JPS60218952A - Automatic frequency control system - Google Patents

Abstract

PURPOSE:To omit a pilot signal and the reproduction of a carrier wave and to attain the automatic frequency control by using a frequency discrimination output signal obtained with the identification timing of a PSK (phase shift keying) signal to control a reception local oscillator. CONSTITUTION:The reception PSK signal sent from an input terminal 31 is mixed with the output signal of a voltage controlled oscillator 37 by a frequency converter 32. This mixed signal is converted into an intermediate frequency, for example, and applied to a demodulator through an output terminal 33. The PSK signal underwent the conversion of frequency through the converter 32 is applied to a frequency discriminator 34. The discriminator 34 has the linear wave detection sensitivity over an occupied band width of the PSK signal, and the frequency discrimination output signal is supplied to a switch circuit 35. The circuit 35 is turned on and off by a clock signal, and the output signal of the circuit 35 is used to the control voltage of the oscillator 37 via an LPF36. Then the oscillator 37 is controlled so that the frequency converted by the converter 32 is set at a fixed level.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to an automatic frequency control method that converts the number of frequencies of a received PSK signal and makes the converted frequency constant.

Prior Art and Problems In satellite communication in which communication is performed between ground stations via an artificial satellite, the local oscillator of the frequency converter on the transmitting side, the local oscillator of the frequency converter on the receiving side, and the local oscillator of the transponder of the artificial satellite. The frequency of the demodulator input signal on the receiving side varies greatly due to frequency variations of the oscillators or Doppler shift based on variations in the relative position between the artificial satellite and the earth.

Also, as a modulation method, P S K (Phase Shi
ftKeying) method, demodulation is generally performed using a synchronous detection method, and therefore it is necessary to regenerate the carrier wave. However, PSK with large frequency fluctuations
It is not easy to configure a carrier recovery circuit so that a carrier wave can be recovered from a signal, and especially in satellite communications, the low signal-to-noise ratio makes carrier wave recovery even more difficult.

Therefore, when performing frequency conversion on the receiving side, an automatic frequency control (AFC) method is adopted to automatically stabilize the frequency, and the modulated signal that has been frequency converted and controlled to a constant frequency is sent to the demodulator. It is proposed to have the user input.

For example, there is a method in which a pilot signal is transmitted from the transmitting side, and the receiving side extracts the biloft signal to control the oscillation frequency of a local oscillator for frequency conversion.

FIG. 1 is a block diagram of the main parts of the automatic frequency control system using this pilot signal, in which 1 is an input terminal, 2 is a frequency converter, 3 is an output terminal, 4 is a bandpass filter for pilot signal extraction, 5 is a reference pilot frequency oscillator, 6 is a phase comparator, 7 is a low-pass filter, and 8 is a voltage controlled oscillator as a local oscillator. The signal applied to input terminal 1 is
In the frequency converter 2, it is mixed with the output signal of the voltage controlled oscillator 8 and converted into, for example, an intermediate frequency. This frequency-converted signal contains a pilot signal, so it is extracted by a bandpass filter 4, and its phase is compared with the output signal of a reference pilot frequency oscillator 5 by a phase comparator 6, and an output signal corresponding to the phase difference is extracted. is used as the control voltage of the voltage controlled oscillator 8 via the low frequency wave generator 7, and the oscillation frequency of the voltage controlled oscillator 8 is controlled to match the phase of the extracted pilot signal and the output signal of the reference pilot frequency oscillator 5. This makes the frequency of the PSK signal frequency-converted from the output terminal 3 constant.

The automatic frequency control method using such a pilot signal has the following drawbacks. That is, in satellite communications, the bandwidth and power capacity of repeaters on the satellite are limited, so the bandwidth allocated to the PSK signal must be reduced only to the pilot signal frequency converter. , and the transmission power of the PSK signal must be reduced by the amount of power allocated to the transmission of the pilot signal, resulting in further deterioration of the signal-to-noise ratio (C/N). Furthermore, if a failure occurs in the reference station that sends the pilot signal, the entire communication system goes down, so a backup reference station is required. Therefore, there is a drawback that the communication system configuration is complicated and expensive.

Therefore, automatic frequency control methods that do not use pilot signals have been proposed. For example, as shown in FIG. 2, a PSK signal without a pilot signal is applied to an input terminal 11 and mixed in a frequency converter 12 with an output signal of a voltage controlled oscillator 21 as a local oscillator, for example at an intermediate frequency. is converted into output terminal 1 via bandpass filter 13.
4 and is applied to, for example, a demodulator (not shown). Further, the output signal of the bandpass filter 13 is converted to N by a multiplier 15.
The multiplied N-phase PSK signal becomes an unmodulated signal, unnecessary frequency components are removed by bandpass filter 16, and frequency divider 1
The frequency is divided by N by 7. The output signal of this frequency divider 17 is
The carrier wave frequency is obtained by multiplying by N and dividing by N, and is added to the phase comparator 18. The phase comparator 18 compares the phase with the output signal of the reference carrier oscillator 19, and sends a signal corresponding to the phase difference to the voltage controlled oscillator 2 via the low-pass filter 20.
1, the oscillation frequency of the voltage controlled oscillator 21 is controlled, and the frequency of the frequency-converted PSK signal is made constant.

