JPS6218982Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to the improvement of an AGC circuit, and particularly aims to improve the response characteristics of the AGC circuit during tuning operation.

Conventionally, an AGC signal generation circuit was connected to the output end of the detection circuit of a radio receiver via an integrating circuit, and the
2. Description of the Related Art An AGC circuit is known that controls the gain of an RF (radio frequency) amplification circuit and an IF (intermediate frequency) amplification circuit using an AGC signal generated from an AGC signal generation circuit. However, in such a conventional AGC circuit, the time constant of the integrating circuit must be increased in order to improve distortion characteristics. Particularly in AM stereo receivers, the levels of the AM component (main signal) and phase component (sub signal) must be made equal, so it is necessary to minimize the fluctuation of the output relative to the input. The time constant of the circuit cannot be made small. However, if the time constant of the integrating circuit is made large, the AGC circuit will not follow the tuning operation that rapidly changes from the detuned state to the tuned state, and a shock noise will occur.

The present invention was developed in view of the above-mentioned points. During normal reception, the time constant of the integrating circuit is increased to improve distortion characteristics, and during tuning operation, the time constant of the integrating circuit is decreased to improve response characteristics. This is what we are trying to measure.

An embodiment of the present invention will be described below with reference to the drawings. In the figure, 1 is the antenna, 2 is the antenna
RF amplifier circuit; 3 is a frequency conversion circuit including a local oscillation circuit 4; 5 is an IF amplifier circuit; 6 is an AM detection circuit; 7 is an integrating circuit consisting of a first resistor 8 and a capacitor 9 ; 11 is an FM circuit that performs FM detection of the IF signal output from the IF amplifier circuit 5;
a detection circuit; 12 is a bandpass filter that passes only a specific frequency component in the output signal of the FM detection circuit 11; 13 is the bandpass filter 12;
A detection circuit 14 is connected in parallel with the first resistor 8 of the integrating circuit 7 .
Variable resistance means consisting of an NPN transistor 15 and a second resistor 16; reference numeral 17 is a drive circuit for driving the variable resistance means 14 .

AM broadcast radio waves received by antenna 1 are RF
Amplified by amplifier circuit 2 and converted to IF by frequency conversion circuit 3
After being converted into a signal, it is amplified by the IF amplification circuit 5,
AM detection circuit 6 performs AM detection. The output signal of the AM detection circuit 6 is transmitted from the output terminal 18 to the subsequent stage, and after being integrated by the integrating circuit 7 , the AGC
The AGC signal applied to the signal generation circuit 10 and generated from the AGC signal generation circuit 10 is applied to the RF amplifier circuit 2.
and is applied to the IF amplifier circuit 5 to perform gain control.

However, if the radio waves received by the antenna 1 are normal AM monaural broadcasting, the IF amplifier circuit 5
The detection output of the FM detection circuit 11 connected to the FM detection circuit 11 is only a DC component, and this DC component is blocked by the bandpass filter 12, so the input signal of the detection circuit 13 becomes zero, and an output is generated from the drive circuit 17. Therefore, the NPN transistor 15 is off, and the integrating circuit 7 has a large time constant determined by the first resistor 8 and the capacitor 9. If the radio waves received by antenna 1 are AM stereo broadcasts,
The output signal of the FM detection circuit 11 contains a DC component and
A 5Hz stereo display signal (ID signal) and an AM stereo difference signal are included. At that time, if the bandpass filter 12 is set to pass signals in a predetermined band including 5Hz, the DC component and the AM stereo difference signal are blocked, and only the ID signal is sent to the input terminal of the detection circuit 13. is applied. The ID signal in AM stereo broadcasting has a relatively low level of about 5mV. Therefore, if the detection level of the detection circuit 13 is set to a predetermined value (for example, 10 mV), the 5Hz stereo display signal will be detected by the detection circuit 13.
Even in the case of AM stereo broadcasting, AM
As in the case of monaural broadcasting, the output signal of the drive circuit 17 becomes zero, and the variable resistance means 14 has no effect on the integrating circuit.

Therefore, whether it is an AM monaural broadcast or an AM stereo broadcast, when the broadcast is being received normally, the time constant of the integrating circuit 7 is large, and the distortion rate can be improved.

Now, if you operate the tuning knob (not shown) to perform tuning, the IF signal frequency will cause a minute change in response to the tuning operation, and this minute change will be detected by the FM detection circuit 11, especially at 5 Hz. Unwanted signals with nearby frequencies are generated. This unnecessary signal is
Although it is close in frequency to the ID signal, it is a large signal with a level several hundred times higher than the ID signal.
Therefore, even in the case of AM stereo broadcast reception, for example, even if the unnecessary signal and ID signal pass through the bandpass filter 12, the ID signal is not detected, only the unnecessary signal is detected by the detection circuit, and the drive circuit A drive signal corresponding to the unnecessary signal is generated from 17. Therefore, the drive signal is generated only during the tuning operation, and at that time the variable resistance means 14
Since the NPN transistor 15 becomes conductive and the second resistor 16 having a smaller value than the first resistor 8 is connected in parallel with the first resistor 8 of the integrating circuit 7 ,
Capacitor 9 of integrating circuit 7 is charged with a small time constant, achieving improved response characteristics.

When the tuning operation is completed and unnecessary signals are no longer generated, the NPN transistor 15 of the variable resistance means 14
is immediately turned off, and the time constant of the integrating circuit 7 becomes large again, which helps improve the distortion characteristics.

As mentioned above, the AGC circuit according to the present invention is
By changing the time constant of the integrating circuit, it is possible to improve the response characteristics during tuning operations, and it is possible to reliably prevent the shock noise that conventionally occurs during tuning operations.

[Brief explanation of the drawing]

The figure is a circuit block diagram showing one embodiment of the present invention. Explanation of main drawing numbers, 6...AM detection circuit, 7 Integration circuit, 11...FM detection circuit, 14 ...Variable resistance means.

Claims (1)

[Scope of utility model registration request] In an AGC circuit that performs AGC by applying a detection output signal to an AGC signal generation circuit via an integrating circuit and applying an AGC signal generated from the AGC signal generation circuit to an RF amplification circuit or an IF amplification circuit, the IF amplification circuit performs AGC. IF signal obtained from the circuit
An FM detection circuit that performs FM detection, a frequency detection means that detects a signal of a specific frequency included in the output signal of the FM detection circuit, a detection circuit that detects the level of the output signal of the frequency detection means, and the detection circuit. variable resistance means that operates in response to an output signal of the integrator circuit and switches the time constant of the integrator circuit to a small value, the time constant of the integrator circuit during tuning operation being smaller than the time constant of the integrator circuit in normal operation. It is characterized by being made small.
AGC circuit.