JPH0681065B2 - FM receiver - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a receiver for receiving an FM broadcast, and more particularly to an FM receiver capable of switching the bandwidth of a reception frequency.

[Background technology and its problems]

Generally, a receiver that receives FM stereo broadcasting has a wideband reception characteristic when there is no interference, and it is in a receiving state where a high-quality audio signal is demodulated. In some cases, a receiving frequency band switching means for narrowing the bandwidth of the receiving frequency to improve the S / N ratio is provided.

FIG. 1 shows a conventional example of an FM receiver having such a switching means. 1 is a front end, 2W is an IF amplification section having a wide band characteristic, and 2N is an IF amplification having a narrow band characteristic. Part 3, demodulation circuit, 4 multiplex circuit, 5a, 5
b is a stereo signal amplifier.

In such an FM receiver, by switching between the switches S ₁ and S ₂ , the pass band of the intermediate frequency amplifier is changed to a wide selection curve A and a narrow selection characteristic B as shown in FIG. 2 to change the bandwidth of the reception frequency. is doing.

However, in such a switching means, the number of IF amplifiers and transformers increases, which is uneconomical. In particular, when switching to a narrow band, the group delay characteristic of the IF amplification unit 2N deteriorates, and distortion or However, there is a problem that deterioration of separation occurs.

[Object of the Invention]

The present invention has been made for the purpose of solving such a problem, and the switching of the reception frequency band is performed by the PLL circuit used for the demodulation circuit, and the separation of the stereo signal is performed even when the receiver is lowered. It is intended to provide an FM receiver that does not deteriorate as much as possible.

[Outline of Invention]

INDUSTRIAL APPLICABILITY The present invention is applied to a demodulator circuit of an FM receiver that receives FM broadcast waves.
The PLL demodulation method is adopted, and the lock range and the capture range of the PLL circuit are set to predetermined values by switching the characteristics of the low-pass filter that constitutes the PLL circuit. As a result, the pass band width of the intermediate frequency amplification unit can be switched to a wide band and a narrow band, and interference with interference and deterioration of the S / N ratio can be dealt with.

〔Example〕

FIG. 3 is a block diagram of the FM receiver of the present invention. As in FIG. 1, 1 is a front end, 2 is an IF amplifier, 3 is a demodulation circuit composed of a PLL circuit, and 4 is a multiplex. The circuits 5a and 5b are amplifiers for demodulated stereo signals.

In this FM receiver, the IF amplifier 2 is the IF amplifier shown in FIG.
It is set to a wideband characteristic like 2W, but when the reception frequency band of the FM receiver is set to a narrow band as described below, the PLL
The switch S of the demodulation circuit 3 composed of a circuit is used for switching.

FIG. 4 shows an embodiment of the demodulation circuit 3 employed in the FM receiver of the present invention. 3a is a phase detector, and 3b surrounded by a one-dot chain line allows variable frequency characteristics and resonance width characteristics. Is a low pass filter, and 3c is a voltage controlled variable oscillator.

Since the demodulation circuit 3 has a PLL circuit configuration as is well known, the oscillation frequency of the voltage controlled variable oscillator 3c is changed by being locked to the IF (10.7MHz) signal, and the control voltage ed at this time is The IF frequency changes, that is, the signal ea obtained by demodulating the FM signal. Therefore, this demodulated signal
The L + R signal, the L-R signal, and the pilot signal are extracted from ea, and the stereo signal (L, L,
R) can be formed.

In this way, the PLL circuit can be used as an FM demodulation circuit by setting the center frequency of the voltage controlled variable oscillator 3c to IF (10.7MHz), but the lock range of this PLL circuit is generally P
It is determined by the loop gain of the LL circuit, and the capture range is related to the loop gain and the time constant of the low pass filter 3b.

Therefore, in the present invention, the range of the lock range is changed by switching the loop gain, the bandwidth of the demodulated signal is controlled, the response frequency characteristic of the PLL circuit is changed, and the bandwidth of the reception frequency is widened as a result. It is intended to change from a narrow band to.

