JPS6247374B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a phase locked circuit with an automatic gain control circuit.

First, the configuration of the phase locked loop will be explained with reference to FIG. 1 is a phase comparator, 2 is a low-pass filter,
3 is a voltage controlled oscillator which is used, for example, as an FM signal demodulator, with 4 as an input and 5 as an output. The phase comparator 1 compares the phase of the signal input from 4 with the output of the voltage controlled oscillator 3 (VCO), and generates an output voltage proportional to the phase difference. Further, the relationship between the amplitude value of the output of the phase comparator and the voltage of the input signal is as follows. In other words, if the voltage of the input signal in step 4 is B, the voltage of the VCO output is A, and the phase difference between A and B is θ, the output of the phase comparator is 2ABsinθ/(A+B)≒2Bsinθ when A≫B. . In this way, the output of the phase comparator changes in proportion to the voltage of the input signal, so the detection sensitivity of the phase comparator
The open loop gain of the phase-locked loop defined by the product F·Vd·Vk of Vd, the gain F of the low-pass filter, and the modulation sensitivity Vk of the VCO is proportional to the input signal level as described above. If the input signal level increases, the loop gain of the phase-locked circuit will also increase, but
Since a phase-locked circuit is essentially a negative feedback loop, if the phase delay at the frequency where the open loop gain is 1 becomes 180 degrees, oscillation will occur and the loop will become unstable. The higher the frequency at which the open-loop gain becomes 1, the larger the phase lag and the more likely oscillation will occur, so the higher the open-loop gain and the higher the stopping frequency of the low-pass filter, the more likely oscillation will occur. .

Conversely, if the loop gain is too low, the phase difference between the input signal and the VCO will not be sufficiently compressed by the loop, and the phase comparator will lose synchronization.

In a phase-locked loop used under such conditions where the signal level fluctuates, the loop gain of the phase-locked loop fluctuates as the signal level fluctuates, making the loop unstable depending on the conditions. To avoid this, it is necessary to use an automatic gain control circuit (AGC) to keep the input signal level of the phase comparator constant.

FIG. 2 is an example of a conventional FM demodulator that demodulates a 70MHz FM modulated wave using a 1700MHz phase synchronization circuit. 1 is a phase comparator, 2 is a low-pass filter, 3 is a voltage controlled oscillator, 6 is an AGC amplifier, 7
is a variable attenuator, 8 is a mixer, 9 is a bandpass filter, 1
0 is an IF amplifier, and 11 is a local oscillator. 1-
A negative feedback loop is formed in 6-7-8-9-10-1, and the amount of attenuation of the variable attenuator 7 is controlled by the magnitude of the detected voltage of the detector in the phase comparator 1, and the signal level at 4 is is kept constant.

As the detector for the AGC circuit, it is desirable to use a quadrature detector that has the same circuit configuration as the phase comparator and has a characteristic that has a phase difference of 90° from that of the phase comparator. The reason for this is that when noise is superimposed on a signal, a normal detector detects the broadband noise along with the signal, resulting in a drop in the actual signal level, whereas a quadrature detector Since it is a wave detector, it does not have the drawbacks mentioned above, and since it uses the same detection circuit as the phase detector, the temperature characteristics of the phase comparator are compensated for. This is especially necessary when applied to devices, where the signal is buried in noise.

However, in conventional AGC circuits using quadrature detectors, once the phase synchronization is lost for some reason, the detector output remains low and positive feedback is applied to the AGC loop, causing the phase comparator to If the input signal level increases, oscillation may occur in the loop as described above, and the unstable state of the phase-locked loop may continue.

This occurs, for example, when the input signal is first applied or when the input signal level rises rapidly.

An object of the present invention is to provide a phase-locked circuit that solves these drawbacks.

To achieve the above object, the phase-locked circuit according to the present invention includes a quadrature detector and a variable attenuator controlled by the output of the quadrature detector in the phase-locked loop.
In the phase-locked circuit equipped with an automatic amplitude control means that keeps the input signal level of the phase-locked loop constant by changing the amount of attenuation of the variable attenuator, a second detection circuit is provided between the variable attenuator and the phase comparator. When the detected voltage of the second detector is below a predetermined value, it does not act on the phase-locked loop, and when the detected voltage of the second detector exceeds the predetermined value, the variable The apparatus includes means for controlling an attenuator to lower the input signal level of the phase-locked loop, and is configured to be able to obtain phase synchronization without causing synchronization even if the signal level in front of the variable attenuator changes. It has been done.

According to the above configuration, synchronization does not occur under any conditions, so a quadrature detector can be used as the AGC detector.

Hereinafter, the present invention will be explained in more detail with reference to the drawings.

FIG. 3 shows an embodiment in which the present invention is applied to the FM demodulator shown in FIG.
is a low-pass filter, 3 is a voltage controlled oscillator, 4 is the input of the phase-locked loop, 5 is the same output, 6 is the AGC amplifier, 7 is the variable attenuator, 8 is the mixer, 9 is the bandpass filter, 10 is an amplifier, 11 is a local oscillator, 12 is a detector, 13 is an amplifier, and 14 and 15 are diodes.

