JPH04156712A - Automatic frequency controller circuit - Google Patents

Abstract

PURPOSE:To prevent the deterioration of the signal error rate of a signal system by avoiding the step-out of a demodulator by adding a low pass filter, an A/D converter, a logic circuit, and a D/A converted, and only executing processing as far as frequency synchronization. CONSTITUTION:A frequency phase compared signal level and the signal level obtained by passing this signal through the low pass filter 6 are converted respectively into digital signals by the analog/digital converter 1, and are supplied to the logic circuit 2. Here, the logic circuit 2 sends just preceeding frequency control information to the digital/analog converter(D/A converter) 3 in the case that the frequency error information of usual frequency fluctuation is below a some threshold DELTAf, and in the case of exceeding DELTAf, it sends the frequency control information in the direction in which frequency error goes to zero to the converter 3, and synchronizes the frequency. By preventing from following to the frequency in such a way in the case of occurrence of the sudden frequency fluctuation, the step-out of the demodulator can be prevented, and the deterioration of the signal error rate of the signal system can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an automatic frequency control circuit used in a local oscillation frequency control circuit of a receiver.

[Conventional technology]

In general, S CP C (Single channel
When communicating using a signal with a narrow required bandwidth such as the per carrier system, the receiver absorbs frequency fluctuations due to secular changes in the local oscillator frequency in the transmitter and repeater or the Doppler effect in satellite communications. , it is necessary to keep the frequency sent to the demodulator constant. For example, in satellite communications, the master station sends out signals other than those used for communication (called pilot signals), the receiving station detects the frequency fluctuations of this pilot signal, and the receiver The output frequency of the local oscillator is changed to control the frequency of the signal sent to the demodulator to be constant.

Conventionally, in this type of automatic frequency control (hereinafter referred to as AFC) circuit, as shown in FIG. 7, the frequency is mixed, and the difference frequency between both signals becomes an output signal. A part of this output signal passes through a band filter 9 that limits the band of noise components, and is supplied to a frequency phase comparator 5.

On the other hand, the output signal of the reference signal generator 4 is also supplied to the frequency phase comparator 5, and the frequency and phase of both are compared. The frequency error resulting from the comparison and the phase error component after the frequency pull-in are negatively fed back to the voltage controlled oscillator 8 to change the frequency or phase of the voltage controlled oscillator 8. This series of operations continues until the error component of the frequency phase comparator 5 becomes zero, and the output frequency of the AFC circuit becomes a constant frequency that absorbs the frequency change of the input signal.

In particular, in the case of satellite communications, the input signal is a signal obtained by partially converting the receiver signal to a lower frequency using a low-noise converter. Additionally, the local oscillator inside the low-noise converter may be one with low short-term stability, and if the frequency of the signal input to the receiver changes instantaneously, the receiver's AFC circuit will lose its phase. Synchronization cannot catch up, resulting in out-of-sync. Therefore, the receiver's AFC
The circuit begins a frequency sweep. Here, the receiver's AFC
Since the output signal of the circuit is demodulated by the demodulator, the demodulator is also out of synchronization. Furthermore, since the capture range of the demodulator is generally narrower than that of the AtC circuit of the receiver, if the AFC circuit of the receiver can follow the fluctuations in the input signal frequency as described above, the demodulator Since the frequency of the input signal to the device also fluctuates greatly instantaneously, the demodulator sometimes loses synchronization.

[Problem to be solved by the invention]

The AFC circuit of the conventional receiver described above has the drawback that when applying frequency synchronization to the received signal, it also causes phase synchronization of the demodulator that demodulates the output signal, which deteriorates the bit error rate of the signal system. .

In view of the above-mentioned problems, an object of the present invention is to avoid the de-synchronization of the demodulator and reduce the deterioration of the bit error rate of the signal system by applying only the frequency synchronization instead of the phase synchronization. The purpose is to prevent it.

[Means to solve the problem]

The automatic frequency control circuit of the present invention includes a first means for outputting a frequency-converted signal using a voltage-controlled oscillator and a frequency mixer that can vary the oscillation frequency of a signal input from the outside, and a reference signal generator for outputting a frequency-converted signal. and the frequency-converted signal to output an error signal of a frequency difference or a phase difference.
an automatic frequency control circuit that negatively feeds back the error signal to the voltage controlled oscillator, the automatic frequency control circuit comprising: passing the output signal of the second means through a low-pass filter, and then converting the error signal into a digital signal; an analog-to-digital converter, and when the frequency error signal corresponding to the digital signal is within a predetermined threshold frequency error Δf, outputting the immediately preceding frequency control information;
A logic circuit that outputs frequency control information that sets the input frequency error signal to zero when the frequency error is greater than or equal to Δf, and a digital-to-analog converter that converts a digital output signal of the logic circuit into an analog signal. , this converted analog signal is fed back to the voltage controlled oscillator.

