JPH069358B2 - Automatic phase control circuit - Google Patents

Description

The present invention relates to a high frequency automatic phase control used in a demodulator for demodulating a four-phase PSK modulated signal in a microwave frequency band in the fields of communication and broadcasting. Regarding the circuit.

[Conventional technology]

The audio signal system in satellite broadcasting uses a 4-phase PSK modulation method which is excellent in terms of occupied bandwidth and transmission efficiency. For demodulation of 4-phase PSK modulated signal, approximately 12 GHz is 14
Performed on the modulated signal converted to 0 MHz. Since the synchronous detection method is used for demodulation, it is necessary to reproduce the carrier wave, but it is suitable to reproduce the carrier wave by the inverse modulation method at a high frequency such as 140 MHz.

The inverse modulation 4-phase PSK demodulator demodulates the 4-phase PSK modulated signal a by the quadrature phase demodulation circuit 1 and outputs demodulated data (b, c), as shown in FIG. At 2, the modulated signal a is delayed by the delay circuit 4 with the demodulated data (b, c), and the phase-adjusted signal a ′ is modulated.
This is inverse modulation, and the output signal d becomes the reproduced carrier signal. The signal d goes through an automatic phase control circuit (APC) to remove excess frequencies and also adjusts the phase with the input modulation signal. The carrier signal e whose phase has been adjusted in this way is used in the quadrature phase demodulation circuit 1.

The frequency of the APC circuit designed by the demodulator is 1
It is considerably severe because it is as high as 40 MHz and the frequency fluctuation of the input modulated wave signal is unavoidable because it requires several steps of frequency conversion from the GHz band to 140 MHz. If the pass band width of the band pass filter included in the APC circuit is narrowed in order to improve the C / N ratio, there is a reciprocal relation that the phase displacement due to the frequency fluctuation becomes large, and therefore some measure is required.

Although the circuit described above is a circuit incorporated in a 4-phase PSK demodulator, in general, at high frequencies of the present invention, C
It is intended for a circuit (APC circuit) having a high / N ratio and capable of maintaining a fixed phase even when the input frequency fluctuates. Therefore, the place of use is not limited to satellite broadcasting.

[Problems to be solved by the invention]

As a high-frequency APC circuit, a Japanese Patent Application No. 60-149 in which the present inventor is one of the inventors, which satisfies the requirements.
The four-phase PSK demodulator of No. 336 uses the circuit shown in FIG.

In this circuit, in order to avoid the difficulty of realizing a high-frequency filter, the input is once converted into a low frequency, and then the original frequency is converted and output again. Input signal 20
The (frequency f) is multiplied by the signal (frequency f ₀ ) 24 of the common voltage controlled oscillator 7 in the multiplier 5, and the output signal 21 thereof is passed through the band pass filter 8 passing through a predetermined conversion frequency to obtain the low frequency signal ( Frequency f−f ₀ ) 22. This signal 22 passes through the band pass filter 9, and is again multiplied as a signal 23 by the multiplier 6 with the signal (frequency f ₀ ) 24 to obtain a signal 25 having the original frequency f component. Frequency f) 26. Since the signal 24 of the voltage controlled oscillator 7 is commonly supplied to the multipliers 5 and 6,
Although the input signal 20 and the signal 26 have the same frequency, the signals 22 and 23 can be moved within a certain frequency width depending on the frequency of the voltage controlled oscillator 7. The band pass filter 9 narrows the band to improve the C / N ratio, but if the frequency of the input signal 20 fluctuates, it shifts from the band center frequency and a phase displacement occurs. FIG. 2 shows the frequency characteristics of gain and phase for the filter (a) with a wide pass band width and the filter (b) with a narrow pass band width.

By the way, in the circuit of FIG. 4, the phase comparison of the signals (signals 22 and 23) at both ends of the band pass filter 9 is performed by the phase comparator 10.
The output is applied to the voltage controlled oscillator 7 through the low pass filter 11 to change its frequency.
L (phase locked loop) is formed. By this PLL, the frequencies of the signals 22 and 23 are automatically changed to the center frequency of the band pass filter 9, so that the phase difference by the band pass filter 9 does not occur. In this way, the input signal 20 and the signal 26 can always have the same frequency and the same phase even if the frequency of the input signal 20 varies, and the C / N ratio of the signal 26 is improved. The signal 26 has a constant amplitude by the limiter 13 and can be output as the signal 27 having a predetermined phase by the variable phase shifter 14.

