JPS5821447B2 - FM receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM receiver that can always receive signals with minimum distortion.

It is known that the sources of distortion in FM receivers are intermediate frequency bandpass filters and demodulators.
The intermediate frequency band pass filter and demodulator are provided in the M receiver to suppress the occurrence of distortion in the intermediate frequency band pass filter and demodulator.

However, these conventional methods are still insufficient and cannot always receive signals with minimum distortion.

Therefore, the present applicant detected the distortion itself, controlled it so that the distortion was always at a minimum, and made it possible to receive signals with minimum distortion at all times.
M receiver (hereinafter, in the present invention, this FM receiver will be referred to as an FM receiver with a distortion detection loop to distinguish it from a conventional FM receiver).

) was filed.

This FM receiver with a distortion detection loop includes means for frequency modulating an intermediate frequency signal of the FM receiver at a predetermined modulation frequency in the FM receiver, and means for demodulating the frequency modulated intermediate frequency signal at the modulation frequency. , comprising means for synchronously detecting the demodulated output from the demodulating means at a frequency twice the modulation frequency, and means for controlling the oscillation frequency of the local oscillator of the FM receiver using the detection output after the synchronous detection. It is characterized by:

An embodiment of this FM receiver with a distortion detection loop includes an antenna 1, a high frequency amplifier 2, a mixer 3, a first
and a second input terminal, the output of which is modulated by the output frequency of the modulation frequency oscillator 11 applied to the first input terminal, and by the DC voltage applied to the second input terminal, that is, the output voltage of the amplifier 16. FM with a local oscillator 4 whose output frequency changes, an intermediate frequency band pass filter 5, an intermediate frequency amplifier 6, a demodulator 7, a low frequency amplifier 8 and a speaker 9
Configure the receiver.

On the other hand, the output of the modulation frequency oscillator 11 is input to the delay circuit 12, and the output of the delay circuit 12 is input to the double multiplier 13.
The output of the multiplier 13 is used as one input of the multiplier 14 which together with the low-pass filter 15 constitutes a synchronous detector, and the other input of the multiplier 14 is the output of the demodulator γ passed through the bypass filter 10. The output of 14 is multiplier 14
The output of the low-pass filter 15 is input to the amplifier 16, and the output of the amplifier 16 is applied to the second input terminal of the local oscillator 4 as described above.

Similarly, the output angular frequency p of the modulation frequency oscillator 11 is selected to be an angular frequency that does not affect the FM reception signal.

In the FM receiver with a distortion detection loop configured as described above, the output of the local oscillator 4 is frequency-modulated by the output angular frequency p of the modulation frequency oscillator 11, and the intermediate frequency signal in the received signal has a constant level of angular frequency p. A modulated signal is generated.

Now let the output of the high frequency amplifier 2 be 5l-cosω1t,
Local oscillator 4 when there is no output of modulation frequency oscillator 11
S2' = cosω2t, and when the output S1 of the high frequency amplifier 2 and the output 82' of the local oscillator 4 are applied to the mixer 3, the output 83' of the mixer 3 has (ω, -ω2) and ( The output of the image frequency component of ω1+ω2) appears, but if we take only the (ω1-ω2) component now, S3'=cosωoo
It becomes t.

Here, ω00'' (ω1-ω2).

Therefore, the output S4 of the modulation frequency oscillator 11 cospt
When the output of the local oscillator 4 is frequency modulated at , the output S3 of the mixer 3 is 53=cos(ω.

ot-S'5inPt) FM wave.

When this FM wave passes through the intermediate frequency band pass filter 5, it is distorted by the amplitude characteristics and phase characteristics of the intermediate frequency band pass filter 5.

The output of demodulating this distorted FM wave by the demodulator 7 is:
If we focus only on the second high frequency, for the fundamental wave Δωcospt, we have 1α3 p2Δω2co s 2 p t− and 2β
2pΔGJ2S Generates a distortion component of 1n2pt.

where p; modulation angular frequency α3; third-order differential coefficient β2 of the amplitude characteristic of the intermediate frequency band-pass filter 5; second-order differential coefficient Δω of the phase characteristic of the intermediate frequency band-pass filter 5; angular frequency deviation 1 ．． K2; constant.

Therefore, the output S6 of the demodulator 7 is S6−Δωcospt+1α3p2Δω2cos2p
t- becomes 2β2pΔω2sin2pt, and if we now pay attention to the overlap of the amplitude characteristics (the positive part of the amplitude characteristic Dct2 is 1α3P2Δω2cos2pt)
, the integral of the phase characteristic Dβ2 is 2β2pΔω2sin2pt
shall be.

) The output of the demodulator 7 is 86', 86'=Δωcos
pt+ becomes 1α3p2Δω2cos2pt.

This 86' is input as one input of the multiplier 14,
Modulation 9 frequency oscillator 11 input to the other side of multiplier 14
The multiplier 14 doubles the output frequency of 86' and multiplies it by the signal S, .

As a result, the DC component in the output 88' of the multiplier 14 is 1α
p2Δω2 is output as the output of the low-pass filter 5)289'.

On the other hand, the third amplitude characteristic of the intermediate frequency bandpass filter 5
The order differential coefficient α3 usually indicates the central angular frequency of the intermediate frequency bandpass filter 5 as ω.

When the closing and operating angular frequency is ω, the linear function of the detuning frequency is α3 knee K near the center angular frequency.

It can be approximated by (ω-ω.).

Ko is a constant. Therefore, the output 89' of the low-pass filter 5 is S, 'knee K.

K1(ω-ω.)p2Δω2, S when the operating angular frequency ω is ω=ω0, S when ′-〇1ω〉ω0, ′〈0
1ω ω.

When S9'>0, the central angular frequency ω is determined by the operating angular frequency ω.

It is a direct current voltage that changes from positive to negative around .

Therefore, the output 89' of the low-pass filter 5 is transmitted to the amplifier 1.
6 and is input to the second input terminal of the local oscillator 4, and the output S,' of the low-pass filter 5 always transmits the angular frequency ωoo of the output S3 of the mixer 3 to the intermediate frequency band-pass filter 5. Frequency ω.

It acts to bring it closer to . Therefore, FM with distortion detection loop
The receiver always overlaps the amplitude characteristics of the intermediate frequency bandpass filter 5.

operates to minimize.

The above also applies to the weight Dβ of the phase characteristics of the intermediate frequency band-pass filter 5, and by simply changing the delay time of the delay circuit 12, the FM receiver with distortion detection loop can always maintain the phase characteristics of the intermediate frequency band-pass filter 5. It can be operated to minimize the weight Dβ.

A detailed explanation thereof will be omitted. In this specification, the local oscillator 4, modulation frequency oscillator 11, delay circuit 12
．． A loop consisting of a doubler 13, a multiplier 14, a low-pass filter 15, and an amplifier 16 is referred to as a distortion detection loop,
An FM receiver not equipped with this distortion detection loop is referred to as a normal receiver.

As described above, the FM receiver with a distortion detection loop always performs the receiving operation in a state of minimum distortion, but it has the disadvantage that it may become unstable depending on the state of the received signal.

This is because (a) if mixing is occurring, (b) the input to the distortion detection loop, that is, the bypass filter 1 to the multiplier 14;
This is a case where the S/N ratio of the output signal of 0 is poor.

First, regarding (a), both the desired signal and the interference signal are present in the output of the demodulator 7, and after passing through the bypass filter 10, which signal's distortion component is extracted and controlled by the distortion detection loop? This is because there may be cases where it cannot be determined.

Regarding (mouth), when the input voltage of the antenna 1 is low and there is a lot of noise after demodulation, a distortion component and a noise component of the signal with the modulation angular frequency p appear at the output end of the bypass filter 10 and are applied to the distortion detection loop. Ru.

Therefore, in the distortion detection loop, in addition to the temporally constant DC level of the distortion, temporally irregular noise is added, causing the voltage at the second input terminal of the local oscillator 4 to fluctuate.

In view of the above, the present invention is intended to eliminate the above-mentioned drawbacks.

The present invention uses an FM receiver with a distortion detection loop to detect noise and the beats of desired waves and interference waves, and to disconnect the distortion detection loop when the level exceeds a certain level.

This is to prevent unstable conditions.

The present invention will be explained below using examples.

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

In FIG. 2, the same components as those of the FM receiver with distortion detection loop shown in FIG. 1 are given the same reference numerals.

It is.

The FM receiver with distortion detection loop of this embodiment is the FM receiver with distortion detection loop shown in FIG.
Detect the output signal of 7.

AM demodulator 18 , a Schmitt circuit 19 that detects the output level of AM demodulator 18 , between the output terminal of modulation frequency oscillator 11 and the first input terminal of local oscillator 4 , and between the output terminal of amplifier 16 and local oscillator 4 Switch elements 20 and 21 each driven by the output of the Schmitt circuit 19 are separately provided between the second input terminal of the Schmitt circuit 19 and the second input terminal of the Schmitt circuit 19 .

Therefore, the noise component and the beat of the desired wave and the interference wave at the input terminal of the distortion detection loop, that is, the terminal that inputs the output of the bypass filter 10 of the multiplier 14, are amplified, and AM detection is performed to detect the noise component or the beat above a certain level. When , the Schmitt circuit 19 generates an output.

The output of this Schmitt circuit 19 turns off switch elements 20 and 21 and disconnects the distortion detection loop.

Therefore, when noise and beats are detected, the distortion detection loop is separated from the FM receiver with distortion detection loop, so the FM receiver with distortion detection loop functions as a normal FM receiver, and the unstable operation of the distortion detection loop is prevented. It will never occur.

Note that in the above embodiment, noise and beats are detected from the FM demodulated output, but there is no need to limit the detection to the FM demodulated output, and detection may be performed from the intermediate frequency signal.

Further, although the distortion detection loop is separated by the switch elements 20 and 21, it is also possible to omit one of the switch elements.

As explained above, according to the present invention, the beat and noise between the desired wave and the interference wave are detected, and when the beat and noise are present above a certain level, the distortion detection loop of the FM receiver with distortion detection loop is disconnected, making it unstable. No more movement occurs.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an FM receiver that the applicant previously applied for. FIG. 2 is a block diagram of one embodiment of the present invention. 2...High frequency amplifier, 3...Mixer,
4...Local oscillator, 5...Intermediate frequency band pass filter, 6...Intermediate frequency amplifier, 7
... Demodulator, 10 ... Bypass filter, 11 ... Modulation frequency oscillator, 12 ...
...Delay circuit, 13...Double multiplier, 14...
... Multiplier, 15 ... Low pass filter, 1
6...Amplifier, 17...Noise amplifier, 18...AM demodulator, 19...Schmitt circuit, 20 and 21...Switch element.

Claims (1)

[Claims] 1 means for frequency modulating an intermediate frequency signal at a predetermined modulation frequency, means for demodulating the intermediate frequency signal frequency modulated at the modulation frequency, and demodulating output from the demodulating means at a frequency twice the modulation frequency. In the FM receiver, the noise level in the demodulated output as an input signal to the means for synchronous detection is provided. a detection circuit that detects the detection circuit; and at least one of a first switch element that is driven by the output of the detection circuit and turns on and off the detection output, or a second switch element that turns on and off the signal of the modulation frequency. A switch element is provided, and when the noise level exceeds a predetermined value, the switch element is turned off to stop controlling the oscillation frequency of the local oscillator by the detection output.