JPH05259820A - Afc circuit - Google Patents

Abstract

PURPOSE:To attain sure AFC locking by setting a highest or a minimum AFC voltage in an AFC operating range outputted from a frequency demodulation section to a prescribed voltage within the operating range so as to decrease a deviation in an AFC voltage in the case of re-input with simple configuration. CONSTITUTION:An input AFC voltage Sdi outputted from a frequency demodulator 5 is inputted to a local oscillator 3 as an output AFC voltage Sdo through an AFC voltage resetting circuit 6. The AFC voltage resetting circuit 6 resets an input AFC voltage Sdi resulting from a change into a highest or a lowest level in the AFC operating range in the case of 2nd input after an FM reception signal Si is interrupted into an output AFC voltage Sdo in the operating range in which the AFC locking is ensured and outputs the result to a local oscillator 3. Thus, the AFC locking at a 2nd input of the FM reception signal Si is ensured and the deviation in the oscillating frequency of a local oscillator 3 is decreased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in a radio receiving apparatus using a frequency modulation (FM) method, etc., and stabilizes the oscillation frequency of a local oscillator of a frequency conversion section by an AFC (Automatic).
Frequency Control) circuit.

[0002]

2. Description of the Related Art Conventionally, this type of automatic frequency control A
In the FC circuit, the AFC voltage from the demodulator is applied to a tuning element that constitutes a tuning circuit of the local oscillator, such as a voltage variable capacitance element (varicap). This AF
The tuning frequency of the local oscillator changes according to the C voltage value,
A so-called closed-loop control is performed in which the AFC is held at a predetermined frequency.

[0003]

In the conventional AFC circuit described above, the AFC voltage changes to the maximum value or the minimum value of the AFC pull-in operation range when the input signal of the received signal is cut off. Therefore, the deviation of the oscillation frequency of the local oscillator is large, and again,
Even if a received signal is input, AFC acquisition may not be possible. For this reason, in the case where the reception signal is disconnected and re-input after that, the AFC voltage is periodically changed by using the sweep circuit so that the AFC pull-in can be surely performed. In this case, there is a problem that the configuration becomes complicated and the device scale increases.

The present invention solves the problems in the prior art as described above. With a simple structure, the input signal of the received signal is interrupted, and the AFC voltage deviation in the case of re-input after that is small. And, A that can certainly pull in AFC
It is intended to provide an FC circuit.

[0005]

In order to achieve the above object, an AFC circuit of the present invention has at least a frequency conversion circuit, a frequency demodulation section, and a local oscillator, and supplies an AFC voltage from the frequency demodulation section. In the AFC circuit that performs closed-loop control of the oscillation frequency by inputting to the local oscillator, the AFC of the highest value or the lowest value of the AFC operation range output from the frequency demodulation unit when the reception signal becomes input interruption and is input again after this. The voltage is set to a predetermined voltage within the operating range and an AFC voltage reset circuit for outputting to the local oscillator is provided between the frequency demodulator and the local oscillator. Furthermore, A
The FC voltage reset circuit includes a first resistor to which the AFC voltage from the frequency demodulation unit is applied at one end and which outputs the reset AFC voltage from the other end, and an AF at the other end of the first resistor.
A first diode that allows a C voltage to flow, and an output terminal of the first diode and a serial connection
A low-voltage setting unit including a series-connected second and third resistor having a voltage applied to one end and the other end grounded, and a series-connected first low-voltage setting unit having a voltage applied to one end and the other end grounded. 4th and 5th
High voltage setting section having a second diode which allows the voltage at the connection section of the fourth and fifth resistors to flow therethrough and whose output end is connected to the other end of the first resistor. It consists of and. In addition, the AFC voltage reset circuit, the first resistor,
It is configured with either a low voltage setting unit or a high voltage setting unit.