As described above, although FIG. 4 has extremely excellent characteristics, when the frequency change of the input signal is too large, the output of the bandpass filter 9 having a narrow passband becomes small and the phase comparator 10 However, the phase comparison cannot be performed normally. Therefore, there may be a case where the above phase lock cannot be performed. However, there is a contradiction that the C / N ratio cannot be improved if the pass bandwidth is increased. This problem is also related to the design of the high-frequency stage before the demodulator. If the frequency variation of the high-frequency stage always stays within a certain range and there is no accidental large frequency variation, FIG. 4 is sufficiently useful. Is.

In view of the above circumstances, the object of the present invention is that the circuit configuration becomes more complicated than the conventional example, but even when the frequency fluctuation of the input signal is large or there is a large frequency fluctuation by accident, this APC circuit is It is to provide a circuit that operates sufficiently.

[Means for solving problems]

The circuit of the present invention has a first frequency converter that reduces the frequency of an input signal that shares a local oscillator, and a second frequency converter that raises the frequency again to generate an output signal.
In the path between the first and second frequency converters, the first and second band pass filters having different pass bands of the same center frequency, wide and narrow, and the voltage control phase shifter are connected in series in that order. Keep it. Then, the voltage control is performed by comparing the phase difference between both ends of the first band pass filter and comparing the phase difference between both ends of the second band pass filter with the means for controlling the frequency of the local oscillator according to the phase difference. And means for controlling the amount of phase shift of the phase shifter.

[Action]

In the present invention, two filters, a wide band pass filter and a narrow band pass filter, are provided as the band pass filter so that they can follow up even when the frequency fluctuation of the input signal is large, and the narrow band pass filter improves the C / N ratio. I am trying.
However, as will be described in detail in the embodiments, the response characteristics of the PLL including the wide band pass filter and the voltage controlled oscillator are not sufficient due to the phase characteristics of the wide band pass filter, and the center frequency of the signal passing through the wide band pass filter is deviated and the A phase difference occurs. This phase difference can be compensated by expanding and detecting the phase difference between the signals at both ends of the narrow band pass filter having the same center frequency, and moving the voltage controlled phase shifter.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings. Although FIG. 1 is a circuit block diagram, the multiplier 101,
The part on the right side of 102 is the part converted to low frequency,
The PLL is shown above the low frequency part and the phase adjusting part is shown below.

The input signal 201 is multiplied by the output signal 202 of the voltage controlled oscillator (hereinafter abbreviated as VCO) 103 in the multiplier 101,
The output signal 203 is passed through a bandpass filter (hereinafter abbreviated as BPF) 104 to obtain a signal 204 that has been frequency-converted to a low frequency f-f ₀ . Where f is signal 20
1, f ₀ is the frequency of the signal 202. The signal 204 is input to the first BPF 105 having a wide pass band width, the phase difference between both ends thereof is obtained by the phase comparator 106, the phase difference is smoothed by the low pass filter (hereinafter abbreviated as LPF) 107, and the control voltage 103a is obtained. To control the VCO 103. Even if the frequency of the input signal 201 varies due to this PLL, the frequency of the VCO 103 follows and the signal 204
Falls within a constant frequency so as to reduce the phase difference. However, in the present invention, the first BPF 1 is designed to be able to follow even when the frequency fluctuation of the input signal 201 is considerably large.
Since the pass band width of 05 is wide, the first BPF1
The phase difference between both ends of 05 is relatively small as shown in FIG. 2 (a), and the control voltage 103a is small. Therefore, when the frequency fluctuation is not so large, the loop gain of the PLL is small and there is a tracking error, and the signals 204 and 205 are the first BPF.
A slight difference from the center frequency of 105 causes a phase difference. This phase difference is due to the second BPF having a narrow pass band in the next stage.
The phase adjustment portion including 108 is used for compensation. Second BPF10
8 has the same center frequency as the first BPF 105,
It is a narrow band, and as shown in FIG.
Even if the frequency of 05 is slightly deviated from the center frequency, the output of the phase comparator 109 for detecting the phase difference between both ends becomes large. This output is smoothed by the LPF 110 and applied to the variable phase shifter 113. The variable phase shifter 113 is a voltage-controlled phase shifter, and is configured to give a constant amount of phase shift with the variable voltage 112 as an initial adjustment. Then, the DC voltage of the LPF 110 is superposed on the voltage fixed by the variable voltage 112 by the adder 111, so that the phase error of the entire first BPF 105 and the second BPF 108 can be compensated.

