JPS61281655A - Automatic frequency control circuit - Google Patents

Abstract

PURPOSE:To execute the automatic frequency control with a high accuracy even when the band of a single tuning circuit is made narrow and a short burst signal is inputted by providing a sample holding circuit at the output side of the phase detector, removing the essential error voltage only by the static frequency deviation of respective burst signals and executing the automatic frequency control. CONSTITUTION:A sample holding circuit 9 is provided at the output side of the phase detecting device, the part only of an essential error voltage 19 by the frequency deviation of respective burst signals is sampled in accordance with the burst detecting signal from a burst detecting circuit 8 out of the phase error detecting voltage which is the output of a phase detecting device 7 by a sample holding circuit 9. The sampled error voltage 19 is held up to the time of the ne4t sampling. The peak value of the positive voltage of the error voltage 19 sampled to the sample holding circuit 9 is held to a positive voltage peak holding circuit 10, and the peak value of the negative voltage is held at a negative voltage peak holding circuit 11. Thereafter, the automatic frequency control is executed as well as the conventional automatic frequency control circuit.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention relates to an automatic frequency control circuit used in a carrier recovery circuit in a demodulator for P8 in a time division multiple access communication system in which a burst signal is input. be.

[Conventional technology]

Figure 2 shows the conventional automatic frequency control ( AFC) circuit is shown. In this figure, (
1) is the input terminal into which the burst modulated wave is input, (2)
is a carrier wave regeneration circuit that reproduces a reference carrier wave from the input modulated wave, (8) is a first frequency converter that converts the frequency of the carrier wave, (4) is a single tuning circuit, and (5) is a frequency conversion circuit. This is a second frequency converter for returning the signal frequency-converted by the converter (8) back to the frequency before conversion. The frequency conversion operation of each of frequency converters +a) and (5) is performed by a signal given from a voltage controlled oscillator C14) to be described later. (7) is a single tuning port M (a phase detector for detecting the phase difference between the input signal and the output signal of 41;
2) is a low-pass filter used to smooth the large phase error voltage at the beginning of the burst generated in the output signal of the phase detector (γ). (6) is a positive voltage peak hold circuit that detects and holds the positive peak voltage of the output voltage of the low-pass filter (6), and a negative voltage peak hold circuit that detects and holds the negative peak voltage of the output voltage. , (the positive voltage peak hold circuit Cl0) and the negative voltage peak hold circuit (6) are adders for obtaining an intermediate value of each output voltage. 01) is a loop filter that allows only low frequency components to pass and determines the response speed of the control loop of the entire automatic frequency control circuit; It is a voltage controlled oscillator that performs frequency conversion on its output signal. In addition (6
) is the output terminal.

Next, the operation of the automatic frequency control circuit having the above configuration will be explained. First, the operation of regenerating a carrier wave will be explained.

A plurality of burst-shaped modulated waves (S) with different frequencies are input to the input terminal (1) at different times. From these plural modulated waves, the carrier regeneration circuit (2) converts each reference carrier wave is played.

This reference carrier wave is frequency-converted by a frequency converter (8) to a frequency near the center frequency of the single-tuned circuit (4) based on the signal given from the voltage-controlled oscillator 0, and then noise contained in the input signal or The signal is input to a single tuning circuit (4) in order to remove noise generated in the process of reproducing the modulated wave and the carrier wave. The carrier wave from which noise has been removed in this way is frequency-converted again by the frequency converter (5) using a signal from the voltage-controlled oscillator CI, and after being returned to the original frequency, it is output from the output terminal (6). .

In the above case, when the frequency of the modulated wave (S) input to the input terminal (1) changes, if the frequency of the output signal of the voltage controlled oscillator α slope is constant, the signal input to the single tuned circuit (4) frequency will move away from its center frequency. As a result, a phase shift occurs in the phase of the output signal of the single tuned circuit (4) compared to when the frequency of the input signal is the center frequency of the single tuned circuit (4). Therefore, the phase difference between the input and output signals of the single tuned circuit (4) is detected as a voltage by the phase detector (7), and the voltage controlled oscillator is operated to control the frequency of the output signal. As a result, even if the frequency of the modulated wave (S) changes, automatic frequency control is performed to maintain the frequency of the signal input to the single tuning circuit (4) at a constant frequency close to the center frequency.

