JPS5916451B2 - FM receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM receiver, and in particular, an object is to stably remove impulse noise that momentarily arrives when receiving an FM broadcast, and to prevent deterioration of the received content of the FM broadcast. It is something.

Generally, when ignition noise from automobiles and other impulse noises generated from various electrical devices are input to an FM receiver, the carrier wave is phase-modulated or suppressed by this impulse noise, resulting in noise interference. This often happens.

For this reason, in the past, the impulse noise, that is, the AM signal component, is detected from the intermediate frequency stage by taking advantage of the fact that the carrier wave becomes AM modulated when impulse noise is received, and based on this, impulse noise is transmitted to the impulse noise section. By holding the previous voltage, disturbance of the FM signal due to this impulse noise is prevented.

Fig. 1 shows an example of an FM receiver that adopts this method, in which 1 is an antenna, 2 is a high frequency amplification circuit, 3 is a mixing circuit, 4 is a local oscillator, and 5, 7, and 9 are intermediates, respectively. 10 is an FM demodulation circuit, 11 is a stereo demodulation circuit, 12 is an impulse noise detection circuit, 14 is a field effect transistor, and 15 is a capacitor. , 16 indicate output terminals. In this case, as is clear from the figure, the output terminal of the amplifier circuit 5 constituting the intermediate frequency stage is connected to the impulse noise detection circuit 1.
The intermediate frequency stage AM signal component is connected to the input terminal of No. 2 and supplied to the impulse noise detection circuit 12.

Further, when the impulse noise detection circuit 12 is supplied with the AM signal component, it operates to generate a control signal at this output terminal, and supplies this control signal to the gate of the field effect transistor 14. This is done so as to turn off the field effect transistor 14. In this case, when the field effect transistor 14 is turned off by the control signal, the capacitor 15 is charged to the voltage just before the transistor 14 was turned off, thereby maintaining the voltage just before the impulse noise. Therefore, according to this conventional FM receiver, when impulse noise momentarily arrives when receiving an FM broadcast, this impulse noise is transmitted to the impulse noise detection circuit 12 from the output terminal of the amplifier circuit 5 in the intermediate frequency stage. The impulse noise detection circuit 12 then operates to generate a control signal, and supplies this control signal to the gate of the field effect transistor 14, thereby turning the field effect transistor 14 off.

On the other hand, the impulse noise part in the FM signal is supplied to the source of the field effect transistor 14 via the FM demodulation circuit 10, the stereo demodulation circuit 11, etc. In this case, when the field effect transistor 14 is turned off, this The impulse noise section is just cut off, and since the field effect transistor 14 is turned off at this time, the capacitor 15 is held, so the voltage immediately before the impulse noise is held in the cut off impulse noise section. , thereby preventing signal disturbance due to impulse noise. However, in this conventional FM receiver, the impulse noise part in the FM signal is filtered through the intermediate frequency stage filters 6 and 8.
Since the pulse noise is supplied to the source of the field effect transistor 14 through the low pass filter of the FM demodulator 10, etc., the duration of the impulse noise becomes longer due to the influence of the delay characteristics of the filter. 1
It becomes necessary to increase the pulse width of the control signal supplied to the gate of 4, and as a result, the hold time due to the hold action of the capacitor 15 becomes longer, and in the case of high frequency signals, it is necessary to increase the pulse width of the control signal supplied to the gate of 4. The hold operation had a negative effect on the subsequent circuit, causing noise. In view of this, the present invention shortens the hold time for noise removal so that this hold operation does not have an adverse effect on the subsequent circuitry, thereby improving the FM
The purpose is to stably remove impulse noise that momentarily arrives during broadcast reception, and to prevent deterioration of the received content of FM broadcasts. Hereinafter, an embodiment of the FM receiver according to the present invention will be described with reference to FIG.

Note that in FIG. 2, parts corresponding to those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted. In this example, a phase lock loop circuit is provided in the intermediate frequency stage, and when impulse noise arrives at the FM receiver, this phase lock loop circuit detects the frequency immediately before the impulse noise of the FM signal during the impulse noise period. Make it hold the signal.