The output signal 207 of the variable phase shifter 113 is returned to the VCO 10 again.
The output signal 202 of No. 3 is multiplied by the multiplier 102. The signal is converted into a signal 208 including the original frequency f by the multiplier 102 and B
An output signal 209 is output via the PF 114 and the limiter 115.
And This signal 209 has the same frequency even if the input signal 201 changes in frequency. Furthermore, the second BPF 108
Since it is a narrow band filter, not only a signal with an improved C / N ratio is obtained, but also its phase holds a specific phase that was initially adjusted.

〔The invention's effect〕

As described in detail above, in the present invention, the high frequency input signal is converted into the low frequency signal, and the APC operation is performed in the low frequency region. Since a common local oscillator is used for frequency conversion, the input signal and the output signal have the same frequency. APC
Circuit elements involved in operation, such as bandpass filters, phase comparators, and voltage-controlled phase shifters, are low-frequency operation, so design is easy. As a bandpass filter included in the APC circuit, two filters having a common center frequency and different bandwidths of wide and narrow are connected in series. Even if the frequency fluctuation of the input signal is large, the wide band pass filter causes the frequency of the local oscillator (voltage controlled oscillator) to follow so that the frequency is converted to the vicinity of the center frequency by the phase locked loop including this filter.

The signal whose frequency is converted to the vicinity of the center frequency as described above increases the C / N ratio by passing through the narrow band pass filter, and is connected in series to the narrow band pass filter by utilizing the change in the phase difference due to the frequency fluctuation. The phase difference caused by the frequency fluctuation can be compensated by the voltage controlled phase shifter. As a result, the phase of the APC circuit can be set to a specific phase by the bias fixedly determined by the voltage control phase shifter.

As described above, according to the present invention, even if the input frequency varies, the phase can be always properly adjusted, and an APC circuit having a high C / N ratio can be obtained.

[Brief description of drawings]

FIG. 1 is a circuit block diagram of an embodiment of the present invention, FIG. 2 is a diagram showing characteristics of a bandpass filter, and FIG. 3 is a 4-phase PSK.
FIG. 4 is a block diagram of a demodulator, and FIG. 4 is a diagram showing a conventional example of an APC circuit. 101, 102 ... Multiplier, 103 ... Voltage controlled oscillator (VC0), 104, 114 ... Band pass filter, 107, 110 ... Low pass filter, 105 ... First band pass filter (wide band), 108 ... Second band pass Filters (narrow band), 106, 109 ... Phase comparators, 113 ... Voltage controlled phase shifters.

Claims (1)

[Claims] 1. A high-frequency automatic phase control circuit capable of improving a C / N ratio by a bandpass filter and maintaining a constant phase regardless of frequency fluctuations of an input signal. It has a first frequency converter that lowers the frequency and a second frequency converter that raises the frequency again to produce an output signal, and passes the same center frequency on the path between the first and second frequency converters. Wide bandwidth
The first and second band-pass filters, which are different from the narrow band, and the voltage-controlled phase shifter are serially connected in that order, and the phase difference between both ends of the first band-pass filter is compared, and according to the phase difference. Means for controlling the frequency of the local oscillator;
A high-frequency automatic phase control circuit comprising: means for comparing the phase difference between both ends of the second band-pass filter to control the amount of phase shift of the voltage controlled phase shifter.