JPH0336101Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to an AGC circuit for an AM radio receiver having an auto-search function, and particularly relates to an AGC circuit that prevents errors in stopping auto-search. . (b) Prior Art As shown in FIG. 2, an IF amplifier circuit 1 that amplifies an AM intermediate frequency (IF) signal, a detection circuit 2 that performs AM detection on the output signal of the IF amplifier circuit 1, and a first and second A two-stage low-pass filter 7 includes two resistors 3 and 4 and first and second capacitors 5 and 6, and smoothes the output signal of the detection circuit 2, and outputs an AGC signal according to the output signal of the low-pass filter 7 . and a control transistor 9 that is turned on in response to a control signal applied to a control terminal 8 indicating that an auto search operation is in progress and short-circuits the first and second resistors 3 and 4. is publicly known. The AGC circuit is described on page 85 of ``'85 Sanyo Semiconductor Handbook Monolithic Bipolar Integrated Circuits,'' published on March 20, 1985. However, when the auto search operation is not performed, the detection output signal is smoothed by the low-pass filter 3 and applied to the AGC signal generation circuit 10, so that the detection output signal from the AGC signal generation circuit 10 is adjusted according to the detection output signal level. AGC signal is generated,
Gain control of the IF amplifier circuit 1 is performed. In that case, the response speed is as follows: first resistor 3 and first capacitor 5
It becomes a predetermined value delayed by a time constant determined by the second resistor 4 and the second capacitor 6, and it is possible to improve the distortion rate during low frequency modulation. By the way, when auto search is performed, the rate of change in the electric field strength of the received signal increases, so
The response speed of the AGC circuit must be increased so that the IFAGC signal can follow it. AGC in Figure 2
In the case of the circuit, this is achieved by short-circuiting the first and second resistor holes 3 and 4 by means of a control transistor 9. That is, when the auto search operation is started, an "H" control signal is applied to the control terminal 8.
Control transistor 9 is turned on. When the control transistor 9 is turned on, the time constant of the low-pass filter 7 is determined by the third resistor 11 and the second capacitor 6, so that the response speed of the AGC circuit can be increased. (c) Problems to be solved by the invention However, even if the response speed of the AGC circuit is increased using the control transistor 9, the first and second capacitors 5 and 6 cannot be discharged. If you perform an auto search while receiving a signal from a station with a medium electric field or higher, the first
Due to the charges accumulated in the second capacitors 5 and 6, the sensitivity of the signal of a weak electric field station near the station with a medium electric field or higher is suppressed, and there is a problem that the auto search does not stop at the weak electric field station. It was hot. (d) Means for solving the problem The present invention has been made in view of the above points, and includes a first control signal indicating that auto search is in progress and a second control signal indicating that there is no received signal. The present invention is characterized in that a means for discharging a capacitor constituting a low-pass filter is provided. (E) Effect According to the present invention, auto search is started and the capacitor constituting the low-pass filter is discharged only when there is no received signal. No suppression of sensitivity to signals occurs. Therefore, auto-search can be reliably stopped even at weak electric field stations near strong electric field stations. Furthermore, since AGC can be applied even during auto-search operation, the linearity of the signal indicating the electric field strength can be improved, and the adjustment range of the auto-search stop sensitivity can be expanded. (F) Embodiment Figure 1 is a circuit diagram showing an embodiment of the present invention.
12 is an input terminal to which an output IF signal of a frequency mixing circuit (not shown) is applied; 13 is an IF amplifier circuit that amplifies the IF signal; 14 is a detection circuit that performs AM detection on the amplified IF signal; Detection circuit 14
16 is an AGC signal generation circuit that generates an AGC signal according to the output signal level of the low-pass filter 15; 17;
18 is a control terminal to which a first control signal indicating that an auto search operation is in progress is applied, and 18 is a control terminal that uses an IF signal obtained from the IF amplifier circuit 13 to generate a second control signal indicating that there is no received signal. a narrowband signal meter circuit, 19, the first and second signal meter circuits;
20 is a discharge circuit that operates when a control signal is present and discharges the capacitor constituting the low-pass filter 15; 20 is a time constant switching circuit that switches the time constant of the low-pass filter 15 in accordance with the first control signal; and 21 is the Narrowband signal meter circuit 1
This is a stop signal generation circuit that generates an auto search stop signal in response to the second control signal from 8. In the receiving state of a certain station, the output IF signal of the frequency mixing circuit is amplified by the IF amplifier circuit 13 and subjected to AM detection by the detection circuit 14. Therefore, an audio signal is obtained at the output terminal 22 and transmitted to the subsequent stage. In addition, since it is not in the auto search state and the first control signal is not applied to the control terminal 17,
The discharge circuit 19 and the time constant switching circuit 20 do not operate, and the output signal of the detection circuit 14 passes through the low-pass filter 15 having the first time constant and is applied to the AGC signal generation circuit 16. As a result, the AGC signal generation circuit 16 generates a signal according to the electric field strength of the received signal.
