JPH06164273A - Agc circuit for fm radio receiver - Google Patents

Abstract

PURPOSE:To eliminate a high frequency component mixed in an AGC circuit and to reduce mutual modulation disturbance by providing a low-pass filter between the level detector of the AGC circuit and an AGC driving circuit. CONSTITUTION:The low-pass filter 22 is inserted between the level detector 21 of the AGC circuit and the AGC driving circuit 20, and the low-pass filter 22 is also made into an IC integrally with a front end circuit. In such a case, the high frequency component superimposed on a level detection signal that is a DC component outputted from the level detector 21 can be surely eliminated by the low-pass filter 22, and a mutual modulation distortion noise mixed from the AGC driving circuit 20 to a mixer stage and an IF stage can be surely eliminated. Also, since the low-pass filter 22 is provided adjacently to the rear stage of the level detector 21, the propagation of the high frequency component due to radiation from a circuit pattern by an IC circuit up to the AGC driving circuit 20 can be surely eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an FM radio receiver AG.
The present invention relates to an improved AGC circuit capable of removing intermodulation distortion generated in a C circuit, particularly an AGC circuit.

[0002]

2. Description of the Related Art In an FM radio receiver, an AGC circuit is provided in its front end circuit, and the gains of an antenna tuning circuit and an RF amplifier circuit are automatically controlled according to the received signal level.

An example of such a front end circuit is shown in FIG. 3, and a desired FM station is selected by the tuning circuit 11 from the FM broadcast wave input from the antenna 10.
It is supplied to the mixer 13 through the RF amplifier circuit 12.

As is well known, the antenna 10 has a PIN.
An attenuator 14 including a diode or the like is connected, and the input signal level can be gain controlled by an AGC circuit described later.

A desired local oscillation signal is supplied from the local oscillator 15 to the mixer 13, and the FM reception signal is mixed with the selected local oscillation signal in the mixer 13, and passes through the bandpass filter 16 as an intermediate frequency signal to the ceramic filter 17. Is supplied to. As is well known, the output of the ceramic filter 17 is intermediate frequency amplified by the IF amplifier 18, and is further supplied to the stereo demodulation circuit through the FM detector 19.

In such a front end circuit,
In order to control the gain of the received signal, the RF amplifier circuit 12
Further, the gain control signal is supplied from the AGC drive circuit 20 to the attenuator 14, and the optimum gain according to the received signal level is set. Conventionally, in order to detect the received signal level, a level detector 21 is provided, the level of the intermediate frequency signal is detected, and this detection signal is supplied to the AGC drive circuit 20.

Therefore, according to this AGC circuit, the level of the FM reception signal can be automatically controlled to an appropriate value by the front end circuit.

[0008]

However, in such a conventional AGC circuit, there is a problem that a high frequency component is output in the level detection signal because the level detector 21 has a non-linear characteristic. It was Such a high frequency component is particularly problematic when signals of different frequencies are output to the output of the mixer 13, and such two types of frequency signals cause intermodulation distortion, and the mutual distortion causes the level detector. However, there is a drawback in that the mutual interference characteristic of the front end portion is significantly deteriorated because it leaks from 21.

When the front end section of the FM receiver is integrated, the high frequency component leaked from the level detector 21 as described above is very easily mixed into the IF stage, and this occurs together with the desired received signal. Interference occurs due to FM demodulation of high frequency components. In fact,
The mixing of the high-frequency component is easily transmitted between the wirings in the integrated front end circuit, and the mixer stage and the I
Since it jumps into the F stage, such deterioration of the intermodulation interference characteristic becomes a problem in a miniaturized portable FM radio receiver, vehicle-mounted FM radio receiver, or the like.

Conventionally, such high frequency components can be removed to some extent by inserting an external capacitor into the output terminal of the level detector 21, for example. In such a case, the IC It is necessary to provide a terminal outside the integrated front end circuit, and there is a drawback that the number of pins is increased and an external capacitor is added.

