JP3059844B2 - AGC circuit of FM radio receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field 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 a front end circuit thereof, and the gains of an antenna tuning circuit and an RF amplifier circuit are automatically controlled in accordance with a received signal level.

FIG. 3 shows an example of such a front-end circuit. A desired FM station is selected by a tuning circuit 11 from an FM broadcast wave input from an antenna 10.
The signal is supplied to the mixer 13 through the RF amplifier circuit 12.

The antenna 10 has a PIN as well known.
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.

The mixer 13 is supplied with a desired local oscillation signal from a local oscillator 15, and the mixer 13 mixes the FM reception signal with the selected local oscillation signal, passes through a band-pass filter 16 as an intermediate frequency signal, and passes through a ceramic filter 17. Supplied to As is well known, the output of the ceramic filter 17 is amplified at an intermediate frequency by an IF amplifier 18 and further supplied to a stereo demodulation circuit through an FM detector 19.

In such a front end circuit,
In order to control the gain of the received signal, the RF amplification circuit 12
The gain control signal is supplied from the AGC drive circuit 20 to the attenuator 14, and the optimum gain is set according to the received signal level. Conventionally, a level detector 21 is provided to detect the level of a received signal, the level of an 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, such a conventional AGC circuit has a problem that a high-frequency component is output in the level detection signal because the level detector 21 has a non-linear characteristic. Was. Such a high frequency component becomes a problem particularly when a signal of a different frequency is output to the output of the mixer 13. Such two types of frequency signals generate intermodulation distortion, and the mutual distortion is a level detector. 21, there is a disadvantage that the mutual interference characteristics of the front end portion are significantly deteriorated.

When the front end of the FM receiver is integrated into an IC, the high-frequency components leaked from the level detector 21 as described above are very easily mixed into the IF stage, and are mixed with the desired received signal. FM interference of high frequency components causes interference. In practice,
The mixing of the high-frequency component easily propagates between the wirings in the front-end circuit formed as an IC, and the
The jump into the F stage causes a problem of such deterioration of the intermodulation interference characteristic in a miniaturized portable FM radio receiver or an in-vehicle FM radio receiver.

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

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an improved FM capable of eliminating intermodulation interference in an AGC circuit with a simple configuration.
An object of the present invention is to provide an AGC circuit of a radio receiver.

[0012]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a level detector for an AGC circuit.
A low-pass filter having a cut-off frequency lower than the intermediate frequency is provided between the C-drive circuit and the C drive circuit to reliably remove a high-frequency signal leaking from the level detector.

[0013]

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

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments 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 of the conventional circuit in FIG.

A feature 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, the low-pass filter 22 is also a front filter. It is integrated with the end circuit.

With the above configuration, in the present invention, the high-frequency component superimposed 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. 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 subsequent stage of the level detector 21, the AGC
The radiation and propagation of high-frequency components from the circuit pattern of the IC circuit up to the drive circuit 20 can also be reliably removed.

As described above, according to the present invention, the low-pass filter 22 is simply constructed from, for example, a combination of a resistor and a capacitor and integrated with the front-end circuit.
By using C, 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 an intermediate frequency or lower and can be built in an IC. As a result, 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 indicates the antenna input voltage, and the vertical axis indicates the FM demodulation output level.

A characteristic 101 indicates a signal level when the FM carrier frequency is set to 98 MHz, for example.
02 indicates the noise level at this time.

In such a receiving state, when the carrier frequency component of the adjacent station is mixed, the signal component and the noise component are generated when the antenna input voltage is 70 dB or more as shown by characteristics 201 and 202. Then, the characteristic 301
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 drawing, the antenna input voltage level becomes 100 dB or more. It is understood that the noise removal effect is functioning sufficiently.

Therefore, according to the present invention, it is preferable that the cut-off 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, high-frequency components mixed into the AGC circuit can be removed and intermodulation interference can be reliably reduced.

Such a low-pass filter has an advantage that it is integrated into a front-end circuit and does not greatly complicate the circuit configuration and does not require pins or capacitors for external connection.

[Brief description of the 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 illustrating an example of a conventional AGC circuit.

[Explanation of symbols]

 Reference Signs List 20 AGC drive circuit 21 Level detector 22 Low-pass filter

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03G 3/20-3/34

Claims (1)

(57) [Claims] A level detector for detecting a level of an intermediate frequency signal obtained by mixing an FM reception signal with a local transmission signal, and a gain of a front end section according to a detection level of the level detector. And an AGC driving circuit for controlling the AGC driving circuit, wherein the AGC driving circuit includes an AGC driving circuit provided between the level detector and the AGC driving circuit.
Cut lower than intermediate frequency by element that can be integrated into IC
An AGC circuit for an FM radio receiver, comprising a low-pass filter for setting an off frequency .