JPH01117421A - Phase comparator - Google Patents

Abstract

PURPOSE:To reduce the oscillation frequency fluctuation of a VCO by providing a waveform shaping circuit and a sample and hold circuit so as to output a phase error two times in one period of a reference signal. CONSTITUTION:The oscillation output of the VCO 2 is frequency-divided into the frequency of a reference signal, outputted at a duty factor of 50% and converted into a trapezoidal wave through a trapezoidal wave generating circuit 4. The trapezoidal wave is given to a switch 6, the output of the trapezoidal wave generating circuit 4 is inputted to a switch 61 while being inverted by an inverting circuit 10. On the other hand, a reference signal is inputted to a differentiation circuit 7 with a duty ratio of 50%, a waveform shaping circuit 8 detects the leading of the input differentiation signal, a waveform shaping circuit 9 detects the trailing of the input differentiation signal to output a pulse respectively. Thus, sampling and holding two times is applied within one period of a reference frequency by means of a capacitor C and the switches 6, 61. Thus, the fluctuation of the oscillation frequency of the VCO is halved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a phase comparator for a PLL circuit (phase locked circuit).

[Conventional technology]

Among PLL circuits, in particular, in PLL circuits where the oscillation frequency is higher than the frequency of the reference signal, the oscillation frequency is divided and compared with the reference signal, so in a digital phase comparator, the oscillation frequency of It changed to ■C
It is not possible to extract a stable oscillation frequency of O, so a sample-and-hold type phase comparator is generally used.

FIGS. 5 and 6 are a block diagram of an example of this sample-and-hold type phase comparator and its operating waveform diagram. This phase comparator 11 is connected to the trapezoidal wave generating circuit 4. The phase comparator 11 is composed of a switch 6, a capacitor C1, a differentiating circuit 7, and a waveform shaping circuit 8. The VCO 2, frequency divider 3, and loop filter 5 constitute a PLL circuit.

A case where this phase comparator 11 is operated as a PLL circuit will be explained. The oscillation output of the VCO 2 is frequency-divided by a frequency divider 3 to the frequency of the reference signal, and further passed through a trapezoidal wave generating circuit 4 and converted into a trapezoidal wave. On the other hand, the reference signal passes through a differentiating circuit 7, the edges of the signal are detected, and a waveform shaping circuit 8
The output of the 0 trapezoid generating circuit 4 through which the rising signal is obtained is input to a switch 6, and a capacitor C is connected between the output terminal of this switch 6 and ground. be done. This sampled and held value is passed through the loop filter 5, and the VC
By applying a control voltage to O2, the oscillation frequency of VCO2 is controlled, forming a PLL loop.

[Problem that the invention seeks to solve]

The conventional phase comparator 11 described above divides the frequency by 1/N when the oscillation frequency of the VCO 2 is N times that of the reference signal, and compares the phase with the reference signal. matches. However, since the oscillation output of VCO2 is only controlled at the rising edge of the reference signal, the disadvantage is that frequency fluctuations, so-called jitter, increase due to the free run characteristics of the VCO until the next control voltage is applied. have.

SUMMARY OF THE INVENTION An object of the present invention is to provide a phase comparator that eliminates such drawbacks and reduces fluctuations in the oscillation frequency of a VCO.

[Means for solving problems]

The present invention has a sample-and-hold phase comparator that detects a phase difference between an input signal and a reference signal, and includes a waveform shaping circuit that generates sample pulses at the rise and fall of the reference signal; and a sample hold circuit that converts the signal into a trapezoidal wave and performs a phase comparison using the sample pulse at the rising slope and falling slope of this trapezoidal wave, and outputs a phase error twice within one period of the reference signal. Features.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIGS. 1 and 2 are a block diagram and an operation waveform diagram of an embodiment of the present invention. The phase comparator 1 in this figure includes a trapezoidal wave generating circuit 4. Switch 6゜61, capacitor C1 differentiation circuit 7. It is composed of a waveform shaping circuit 8, 99 and an inversion circuit 10, and a PLL circuit is composed of this phase comparator 1, a VCO 2, a frequency divider 3 and a loop filter 5.

The operation of this phase comparator will be explained using a PLL circuit. V
The oscillation output of CO2 is frequency-divided by the frequency divider 3 to the frequency of the reference signal, outputted with a duty of 50%, and further passed through the trapezoidal wave generation circuit 4 and converted into a trapezoidal wave. This converted trapezoidal wave is input to switch 6.

On the other hand, the output of the trapezoidal wave generating circuit 4 is inverted by the inverting circuit 10 and input to the switch 61. The outputs of these switches 6 and 61 are connected in common, and a capacitor C is connected between the output terminal and ground.

On the other hand, the reference signal is input to the differentiating circuit 7 with a duty ratio of 50%, and is further input to the waveform shaping circuits 8 and 9. The waveform shaping circuit 8 detects the rising edge of the input differential signal and outputs a positive pulse, and the waveform shaping circuit 9 detects the falling edge of the input differential signal and outputs a pulse (Fig. 2). Therefore, the reference Sample and hold can be performed twice within one frequency period using capacitor C and switch 6.61. This sample-and-hold voltage passes through the loop filter 5, controls the VCO, and operates the PLL circuit.

FIGS. 3 and 4 are block diagrams and operational waveform diagrams of a second embodiment of the present invention. The trapezoidal wave output from the frequency divider 3 is inputted to one input terminal of the switch 62, and this trapezoidal wave is inputted to the inverting circuit 10 and inverted.
It is input to the other input terminal of 2. The output of this switch 62 is switched by the output of the frequency divider 3, and the output is input to the switch 6. The output of switch 6 is loop filter 5
It is connected to the.

On the other hand, the outputs of the waveform shaping circuits 8 and 9 are input to an OR circuit 13, and the switch 6 is controlled by the output of the OR circuit 13. In this case as well, sample and hold is performed twice in -periods.

[Effects of the Invention] As explained above, the present invention generates sample pulses at the rise and fall of a reference signal in a sample-and-hold type phase comparator that detects the phase difference between two signals, and further generates a comparison signal. By converting it into a trapezoidal wave and performing phase comparison using the above-mentioned sample pulse on its rising slope and falling slope, the phase error can be reduced once per period by outputting the phase error twice within one period. P of output configuration
Compared to L L', the fluctuation of the oscillation frequency of vCO is reduced to 1/
It has the effect of being able to do two things.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the first embodiment of the present invention, FIG. 2 is a waveform diagram of each part in FIG. 1, FIG. 3 is a block diagram of the second embodiment of the present invention, and FIG. 3 is a waveform diagram of each part, FIG. 5 is a block diagram of an example of a conventional PLL circuit, and FIG.
It is a waveform diagram of each part in the figure. 1.11... Phase comparator, 2... vCo, 3...
Frequency divider, 4... Trapezoidal wave generation circuit, 5... Loop filter, 6.61.62... Switch, 7... Differentiator circuit, 8°9... Waveform shaping circuit, 10...・Inversion circuit,
C... Capacitor.

Claims (1)

[Claims] A sample-and-hold phase comparator that detects a phase difference between an input signal and a reference signal includes a waveform shaping circuit that generates sample pulses at the rise and fall of the reference signal, and a waveform shaping circuit that converts the input signal into a trapezoidal wave. The sample-and-hold type is characterized by comprising a sample-and-hold circuit that performs phase comparison using the sample pulse at the rising slope and falling slope of the trapezoidal wave, and outputting a phase error twice within one cycle of the reference signal. Phase comparator.