Since an AGC signal is generated and applied to the IF amplifier circuit 13, the circuit of FIG. 1 performs normal AGC operation. At this time, since the IF signal whose level has been controlled by the AGC operation is applied to the narrow band signal meter circuit 18, the linearity of the output signal of the signal meter circuit 18 can be maintained well. Next, in order to perform an auto search, when an auto search button (not shown) is pressed while a certain station is receiving, the first control signal is first applied to the control terminal 17, and the time constant switching circuit 20 is activated. Then, the time constant of the low-pass filter 15 is switched to the second time constant, and the discharge circuit 19 enters the standby state. If auto search is performed in this state, the tuning state will shift from the reception state of the certain station, so
A second control signal indicating that there is no received signal is generated from the signal meter circuit 18, and the discharge circuit 19 starts operating in response to the second control signal. When the discharge circuit 19 operates, the charges accumulated in the capacitor constituting the low-pass filter 15 are rapidly discharged through the discharge circuit 19. Therefore, even if the certain station is a strong station with a medium electric field or higher, the signal of a nearby weak electric field station is prevented from being suppressed by the residual charge of the capacitor. As the auto search progresses and tuning is performed at the frequency where the signal exists (second station), the second control signal generated from the signal meter circuit 18 is stopped, and the operation of the discharge circuit 19 is stopped. For that reason,
The detection output signal of the signal from the second station is passed through the low-pass filter 15 having a second time constant to the AGC.
The AGC signal that is applied to the signal generation circuit 16 and generated from the AGC signal generation circuit 16 is applied to the IF amplifier circuit 1.
3, a rapid AGC operation is performed, and the output signal of the IF amplifier circuit 13 becomes a suppressed predetermined value. Therefore, the IF amplifier circuit 13
The output signal is applied to the stop signal generation circuit 21 via the signal meter circuit 18, so if the level of the output signal is higher than the voltage of the reference power supply 22, a stop signal is generated at the output terminal 23, and the auto Search operation stops. Further, if the output voltage is lower than the voltage of the reference power source 22, no stop signal is generated and the auto search operation continues. As described above, according to the present invention, the capacitor constituting the low-pass filter 15 is rapidly discharged using the first control signal indicating that the auto search operation is in progress and the second control signal indicating that there is no received signal. Therefore, it is possible to prevent signals from weak electric field stations near medium-strong electric field stations from being unnecessarily suppressed. In addition, since the operation of the discharge circuit 19 is stopped depending on the presence of a received signal, even during auto search operation, the operation of the discharge circuit 19 is stopped.
Since the AGC operation can be performed and the level of the received signal can be suppressed, the linear range of the output signal of the signal meter circuit 18 can be expanded, and the selection range of the stop level of the auto search operation can be expanded. You can. Figure 3 shows a specific example of the circuit shown in Figure 1.
In the normal reception state, the switch 24 is closed;
Since current is flowing through the current mirror circuit 25 , the first transistor 26 forming the discharge circuit 19 and the second transistor 27 forming the time constant switching circuit 20 are turned off. Therefore, the discharge circuit 19 and the time constant switching circuit 20 do not operate, and the output signal of the detection circuit 14 is passed through the low-pass filter 15 consisting of first and second resistors 28 and 29 and first and second capacitors 30 and 31. The signal is applied to the AGC signal generation circuit 16 via the signal line, and normal AGC is performed. When the auto search operation is started, the switch 24 is opened and the current inversion circuit 25 is turned off, so the first and second transistors 26 and 27 are biased by the bias circuit 32 and turned on. Therefore,
First, the time constant of the low-pass filter 15 is switched. When the auto search progresses and the output signal of the signal meter circuit 18 disappears (generation of the second control signal), the third and fourth transistors 33 and 34
turns on, the discharge circuit 19 becomes operational, and the second
The charge accumulated in the capacitor 31 is transferred to the collector-emitter path of the fifth transistor 35, the collector-emitter path of the first transistor 26, and the discharge resistor 3.
6. It is rapidly discharged via the collector-emitter path of the third transistor 33. Further, when a signal is present at a certain frequency, an output signal of the signal meter circuit 18 is generated, and the third and fourth transistors 3
3 and 34 are turned off, and the discharge circuit 19 stops its operation. Therefore, a rapid AGC operation with a short time constant using the second transistor 27 is performed. When a signal large enough to stop the auto search operation is received, the stop signal generation circuit 21 is activated to stop the auto search, and the switch 24 is closed accordingly, causing the first and second transistors to close. 2
6 and 27 are turned off and normal AGC operation is performed again. (g) Effects of the invention According to the invention, the discharge circuit is operated using a first control signal indicating that an auto search operation is in progress and a second control signal indicating that there is no received signal, Since the capacitors that make up the low-pass filter are rapidly discharged, the signals of weak electric field stations near medium-strong electric field stations are not unnecessarily suppressed, and mistakes in stopping auto search can be prevented. I can do it. In addition, even during auto search operation, if there is a received signal, the AGC circuit can be activated, so the linear range of the signal meter circuit can be expanded, and the adjustment range of auto search stop sensitivity can be expanded. It can be measured. Furthermore, as in the embodiment, if a low-pass filter time constant switching circuit is provided and the time constant switching circuit and the discharge circuit are operated in combination, AGC operation can be performed according to the state, and AM radio reception is possible. The added value of the machine can be increased.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing a conventional AGC circuit, and FIG.
The figure is a circuit diagram showing a specific example of the circuit shown in FIG. 15...Low pass filter, 16...AGC signal generation circuit, 18...Signal meter circuit, 19
...Discharge circuit, 20...Time constant switching circuit.

Claims (1)

[Scope of utility model registration request] A low-pass filter that passes the output signal of the detection circuit, and a
In the AGC circuit including an AGC signal generation circuit that generates an AGC signal, the low-pass An AGC circuit characterized in that a discharge circuit is provided to discharge a capacitor constituting a filter, and the AGC operation is stopped when an auto search operation is in progress and there is no received signal.