The present invention has been made in view of the above conventional problems, and an object thereof is an improved FM capable of removing intermodulation interference in an AGC circuit with a simple structure.
It is to provide an AGC circuit of a radio receiver.

[0012]

In order to achieve the above object, the present invention provides a level detector and an AG in an AGC circuit.
A low pass filter having a cutoff frequency lower than the intermediate frequency is provided between the C drive circuit and the high frequency signal leaking from the level detector is reliably removed.

[0013]

Therefore, according to the present invention, the high-frequency component leaked to the output of the level detector 21 can be removed very easily by the low-pass filter, and the intermodulation interference due to the frequency of the adjacent station can be surely removed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a preferred embodiment of the present invention.
The same members as those in the conventional circuit shown in FIG.

A characteristic of the present invention is that a low-pass filter 22 is inserted between the level detector 21 of the AGC circuit and the AGC drive circuit 20. In the embodiment, this low-pass filter 22 is also front. IC integrated with the end circuit.

With the above configuration, in the present invention, the high frequency component superposed on the level detection signal which is the DC component output from the level detector 21 is reliably removed by the low pass filter 22, and the mixer from the AGC drive circuit 20 is removed. The intermodulation distortion noise mixed in the stage and the IF stage can be reliably removed. Further, since the low-pass filter 22 is provided close to the latter stage of the level detector 21, the AGC
Radiation and propagation of high frequency components from the circuit pattern of the IC circuits up to the drive circuit 20 can be reliably eliminated.

As described above, according to the present invention, the low-pass filter 22 is simply constructed, for example, from a combination of a resistor and a capacitor and integrated into the front end circuit.
By changing to C, the intermodulation interference can be removed very easily. In that case, for example, the resistance value is 100 kΩ
If a low-pass filter is constituted by the resistor and the capacitor having a capacitance of 50 pF, the cut-off frequency can be set to the intermediate frequency or lower and can be built in the IC. As a result, the elimination of intermodulation interference can be achieved, and I
The number of pins for external connection of C does not increase.

FIG. 2 shows the effect of the low-pass filter according to the present invention. In the figure, the horizontal axis shows the antenna input voltage and the vertical axis shows the FM demodulation output level.

Characteristic 101 shows a signal level when the FM carrier frequency is 98 MHz, for example, and characteristic 1
02 indicates the noise level at this time.

When a carrier frequency component of an adjacent station is mixed in such a receiving state, a signal component and a noise component are generated when the antenna input voltage is 70 dB or more as shown by characteristics 201 and 202. And the characteristic 301
Reference numerals 302 and 302 show signal characteristics and noise characteristics when the low-pass filter 22 is inserted between the level detector 21 and the AGC drive circuit 20 according to the present invention. As is clear from the figure, the antenna input voltage level is 100 dB or more. It can be seen that the denoising effect is working well.

Therefore, according to the present invention, it is preferable that the cutoff frequency of the low-pass filter is set to a band lower than the intermediate frequency.

[0023]

As described above, according to the present invention,
By providing a low-pass filter between the level detector of the AGC circuit and the AGC drive circuit, it is possible to remove high frequency components mixed in the AGC circuit and reliably reduce intermodulation interference.

Further, such a low-pass filter has an advantage that it is integrated into a front end circuit as an IC, does not significantly complicate the circuit structure, and does not require a pin or a capacitor for external connection.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing a preferred embodiment of an AGC circuit of an FM radio receiver according to the present invention.

FIG. 2 is an antenna input-FM demodulation output characteristic diagram in FIG.

FIG. 3 is a block circuit diagram showing an example of a conventional AGC circuit.

[Explanation of symbols]

 20 AGC drive circuit 21 Level detector 22 Low pass filter

Claims (1)

[Claims] 1. A level detector for detecting the level of an intermediate frequency signal obtained by mixing an FM reception signal with a local oscillation signal, and an AGC drive circuit for controlling the gain of a front end section according to the detection level of the level detector. In the AGC circuit of the FM radio receiver including, a low pass filter having a cutoff frequency lower than the intermediate frequency is provided between the level detector and the AGC drive circuit.