JPS6129575B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in AFC (automatic frequency control) circuits, and particularly aims to provide a new circuit for stopping AFC during channel selection. In today's broadcasting environment where many broadcasting stations are placed adjacent to each other, if the AFC circuit is operating during channel selection, it may be difficult to select the desired station due to the capture characteristics of the AFC circuit. It has the disadvantage of being For example, even if the tuning operation is stopped near the center frequency of a certain broadcasting station, the tuning point will be pulled to the center frequency of the certain broadcasting station due to the action of the AFC circuit, and reception of the certain broadcasting station will be achieved. . Therefore, if the AFC circuit is always working, even if a slight tuning operation is performed to receive the neighboring station of the certain broadcasting station, the tuning point will not shift and the neighboring station cannot be received. do not have. Therefore, it is necessary to perform a sufficient tuning operation to remove the station from the capture range of the certain broadcasting station, and then perform a tuning operation so that the adjacent station can be received, which has the drawback that the tuning operation is troublesome. In addition, when tuning is stopped within the capture range of the center frequency of a certain broadcasting station to receive the certain broadcasting station, the tuning point is pulled to the center frequency of the certain broadcasting station by the operation of the AFC circuit, and the reception is in a completely tuned state. is achieved. However, if the power is cut off in that state and then turned on again, the tuning returns to the state where AFC is not applied. At that time, if the characteristics of the element change due to temperature drift, etc., it will deviate from the capture range of the certain broadcasting station, and the tuning will return to the state where AFC is not applied. There is a danger that a situation may arise in which it will not be possible to receive the broadcast station. Conventionally, in order to prevent the above-mentioned drawbacks, various means have been used to stop AFC during a channel selection operation. For example, it is known that a dial knob (synchronization knob) is equipped with a touch switch, and the AFC is stopped when the operator's hand touches the dial knob. However, in such a case, an expensive touch switch must be used, resulting in high costs. In addition, the dial knob is placed inside the hidden box, and when the door of the hidden box is opened, the AFC
There are some that stop. However, in such a case, there is a problem that the appearance is restricted, and the cost is high because the door must be modified. The present invention has been made in view of the above points, and will be described below based on embodiments with reference to the drawings. In FIG. 1, 1 is a receiving antenna, 2 is a radio frequency amplification circuit, and 3 is an intermediate frequency signal generated from an output signal from the radio frequency amplification circuit 2 and a local oscillation signal from a local oscillation circuit 4. A mixing circuit, 5 is an intermediate frequency amplification circuit that amplifies the intermediate frequency signal, 6 is a detection circuit for detecting the intermediate frequency signal and generating a low frequency signal, and 7 is amplifying the detection output signal to generate an AFC signal. 8 is a level detection circuit that generates an output according to the output level of the AFC amplifier 7; 9 is a resistor 10 connected between the AFC amplifier 7 and the local oscillation circuit 4;
and a capacitor 11; and 12 , a first switching transistor 13 that switches in response to a muting signal from the intermediate frequency amplifier circuit 5, and a second switching transistor that responds to the operation of the first switching transistor 13. 14, and a third switching transistor 15 responsive to the output signal of the level detection circuit 8 ;
the first and third switching transistors 13;
and a fourth switching transistor 16 that responds to the operation of the fourth switching transistor 15, a capacitor 17 that is discharged when the fourth switching transistor 16 is on and charged when the fourth switching transistor 16 is off, and the integrating circuit 9.
This is an AFC stop circuit consisting of a fifth switching transistor 18 for fixing the output voltage of the AFC. Next, the operation will be explained. The muting signal from the intermediate frequency amplification circuit 5 is as shown in FIG.
It is set in advance according to the electric field strength so that it is "L" in the capture range A of the AFC circuit and "H" in the capture range B outside the AFC circuit capture range. In addition, the level detection circuit 8 is connected to the AFC as shown in FIG.
The output signal of amplifier 7 is within a predetermined range (X±△XV) centered on the reference voltage (XY) given from point X.
(When the output signal is "L" in the tuning range C,
Further, it is configured to generate an output signal that becomes "H" outside the predetermined range (out-of-tuning range D) at its output terminal. Now, if the tuning state is outside the capture range B, the muting signal becomes "H", the first switching transistor 13 is turned on, the second switching transistor 14 is turned off, and the power supply voltage (+Vcc) increases. Resistor 19 and diode 20
is applied to the base of the fifth transistor 18 via the fifth switching transistor 1.
8 is turned on, and the output voltage of the integrating circuit 9 is fixed to the reference voltage (XV) given from point X. Therefore, the AFC circuit becomes unlocked and the AFC operation stops. At that time, since it is naturally in the non-tuning range D, the output signal of the level detection circuit 8 becomes "H", and the third switching transistor 15 is turned on.
The fourth switching transistor 16 is turned off, and the tuning display light emitting diode 21 is turned off.
