JPS5880904A - Fm demodulating circuit - Google Patents

Abstract

PURPOSE:To eliminate the generation of an impulse noise when the degree of control is large by applying a control voltage generated in accordance with the degree of modulation of an FM signal from the output of a PLL demodulating circuit to a loop filter, and controlling a synchronism range when an electric field is weak by said control voltage. CONSTITUTION:At the output of a PLL demodulating circuit 6, a control voltage corresponding to the degree of modulation of the carrier in an FM signal is generated and then applied to the loop filter 7 in the PLL demodulating circuit 6 to control the synchronism range of the PLL when an electric field is weak by said control voltage. Consequently, while an S/N ratio when the electric field is weak is improved by sufficient band compression, the synchronism range of the PLL is controlled in accordance with the level when the degree of modulation is large, thereby suppressing the generation of distortion due to an impulse noise.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of an FM demodulation circuit including a phase locked loop (hereinafter abbreviated as PLL).

Although the FM reception system generally has a larger SIN ratio than the AM reception system, there is a so-called threshold phenomenon in which the noise suddenly increases when the signal input to the demodulator reaches the same level as the noise level. Moreover, when the input FM signal is below this threshold level, 1-) M8ai saving S
/Worse than N. PLL demodulation is a good method of performing demodulation while compressing the band in order to improve the above-mentioned threshold level, and is often used in FM receivers.

FIG. 1 is a schematic block diagram of an FM@tuning circuit using a conventional PLL. The input terminal noise is the frequency conversion stage (
An FM intermediate frequency signal from (not shown) is given, and this F
After the M intermediate frequency signal is band-limited and amplified by an intermediate frequency band filter (2) and an intermediate frequency amplifier (3), it is passed through a phase comparator (4), a voltage controlled oscillator (hereinafter referred to as ■co) (5), and PL consisting of loop filter (7)
The signal is input to the L demodulation circuit (6).

The FM intermediate frequency signal input to the phase comparator (4) is v
It is compared with the output from CO (51), and the output corresponding to the phase difference is given to the loop filter (7). The low-pass output of the roof filter (7) is cO (5HC:I
:/The oscillation frequency of V COi51 is changed so that the phase difference becomes 0. In other words, from this loop number, the FM intermediate frequency signal and V COi5
The + output is phase-compared by the phase comparator (4), and when they become equal, the C voltage is applied to the resistor (8) and capacitor (9).
It passes through a de-emphasis circuit consisting of, and appears at the output terminal (IG). Therefore, a detection output corresponding to the frequency shift of the FM intermediate frequency signal input from the input terminal (1) can be taken out from the output terminal body.

However, in order to improve the deterioration of the S-ratio in a weak electric field by PLL demodulation, the greater the effect can be obtained as the tuning range of the PLL is made narrower and the band is compressed. However, as band compression progresses, subcarrier 3 in stereo mode will decrease in the vicinity of a weak electric field where a threshold phenomenon occurs.
8 Bandwidth consideration for KHm etc. is not necessary since switching to monaural reception is normally done, but if the degree of modulation, that is, the frequency deviation becomes large, there will be a significant improvement in the S-ratio defined by impulse at the upper and lower limits of the tuning range. However, when the degree of modulation becomes large, distortion increases due to the impulse noise, which becomes a practical problem.

SUMMARY OF THE INVENTION An object of the present invention is to provide an FM demodulation circuit that eliminates the generation of impulse noise when the degree of modulation increases while obtaining the effect of improving the threshold through sufficient band compression in PLL demodulation.

FIG. 2 is a schematic block diagram showing one embodiment of the present invention.

The configuration of this embodiment is similar to the circuit shown in FIG. 1 except for the points described below, so corresponding parts are given the same reference numerals and their explanation will be omitted.