〔Example〕

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

FIG. 1 is a block diagram of one embodiment of the present invention. In the embodiment shown in FIG. 1, a signal frequency-converted by a frequency mixer 7 is supplied to a bandpass filter 9, and a frequency-phase comparator 5 converts the band-limited signal and the output signal of a reference signal generator 4 into a frequency phase comparator 5. The process is the same as before until the comparison is made. In the present invention, the signal level whose frequency and phase have been compared and the signal level of this signal passed through a low-pass filter 6 are each converted into digital signals by an analog-to-digital converter 1 and supplied to a logic circuit 2. Here, the logic circuit 2 has a history of frequency control information, and when the frequency error information, which is normal frequency fluctuation, is below a certain threshold value Δf,
The previous frequency control information is transferred to the digital-to-analog converter (
If Δf is exceeded, frequency synchronization is performed by sending frequency control information in the direction in which the frequency error becomes zero to the D/A converter 3.

Furthermore, if the frequency error information instantaneously exceeds Δf,
The frequency control information sent just before is sent to the D/A converter 3. Such processing eliminates the problem of demodulator loss of synchronization due to sudden frequency fluctuations. Here, the cut-off frequency of the low-pass filter 6 is Δf
If you select the output level of the low-pass filter 6 in advance according to the frequency difference between the input and output signals of the AFC circuit,
It can be determined from the output level of the low-pass filter 6 whether the frequency difference between the input and output signals of the AFC circuit is within the range of Δf. Note that Δf needs to be selected within a range that does not cause the demodulator to lose synchronization. The D/A converter 3 is a logic circuit 2
The voltage controlled oscillator 8 converts the signal from the
Add to the control terminal of

The relationship between the input and output signals of the AFC circuit when this embodiment is used is shown in the operation explanatory diagram of FIG. A, B.

When the frequency difference between the input signal and the output signal exceeds Δf, as at point 0, the frequency of the output signal is offset by Δf. When the frequency of the input signal changes instantaneously and greatly as at points D and E, the frequency of the output signal does not change.

〔Effect of the invention〕

As described above, the present invention adds a low-pass filter, an A/D converter, a logic circuit, and a D/A converter, so that when the input signal exceeds a predetermined threshold value Δf, By setting the frequency within the phase pull-in range by offsetting and not following the frequency when sudden frequency fluctuations occur, the effect is to prevent the demodulator from losing synchronization and prevent deterioration of the bit error rate of the signal system. be.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a block diagram of a conventional automatic frequency control circuit. FIG. 3 is an explanatory diagram of the operation of this embodiment. DESCRIPTION OF SYMBOLS 1... Analog-digital converter, 2... Logic circuit, 3... Digital-analog converter, 4... Reference signal generator, 5... Frequency phase comparator, 6... Low bandpass filter, 7... frequency mixer, 8... voltage controlled oscillator, 9... bandpass filter.

Claims (1)

[Claims] 1. A first means for outputting a frequency-converted signal using a voltage-controlled oscillator and a frequency mixer that can vary the oscillation frequency of a signal input from the outside, and a reference signal of a reference signal generator. and a second means for comparing the frequency-converted signal and outputting a frequency difference or phase difference error signal, the automatic frequency control circuit configured to negatively feed back the error signal to the voltage controlled oscillator. an analog-to-digital converter for converting the error signal into a digital signal after passing the output signal of the means through a low-pass filter, and a frequency error signal corresponding to the digital signal having a frequency error Δf of a predetermined threshold If within, output the previous frequency control information,
A logic circuit that outputs frequency control information that sets the input frequency error signal to zero when the frequency error is greater than or equal to Δf, and a digital-to-analog converter that converts a digital output signal of the logic circuit into an analog signal. , an automatic frequency control circuit characterized in that the converted analog signal is fed back to the voltage controlled oscillator. 2. The automatic frequency control circuit according to claim 1, wherein when the frequency error signal input to the logic circuit instantaneously exceeds the frequency error Δf, the immediately preceding frequency control information is output.