Further, since the voltages across the capacitor 17 are approximately equal, the capacitor 17 is neither charged nor discharged. When the tuning knob 22 is operated from the above state to move the tuning state to within the capture range A (but not to the tuning range C), the muting signal becomes "L" and the first switching transistor 1
3 is off, and the second switching transistor 14 is on. Therefore, the power supply voltage (+Vcc) is no longer applied to the base of the fifth switching transistor 18 via the resistor 19 and the diode 20, but the charging current of the capacitor 17 is supplied to the base of the fifth switching transistor 18. Therefore, the fifth switching transistor 18 remains on, and the AFC circuit also remains unlocked. The charging time constant of the capacitor 17 needs to be sufficiently large. Because the fifth switching transistor 1
8 remains on only while the capacitor 17 is being charged after the second switching transistor 14 is turned on. However, if the charging time constant is short, the switching transistor 8 remains on before the tuning point is reached by the tuning operation. , the fifth switching transistor 18 is turned off,
This is because there is a risk that the AFC circuit will become locked. While the capacitor 17 is being charged,
When the tuning operation is performed to bring the tuning state into the tuning range C, the output signal of the level detection circuit 8 becomes "L", the third switching transistor 15 is turned off, and the fourth switching transistor 16 is turned on. Therefore, the light emitting diode 21 is turned on, indicating that the tuning range is C, and the capacitor 17 starts discharging. Therefore, the fifth switching transistor 18 is turned off and the AFC
The circuit becomes locked and full reception of the desired station is achieved. By the way, even if the tuning operation is stopped in the state where the aforementioned tuning is within the capture range A, complete reception can be performed by the operation of the AFC circuit. In other words, when the tuning operation is stopped at A within the capture range, the muting signal is "L", so
The first switching transistor 13 is off, the second
Switching transistor 14 is turned on.
Also, since the output of the level detection circuit 8 is "H", the switching transistor 15 is off, and the fourth switching transistor 16 is off, so
The capacitor 17 continues to be charged, the fifth switching transistor 18 remains on, and the AFC circuit also remains unlocked. When the time determined by the charging time constant of the capacitor 17 has elapsed, the capacitor 17 is fully charged and the base current of the fifth switching transistor 18 becomes zero. Therefore, the fifth switching transistor 18 is turned off and the AFC circuit starts operating. As the AFC circuit works and the tuning frequency is corrected, the output voltage of the AFC amplifier 7 becomes
It approaches the reference voltage (XV). and said
When the output voltage of the AFC amplifier 7 reaches the vicinity of the reference voltage, the output signal of the level detection circuit 8 becomes "L", the third switching transistor 15 is turned off, and the fourth switching transistor 16 is turned on, causing light emission. The diode 21 is turned on, indicating a state of complete tuning, and the capacitor 17 is discharged. It is desirable that the discharge time constant of the capacitor 17 is small. This is because, if the discharge time constant is long, immediately after reaching the fully tuned state,
This is because when a tuning operation is performed, there is a charge remaining in the capacitor 17, which shortens the charging time of the capacitor 17, and there is a risk that the AFC circuit will not be unlocked. Furthermore, when the AFC circuit according to the present application enters the next tuning operation from the complete tuning state, the AFC circuit immediately starts the AFC circuit.
This has the advantage that the circuit is unlocked and the tuning state reliably follows the tuning operation. That is, in the fully tuned state, the first, third and fifth switching transistors 13, 15 and 18 are off, and the second and fourth switching transistors 14 are off.
and 16 are on, the light emitting diode 21 is on, and the capacitor 17 is completely discharged. However, when the tuning knob 22 is operated, the light emitting diode 21 is on and the capacitor 17 is completely discharged.
The output voltage of AFC amplifier 7 changes (integrator circuit 9
(because of the time constant), an "H" output signal is generated from the level detection circuit 8 , the third switching transistor 15 is turned on, and the fourth switching transistor 16 is turned off. Therefore, the capacitor 17 is charged again, the fifth switching transistor 18 is turned on, and the AFC circuit enters the AFC stop state. Therefore, until the charging of the capacitor 17 is completed, tuning operations can be performed freely. As described above, the AFC circuit according to the present invention is
Since the AFC stop can be achieved purely electronically without using a special mechanical switch, a highly reliable and inexpensive AFC circuit can be provided.
Furthermore, since the muting signal and the signal from the level detection circuit are used as control signals, the operation is reliable and various measures are possible. especially,
Since the tuning display is a display of the tuning range rather than the capture range, you can reliably find the perfect tuning point during tuning operations, and the capture range will not go out even if the power is turned on and off repeatedly. It is an excellent device that has many advantages, such as being able to always obtain good reception conditions.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figure 2 is a circuit diagram showing an embodiment of the present invention.
Figures A and B are characteristic diagrams for explaining the present invention. Explanation of main drawing numbers, 7...AFC amplifier, 8 ...Level detection circuit, 9 ...Integrator circuit, 12 ...AFC stop circuit, 17...Capacitor.

Claims (1)

[Claims] 1. AFC that changes the tuning frequency according to changes in the received signal and maintains a completely tuned state.
The circuit includes a switching transistor for stopping the operation of the AFC circuit, a first control circuit that operates according to the capture range of the AFC circuit, and a second control circuit that operates according to the tuning state of the receiver; The switching transistor includes a first drive circuit that drives the switching transistor according to the operation of the first control circuit, and a capacitor, and the switching transistor is driven by a charging/discharging current flowing to the capacitor according to the operation of the second control circuit. second to drive
a first control circuit and a first drive circuit to drive the switching transistor when the received signal is outside the capture range;
When the AFC circuit is stopped and the received signal is within the capture range and outside the tuning range,
An AFC circuit comprising: an AFC stop circuit that drives the switching transistor using a second control circuit and a second drive circuit to stop the AFC circuit for a predetermined period of time. 2. The AFC circuit according to claim 1, wherein the capacitor included in the second drive circuit has a sufficiently long charging time constant and a sufficiently short discharging time constant.