Now, as a result of band compression by PLL in a weak electric field, when a large modulation that generates impulse noise is applied, the demodulated output taken out from the demodulated output terminal αe via the coupling capacitor at+ increases, and the output from the diode α9 and the resistor increases. a Suddenly, the DC voltage corresponding to the level of the demodulated output, that is, the degree of modulation, which is rectified and smoothed by the capacitor α4, also increases, and is inverted by the inverter αG and sent to the loop filter (7).
The control voltage applied to is decreased to widen the synchronization range. In other words, (to) in the figure is the FM signal from the output of the PLL demodulation circuit (6).
A control voltage corresponding to the degree of modulation of the carrier wave in the signal is generated, and the control voltage is applied to the loop filter (7) in the PLL demodulation circuit (6) to control the synchronization range in the weak electric field according to the degree of modulation. Functions as a modulation degree detection circuit.

Figure 4 shows the loop filter (7) in the 2nd $J circuit.
This is a circuit example of a lag-lead type low-pass filter.

The output of the phase comparator (4) is connected to the filter input terminal αat, and the signal at the filter output terminal is given to the de-emphasis circuit as a demodulated output and also to CO as a control voltage. The synchronization range of the PLL is determined by the impedance between the collector and emitter of the resistor αl, the capacitor 2, and the transistor.

In Fig. 2, the inverter (II small output of this loop filter (7) decreases as the modulation degree increases)
The control voltage is applied to the terminal (c) through the current limiting resistor (2) to the base of the transistor (2), increasing the impedance between the collector and emitter. Therefore, the synchronization range, which is narrow band in the case of a weak electric field and an average modulation degree, is changed by the operation of this circuit as the RC time constant of the loop filter (7) changes as the modulation degree increases. The synchronization range is expanded and distortion caused by impulse noise is suppressed.

Note that the PLL return circuit filter may be an active filter using an operational amplifier in addition to the above-mentioned lag-lead type filter, and in this case, the synchronization range can be controlled by gain control of the liquid active filter, etc. Possible.Also, the time constant of the control is set by resistor a3αω so as not to cause abnormality in PLL operation.
, the capacitor α is set optimally.

Figure 3 is a characteristic diagram showing changes in the PLL tuning range with respect to the input electric field level, and the vertical axis represents the input electric field level.
The horizontal axis shows the PLL tuning range.

According to conventional PLL demodulation, as the electric field level decreases, the synchronization range also decreases in accordance with the overall characteristics and the band characteristics of the intermediate frequency band filter. On the other hand, in the device of the present invention, when the circuit constants are set so that the band compression effect is optimized, the synchronization range that can be controlled for a certain weak electric field input level is A-N as shown in the figure, when there is no modulation and low The modulation depth makes use of this band to improve the S/N ratio, and at a high modulation depth, the synchronization range is widened to B - B/ according to the level, thereby eliminating the influence of distortion caused by impulse noise. , can be reduced.

As described above, according to the present invention, a control voltage according to the modulation degree of the FM signal is generated from the output of the PLL demodulation circuit, and this is added to the loop filter of the PLL demodulation circuit to control the synchronization range of PtL in a weak electric field. By configuring the PLL to be controlled by the control voltage, the S/C ratio, which is caused by a weak electric field, can be improved by sufficiently compressing the band. Therefore, it is possible to provide an FM demodulation circuit that can control distortion caused by impulse noise and suppress the occurrence of distortion due to impulse noise.

[Brief explanation of drawings]

FIG. 1 is a schematic block diagram showing a conventional FM demodulation circuit, FIG. 2 is a schematic block diagram showing an embodiment of the FM demodulation circuit according to the present invention, and FIG. 3 is a characteristic showing changes in synchronization range with respect to input electric field level. FIG. 4 is a circuit diagram showing an example of the loop filter of FIG. 2. (4)...Phase comparator, (5)...Voltage controlled oscillator, (6)...PLL demodulation circuit, (7)...Loop filter, Life...Variation detection circuit . Agent Shin Kuzuno −

Claims (1)

[Claims] <11 A PLL demodulation circuit that has a phase-locked loop and demodulates the FM signal, generates a control voltage corresponding to the modulation degree of the carrier wave that can be transmitted to the FM signal from the output of this PLL demodulation circuit, and uses the control voltage as the PLL demodulation circuit. An FM demodulation circuit comprising: a modulation degree detection circuit that is applied to a loop filter in the demodulation circuit to control a synchronization range in a weak electric field by the modulation degree.