AGC loop consisting of 7-12-8-9-10-1-6-15-7 and 7-12-13-1
Both of the loops 4-7 are negative feedback loops, and when the signal level at 4 becomes high, the variable attenuator 7 is controlled to increase the amount of attenuation.
The amplifier 13 operates as an amplifier when the detected voltage of the detector 12 exceeds a predetermined threshold, but while the detected voltage is below the threshold, the output voltage of the amplifier 13 is maintained at a sufficiently high potential. Therefore, the diode 14 is not conductive.

14 and 15 are diodes that serve as selector switches; of the two amplifiers 6 and 13, only the diode connected to the one with the lower output voltage is conductive, and the diode connected to the variable attenuator 7 is conductive. is controlled by a loop containing Therefore, while the variable attenuator 7 is controlled by one loop, the other control circuit is separated from the loop and does not affect any other loop.

Next, to make this work, 7-12-13
- The signal level of 4 determined by the loop of 14-7,
7-12- within the range where the loop does not cause oscillation.
Consists of 8-9-10-1-6-15-7
Set it in advance higher than the signal level 4 determined by the AGC loop. Since it is set like this, the signal level of 4 is 7-12-13-1
While the signal level is lower than the one determined by loop 4-7,
Since the output voltage of the AGC amplifier 6 is lower than the output voltage of the amplifier 13, only the diode 15 is conductive, and the variable attenuator 7 is 7-12-8-9-10-
The phase locked loop is controlled by a 1-6-15-7 loop and is synchronized.

When the signal level of 4 becomes higher than the signal level determined by the loop 7-12-13-14-7, the amplifier 13 starts operating, and the output voltage of the amplifier 13 becomes lower than the output voltage of the AGC amplifier 6. Contrary to the previous case, only the diode 14 is conducting and the variable attenuator 7 is 7-12-13-14
-7 loop only.

Because of this configuration, when the above-mentioned quadrature detector is used as the detector 1, even if an input signal is input or the input signal level suddenly increases and the signal level of 4 attempts to rise, 7-12-13
The rise in the signal level of 4 is suppressed by the loop of -14-7 to a level that does not cause oscillation of the phase-locked loop, thereby preventing the phase lock from being lost. 1
-While the phase of the loop of 2-3-1 is synchronized, the level of 4 is 7-12-8-9-10-1-6-
It is kept constant by the 15-7 AGC loop,
From the above, phase synchronization will not be lost under any conditions.

FIG. 4 is a block diagram showing a second embodiment of the present invention. In the embodiment shown in FIG. 3, the diode switch is connected to the amplifier 6 or 13 by making it conductive or non-conducting.
The variable attenuator 7 was connected, but as shown in Figure 4,
The comparator 16 is connected to the output of the detector, and the output of 13 is connected to the input of 6. When the signal level rises and the detection voltage of the detector 12 exceeds a predetermined threshold, the bias voltage changes from 16 to 6. give,
The same effect can be obtained by increasing the amount of attenuation of the variable attenuator 7.

As explained above, the phase-locked circuit according to the present invention does not lose phase lock even if the signal level suddenly increases, so it is possible to use a quadrature detector with good characteristics as described above for the AGC loop detector. There are advantages that can be achieved.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of a phase-locked circuit, Figure 2 is a conventional configuration diagram in which a phase-locked circuit is applied to an FM demodulator,
FIG. 3 is a block diagram showing a first embodiment in which the present invention is applied to the FM demodulator shown in FIG. 2, and FIG. 4 is a block diagram showing a second embodiment in which the present invention is applied to the FM demodulator shown in FIG. FIG. 1... Phase comparator, 2... Low-pass filter, 3... Voltage controlled oscillator, 4... Phase locked loop input, 5... Same output, 6... AGC amplifier, 7... Variable attenuator, 8... Mixer, 9 ...bandpass filter, 10...amplifier, 11...local oscillator, 12...detector, 13...amplifier, 14,
15...Diode, 16...Comparator.

Claims (1)

[Claims] 1 The phase-locked loop is equipped with a quadrature detector and a variable attenuator controlled by the output of the quadrature detector, and an automatic amplitude system that maintains the input signal level of the phase-locked loop always constant by changing the amount of attenuation of the variable attenuator. A phase locked circuit including a control means, comprising a second detector between the variable attenuator and the phase comparator,
When the detected voltage of the second wave detector is below a predetermined value, it does not act on the phase locked loop, and when the detected voltage of the second wave detector exceeds a predetermined value, the variable attenuator is controlled. and a means for lowering the input signal level of the phase-locked loop, and is configured to be able to obtain phase synchronization without causing synchronization even if the signal level in front of the variable attenuator changes. circuit.