JPH0125453B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a distortion correction circuit for an FM receiver.

It has been known that when a modulated wave with an excessive frequency shift is applied to an FM receiver, extremely large overmodulation distortion occurs. Therefore, when receiving FM waves from a simplified transmitter without an overmodulation prevention circuit, such as a small wireless microphone, large overmodulation distortion may occur.

An object of the present invention is to provide a receiving circuit that effectively prevents overmodulation distortion on the transmitting side without widening the demodulation band.

The present invention will be described in detail below with reference to Examples.

FIG. 1 shows an embodiment of the present invention. In the figure, an FM signal 1 from a transmitter (not shown) is converted into a demodulated output 9 by an FM receiving and demodulating circuit 11 consisting of a frequency converter 2, an intermediate frequency amplifier 3, an FM demodulator 4, and a local oscillator 6. . The demodulated output 9 is applied to an adder 7 and a nonlinear amplifier 5 , and the output of the nonlinear amplifier 5 is applied to a local oscillator 6 and also to the adder 7 via a level adjuster 8 .

In the above configuration, as is well known, FM signal 1 is
The signal is demodulated by the FM reception and demodulation circuit 11 with a demodulation sensitivity of _K1 , resulting in an FM demodulation output 9. As shown in Figure 2, the characteristics of the nonlinear amplifier 5 are that the amplification level is almost zero up to the demodulation output level ±V ₁ corresponding to the normal FM modulation degree m, and for the demodulation output 9 exceeding ±V ₁ . is designed to have a large degree of amplification. Therefore, since the amplification degree of the nonlinear amplifier 5 is small for an FM wave with a normal modulation degree m, the demodulated output 9 is delivered almost as is to the output terminal 10, and there is almost no influence of the correction circuit, and the conventional FM receiver Similarly, it is demodulated with high demodulation sensitivity. Next, the modulation degree exceeds m
For FM waves, the part of the demodulated output 9 that exceeds ±V ₁ is fed back to the local oscillator 6 as negative FM feedback, and the modulation depth becomes equivalently shallower, allowing it to be within the demodulation band and reducing overmodulation distortion. Since the demodulated output level that generates this signal is never reached, overmodulation distortion does not occur.
On the other hand, for the portion of the demodulated output 9 that exceeds ±V ₁ , demodulation sensitivity decreases and non-linear distortion occurs, so the level of the output of the non-linear amplifier 5 is adjusted by the level adjuster 8 and added to the demodulated output 9, causing distortion. Corrections are made.

Now, the demodulation sensitivity of the FM demodulator 4 is K ₁ The local modulation sensitivity of the local oscillator 6 is K ₃ The input/output characteristics of the non-waveform amplifier circuit 5 is K ₂ (V) The frequency deviation of the input signal is i The demodulation output 9's If the level is V and the correction output of the output terminal 10 is V ₀ , the demodulated output V will be V=K ₁・i/1+K ₁・K ₃・K ₂ (V) due to FM negative feedback, and on the other hand. V ₀ =V(1+K ₂ (V)·K ₄ ). Therefore, if K ₄ =K ₁ ·K ₃ is adjusted, V ₀ =K ₁ ·i, and the influence of the characteristic K ₂ (V) of the nonlinear amplifier 5 does not appear.

Since the demodulated signal demodulated with the modulation degree limited in this manner is accurately expanded and corrected by the same nonlinear circuit used to limit the modulation degree, distortion is not corrected.

The nonlinear amplifier having the characteristics shown in FIG. 2 can be realized by a well-known circuit such as connecting two Zener diodes with the same characteristics in series with opposite polarities, so a detailed description thereof will not be given here.

As described above, according to the present invention, FM with deep modulation
Even when waves are received, overmodulation distortion does not occur, and effects such as being able to obtain the same demodulation sensitivity as conventional methods for FM waves with a normal modulation degree can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention;
FIG. 2 is an explanatory diagram of the operation. 4...FM demodulator, 5...nonlinear amplifier, 7...adder.

Claims (1)

[Claims] 1: an FM demodulation circuit; means for negative FM feedback of the demodulation output of the FM demodulation circuit to a local oscillator of the FM demodulation circuit via a nonlinear amplifier with a high amplification degree for input signals of a predetermined level or higher; A distortion correction circuit for an FM receiver that obtains an output signal by adding the output of the above nonlinear amplifier.