That is, the output terminal of the filter 8 in the intermediate frequency stage is connected to the input terminal of the phase comparator 17, and the output terminal of the phase comparator 17 is connected to the source of the field effect transistor 14 via the low-pass filter 18. Furthermore, the drain of this field effect transistor 14 is grounded via the capacitor 15 as described above, and this drain is connected to the voltage controlled oscillator 20.
The output terminal of the voltage controlled oscillator 20 is connected to the oscillation signal supply terminal of the phase comparator 17. In this case, the phase comparator 17, low-pass filter 18, and voltage-controlled oscillator 20 constitute a phase lock loop circuit as usual, and the FM signal is supplied to the phase comparator 17 from the filter 8 of the intermediate frequency stage. The phase of this FM signal and the oscillation signal from the voltage controlled oscillator 20 are compared in the phase comparator 17, and at this time, the output signal of the phase comparator 17 is equal to the frequency of the oscillation signal of the voltage controlled oscillator 20. Try to match the frequency of the FM signal. That is,
The oscillation frequency of the voltage controlled oscillator 20 is controlled to lock this phase lock loop circuit. Furthermore, the output terminal of the voltage controlled oscillator 20 is connected to the input terminal of the amplifier circuit 9 via a filter 22 forming an intermediate frequency stage.

The rest of the structure is the same as in FIG. 1. Since one embodiment of the FM receiver according to the present invention is configured as described above, the FM
During broadcast reception, when impulse noise does not arrive, the FM signal received by the antenna 1 is passed through a high frequency amplification circuit 2, a mixing circuit 3, an amplification circuit 5 constituting an intermediate frequency stage, and a filter 6 as usual. , an amplifying circuit 7, and a filter 8, respectively, to a phase comparator 17.

At this time, since the field effect transistor 14 is turned on, the phase comparator 17 operates as described above, and the voltage controlled oscillator 2 is activated to lock the phase lock loop circuit.
The oscillation frequency of the voltage controlled oscillator 20 is controlled to match the frequency of the FM signal, that is, the intermediate frequency of 10.7 MHz. Therefore, an FM signal substantially similar to the FM signal supplied to the phase comparator 17 is obtained at the output terminal of the voltage controlled oscillator 20, that is, the output terminal of the phase lock loop circuit, and this FM signal is transmitted to the intermediate frequency stage. FM demodulation circuit 10 via filter 22 and amplification circuit 9
and the stereo demodulation circuit 11, and at this time, right and left audio signals are obtained at the output terminal 16 as usual. On the other hand, when impulse noise momentarily arrives during FM broadcast reception, this impulse noise is detected by the impulse noise detection circuit 12 from the output terminal of the amplifier circuit 5 of the intermediate frequency stage, and then this impulse noise detection circuit 12 is operative to generate a control signal and apply this control signal to the gate of field effect transistor 14, which then turns off field effect transistor 14.

Therefore, the phase lock loop circuit is cut off and the output signal from the phase comparator 17 is no longer supplied to the voltage controlled oscillator 20. However, in this case, since the capacitor 15 is held, the voltage controlled oscillator 20 is affected by the electric field effect. A signal of approximately the same level as the output signal of the phase comparator 17 immediately before the transistor 14 is turned off, that is, immediately before the impulse noise is supplied from the capacitor 15, and therefore the oscillation frequency of the voltage controlled oscillator 20 is equal to that of the field effect transistor 14. Even during the off period, that is, the impulse noise period, the frequency is maintained at the frequency immediately before the impulse noise, and the FM signal immediately before the impulse noise is obtained at the output terminal of the voltage controlled oscillator 20 during this period. Then, as described above, this FM signal is supplied to the FM demodulation circuit 10 and the stereo demodulation circuit 11 via the intermediate frequency stage filter 22 and amplification circuit 9.
At the output terminal 16, even during the impulse noise period, the right and left audio signals are obtained as usual, thereby eliminating the impulse noise. However, in this case,
A phase lock loop circuit is provided before the FM demodulator 10, and this phase lock loop circuit holds the frequency signal immediately before the impulse noise of the FM signal during the impulse noise period, so that the noise of the impulse noise is reduced. The duration is shorter than in the past, and therefore the hold time for noise removal can be shortened accordingly, and this hold operation will no longer have an adverse effect on subsequent circuits.

Therefore, it is possible to stably remove impulse noise that momentarily arrives when receiving an FM broadcast, and it is possible to prevent deterioration of the received content of the FM broadcast. Note that the present invention is not limited to the above-described embodiments, and various other configurations may be adopted without departing from the gist of the present invention.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing an example of a conventional FM receiver;
FIG. 2 is a schematic block diagram showing an embodiment of the FM receiver according to the present invention. 12 is an impulse noise detection circuit, 14 is a field effect transistor, 15 and 21 are capacitors, 17 is a phase comparator, 18 is a low-pass filter, 20 is a voltage controlled oscillator, and 22 is a filter forming an intermediate frequency stage.

Claims (1)

[Claims] 1 A phase-locked loop circuit is provided in the front stage of the FM demodulator, and impulse noise in the FM signal is detected, and during the noise period, the frequency signal immediately before the impulse noise of the FM signal is held by the phase-locked loop circuit. An FM receiver characterized by: