JPS6336568B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention is an FM system that reproduces a frequency modulated signal (hereinafter referred to as an FM signal) in which a carrier signal is frequency modulated.
Regarding the playback device, in particular, the amplitude suppression degree in the limiter circuit stage of the intermediate frequency amplification circuit of the FM playback device is improved, and the distortion of the playback signal is improved.

Generally, in an FM playback device, the FM signal from the FM tuner is supplied to the FM detection circuit via an intermediate frequency amplification circuit, an intermediate frequency signal passing filter, and a limiter circuit, and the limiter circuit detects the amplitude fluctuation component of the carrier signal. is removed to improve the distortion of the reproduced signal due to this amplitude fluctuation.

Conventionally, such a limiter circuit as shown in FIG. 1 has been used, which has very good characteristics in terms of amplitude limiting operation.

That is, in FIG. 1, 1 indicates an FM signal input terminal to which an FM signal from an intermediate frequency amplifier circuit is supplied, and the FM signal supplied to this FM signal input terminal 1 is passed through an intermediate frequency signal passing filter 2. and supplies it to the limiter circuit 3. In this limiter circuit 3, the output side of the intermediate frequency signal passing filter 2 is connected via a capacitor to the base of one npn type transistor 3a constituting a differential amplifier, and the emitter of this transistor 3a is connected to the base of the other npn type transistor 3a constituting the differential amplifier. of
The connection point between the emitters of transistors 3a and 3b is connected to the collector of an npn transistor 3c constituting a constant current source.
The base of the resistor 3 is connected to a power supply terminal 4 to which a positive DC voltage is supplied through a series circuit of resistors 3d and 3e, and the connection point of the resistors 3d and 3e is connected to the resistor 3.
The base of transistor 3b is connected to the connection point of resistors 3e and 3f via resistor 3g, and the base of transistor 3b is grounded via capacitor 3h. are connected to the power supply terminal 4 through resistors 3i and 3j, respectively, and from the collectors of each of the transistors 3a and 3b, respectively.
Output terminals 3k and 3l are respectively derived to supply signals to the FM detection circuit, and the power supply terminal 4 is grounded through a series circuit of a resistor 3m and a parallel circuit of a resistor 3n and a capacitor 3o. and 3n
Connect the ground point of to the base of transistor 3c,
The emitter of this transistor 3c is grounded. Although the limiter circuit 3 has very good characteristics in terms of amplitude limiting operation, it has the disadvantage that distortion due to amplitude fluctuations cannot be sufficiently improved in the FM reproducing machine shown in FIG.

In view of this point, various studies were conducted and it was confirmed that the limiter circuit 3 has a parasitic capacitance C _E between the common emitter line of the transistors 3a and 3b and the ground. Moreover, this parasitic capacitance C _E is
It was found that it was difficult to reduce the value to about 3 pF or less. Due to the existence of this parasitic capacitance C _E , the phase of the output signal So obtained at the output terminals 3k and 3l changes with respect to the level change of the input signal Si supplied to the input side of the limiter circuit 3, that is, the base of the transistor 3a. It was confirmed that it varies with the phase of Si. That is, the input signal Si of the limit circuit 3
If , contains an amplitude fluctuation component, this amplitude fluctuation component will phase modulate the output signal So obtained at the output terminals 3k and 3l, and for this reason, even if the amplitude limiting operation characteristics of the limit circuit 3 are good, It has been found that distortion occurs in the reproduced signal regardless of the method.
Moreover, in normal FM playback devices, it is unavoidable that cross-modulation distortion occurs in the front end section, generating amplitude modulation components, or that the FM signal is subjected to amplitude modulation by passing through an intermediate frequency signal passing filter. That's true.

In view of this point, the present invention is designed to improve the distortion of the reproduced signal due to the phase modulation effect of the limiter circuit as described above.

Hereinafter, one embodiment of the FM intermediate frequency amplification circuit of the present invention will be explained with reference to FIG. This second
In the figure, parts corresponding to those in FIG. 1 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

In the example shown in FIG. 2, the output signal of the intermediate frequency signal passing filter 2 is supplied to the limiter circuit 3 via the amplitude modulation circuit 5, and the amplitude fluctuation component of the input signal supplied to the limiter circuit 3 is reduced. do as you like. That is, the output signal of the intermediate frequency signal passing filter 2 is supplied to the amplitude modulation circuit 5, and the output signal of this intermediate frequency signal passing filter 2 is supplied to the negative amplitude detection circuit 7 via the variable gain amplification signal circuit 6. The AC component obtained at the output side of the negative amplitude detection circuit 7 is supplied to the modulation signal input terminal of the amplitude modulation circuit 5, and the DC component obtained at the output side of the negative amplitude detection circuit 7 is supplied via the automatic gain control circuit 8. and is supplied to the control terminal of the variable gain amplifier circuit 6.
The output signal of this amplitude modulation circuit 5 is supplied to a limiter circuit 3 similar to that shown in FIG. 1, and the other configuration is the same as the conventional one.

Since this example is configured as described above, the output signal e _i of the intermediate frequency signal passing filter 2 is now set to e _i =A×{1+a×f(t)}sinωt. Here, A is the signal level, a is the amplitude modulation level, and f(t) is the amplitude envelope. this signal
When e _i is amplitude-detected, an AC component of A×a×f(t) is obtained, but in the above example, since the automatic gain control circuit 8 is provided in the amplitude detection circuit 7 section, this amplitude detection circuit As the alternating current component of the output signal of No. 7, a detected output signal −a×f(t) independent of the input signal level A can be obtained. This detection output signal -a×f
(t), the output signal of the intermediate frequency signal passing filter 2 is inversely modulated in amplitude by the amplitude modulation circuit 5, and the output signal e _p of the amplitude modulation circuit 5 is e _p =e _i ×{1−a× f(t)}=A{1+a*f(t)}sinωt*{1-a*f(t)}=A{1- ^a2 *f(t) ² }sinωt. Here, since the amplitude modulation level a normally satisfies a<<1, the amplitude modulation level can be made significantly shallower.

Since this signal with a significantly reduced amplitude modulation level is supplied to the limiter circuit 3, the phase modulation component in the limiter circuit 3 can be reduced.

Therefore, according to the present invention, the phase modulation components in the three stages of limiter circuits can be reduced, so that the degree of amplitude suppression can be favorably improved and the distortion of the reproduced signal can be improved.

Further, FIG. 3 shows another embodiment of the present invention, and this third embodiment shows another embodiment of the present invention.
In the figure, parts corresponding to those in FIG. 2 are designated by the same reference numerals. Although FIG. 2 above shows an example of feedback-type amplitude inverse modulation, FIG. 3 shows an example in which the amplitude fluctuation component of the input signal to the limiter circuit 3 is reduced by feedback-type amplitude inverse modulation. show.

That is, in FIG. 3, the output signal of the intermediate frequency signal passing filter 2 is supplied to the amplitude modulation circuit 5 via the variable gain amplifier circuit 9 constituting the automatic gain control circuit, and the output signal of the amplitude modulation circuit 5 is This output signal is supplied to the limiter circuit 3, and this output signal is supplied to the negative amplitude detection circuit 7, and the AC component of the output signal of this negative amplitude detection circuit 7 is supplied as a modulation signal to the amplitude modulation circuit 5, and this negative amplitude detection circuit 7 The output signal is supplied to the gain control terminal of the variable gain amplifier circuit 9 via the low-pass filter 10. The rest of the structure is the same as in FIG. 2.

In the example shown in FIG. 3, the amplitude of the output signal of the amplitude modulation circuit 5 is detected and used as a control signal for the automatic gain control circuit, as well as as an amplitude inverse modulation signal for the amplitude modulation circuit 5. It is easy to understand that the amplitude fluctuation component of the signal supplied to the limiter circuit 3 can be reduced and the same effect as that shown in FIG. 2 can be obtained. Furthermore, since an automatic gain control circuit is provided in this example in FIG. 3, the level of the signal supplied to the amplitude modulation circuit 5 is
This has the effect that the amplitude modulation circuit 5 can be set in a region where it operates linearly, and the modulation level of the amplitude modulation circuit 5 can be limited to a fairly narrow range.

It goes without saying that the present invention is not limited to the above-described embodiments, and can take various other configurations without departing from the gist of the present invention.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the main parts of an example of a conventional FM intermediate frequency amplification circuit, Fig. 2 is a block diagram showing the main parts of an embodiment of the FM intermediate frequency amplification circuit of the present invention, and Fig. 3 is a block diagram showing the main parts of an example of the FM intermediate frequency amplification circuit of the present invention. It is a block diagram which shows the principal part of another Example. 2 is an intermediate frequency signal passing filter, 3 is a limiter circuit, 5 is an amplitude modulation circuit, and 7 is a negative detection circuit.

Claims (1)

[Claims] 1. An FM intermediate frequency, characterized in that an inverse amplitude modulation circuit is provided before a limiter circuit to which the FM intermediate frequency signal is supplied, and the intermediate frequency signal is inversely modulated by the negative amplitude detection output of the intermediate frequency signal. Amplification circuit.