[0006]

In the AFC circuit of the present invention having such a configuration, the input signal of the received signal is interrupted, and the AFC voltage of the highest value or the lowest value of the AFC operating range output from the frequency demodulation unit in the case of re-inputting after that. Is reset to a predetermined voltage within the operating range and is output to the local oscillator, and the AFC voltage reset circuit includes the first, second, third, fourth and fifth resistors,
Since it is composed of only the first and second diodes, the input signal of the received signal is interrupted and the AFC voltage deviation at the time of re-inputting after this is reduced with a simple structure to ensure A
FC can be pulled in.

[0007]

Embodiments of the AFC circuit of the present invention will be described below with reference to the drawings. FIG. 1 shows the overall configuration of the embodiment. In FIG. 1, in this example, an FM reception signal Si is input from an input terminal 1, a mixer 2 that outputs an intermediate frequency band signal (hereinafter, referred to as an IF signal), a local oscillator 3 that outputs a local oscillation signal, Bandpass filter (BPF) that passes only IF signal, AG that amplifies IF signal
It has a C circuit 4.

Further, the frequency demodulator 5 for outputting the demodulated baseband signal So from the output terminal 7 and the input AFC voltage Sdi from the frequency demodulator 5 are reset to an operating range in which the AFC pull-in is sufficiently possible. An AFC voltage reset circuit 6 that outputs the output AFC voltage Sdo is provided.

FIG. 2 shows a detailed configuration of the AFC voltage reset circuit 6. The AFC voltage reset circuit 6 has a resistor R1 to which the input AFC voltage Sdi from the frequency demodulator 5 is input, and a low or high input AFC voltage Sdi when the input AFC voltage Sdi is high or low. It has a low voltage setting part and a high voltage setting part which reset the voltage and output the output AFC voltage Sdo in the operating range where the AFC pull-in can be surely performed.

This low voltage setting unit has a voltage V + applied to one end and a resistor R2 and a resistor R3 connected in series, the other end of which is grounded, and the input AFC voltage Sdi from the resistor R1 in the forward direction. Diode D1 that outputs to the connection of R2 and 3
Is provided. The high-voltage setting unit inputs the voltage from the resistor R1 to the resistor R4 and the resistor R5 connected in series, in which the voltage V + is applied to one end and the other end is grounded, and the voltage at the connection between the resistor R4 and the resistor R5. A diode D2 for outputting and applying the AFC voltage Sdi in the forward direction is provided.

Next, the operation of the AFC circuit having such a configuration will be described. In FIG. 1, in the mixer 2 to which the FM reception signal Si is input from the input terminal 1, the FM reception signal S
i and the local oscillation signal from the local oscillator 3 are mixed,
It is converted into an IF signal. The IF signal is amplified to a constant level by the AGC circuit 4, then demodulated by the frequency demodulator 5, and the baseband signal So is output from the output terminal 7. In this case, the input AFC voltage S output from the frequency demodulator 5
The di is input to the local oscillator 3 as the output AFC voltage Sdo through the AFC voltage reset circuit 6.

In the AFC voltage reset circuit 6, the FM reception signal Si is disconnected, and the AFC in the case of subsequent re-input.
Input AFC changed to the highest or lowest operating range
The voltage Sdi is reset to the output AFC voltage Sdo in the operating range in which the AFC pull-in is surely possible and output to the local oscillator 3, and the AFC pull-in is surely performed when the FM reception signal Si is input again, and the local oscillator 3 The deviation of the oscillation frequency is reduced.

Next, the operation of the AFC voltage reset circuit 6 will be described. In the case where the FM reception signal Si is disconnected and the signal is input again thereafter, the frequency demodulator 5 may output the input AFC voltage Sdi having the highest value in the AFC operation range. This highest value of the input AFC voltage Sdi flows through the resistor R1, the diode D1, and the resistor R3, and is set to a voltage value obtained by dividing the voltage V + by the resistor R1 and the resistor R3.
It is output to the local oscillator 3 as the output AFC voltage Sdo in the operating range in which the FC pull-in is surely possible.

