JPH0421374B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an intermediate frequency signal processing device that improves distortion generated by an IF filter in an IF (intermediate frequency) amplification stage of an FM receiver.

A general block diagram of the IF signal processing stage is shown in FIG.
This amplified output is detected by the FM detector 4. The IF filter 2 needs to satisfy both selection characteristics and low distortion characteristics in order to eliminate interfering signals, and therefore a filter with emphasis on amplitude characteristics and phase characteristics, such as a Betssel type LC filter, is used. However, since there is a limit to the improvement of the characteristics of the IF filter 2, it is not possible to fully satisfy the above characteristics, and it is difficult to achieve a low distortion rate.

FIG. 2 shows an FM negative feedback type IF signal processing circuit that has been put into practical use to improve the characteristics of the device shown in FIG. By RF (high frequency) amplifier 5
The RF signal is mixed with the output of a VCO (voltage controlled oscillator) 6 in a mixer 7 and converted into an IF signal.
This IF signal is passed through IF filter 2, IF amplifier 3 and
The signal is input to the LPF 8 via the FM detector 4. this
Feedback the output of LPF8 to use as a control signal for VCO6 to control the oscillation frequency of VCO6,
The FM detection output is frequency modulated by this VCO 6 and is used as a mixing input of a mixer 7.

With this configuration, the frequency deviation (±75 KHz) of the RF signal is compressed (for example, ±7.5 KHz) by mixing with the output of the VCO 6, and the bandwidth of the IF signal becomes narrow. As a result, the apparent passband width of the IF filter 2 becomes wider than that shown in FIG. 1, and distortion can be improved.

However, in this FM negative feedback method, the frequency shift is compressed, so the detection efficiency is reduced accordingly, and the band of the interfering signal is also compressed at the same time, resulting in poor selectivity. Furthermore, because of the negative feedback method, there is a drawback that the circuit becomes unstable and there is a risk of oscillation.

The purpose of the present invention is to improve distortion generated in an IF filter without deteriorating selection characteristics.
The purpose is to provide an IF signal processing device.

The IF signal processing device of the receiver according to the present invention is based on the FM
- means for modulating the RF signal into a first IF signal; means for detecting the signal that has passed through the bandpass filter of the first IF signal; and FM modulation means for obtaining an FM modulated signal in accordance with the detection output; The modulation signal and the first
means for mixing with the IF signal to obtain a second IF signal;
This second IF signal and the modulation signal are mixed and the first
The device is characterized in that it includes means for modulating the IF signal into an IF signal and sending it to the FM detector.

In order to better understand the features and effects of the present invention, the present invention will be explained below with reference to the drawings.

FIG. 3 is a block diagram of an embodiment of the present invention;
First IF signal (10.7M
Hz), interference waves are removed by an IF filter 31 and FM detected by an FM detector 32. this
Transmission distortion will occur in the IF filter 31 and the detector 32. This detection output is LPF3
3 and becomes a control signal for the VCO 34, but the LPF 33 may be designed to pass up to 53 KHz (the upper limit frequency of the sub signal) in the case of a so-called pilot tone FM broadcast wave. The detection output by VCO34 becomes an FM signal again, but the free-running oscillation frequency of VCO34 is changed to 6.2M.
Hz, and the frequency is modulated around it according to the detection output. The spectrum of the FM signal from this VCO 34 is calculated by assuming that the original signal component is S, and the distortion component generated by the transmission system of the IF filter 31, detector 32, LPF 33, and VCO 34 is (S+
D) Contains the following ingredients.

In other words, if the frequency deviation of the signal component S of the first IF signal is ±Δf _S , the first IF signal is 10.7 M
Hz±Δf _S. Also, output from VCO34
If the sensitivity of the VCO 34 is set so that the frequency deviation of the signal component S of the FM signal is ±Δf _S , and the frequency deviation of the distortion component D is ±Δf _D , then the VCO 34
The FM signal output from is 6.2MHz±Δf _S ±Δf _D. Note that since the distortion component D is originally smaller than the signal component S, Δf _D also has a smaller value than Δf _S.