The frequency control method of the voltage controlled oscillator 0 is performed as follows. In a time division multiple access communication system, a plurality of signals having different frequencies are periodically input at different times. In this case, ■ When the frequency difference between signals is large, automatic frequency control is performed for each signal. ■ When the frequency between signals is small, the intermediate value between the maximum and minimum frequencies of the signals is set to the center frequency of the single tuning circuit (4). One of two methods is required: automatic frequency control to match. Since the frequency difference between the signals is usually small, method (2) is adopted in the above control.

This will be explained in detail with reference to FIG. Figure 3 shows the signal processing state when each burst signal has a different frequency and the time interval is small compared to the response speed of the single-tuned circuit (4), and from the top, the input state of the single-tuned circuit (4). , output state,
It shows the output state of the phase detector (γ) and the output state of the low-pass filter (5). In this case, there is a preceding burst signal (16) at the output of the single-tuned circuit (4), and a burst signal (7) following its input, so that both burst signals α6) are present.

Since the phase of C17) is uncorrelated, the phase detector (7)
A large phase error voltage (total) may be temporarily output. This phase error voltage (to) changes depending on the phase relationship between the preceding burst signal α6) and the next burst signal α7). 09) before and after phase error voltage (g)
is the original error voltage due to the frequency deviation of each burst signal.The peak phase error voltage (18) generated at the beginning of the zero burst signal αη is smoothed by the low-pass filter (15), When the steady state is reached after the start of the burst, a voltage α9) corresponding to the input frequency deviation of each burst signal is output. The positive peak value (a) and negative peak value (211) in the output voltage of the low-pass filter, that is, the maximum values of the frequency deviation in the positive direction and the frequency deviation in the negative direction, are determined by the positive voltage peak hold circuit σQ and the negative voltage The signal is held by the peak hold circuit (1) and input to the adder (2).

As a result, the intermediate value of the two input voltages is output as the output voltage of the adder. This intermediate voltage is used to filter the loop filter (1B) that determines the response speed of the automatic frequency control circuit. After passing through the signal, it is added to the voltage controlled oscillator a.In this way, the intermediate value between the maximum and minimum frequency of the signal input to the input terminal (1) becomes the center frequency of the single-tuned circuit (4). The voltage-controlled oscillator 0 clause is controlled at 0. As a result, unless the automatic frequency control is performed for each burst signal, the frequency deviation generated by the single-tuned circuit (4) is small when the frequency deviation for each burst signal is small. The phase error caused by this can be minimized.

[Problem that the invention seeks to solve]

In the conventional automatic frequency control device as described above, when the band of the single tuning circuit (4) is narrowed, the duration of the phase error voltage (G) due to the preceding burst signal generated at the beginning of the burst signal becomes longer; A low-pass filter (15) with a larger time constant is required for smoothing. However, if the time constant of the low-pass filter (to) is increased, the response speed becomes slow and it becomes impossible to detect the phase error voltage due to the original frequency deviation for short burst signals. However, it is impossible to completely eliminate the phase error voltage (■), so automatic frequency control cannot be performed accurately.

This invention was made to solve the above problem, and even if the band of the single-tuned circuit is narrowed, it can be short-circuited regardless of the large phase error voltage of the phase detector output due to the preceding burst that occurs at the beginning of the burst. It is an object of the present invention to provide an automatic frequency control circuit capable of detecting only a phase error voltage due to an original frequency deviation after the beginning of a burst even in the case of a burst, and thereby correctly performing an automatic frequency control operation.

