JP3289693B2 - Automatic gain control circuit for FM tuner front end - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic gain control circuit (AGC) for a front end of an FM tuner.
The interference rejection capability such as the intermodulation interference ratio is improved without sacrificing the effective selectivity characteristics.

[0002]

2. Description of the Related Art FIG. 2 shows a signal processing system of a conventional FM tuner. The FM signal received by the antenna 10 is amplified by the high-frequency amplifier 14 at the front end 12 and mixed with a local oscillation signal oscillated from the local oscillator 18 by the frequency mixer 16, and centered on an intermediate frequency of 10.7 MHz. To a relatively wideband signal. The AGC circuit 20 amplifies the converted signal by the amplifier 22,
The signal is detected by the detection circuit 24 to detect the level of the signal.
The control circuit 26 controls the gain of the high-frequency amplifier 14 by controlling the bias of the elements of the high-frequency amplifier 14 according to the detected signal level.

A signal output from the front end 12 is input to an intermediate frequency section 28, and a signal component (intermediate frequency signal) of a required band (intermediate frequency band) centered on 10.7 MHz by a filter 30 such as a ceramic filter. Is extracted and amplified by the amplifier 32. The intermediate frequency signal output from the intermediate frequency unit 28 is FM-detected by the detector 34 and output as an audio signal.

[0004]

With respect to the frequency f _{0 of the} desired wave, the interfering waves at the frequencies f ₁ and f ₂ having the relationship of 2f ₁ −f ₂ = f ₀ or 2f ₂ −f ₁ = f ₀ are strong. When received at a level, intermodulation interference occurs that causes distortion of the desired signal. On the other hand, the detection band of the AGC circuit 20 has a relatively wide band centered on 10.7 MHz as shown in FIG. It works to suppress. FIG. 4 shows this state. That is, for example, for a desired wave of 98 MHz,
And the 96MHz interference wave is received, the AGC circuit 2
When 0 is off, the output characteristic of the frequency mixer 16 becomes the characteristic shown by the solid line in FIG. 4, and at this time, the level of the intermodulation interference wave becomes the level shown by "AGC off". On the other hand, while the output characteristics of the frequency mixer 16 when the AGC circuit 20 is turned on are reduced as shown by the dotted line in FIG. 4, the level of the intermodulation interference wave is greatly reduced as shown by "AGCon". I do. As a result, intermodulation interference is suppressed.

However, the AGC circuit 20 shown in FIG.
According to the above detection band characteristic, there is a problem that the effective selectivity characteristic deteriorates. That is, if the effective selectivity characteristic when the AGC circuit 20 is off is shown by a solid line in FIG. 5, the effective selectivity characteristic when the AGC circuit 20 is on is shown by a dotted line in FIG. For this reason, interference of an interference wave adjacent to the desired wave is easily caused. In order to improve the intermodulation interference ratio without sacrificing the effective selectivity characteristics, conventionally, it was necessary to increase the number of tuning stages, which greatly increased the cost.

The present invention solves the above-mentioned problems in the prior art, and can improve the interference rejection capability such as the intermodulation interference ratio without increasing the number of tuning stages and without sacrificing the effective selectivity characteristics. It is intended to provide an automatic gain control circuit for a tuner front end.

[0007]

SUMMARY OF THE INVENTION According to the present invention, a received F
The M signal is amplified by a high-frequency amplifier, and the amplified signal is mixed with a local oscillation signal corresponding to a desired wave by a frequency mixer to be converted into a signal of a relatively wide band centered on a predetermined intermediate frequency. In an FM tuner front end for controlling the gain of the high-frequency amplifier by an automatic gain control circuit according to the level of the signal, the automatic gain control circuit includes:
A filter for removing a signal component of a relatively narrow band including an intermediate frequency band from the converted signal is disposed, a level of an output signal of the filter is detected, and a gain of the high-frequency amplifier is determined in accordance with the detected level. The control is performed.

According to the present invention, since the AGC is less likely to be applied to the desired wave and the interfering wave adjacent thereto (the gain of the high-frequency amplifier is not reduced), it is possible to prevent a decrease in the effective selectivity characteristic. . In addition, since AGC is applied to an interfering wave distant from the desired wave, it is possible to maintain the interference rejection capability such as an intermodulation interference ratio as before.

[0009] By forming the filter with, for example, a ceramic filter, a steep and narrow band characteristic can be obtained. The band to be removed by the filter is a band including the intermediate frequency band, and can be set to, for example, about the intermediate frequency ± 400 kHz.

[0010]

FIG. 1 shows an embodiment of the present invention. 2 are denoted by the same reference numerals.

