JPH06291656A - Automatic frequency control circuit - Google Patents

Abstract

PURPOSE:To continuously perform automatic frequency control over the output frequency variable range of a voltage controlled crystal oscillator at the automatic frequency control circuit provided with a frequency synthesizer and the voltage controlled crystal oscillator. CONSTITUTION:This automatic frequency control circuit to generate the reference frequency of a frequency synthesizer 11 with a voltage controlled crystal oscillator 10 is provided with a control circuit 13 for monitoring the controlled voltage of the voltage controlled crystal oscillator 10 and a phase locked loop(PLL) circuit 14 to be locked at the output frequency of the frequency synthesizer 11. When the controlled voltage of the voltage controlled crystal oscillator 10 gets out of a prescribed range, the control circuit 13 changes the frequency channel of the frequency synthesizer 11, changes the controlled voltage corresponding to this frequency channel and simultaneously sets a band variable loop filter 142 of the PLL circuit 14 to a narrow band.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic frequency control circuit suitable for use in communication devices and the like, and more particularly to an automatic frequency control circuit capable of expanding a control frequency range.

[0002]

2. Description of the Related Art FIG. 2 shows an example in which a conventional automatic frequency control circuit is applied to a receiver. The received signal 1 including the pilot signal is input to the mixer 2, where it is mixed with the frequency from the frequency synthesizer 11 to undergo frequency conversion, after which unnecessary waves are removed by the bandpass filter 3 and supplied to the distributor 4. To be done. One of the outputs of the distributor 4 is the output signal 5
Is sent to the next stage circuit. Further, the other output of the distributor 4 is extracted by the band pass filter 6 and only the pilot signal is supplied to the mixer 7. In this mixer 7, it is compared with the reference frequency from the local reference oscillator 8,
The error frequency is detected. The output of the mixer 7 is supplied to the voltage controlled oscillator 10 through the loop filter 9,
The oscillation frequency of the voltage controlled crystal oscillator 10 is controlled. The output of the voltage controlled crystal oscillator 10 is used as the reference frequency of the frequency synthesizer 11, and the output of the frequency synthesizer 11 is supplied to the mixer 2 as described above.

Here, the frequency synthesizer 11 is constructed so that the frequency can be changed stepwise by the channel setting switch 12. That is, the reference frequency of the frequency synthesizer 11 is obtained from the voltage-controlled crystal oscillator 10 in order to stabilize the frequency. However, since the voltage-controlled crystal oscillator 10 has a narrow control frequency range, the channel setting is made to cope with this. The switch 12 is provided to expand the frequency variable range of the frequency synthesizer 11. The channel setting switch 12 is configured, for example, as a switch that switches a variable frequency divider provided in the frequency synthesizer 11. Therefore, when the received pilot signal deviates beyond the variable frequency range of the output of the frequency synthesizer 11, by switching the channel setting switch 12, even if the frequency range of the voltage controlled crystal oscillator 10 is narrow, the output frequency of the frequency synthesizer 11 is stepped. The frequency variable range can be expanded apparently, and the deviation of the received pilot signal can be dealt with.

[0004]

This conventional automatic frequency control circuit is capable of automatic frequency control within the output frequency variable range of the frequency synthesizer by the voltage controlled crystal oscillator, but the received pilot signal exceeds this frequency variable range. If there is a deviation, it is necessary to switch the channel setting switch, and there is the problem that automatic frequency control cannot be performed continuously over this frequency variable range. In addition, it is possible to control the frequency over the variable frequency range by switching the channel setting switch, but at this time, the frequency synthesizer needs a certain amount of time to follow the output of the voltage controlled crystal oscillator. There is a problem that the output signal of the automatic frequency control circuit is temporarily cut off. An object of the present invention is to provide an automatic frequency control circuit capable of performing automatic frequency control in a variable output frequency range of a frequency synthesizer or above, and in which the frequency control is not interrupted. .

[0005]