That is, when the switch S is provided to change the loop gain in the demodulation circuit 3 of FIG. 4 and the resistor R ₃ is connected in parallel with the resistor R ₂ , the gain of the amplifier A becomes small and the lock range of FIG. It becomes narrower from the solid line A to the one-dot chain line B.

In this case, the capture range is also narrower than the lock range, but the capture range is also related to the time constant of the low-pass filter. Therefore, it is preferable to add a capacitor C _D to maintain the predetermined time constant for correction.

Note that the capacitors that make up the low-pass filter 3b
C ₀ and the resistor r ₀ are impedance elements for phase compensation of the amplifier A, and the capacitor C ₁ is a capacitor that mainly sets the cutoff frequency of the low pass filter 3b.

FIG. 6 is a curve showing the band of the demodulated FM modulated wave,
A shows a demodulation band having a wide band characteristic, and B shows a demodulation band having a narrow band characteristic.

In the case of narrow band characteristics, subcarrier of stereo signal 38 KHz A
The capacitor C ₂ is connected in parallel with the capacitor C ₃ by the switch S ′ that works in conjunction with the switch S in order to prevent the demodulation characteristics around 53 KHz, which is the high frequency of the M-modulated sideband, from deteriorating. It is preferable to raise the characteristics as shown by the dotted line C to improve the separation.

The switching of the loop gain by the switch S can also be performed by electronically controlling the variable resistance element formed of the field effect transistor. In addition to the manual switching, this switching control detects the IF signal level with the detector 6 as shown by the dotted line in FIG. 3, and when there is an interference wave or the S / N ratio is bad, Microcomputer 7
Alternatively, the switch S may be switched to automatically control the narrow band characteristic.

〔The invention's effect〕

As described above, the FM receiver of the present invention adopts the PLL circuit as the demodulation circuit to switch the bandwidth of the reception frequency,
Since the pass band of the intermediate frequency amplifier is apparently switched to the wide band and the narrow band by switching the loop gain and the response frequency characteristic of the PLL circuit, the switching of the reception band becomes extremely simple. Also,
Since the IF characteristics are always constant, there is the advantage that there is no distortion due to group delay characteristics or deterioration of separation.

Further, since the band switching can be performed by electronic control, there is an effect that it is possible to easily perform continuous variable and automatic switching by the received radio wave.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a conventional FM receiver having a bandwidth switching means, FIG. 2 is a characteristic diagram of an IF amplifying section, and FIG. 3 is an FM showing an outline of the present invention. FIG. 4 is a block diagram of the receiver, FIG. 4 is a circuit diagram showing an embodiment of a PLL circuit constituting a demodulation circuit, FIG. 5 is a characteristic diagram showing a lock range of the PLL circuit, and FIG. 6 is a band of a demodulation signal. It is a characteristic diagram. In the figure, 1 is a front end, 2 is an IF amplifier, 3 is a demodulation circuit consisting of a PLL circuit, 3a is a phase detector, 3b is a low-pass filter, and 3c is a voltage controlled variable oscillator.

Claims (1)

[Claims] 1. An FM receiver comprising a front end for receiving a frequency-modulated broadcast wave, an intermediate frequency amplification section, and a demodulation circuit having a PLL circuit, wherein the demodulation circuit constitutes a PLL circuit. A first switching means for switching the closed loop gain characteristic of the PLL circuit by changing the amplitude characteristic of the low-pass filter, and the closed loop characteristic is controlled so as to decrease in conjunction with the first switching means. And a second switching means for switching the response frequency characteristic of the PLL loop so that the response frequency characteristic becomes high at a high frequency, and by operating the first and second switching means in conjunction, The response characteristics are not switched, and the bandwidth of the received signal demodulated by the demodulation circuit is limited by the PLL circuit. FM receiver and features.