The FM signal received by the antenna 10 is amplified at the front end 12 by a high-frequency amplifier 14 and mixed with a local oscillation signal oscillated from a local oscillator 18 by a frequency mixer 16 to obtain an intermediate frequency of 10.7 MHz. Is converted into a signal of a relatively wide band. AGC
The circuit 20 amplifies the converted signal by the amplifier 22, and a relatively narrow band centered on the intermediate frequency by the filter 36 (a band including the intermediate frequency band, for example, the intermediate frequency ± 4
00 kHz), and the remaining signal components (the signal components in the bands on both sides of the removed band) are detected by the detection circuit 24 to detect the level of the signal. Control circuit 26
Corresponds to the high-frequency amplifier 14 according to the detected signal level.
The gain of the high-frequency amplifier 14 is controlled by controlling the bias of the element described above. Thereby, the front end 1
The level of the signal in the band centered on the intermediate frequency output from 2 is constant regardless of the strength of the radio wave.

A signal output from the front end 12 is input to an intermediate frequency section 28, and a required band component centered on 10.7 MHz is extracted by a filter 30 such as a ceramic filter and amplified by an amplifier 32. The intermediate frequency signal output from the intermediate frequency unit 28 is FM-detected by the detector 34 and output as an audio signal.

FIG. 6 shows a detection band characteristic of the AGC circuit 20 shown in FIG. The band component of the intermediate frequency ± 400 kHz has been removed. FIG. 7 shows the output characteristics of the frequency mixer 16 under the same conditions as in FIG. The characteristics when the AGC circuit 20 is turned on are shown by the dotted lines, and the level is increased in the band of the intermediate frequency ± 400 kHz as compared with the case where the filter 36 is not provided (the dotted line in FIG. 4). At this time, the interference waves f ₁ ,
Intermodulation interference exclusion capability for f _2, the cases equivalent capacity of 4 is obtained. The effective selectivity characteristics are shown in FIG.
, And the characteristic (dotted line) when the AGC circuit 20 is turned on is less reduced than the characteristic when the filter 36 is not provided (FIG. 5).

FIG. 9 shows a specific example of the AGC circuit 20 shown in FIG. The output signal of the frequency mixer 16 has a DC component removed by a capacitor C1 and is amplified by an amplifier 22 including a transistor Tr1. The amplified signal is input to a filter 36 including a ceramic filter SF. The ceramic filter SF has a frequency of ± 40 around 10.7 MHz.
It has a band of about 0 kHz, cuts off signal components within the band of the input signal, and passes signal components outside the band.
The output signal of the ceramic filter SF is a diode D
, D2 and a capacitor C2.
6, the gain of the high-frequency amplifier 14 is controlled by controlling the bias of the element of the high-frequency amplifier
Implement the operation.

Note that a ceramic filter SF 'having an inverse characteristic
FIG. 10 shows the connection in the case of using.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing a signal processing system of a conventional FM tuner.

FIG. 3 is a characteristic diagram showing a detection band of the AGC circuit 20 of FIG.

FIG. 4 is an output characteristic diagram of the frequency mixer 16 of FIG.

FIG. 5 is an effective selectivity characteristic diagram of the tuner of FIG. 2;

FIG. 6 is a characteristic diagram showing a detection band of the AGC circuit 20 of FIG.

FIG. 7 is an output characteristic diagram of the frequency mixer 16 of FIG.

FIG. 8 is an effective selectivity characteristic diagram of the tuner of FIG. 1;

9 is a circuit diagram showing a specific example of the AGC circuit 20 of FIG.

10 is a circuit diagram showing a specific example of the AGC circuit 20 of FIG. 1 when a ceramic filter having a characteristic opposite to that of FIG. 9 is used. 12 front end, 14 high frequency amplifier, 16 frequency mixer, 18 local oscillator, 20 AGC (automatic gain control) circuit, 36 filter, SF, SF 'ceramic filter.

Claims (3)

(57) [Claims] 1. A received FM signal is amplified by a high-frequency amplifier, and the amplified signal is mixed with a local oscillation signal corresponding to a desired wave by a frequency mixer to form a relatively wide band centered on a predetermined intermediate frequency. A FM tuner front end that converts the converted signal into a signal and controls the gain of the high-frequency amplifier with an automatic gain control circuit in accordance with the level of the converted signal; A filter for removing signal components of a relatively narrow band including a band is arranged, and the level of an output signal of the filter is detected
And an automatic gain control circuit of the FM tuner front end which performs gain control of the high-frequency amplifier according to the detected level. 2. An automatic gain control circuit for an FM tuner front end according to claim 1, wherein said filter comprises a ceramic filter. 3. The band removed by said filter is a band of ± 400 kHz centered on said intermediate frequency.
Or the automatic gain control circuit of the FM tuner front end according to 2.