JPH088863A - Fm receiver - Google Patents

Abstract

PURPOSE:To improve S/N at the time of stereo reproduction and to enlarge the FM stereo reception area for an FM stereo receiver. CONSTITUTION:FMIF signals are separated into L and Rch by a distributor 3, switched in synchronism with sub carrier waves respectively in switching circuits 5 and 5' and FM demodulated through cyclic filters 16 and 16' and impulse noise is eliminated by noise elimination circuits 11 and 11' by the demodulated output. The output of the circuits 11 and 11' is taken out as L and R signals through de-emphasis circuits 14 and 14' and LPFs 15 and 15', also becomes the control signals of variable delay Lines 7 and 7' and controls the center frequency of the cyclic filters 16 and 16'.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement for improving S / N in stereo demodulation of an FM receiver.

[0002]

2. Description of the Related Art FIG. 2 shows an example of a stereo reproduction circuit of a conventional FM stereo receiver. In the figure, 1 is an input terminal, 2 is an IF stage band pass filter (BPF), 9 is an FM demodulator, 14 and 14 'are de-emphasis circuits, 1
Reference numeral 7 is a stereo demodulator. In FIG. 2, the input terminal 1
To the IF signal of the FM, the band is limited by the BPF (band pass filter) 2 in the IF stage, and the composite signal is obtained by the FM demodulator 9. After FM demodulation, the stereo demodulator 17 separates the stereo left and right signals from the composite signal, and the left and right audio signals L and R are obtained through the de-emphasis circuit 14. The stereo demodulator 17 includes a matrix type circuit and a switching type circuit.

[0003]

Now, the composite signal of FM is represented by the following equation (1), and its spectrum is as shown in FIG.

## EQU00001 ## S = (L + R) + (LR) cos .omega.st + Pcos (.omega.s / 2) t (1) (.omega.s: angular velocity of subcarrier) The composite signal band is up to 53 KHz. Therefore, when performing stereo reproduction, FM demodulation is required up to 53 KHz. By the way, in the case of monaural reproduction, up to the main signal (L + R), that is, up to 15 KHz. As the FM demodulation noise spectrum becomes triangular noise as shown in Fig. 4, the S / N deterioration of the stereo reproduction output is larger than that of the monaural reproduction output, and the theory is taken into consideration when performing de-emphasis of the reproduction output. Top 2
The deterioration is 1.7 dB. Especially, this effect becomes remarkable in the weak electric field.

An object of the present invention is S / N during stereo reproduction.
To improve the stereo reception area of FM.

[0005]

To achieve the above object, an FM receiver of the present invention switches an IF signal of an FM reception signal in synchronization with a subcarrier to separate FM stereo L and R signals. The stereo separation means and the separated FM stereo L and R signals are passed through a cyclic filter to F
An FM demodulation unit for M demodulation, a noise removal unit for removing noise from an FM demodulation output, and a control unit for controlling the center frequency of the cyclic filter according to the output of the noise removal unit are summarized. To do.

In the present invention, the noise removing means may be configured to control the noise removing operation in response to the subcarrier.

The cyclic filter may include an adder, a variable delay line and a gain control circuit, and the control means may include a loop filter and a variable delay line control amplifier.

[0008]

In the FM receiver of the present invention, since the stereo L and R signals are separated at the IF stage, FM demodulation is performed up to the audio frequency band, and the S / N at stereo demodulation is improved. It In this case, deterioration of S / N due to IF switching can be suppressed by using a cyclic filter, and a high S / N close to that of monaural reception can be obtained.

