JPH0311581B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic phase synchronizer used in a phase demodulator in a phase modulation system, and more particularly to an automatic phase synchronizer having a free-running frequency stabilizing circuit.

An automatic phase synchronization device, to explain a carrier wave synchronization device as an example, is a device that performs free-running frequency stabilization of voltage-controlled oscillation to synchronize the phase or frequency of a carrier wave. The basic configuration of this type of carrier synchronizer is a phase-controlled voltage-controlled oscillator, a phase detection circuit that phase-detects the input modulated signal using the free-running frequency output of this voltage-controlled oscillator, and a phase detection circuit that detects the phase of the input modulated signal using the free-running frequency output of this voltage-controlled oscillator. It is equipped with a low frequency filter that removes the demodulated signal and outputs a demodulated signal, and an automatic phase control circuit that uses a part of the outputted demodulated signal to generate a signal for performing the above-mentioned phase (frequency) control.
However, in such a configuration, the gain of the DC amplifier used in the automatic phase control circuit must be set very high, so that once the input signal disappears, the output of the automatic phase control circuit will go to a high or low level. There is a drawback that the device is fixed and the phase synchronization pull-in operation of the device is no longer performed.

In order to avoid the above-mentioned problems, conventional devices have taken measures by, for example, adding a limiter to the automatic phase control circuit to limit fluctuations in the free-running frequency of the voltage-controlled oscillator. However, in such a conventional carrier synchronization device, as will be explained in detail later,
During small-capacity transmission where the band of the low-band transmitter becomes narrow, the gain of the automatic phase control circuit decreases, resulting in deterioration of demodulation characteristics. Another method is known in which a waver similar to the broadband low-frequency waver described above is provided separately from the main signal.
The disadvantage was that the circuit size increased and at the same time the deterioration in response to unnecessary waves was significant.

Furthermore, as another conventional method, when the level of the input modulation signal is below a certain value, a fixed frequency signal with the same frequency and level as the input modulation signal is sent to the phase comparison circuit of the loop circuit instead of the input modulation signal, and the voltage is controlled. There is a method in which the oscillator is operated at a value close to the normal level to maintain the device operation in a substantially steady state (Japanese Patent Application Laid-Open No. 17237-1983). However, this method has the disadvantage that the oscillator that oscillates an accurate fixed frequency is relatively expensive, and in reality, it is necessary to prepare multiple fixed frequencies according to the number of partner stations, making it quite expensive. Yes.

Therefore, the object of the present invention is to provide simple and inexpensive carrier synchronization that maintains the oscillation frequency of a voltage oscillator near the input signal center frequency and exhibits good automatic phase synchronization characteristics even when there is no input signal, regardless of the size of the transmission capacity. ie, an automatic phase synchronization device.

In order to achieve the above object, the present invention adds an alternating current signal to replace the output signal of the automatic phase control circuit when the input modulation signal falls below a certain level and the output signal of the automatic phase control circuit becomes small. The magnitude of the phase control signal is maintained at an appropriate value. That is, according to the present invention, a voltage controlled oscillator that receives an automatic phase control signal and generates a free-running frequency output, a phase detection circuit that phase-detects an input modulation signal using the free-running frequency output, and a noise component of the phase-detected input modulation signal. automatic phase synchronization, comprising: a low frequency filter that removes the demodulated signal and sends the demodulated signal as a device output; and an automatic phase control circuit that uses a part of the demodulated signal to generate an output signal for obtaining the automatic phase control signal. The device includes an input detection circuit that detects the input modulation signal and generates a first signal if the value is larger than a reference level and a second signal if the value is smaller than the reference level; an alternating current signal generator that emits an alternating current signal indicating a steady average level; and an alternating current signal generator that transmits the alternating current signal when it receives the second signal, and does not send out the alternating current signal when it receives the first signal; and an addition circuit that adds the sent out alternating current signal and the output signal generated by the automatic phase control circuit to generate the automatic phase control signal. An automatic phase control device having a free-running frequency stabilizing circuit is obtained.

Next, a detailed explanation will be given with reference to the drawings.

FIG. 1 is a diagram showing the configuration of a conventional example of a carrier synchronization device, which is a type of automatic phase synchronization device, in which 1 is a phase detector, 2 is a low frequency filter, 3 is an automatic phase control circuit including a limiter, 4 is a high gain DC amplifier, 5
is a voltage controlled oscillator, I is a modulation signal input, and P is a demodulation signal output. The operation and problems of this device have been outlined above, but detailed explanations will be given later.

First, for the sake of explanation, the basic configuration of the automatic phase control circuit 3 that does not include a limiter will be described. The oscillation frequency variable range of the voltage controlled oscillator 5 is as follows.
Noise limiting low-pass filter 2 in the phase-locked loop
In order to increase the loop gain of the phase-locked loop, it is necessary to make it considerably wider than the band of . For this purpose, the gain of the DC amplifier is very high, so that when there is no input signal, the output of the amplifier 4 is fixed at either a high level or a low level. Therefore, the free-running frequency of the voltage controlled oscillator 5 deviates significantly from the input signal center frequency, and even if the input signal is applied again, the demodulated signal will be out of the band of the low frequency converter 2.
No signal is obtained at the output of wave generator 2, and as a result, the output signal of automatic phase control circuit 3 remains fixed at either a high level or a low level. As a result,
Phase synchronization pull-in operation becomes impossible.