Further, the FM reception signal Si is disconnected,
In the case of subsequent re-input, the frequency demodulator 5 may output the input AFC voltage Sdi having the minimum value in the AFC operating range. This minimum value of the input AFC voltage Sdi is obtained by adding the voltage value obtained by dividing the voltage V + by the resistor R4 and the resistor R5 by the input AFC voltage Sdi through the resistor 1 through the forward direction of the diode D2, and thereby the AFC pull-in. The local oscillator 3 is used as the output AFC voltage Sdo in a surely possible operating range.
Is output to. As a result, the FM reception signal Si
Is input, the AFC is reliably pulled in, and a stable oscillation signal can be supplied from the local oscillator 3 to the mixer 2.

In this way, the output AFC voltage Sdo in the operating range in which the AFC pull-in can be surely performed is output to the local oscillator 3 with a simple structure including the resistors 1 to 5 and the diodes D1 and D2. In this case, the frequency stability of the local oscillator of the recent radio receiver is improved, and the variable range of the AFC voltage is not particularly required to be wide. Therefore, the AFC voltage reset circuit 6 having such a simple structure is sufficient. The effect is obtained.

The FM reception signal Si is disconnected,
In the case of re-inputting thereafter, if it is known in advance that the AFC operating range deviates to only one of the highest value and the lowest value, the high voltage setting unit and the low voltage setting unit of the AFC voltage reset circuit 6 Only one of the above may be provided.

[0017]

As described above, the AFC circuit of the present invention is
When the input signal is cut off and the signal is re-input after that, the maximum or minimum AFC voltage of the AFC operating range output from the frequency demodulator is reset to a predetermined voltage within the operating range and output to the local oscillator. In addition, the AFC voltage reset circuit includes a first resistor, a second resistor, a third resistor, a fourth resistor, and a first resistor.
Also, since it is composed of only the second diode, the received signal is disconnected due to a simple structure, and the AFC voltage deviation in the case of re-input after that can be made small to surely pull in the AFC. Have an effect.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration in an embodiment of an AFC circuit of the present invention.

FIG. 2 is a circuit diagram showing a detailed configuration of an AFC voltage reset circuit shown in FIG.

[Explanation of symbols]

 2 mixer 3 local oscillator 4 AGC circuit 5 frequency demodulator 6 AFC voltage reset circuit D1, D2 diode R1, R2, R3, R4, R5 resistor Si FM received signal So baseband signal Sdi input AFC voltage Sdo output AFC voltage

Claims (3)

[Claims] 1. An AFC circuit having at least a frequency conversion circuit, a frequency demodulation unit, and a local oscillator, wherein the AFC voltage from the frequency demodulation unit is input to the local oscillator to perform closed-loop control of the oscillation frequency. When the signal is cut off and re-inputted thereafter, the AFC voltage of the highest value or the lowest value of the AFC operation range output from the frequency demodulation unit is set to a predetermined voltage within the operation range, and the local oscillator is An AFC circuit, wherein an AFC voltage reset circuit for outputting is provided between the frequency demodulation unit and a local oscillator. 2. An AFC voltage reset circuit is reset by applying the AFC voltage from the frequency demodulation unit to one end of the AFC voltage reset circuit.
A first resistor for outputting the FC voltage from the other end, and
The first diode that allows the AFC voltage at the other end of the resistor to flow, and the output end of the first diode are connected to the series connection part, and the voltage is applied to one end and the other end is grounded. A low-voltage setting unit including second and third resistors connected in series, fourth and fifth resistors connected in series in which a voltage is applied to one end and the other end is grounded, and the fourth, And a high voltage setting section having a second diode connected to the other end of the first resistor, the second end of which is connected to the fifth resistor and the output of which is passed through. The AFC circuit according to claim 1, which is characterized in that. 3. The AFC voltage reset circuit comprises the first resistor according to claim 2 and either a low voltage setting part or a high voltage setting part.
The AFC circuit described.