JPS619021A - Fm receiver - Google Patents

Abstract

PURPOSE:To simplify the switching of a reception band and to prevent crosstalk disturbance and deterioration of S/N by adopting a PLL circuit for a demodulation circuit and varying a loop gain of said PLL circuit to constitute the receiver that a passing frequency band of an intermediate frequency amplifier section is switched apparently. CONSTITUTION:The IF amplifier section 2 is set to the broad band characteristic in the FM receiver and when the reception frequency band of the FM receiver is brought into the narrow band, it is switched by a switch S of a demodulation circuit 3 comprising a PLL circuit. The switch S is provided to change the loop gain in the demodulation circuit 3 and a resistor R3 is connected in parallel with a resistor R2, the gain of an amplifier A is decreased and the lock range is narrowed as shown in solid lines A to alternate long and short dash lines B in figure. In this case, although the capture range is narrowed than the lock range, since the capture range relates to a time constant of a low-pass filter, it is preferable to keep a prescribed time constant by adding a capacitor CD for the correction.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a receiver for receiving FM broadcasting, and particularly relates to an FM receiver capable of switching the reception frequency bandwidth.

[Background technology and its problems]

Receivers that receive FM stereo broadcasts generally have wideband reception characteristics when there are no interference waves, and the reception condition is such that high-quality audio signals are demodulated. In some cases, reception frequency band switching means is provided to narrow the reception frequency bandwidth and improve the S/N ratio.

Figure 1 shows a conventional example of an FM receiver with such a raw switching means, where 1 is the front end, 2W is an IF amplifier with wideband characteristics, and 2N is an IF amplifier with narrowband characteristics. 3 is a demodulation circuit, 4 is a multiplex circuit, and 5a and 5b are stereo signal amplification sections.

In such an FM receiver, by switching the switches S1 and S2, the passband of the intermediate frequency amplification section is changed to a wide selection curve A and a narrow selection characteristic B, as shown in Figure 2, and the bandwidth of the receiving frequency is changed. There is.

However, such switching means not only increases the number of IF amplifiers and transformers and is uneconomical, but also
When switching to a narrow band, the group delay characteristic of the IF amplifying section 2N deteriorates, causing problems such as generation of distortion and deterioration of separation.

[Purpose of the invention]

The present invention was made with the aim of solving this problem, and provides an FM receiver in which switching of reception frequency bands is performed by a PLL circuit used in a demodulation circuit.

[Summary of the invention]

This invention employs a PLL demodulation method in the demodulation circuit of an FM receiver that receives FM broadcast waves, and by switching the characteristics of the low-pass filter that constitutes the PLL circuit, the PLL
The lock range and capture range of the circuit are set to predetermined values. As a result, the passband width of the intermediate frequency amplification section can be switched between a wide band and a narrow band, making it possible to deal with interference and deterioration of the S/N ratio.

〔Example〕

I Fig. 3 shows a block diagram of the FM receiver of the present invention.Similar to Fig. 1, 1 is a front end, 2 is an IF amplification section, 3 is a demodulation circuit consisting of a PLL circuit, and 4t*multi Plex circuit, 5a.

5b is an amplification section for demodulated stereo signals.

In this FM receiver, the IF amplifying section 2 is the IF amplifier shown in FIG.
Like the amplifier section 2W, it is set to wideband characteristics, but as will be described later, when the receiving frequency band of the FM receiver is set to a narrow band, it is switched by the switch S of the demodulation circuit 3, which is composed of a PLL circuit. has been done.

FIG. 4 shows a demodulation circuit 3 employed in the FM receiver of the present invention.
3a is a phase detector, 3b surrounded by a dashed line is a low-pass filter whose frequency characteristics and resonance amplitude characteristics can also be varied, and 3c is a voltage-controlled variable oscillator.

