 is a block diagram showing a configuration example of the noise detection circuits 17 and 18, and FIG. 6 shows operation waveforms of each block of FIG. In FIG. 3, an FM signal (BPF15, 1 in FIG.
6 output) is input. The AM detector 31 performs envelope detection of the amplitude fluctuation of the FM signal.

This amplitude fluctuation component (FIG. 6 (a)) is differentiated by the differentiator 32 (FIG. 6 (b)), and the limiter 33 is further provided.
After the pulse waveform having a constant amplitude (FIG. 6 (c)) is obtained, the signal is input to the Schmitt triggers 34 and 35. At the Schmitt trigger 34, the Schmitt trigger 35
Then, a pulse is generated at the rising edge of the rectangular wave. Gate 3
Envelope detection outputs 6 and 37 (output of AM detector 31)
Are input respectively, and these gates are controlled by the output pulses of the Schmitt triggers 34 and 35 described above.

When the output from the Schmitt trigger 34 is at the high level, the envelope detection waveform is output from the gate 36. At this time, the level of the amplitude of the envelope detection waveform that is output is the maximum value (maximum value). Similarly, gate 3
From 7, the minimum value (minimum value) of the amplitude of the envelope detection waveform is obtained.

The outputs of the above two gates are input to the sample hold circuits (S / H) 38 and 39, and the maximum amplitude of the FM wave subjected to amplitude fluctuation due to noise as shown by the broken line in FIG. Get the value, the minimum value. The difference obtained by inputting these to the subtractor 40 becomes the absolute amplitude value of noise.

Next, the AM detector 31 is divided by the divider 41.
If the envelope detection output from is divided by the noise amplitude (output of the subtractor 40), the instantaneous phase φ of the noise as shown in FIG.
(Cos φ (t)) is determined. This phase component is
I can take it out from 5. In FIG. 5B, Nc indicates the envelope detection output, and N indicates the output of the subtractor 40.

[0026]

As described above, according to the present invention, F
Since the noise phase component can be detected with respect to the instantaneous frequency change of the M signal, an FM signal reduced by noise can be obtained by removing this noise phase component from the input FM signal, and the S / N can be improved. .

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of an FM signal noise elimination system of the present invention.

FIG. 2 is a block diagram showing a configuration example of a switch control circuit for controlling a switch.

FIG. 3 is a block diagram showing a configuration example of a noise detection circuit.

FIG. 4 is a vector diagram showing a relationship between an FM signal and a noise component.

FIG. 5 is an explanatory diagram for obtaining an instantaneous phase φ of noise.

FIG. 6 is an operation waveform diagram of each block in FIG.

[Explanation of symbols]

11 mixer (first frequency conversion means) 13 local carrier oscillator (first frequency conversion means) 15 BPF (first frequency conversion means) 17 noise detection circuit (first noise detection means) 19 mixer (first frequency conversion means) Multiplier means 23 Limiter (first demodulation means) 25 FM detector (first demodulation means) 12 Mixer (second frequency conversion means) 14 Local carrier oscillator (second frequency conversion means) 16 BPF (second) Frequency conversion means) 18 noise detection circuit (second noise detection means) 20 mixer (second multiplication means) 24 limiter (second demodulation means) 26 FM detector (second demodulation means) 27 inverting circuit ( Inversion means) 28 switch 50 switch control circuit (switch control means) 51 inversion circuit (first modulation means) 52 voltage controlled oscillator (first Modulating means) 54 mixer (third multiplying means) 53 voltage controlled oscillator (second modulating means) 55 mixer (fourth multiplying means) 56, 57 LPF (level comparing means) 58 voltage comparator (level comparing means) )

Claims (2)

[Claims] 1. A first and second frequency conversion means for extracting a difference frequency component from each of the FM signals branched to the first and second paths, and a phase of noise with respect to a carrier signal from each of the difference frequency components. First and second noise detecting means for detecting a component, first and second multiplying means for multiplying the outputs of the first and second noise detecting means by the FM signal, respectively, and the first and second First and second demodulating means for FM demodulating the output of the second multiplying means, inverting means for inverting the output of the second demodulating means, and output of the first demodulating means based on a control signal And a switch means for selecting and outputting an FM demodulated signal from which noise has been removed from any of the outputs of the inverting means, and a switch control means for outputting the control signal. Noise removal method of the issue. 2. The noise elimination method for FM signals according to claim 1, wherein the switch control means FM-modulates the first modulation means for FM-modulating the output from the switch means and the inverted output of the output of the switch means. A second modulation means and a first
And third and fourth multiplication means for multiplying the respective signal components obtained by removing the amplitude fluctuation component from the output from the second multiplication means by the outputs of the first and second modulation means, and the third and fourth multiplication means. An FM signal noise elimination method, comprising: an output level comparing means for comparing the output levels of the fourth multiplying means to detect which output level is lower and outputting a control signal.