[0009]

Embodiments of the present invention shown in the drawings will be described below.
FIG. 1 is an embodiment of an FM receiver according to the present invention. In the figure, 1 is an input terminal, 2 is a BPF of an IF stage, 3 is a distributor, 4 is a subcarrier synchronizing signal generator, 5 and 5'is a switching circuit, 6 and 6'is an adder, and 7 and 7 '. Is a variable delay line, 8 and 8 ′ are gain control circuits, and 9 and 9 ′ are F
M demodulator, 10 and 10 'are impulse noise removing pulse signal generators, 11 and 11' are impulse noise removing circuits, 12 and 12 'are loop filters (LPF), 13,
13 'is a variable delay line control amplifier, 14 and 14' are de-emphasis circuits, 15 and 15 'are audio low-pass filters (LPF), and 16 and 16' are cyclic filters.

In FIG. 1, F input from the input terminal 1
The IF signal of M is passed through the BPF 2 of the IF stage and the distributor 3 outputs L,
It is distributed to two lines of R. The respective IF signals are switched by the switching circuits 5 and 5 '. The switching operation is the opposite of the switching circuits 5 and 5 ', that is, when the IF signal is ON in the switching circuit 5, the switching circuit 5'is turned off and the switching circuit 5 is turned off.
At the time of, the switching circuit 5 ′ operates so as to be turned on. This switching is performed by the subcarrier synchronization signal generator 4
The IF signal is changed to the left and right IF signals IF _L and IF _R by performing at the timing of the 38 KHz square wave pulse output from
Can be separated into Hereinafter, the Lch will be described as an example. FIG. 5 shows the waveform of each part of Lch. (A) is the deviation of the FM signal before switching. The switching pulse is shown in (b). In this case, the switch is turned on in the Hi section of the square wave pulse, and the switch OF in the Low section.
It becomes F. As a result, the deviation on the L side can be extracted as shown in (c). IF switched
The time waveform of the signal is a burst-like waveform as shown in (b). This burst IF signal is applied to the cyclic filter 1
By passing through 6 ', the waveform (E) in which the non-signal section is interpolated is obtained. The recursive filter 16 ′ includes an adder 6, a variable delay line 7, and a gain control circuit 8.

The IF signal passed through the cyclic filter 16 'is F
By performing FM demodulation by the M demodulator 9, an audio signal in which stereo left and right signals are separated can be obtained.
However, impulse noise occurs in the FM demodulation waveform at the point where the IF signal is turned on by switching. This is because the cyclic filter 16 'is used. That is, it is caused by the phase shift between the IF signal interpolated by the cyclic filter 16 'and the input signal. When the FM demodulated signal is passed through the audio LPF as it is, distortion occurs. Therefore, impulse noise removal circuit 11 '
After removing the impulse noise with, pass through the de-emphasis circuit 14 'and then pass through the audio LPF 15' to L
Get the signal output.

FIG. 6 shows how impulse noise is removed. Although (a) is a switching pulse, impulse noise occurs at the point where switching is turned on as in (g). In order to remove this, a pulse signal (f) synchronized with the rising edge of the switching pulse is obtained by the impulse noise removing pulse signal generator 10 '. Since the width of impulse noise is about 5 μs, the pulse width (section H) of (f) is set to be larger than that. This pulse signal (f)
Controls the operation of the impulse noise elimination circuit 11 '. H
By holding the input signal in the section of i, the waveform (h) from which impulse noise is removed can be obtained. FIG. 7 shows an example of the impulse noise removing circuit. This is an example of a sample and hold circuit, 20 and 21 are differential amplifiers, 22 is a sampling FET, D ₁ and D ₂ are diodes, C ₁ and C ₂ are capacitors, and R _{1 to} R ₄ are resistors. The signal S ₁ is input from the input terminal (A), and a sampled output is obtained from the output terminal (B) by the timing pulse input to the terminal (C). In this circuit, the signal S is obtained by reversing Hi and Low of the timing pulse of sample and hold, cutting the direct current component and shifting to the center of the ground.
Perform sample hold with ₂ . In this case, the sampling operation is performed when the voltage is positive, and the holding operation is performed when the voltage is negative.