Therefore, as mentioned at the beginning, in the conventional equipment actually used as a solution to this problem,
This was dealt with by including a limiter or the like in the automatic phase control circuit 3 to limit fluctuations in the free-running frequency of the voltage-controlled oscillator 1, but when the signal capacity is small and the band of the low-pass filter 2 becomes narrow, In order to narrow the limiter of the automatic phase control circuit 3 accordingly, the gain of the automatic phase control circuit 3 is reduced, and as a result, the round-trip gain of the phase-locked loop is also reduced, resulting in a disadvantage that the demodulation characteristics deteriorate. .

FIG. 2 is a block diagram showing the configuration of one embodiment of the present invention. In this Figure 2, 1, 2,
4 and 5 indicate the same components as those with the same numbers in FIG. 10 is an automatic phase control circuit, which differs from 3 in Figure 1 in that it does not include a limiter, 11 is an input/output detection circuit, 12 is an AC signal control circuit, and 13 is an automatic phase control circuit that automatically adjusts the average DC voltage. The alternating current signal generator generates a signal having the normal average level of the phase control signal, and in this case uses a clock oscillator, and 14 is an adder circuit. In such a configuration, the input modulation signal I enters the phase detector 1, is synchronously detected with the regenerated carrier wave of the voltage controlled oscillator 5, is demodulated through the low frequency converter 2, and is output as a baseband data signal P. At this time, the input modulation signal I is simultaneously input to the input detection circuit 11.

Here, if the input signal level is higher than a certain reference level, it is determined to be normal and a first signal is output, and this signal enters the control circuit 12 and the AC signal from the AC signal generator 13 enters the adder circuit 14. prohibited. Therefore, the function as a phase demodulator is equivalent to that of the conventional device shown in FIG.

Next, when the input modulation signal I is smaller than a certain reference level, for example, when it is cut off, the input detection circuit 11 determines that it is abnormal and generates a second signal (which may be zero), and the AC signal control circuit 12 outputs an AC The AC signal from the signal generator 13 is controlled to be applied to the adder circuit 14. As a result, the automatic phase control voltage given to the voltage controlled oscillator 5 (output of the amplifier 4)
, the average value of the output of the AC signal generator 13 appears by a low-pass filter (PLL loop filter) not shown included in the automatic phase control circuit 10, and this is the automatic phase control voltage at the time of phase synchronization. Since the voltage controlled oscillator 5 operates at a voltage close to the input signal center frequency, the voltage controlled oscillator 5 can oscillate at a frequency close to the input signal center frequency.

When a modulation signal higher than a certain reference level is input in such a state, the input detection circuit 11 determines that it is normal, and the AC signal control circuit 12 prohibits the AC signal from being applied to the addition circuit 14. At the same time, a demodulated signal is output from the low-pass filter 2, a synchronization pull-in process begins, and normal operation returns.

From the above, in carrier synchronization circuits for small capacity transmission where it is necessary to narrow the band of the low frequency converter 2, the automatic phase control voltage, which affects the demodulation characteristics, can be used without limiting it with a limiter or the like. In addition, the desired characteristics can be obtained by adding a simple and inexpensive fixed oscillator without separating the main circuit for baseband output and automatic phase control.

[Brief explanation of the drawing]

FIG. 1 is a basic block diagram of a conventional carrier synchronizer, and FIG. 2 is a block diagram of an embodiment of the present invention. Explanation of symbols: 1 is a phase detector, 2 is a low frequency detector, 3 is an automatic phase control circuit, 4 is an amplifier, 5 is a voltage controlled oscillator, 10 is an automatic phase control circuit, 11 is an input detection circuit, 12 is an AC Signal control circuit, 13
14 represents an AC signal generator, 14 represents an adder circuit, I represents a modulation signal input, and P represents a demodulation signal output (baseband data).

Claims (1)

[Claims] 1. A voltage controlled oscillator that receives an automatic phase control signal and generates a free-running frequency output, a phase detection circuit that phase-detects an input modulated signal using the free-running frequency output, and demodulates the phase-detected input modulated signal by removing its noise component. In an automatic phase synchronization device, the automatic phase synchronization device includes a low frequency generator that sends out a signal as a device output, and an automatic phase control circuit that uses a part of the demodulated signal to generate an output signal for obtaining the automatic phase control signal. an input detection circuit that detects a modulation signal and outputs a first signal if the value is larger than a reference level and a second signal if the value is smaller than a reference level; an alternating current signal generator that emits an alternating current signal indicating the following: an alternating current signal generator that controls the alternating current signal so as to transmit the alternating current signal when receiving the second signal, and not transmitting the alternating current signal when receiving the first signal; A self-running device characterized in that it is provided with a signal control circuit and an addition circuit that adds the sent out alternating current signal and the output signal generated by the automatic phase control circuit to generate the automatic phase control signal. Automatic phase synchronizer with frequency stabilization circuit.