SUMMARY OF THE INVENTION An automatic frequency control circuit of the present invention is a frequency synthesizer capable of changing a frequency channel stepwise, a voltage controlled crystal oscillator for generating a reference frequency of the frequency synthesizer, and a voltage controlled crystal oscillator. Is provided with a control circuit for monitoring a control voltage supplied to the frequency synthesizer and a phase-locked circuit (hereinafter referred to as a PLL circuit) locked to the output frequency of the frequency synthesizer. When the frequency is out of the predetermined range, the frequency channel of the frequency synthesizer is changed, the control voltage is changed corresponding to this frequency channel, and at the same time, the PLL circuit is set to a narrow band. For example, the control circuit controls an A / D converter that converts the control voltage supplied to the voltage controlled crystal oscillator into a digital signal, a controller that monitors the converted digital signal, and a digital signal that passes through the controller. And a D / A converter for converting the voltage. The PLL circuit also includes a voltage-controlled oscillator, a phase detector for phase-comparing the output frequency of the frequency synthesizer and the oscillation frequency of the voltage-controlled oscillator, and a variable band that uses the output of this phase detector as the control voltage of the voltage-controlled oscillator. And a loop filter. Then, the controller changes the channel of the frequency synthesizer by one step by the control signal for channel setting when the digital signal exceeds a predetermined value and the control voltage is out of the control frequency range of the voltage controlled crystal oscillator, and The control voltage of the voltage controlled crystal oscillator is changed by an amount corresponding to one step frequency of the frequency synthesizer, and the band variable loop filter is set to a narrow band by the band variable control signal. In this case, the frequency control range of the voltage controlled crystal oscillator and the frequency step (frequency range) of the frequency synthesizer are set to be equal, and the controller controls the voltage of the voltage controlled crystal oscillator when the channel of the frequency synthesizer is changed by one channel. Is set to a voltage corresponding to the center frequency.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. 1 is a block diagram of an embodiment of an automatic frequency control circuit of the present invention, and the same parts as those in FIG. 2 are designated by the same reference numerals. The reception signal 1 including the pilot signal is input to the mixer 2, where it is mixed with the frequency from the PLL circuit (phase synchronization circuit) 14 that operates based on the output frequency of the frequency synthesizer 11, and the frequency is converted thereafter. Unwanted waves are removed by the pass filter 3 and supplied to the distributor 4. One of the outputs of the distributor 4 becomes an output signal 5 and is sent to the circuit of the next stage. Further, the other output of the distributor 4 is extracted by the band pass filter 6 and only the pilot signal is supplied to the mixer 7. In this mixer 7, the error frequency is detected by comparison with the reference frequency from the local reference oscillator 8. The output of the mixer 7 is supplied to the control circuit 13 through the loop filter 9, and the output of the control circuit 13 is supplied to the voltage controlled crystal oscillator 10 to control the oscillation frequency of the voltage controlled crystal oscillator 10. The output of the voltage controlled crystal oscillator 10 is used as the reference frequency of the frequency synthesizer 11, and the PLL circuit 14 is operated by the output of the frequency synthesizer 11.

The control circuit 13 has an A / D converter 131 for analog-digital converting the output of the loop filter 9 into a digital signal of arbitrary bits, and a controller 132 for constantly monitoring the digital signal. D for digital-to-analog conversion of the output from the controller 132
And / A converter 133. The PLL circuit 14 is composed of a phase detector 141, a band variable loop filter 142, and a voltage controlled oscillator 143. The phase detector 141 has a phase between the output of the frequency synthesizer 11 and the voltage controlled oscillator 143. Detect the error. The output of the phase detector 141 passes through the band variable loop filter 142 and becomes the control voltage of the voltage controlled oscillator 143. As a result, the voltage controlled oscillator 143 is locked to the output frequency of the frequency synthesizer 11, and its output is supplied to the mixer 2.

Further, the controller 132 of the control circuit 13
When the signal from the A / D converter 131 is within a certain value, this signal is sent to the D / A converter 133 as it is,
It is output as the control voltage of the voltage controlled crystal oscillator 10.
On the other hand, when the signal exceeds a certain value, the frequency setting control signal 1 for the frequency synthesizer 11 is set.
5 is output to change the channel in the frequency synthesizer 11 by one channel. At the same time, the band variable control signal 16 is output to the band variable loop filter 142 of the PLL circuit 14 to control the band variable loop filter 142 from the wide band state until then to the narrow band state.

According to this structure, when the received signal is within the controllable range of the voltage controlled crystal oscillator 10, the control circuit 13 simply has a function of relaying the output of the loop filter 9 to the voltage controlled crystal oscillator 10. Yes and PL
The L circuit 14 supplies the output frequency of the voltage controlled oscillator 143 to the mixer 2 based on the output frequency of the frequency synthesizer 11. As a result, automatic frequency control is performed within the frequency variable range of the frequency synthesizer 11 or the PLL circuit 14 by the voltage controlled crystal oscillator 10.

On the other hand, if the received signal deviates by more than the controllable range of the voltage controlled crystal oscillator 10, the controller 132
Detects this and inputs the band variable control signal 16 to the band variable loop filter 142 of the PLL circuit 14 to narrow the band of this filter. At the same time, the controller 1
32 inputs the channel setting control signal 15 to the frequency synthesizer 11 and changes the channel by one channel. Further, the control voltage of the voltage controlled crystal oscillator 10 is also controlled through the D / A converter 133 at the same time. For example, when the frequency control range of the voltage controlled crystal oscillator 10 and the frequency step (frequency range) of the frequency synthesizer 11 are set to be equal and one channel of the frequency synthesizer 11 is changed, the voltage controlled crystal oscillator 10 is changed.
The control voltage of is set to a control voltage corresponding to the center frequency.