[Means for solving the problem] The automatic frequency control circuit according to the present invention is an automatic frequency control circuit for a time division multiple access communication system in which a burst signal is input, and includes a sample and hold circuit on the output side of a phase detector. A burst detection circuit that separately detects a burst signal is prepared, and based on the burst detection signal from the burst detection circuit, the sample and hold circuit samples only the phase error voltage generated due to the original frequency deviation, and this voltage is The system is configured to perform automatic frequency control.

[Effect]

In this invention, the influence of the large phase error voltage caused by the preceding burst signal occurring at the head position of the burst signal is eliminated, and only the error voltage due to the original frequency deviation is extracted and used for automatic frequency control, thereby improving accuracy.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an automatic frequency control circuit according to an embodiment of the present invention, and the lines (1) to (a) and the lines .alpha. to .alpha. are exactly the same as those of the conventional circuit. (8) is a burst detection circuit for detecting the burst position from the burst modulated wave (S) input to the input terminal (1). (
9) is a sample hold circuit that samples a specific portion of the output voltage of the phase detector (γ) based on the burst detection signal given from the burst detection circuit (8), and holds the sampled voltage until the next sampling point. The peak voltage in the positive and negative directions of the output voltage of this sample and hold circuit (9) is held in the positive voltage peak hold circuit and the negative voltage peak hold circuit.

In the automatic frequency control circuit configured as described above, the phase detector (γ
According to the burst detection signal from the burst detection circuit (8), only a part of the original error voltage α due to the frequency deviation of each burst signal is sampled out of the phase error detection voltage that is the output of the one is retained until the next sampling point. The sampled error voltage (1
91 positive voltage peak hold circuit σ
The peak value of the negative voltage is held in the negative voltage peak hold circuit (6). After that, automatic frequency control is performed in the same manner as in the conventional automatic frequency control circuit.

Therefore, regardless of the phase error voltage (18) generated due to the preceding burst signal at the head position of the burst signal, the original error voltage <191 due to the steady frequency deviation of each burst signal that occurs in the burst signal. Since only that part is detected, the influence of the phase error voltage (to) can be eliminated. The timing of sampling in the sample and hold circuit (9) is set by the burst detection circuit (8).

〔Effect of the invention〕

As explained above, according to the present invention, in an automatic frequency control circuit of a time division multiple access communication system to which burst signals are input, a sample and hold circuit is provided on the output side of a phase detector, and a steady frequency deviation of each burst signal is determined. Since automatic frequency control is performed by extracting only the original error voltage caused by the error voltage, there is an effect that highly accurate automatic frequency control can be performed even when a narrow and short burst signal is input to the single tuning circuit band.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an automatic frequency control circuit according to an embodiment of the present invention, Fig. 2 is a block diagram of a conventional automatic frequency control circuit, and Fig. 3 is a signal waveform of each part of the circuit explaining the operation of automatic frequency control. In Figure 0, (2) is the carrier wave regeneration circuit % (8) # (5) is the frequency converter, (
4) is a single tuning circuit, (γ) is a phase detector, (8) is a burst detection circuit, (9) is a sample and hold circuit, (inversion)
is a positive voltage peak hold circuit, (2) is a negative voltage peak hold circuit, 4 is an adder, Ql) is a loop filter, α
The slope is a voltage controlled oscillator, α6), and αη is a burst signal.

Claims (1)

[Claims] A carrier wave regeneration circuit extracts a carrier wave from a burst input signal, a frequency converter converts the frequency of the carrier wave, a single tuning circuit inputs the output of the frequency converter, and a frequency converting circuit converts the output signal of the single tuning circuit. a frequency converter that returns the frequency to A sample and hold circuit samples and holds the output voltage of the phase detector at the time when the burst detection signal is output from the detection circuit, and holds the positive voltage peak and negative voltage peak of the output of this sample and hold circuit, respectively. two peak hold circuits,
The frequency is determined by the adder that outputs the center value of the voltage peak output from these two peak hold circuits, the loop filter that passes only the low frequency component of the output of this adder, and the output voltage of the loop filter. An automatic frequency control circuit comprising: a voltage controlled oscillator that applies an output signal to each of the frequency converters to perform a frequency conversion operation.