Since this demodulation circuit 3 has a PLL circuit configuration as is well known, it is locked to the IF (10.7 MHz) signal and the oscillation frequency of the voltage controlled variable oscillator 3C changes, and the control voltage 1 at this time changes. zeed changes the IF frequency, that is, the FM signal.
1 demodulated signal ea. Therefore, from this demodulated signal ea, an L+R signal and an LR signal.

The pilot signal can be extracted and a stereo signal (L, R) formed in the multiplex circuit 4.

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-30 to I F (10.7MHz), but the lock range of this PLL circuit is generally the same as the loop of the PLL7 circuit. The capture range is determined by the gain and is related to the loop gain and the time constant of the low-pass filter 3b.

Therefore, in this invention, the range of the lock range is changed by switching the loop gain, the bandwidth of the demodulated signal is limited, and as a result, the bandwidth of the receiving frequency is changed from a wide band to a narrow band. It is.

That is, in the demodulation circuit 3 of FIG. 4, a switch S is provided to change the loop gain, and the resistor R3 is replaced by the resistor R2.
When connected in parallel with , the gain of amplifier A becomes smaller,
The lock range becomes narrower as shown from the solid line A to the dashed line B in FIG.

In this case, the capture range is also narrower than the lock range, but since the capture range is also related to the time constant of the low-pass filter, it is preferable to add a capacitor CD to correct it and maintain a predetermined time constant.

Note that the capacitor Coo resistor ro constituting the low-pass filter 3b is an impedance element for phase compensation of the amplifier A, and the capacitor CI 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, where A shows the demodulation band with wideband characteristics and B shows the demodulation band with narrowband characteristics.

In the case of narrowband characteristics, in order to prevent the demodulation characteristics from deteriorating around 53 KHz, which is the high frequency of the sideband AM modulated by the subcarrier of the stereo signal, the capacitor C2 is connected to the capacitor C2 by the switch S' which works with the switch S. C
It is preferable to improve the separation by connecting the power source 3 in parallel to raise the high frequency characteristics as shown by the dotted line C.

Note that the switching of the loop gain by the switch S can also be performed by electronically controlling a variable resistance element formed by a field effect transistor. In addition to manual switching, this switching control also uses a detector 6 to detect the fF signal level as shown by the dotted line in Figure 3. The microcomputer 7 may switch the switch S to automatically control the narrowband characteristic.

〔Effect of the invention〕

As mentioned above, the FM receiver of this invention employs a PLL circuit in the demodulation circuit to switch the bandwidth of the reception frequency, and by switching the loop gain of this PLL circuit, the apparent intermediate frequency amplification section Since the configuration is such that the band can be switched between a wide band and a narrow band, switching the receiving band becomes extremely easy. Furthermore, since the IF characteristics are always constant, there is an advantage that there is no distortion due to group delay characteristics or deterioration of separation.

Furthermore, since the switching of the band can be performed by electronic control, it is possible to easily change the band continuously and to switch automatically according to received radio waves.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an FM receiver with conventional bandwidth switching means, Fig. 2 is a characteristic diagram of an IF amplification section, Fig. 3 is a block diagram of an FM9 receiver showing an overview of the present invention, and Fig. 4
FIG. 5 is a circuit diagram showing one embodiment of a PLL circuit constituting a demodulation circuit, FIG. 5 is a characteristic diagram showing the lock range of the PLL circuit, and FIG. 6 is a characteristic diagram showing the band of the demodulated signal. In the figure, 1 is the front end, 2 is the IF amplification section, and 3 is the PL
3a is a phase detection Ha, 3b is a low-pass filter, and 3C is a voltage-controlled variable oscillator. Figure 4 Figure 5 1000 500 f+p 500 10
00°- to C+ to C Figure 6-(to Hz) ・i

Claims (1)

[Claims] FM equipped with a front end for receiving frequency modulated broadcast waves, an intermediate frequency amplification section, and a demodulation circuit
In the receiver, a PLL circuit is adopted as the demodulation circuit,
An FM receiver characterized in that the loop gain of the PLL circuit is made variable to apparently switch the pass frequency band of the intermediate frequency amplification section.