On the other hand, the first IF signal from the front end 1 passes through the mixer 36 through the delay device 35 that corrects the delay time due to the transmission system of the IF filter 31 to VCO 34.
and is mixed with the FM signal output from the VCO 34. From this mixed output, the BPF 37 extracts a difference component, that is, a second intermediate frequency signal (second IF signal) of 10.7MHz-6.2MHz=4.5MHz. In this case, if we include the modulation component, we get
The second IF signal output from the BPF37 is (10.7MHz±Δf _S ) −(6.2MHz±Δf _S ±Δf _D ) =4.5MHz〓Δf _D , and the signal component S is canceled, and S−(S
+D)=-D, that is, only the distortion component exists. A second waveform containing only this distortion component -D
The IF signal has a small frequency deviation and has a BPF of 37.
Even if the interference components are removed by using a narrow band, no distortion occurs in the BPF 37.

The second IF signal output from this BPF37 is
It is mixed with the output signal of the VCO 34 in the mixer 38, the sum component is extracted in the IF filter 39, and it becomes the first IF signal of 10.7MHz again.
It is input to the FM detector 4. In the modulated output from this mixer 38, (4.5MHz〓∆fD ₎ + (6.2MHz± _∆fS ± _∆fD ) = 10.7MHz± _∆fS , and -D+(S+D)=S component, that is, distortion component - D is removed, resulting in a spectrum of only the signal component S. Therefore, the IF filter 39 can be a broadband filter that does not cause distortion.

By doing so, signal distortion caused by improving the selectivity characteristics of the IF filter 31 can be canceled out, so that the characteristics of the IF processing circuit can be improved. In particular, since negative feedback is not used, there is no risk of oscillation and the circuit can be stabilized. Furthermore, by inserting an LPF between the VCO and the FM detector that drives it, it is possible to control the band necessary for distortion improvement.

Note that the delay device 35 may be omitted if the signal delay caused by the transmission line from the filter 31 to the VCO 34 can be ignored. Further, the filter 39 may be an HPF (high-pass filter) if the mixer 38 is an ideal multiplier, and the LPF 33 determines the frequency range in which distortion can be corrected.
It may be omitted if the frequency characteristics of the FM detector 32 can be set appropriately. Further, the output frequency deviation of the VCO may be made smaller than that of the first IF signal, but in this case, the degree of improvement in distortion will decrease accordingly. The above numerical examples are not limited to these, and various modifications can be made.

[Brief explanation of drawings]

1 and 2 are block diagrams of an IF signal processing circuit of a conventional FM receiver, and FIG. 3 is a block diagram of an embodiment of the present invention. Explanation of symbols of main parts, 31, 39...IF filter, 32...Detector, 33...LPF, 34
...VCO, 36, 38...Mixer, 37...
B.P.F.

Claims (1)

[Claims] 1. A means for converting a frequency-modulated received signal into a first intermediate frequency signal, a means for detecting a signal of the first intermediate frequency signal that has passed through a bandpass filter, and a means for detecting a signal that has passed through a band filter of the first intermediate frequency signal, and a modulating means for obtaining a frequency modulated signal, a means for obtaining a second intermediate frequency signal by mixing the frequency modulated signal and the first intermediate frequency signal, the second intermediate frequency signal and the frequency modulated signal; An intermediate frequency signal processing device for a receiver, comprising means for mixing the first intermediate frequency with the first intermediate frequency, and transmitting the first intermediate frequency to an FM detector. 2. Claim 1, wherein the modulation means comprises a low-pass filter that receives the detected output as an input, and a voltage-controlled oscillator whose oscillation frequency is controlled by the output of the low-pass filter. An intermediate frequency signal processing device for the receiver as described.