Next, the control of the cyclic filter 16 'will be described. S by IF switching in this system
In order to cover the / N deterioration, the loop gain of the cyclic filter 16 'needs to be about 0.99, and 3d at that time is required.
The B band is about 34 KHz. Naturally, in order to pass the FM signal, it is necessary to make the center frequency of the recursive filter 16 'follow the deviation. In this method, since the cyclic filter 16 'is used for the signal that has already been separated into L and R after IF switching, it is not necessary to follow the 38 KHz component, and the Lch component (the Lch component of the dotted line in FIG. 5C)
Should follow. The control of the center frequency of the cyclic filter 16 'is performed by controlling the delay time of the variable delay line 7'. The output of the impulse noise removing circuit 11 'is used as the control signal. The output of the circuit 11 'is a loop filter (LP
F) The signal is input to the variable delay line drive amplifier 13 'through 12', and the output voltage controls the delay time of the variable delay line 7 '. FIG. 8 shows the relationship between the control voltage of the variable delay line and the delay time. The variable range of the variable delay line 7'is set to a range capable of covering the reciprocal of the FM instantaneous frequency (10.625 to 10.775 MHz).

The operation of the present system has been described above taking the Lch as an example, but the same applies to the case of the Rch. However, in the case of Rch, the switching of IF is reversed from the case of Lch. Further, since impulse noise is generated at the trailing edge of the square wave pulse in FIG. 6B, the impulse noise removal pulse signal generator 10 outputs a pulse synchronized with the trailing edge of the square wave pulse to remove the impulse noise. To do. That is, the impulse noise removal timing may be shifted by 13 μsec (a half cycle of 38 KHz) with respect to the case of Lch.

[0015]

As described above, according to the present invention, I
By combining F switching and a cyclic filter, S / N deterioration after FM demodulation due to IF switching can be suppressed. Therefore, the S / N at the time of stereo reproduction can be improved by separating the stereo left and right signals at the IF stage and performing FM demodulation up to the audio band. Also,
Due to the effect of narrowing the band by the cyclic filter, the threshold level can be improved and the stereo reception sensitivity can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing a conventional stereo demodulation circuit.

FIG. 3 is a spectrum diagram of an FM composite signal.

FIG. 4 is a spectrum diagram of FM demodulation noise.

FIG. 5 is a waveform diagram of each part at the time of IF switching in the above embodiment.

FIG. 6 is a waveform diagram showing how impulse noise is generated and how it is removed.

FIG. 7 is a circuit diagram showing an example of an impulse noise elimination circuit.

FIG. 8 is a diagram showing a relationship between a control voltage of a variable delay line and a delay time.

[Explanation of symbols]

1 Input terminal 2 IF stage BPF 3 Distributor 4 Subcarrier synchronization signal generator 5, 5'Switching circuit 6, 6'Adder 7, 7 'Variable delay line 8, 8' Gain control circuit 9, 9'FM demodulation Pulse generator 10, 10 'Pulse signal generator for impulse noise elimination 11, 11' Impulse noise elimination circuit 12, 12 'Loop filter (LPF) 13, 13' Variable delay line control amplifier 14, 14 'De-emphasis circuit 15, 15' Audio LPF 16,16 'Cyclic filter 17 Stereo demodulator

Claims (3)

[Claims] 1. A stereo separation unit for switching an IF signal of an FM reception signal in synchronization with a subcarrier to separate an FM stereo L, R signal, and a cyclic filter for the separated FM stereo L, R signal. FM demodulated via FM
An FM receiver comprising demodulation means, noise removal means for removing noise from the FM demodulation output, and control means for controlling the center frequency of the cyclic filter according to the output of the noise removal means. . 2. The FM receiver according to claim 1, wherein the noise removing unit is configured to control a noise removing operation in response to the subcarrier. 3. The FM according to claim 1, wherein the cyclic filter comprises an adder, a variable delay line and a gain control circuit, and the control means comprises a loop filter and a variable delay line control amplifier. Receiving machine.