In this way, if the controller 132 changes the channel of the frequency synthesizer 11 and at the same time controls the control voltage of the voltage-controlled crystal oscillator 10, the output of the frequency synthesizer 11 is instantaneously changed when the channel is changed. Although it is cut off, immediately after that, it is output with the frequency changed. The output frequency of the frequency synthesizer 11 is input as the reference frequency of the PLL circuit 14. However, in the PLL circuit 14, the band variable loop filter 142 has a narrow band at this time, so that the output of the frequency synthesizer 11 is instantaneous. The output of the voltage controlled oscillator 143 is locked to the output frequency of the frequency synthesizer 11 with almost no frequency change even when the output is cut off, and the output is input to the mixer 2. Therefore, it becomes possible to continuously control the frequency of the received signal. After the control by the channel setting control signal 15 and the band variable control signal 16 from the controller 132, the PLL is again controlled by the band variable control signal 16.
The band variable loop filter 142 in the circuit 14 is returned to the original wide band loop band, and normal automatic frequency control is performed.

[0012]

As described above, according to the present invention, in addition to the voltage controlled crystal oscillator and the frequency synthesizer, the control circuit for controlling the control voltage of the voltage controlled crystal oscillator and the PLL circuit locked to the output frequency of the frequency synthesizer are provided. When the control voltage of the voltage controlled crystal oscillator is out of a predetermined range, the frequency channel of the frequency synthesizer is changed, the control voltage is changed, and at the same time, the PLL circuit is set to a narrow band. The frequency control is instantaneously interrupted only while the channel is changed, but this instantaneous disconnection is substantially eliminated by setting the PLL circuit to a narrow band, and as a result, the voltage controlled crystal oscillator. It is possible to continuously and automatically perform frequency control over the control frequency possible range. Therefore, unlike the conventional control circuit, the frequency control is not interrupted until the frequency synthesizer follows up, and continuous automatic frequency control in a wide frequency range can be realized.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of an automatic frequency control circuit of the present invention.

FIG. 2 is a block diagram of an embodiment of a conventional automatic frequency control circuit.

[Explanation of symbols]

 1 Received Signal (Input Signal) 2 Mixer 4 Distributor 7 Mixer 8 Reference Oscillator 10 Voltage Control Crystal Oscillator 11 Frequency Synthesizer 13 Control Circuit 14 PLL Circuit 132 Controller 141 Phase Detector 142 Band-Variable Loop Filter 143 Voltage Control Oscillator

Claims (3)

[Claims] 1. A frequency synthesizer capable of changing a frequency channel stepwise, a voltage controlled crystal oscillator for generating a reference frequency of the frequency synthesizer, and a control circuit for monitoring a control voltage supplied to the voltage controlled crystal oscillator. A phase synchronization circuit locked to the output frequency of the frequency synthesizer, wherein the control circuit changes the frequency channel of the frequency synthesizer when the control voltage of the voltage controlled crystal oscillator is out of a predetermined range, An automatic frequency control circuit having a function of changing the control voltage corresponding to the frequency channel and simultaneously setting the phase lock circuit to a narrow band. 2. The control circuit includes an A / D converter that converts a control voltage supplied to the voltage controlled crystal oscillator into a digital signal, and a controller that monitors the converted digital signal.
D for converting the digital signal passing through the controller into the control voltage
/ A converter, the phase-locked loop circuit includes a voltage-controlled oscillator, a phase detector that compares the output frequency of the frequency synthesizer with the oscillation frequency of the voltage-controlled oscillator, and the output of the phase detector is voltage-controlled. And a band variable loop filter which is used as a control voltage of the oscillator, wherein the controller controls the channel setting control when the digital signal exceeds a predetermined value and the control voltage is out of the control frequency range of the voltage controlled crystal oscillator. The signal changes the channel of the frequency synthesizer by one step, and the control voltage of the voltage controlled crystal oscillator is changed by the amount corresponding to one step frequency of the frequency synthesizer, and the band variable loop filter is set to a narrow band by the band variable control signal. The automatic frequency control circuit according to claim 1. 3. The frequency control range of the voltage controlled crystal oscillator and the frequency step (frequency range) of the frequency synthesizer are set to be equal, and when the controller changes one channel of the frequency synthesizer, 3. The automatic frequency control circuit according to claim 2, wherein the control voltage is set to a voltage corresponding to its center frequency.