In this way, when a pilot signal is not used, the carrier wave is regenerated, the regenerated carrier frequency is compared with the reference carrier frequency, and the frequency-converted Ps is controlled so that the phase difference is zero. "The frequency of the K signal is made constant, and as the carrier wave reproducing means, in addition to the above-mentioned N multiplication and N frequency division methods, a method of reproducing the carrier wave by baseband signal processing can be considered.

As mentioned above, automatic frequency control to keep the frequency of the received PSK signal constant is to facilitate carrier wave regeneration in the demodulator. Playing P with large frequency fluctuations
Since the carrier wave is regenerated from the SK signal, it is practically difficult to regenerate the carrier wave for automatic frequency control.Especially in satellite communications with a low signal-to-noise ratio, automatic frequency control that does not use a pilot signal is difficult. It was not possible to realize a frequency control method.

OBJECTS OF THE INVENTION It is an object of the present invention to make it possible to automatically control the conversion frequency of a received PSK signal without requiring pilot signal and carrier wave regeneration.

Structure of the Invention The present invention provides an automatic frequency control system that performs frequency conversion by mixing a received PSK signal and an output signal of a receiving local oscillator, and controls the receiving local oscillator to keep the frequency of the frequency-converted PSK signal constant. In this method, the frequency-converted PSK signal is input, a frequency discriminator having linear detection sensitivity over the occupied bandwidth of the PSK signal, and an output signal of the frequency discriminator are intermittent in accordance with a reproduced clock signal. a switch circuit; and a low-pass filter that uses a low-frequency component of the output signal of the switch circuit as a control voltage for controlling the oscillation frequency of the receiving local oscillator, and outputs frequency discrimination at the identification timing of the PSK signal. The receiving local oscillator is controlled using a signal, and automatic frequency control is possible without the need for a pilot signal and carrier wave regeneration. Examples will be described in detail below.

Embodiment of the invention FIG. 3 is a block diagram of main parts of an embodiment of the invention.
1 is an input terminal for the received PSK signal, 32 is a frequency converter,
33 is an output terminal, 34 is a frequency discriminator, 35 is a switch circuit, 36 is a low-pass filter, 37 is a voltage controlled oscillator as a local oscillator, and 38 is a clock regeneration circuit. In the frequency converter 32, the received PSK signal from the input terminal 31 is mixed with the output signal of the voltage controlled oscillator 37, converted to, for example, an intermediate frequency, and applied to a demodulator (not shown) from the output terminal 33. The PSK signal frequency-converted by this frequency converter 32 is applied to a frequency discriminator 34.

This frequency discriminator 34 has linear detection sensitivity over the occupied bandwidth of the PSK signal, and a frequency discrimination output signal is applied to a switch circuit 35. This switch circuit 35 is controlled on and off by a clock signal, and its output signal becomes a control voltage for a voltage controlled oscillator 37 via a low-pass filter 36, so that the frequency converted by the frequency converter 32 remains constant. , voltage controlled oscillator 37
is controlled.

Generally, PSK signals use carrier waves as digital information “1”.
For example, in a two-phase PSK signal, the carrier wave phase is changed to 0 degrees and 180 degrees corresponding to digital information 1'' and 0.
It is a modulation signal that switches between
It is obtained by orthogonally synthesizing two-phase PSK signals, and has four angles of θ degrees, 90 degrees, 180 degrees, and 270 degrees.
This is a modulated signal that takes one of two phases.

Such a PSK signal does not have a line spectrum at the carrier frequency. That is, there is no energy in the carrier frequency component, and therefore, it was thought that frequency error information could not be obtained even if the frequency of the PSK signal was directly discriminated.

However, by using a frequency discriminator that has linear discrimination sensitivity over the signal bandwidth occupied by the PSK signal, frequency error information can be obtained without a line spectrum of the carrier frequency. This principle can be explained as follows.

In the 2-phase PSK signal, 0 corresponds to digital information.
A carrier wave, that is, a modulated wave, with a phase of 180 degrees is obtained, and the reason why there is no line spectrum in this carrier wave is because the two phases are opposite to each other and cancel each other out.

As a frequency discriminator, in a configuration that combines differentiation and envelope detection, the input signal is E. If A is the amplitude, ω is the angular frequency, and θ is the phase, then E6 = Asin(ωt+θ
)...(1), and when differentiated, it becomes go'=Aωcosωt −−・−(2). That is, by envelope-detecting the signal of equation (2), a signal with an amplitude proportional to the angular frequency ω is obtained.

On the other hand, when a PSK signal that is not band-limited is differentiated, an impulse is generated at the code conversion point, and a signal proportional to the angular frequency is obtained at other parts.

That is, when looking at a two-phase PSK signal, the conversion point between 0 and 180 degrees of the carrier wave corresponds to a large frequency change, and the other parts have the same frequency, so the discrimination output at the code conversion point is This becomes an impulse, and the discrimination output in other parts becomes a DC signal with a value proportional to the angular frequency ω. In other words, frequency discrimination can be performed.

Band-limiting a PSK signal also causes the phase change to occur over time rather than instantaneously, which means that the instantaneous angular frequency changes, but this change depends on the modulation pattern and is generally Since the PSK signal is spread by a pseudorandom code, etc., the change in the instantaneous angular frequency mentioned above is averaged, and the average DC voltage of the discrimination output becomes proportional to the input frequency, and frequency discrimination is performed in this case as well. be able to.

According to the above-mentioned principle, the frequency of the frequency-converted PSK signal can be discriminated by the frequency discriminator 34, and the oscillation frequency of the voltage-controlled oscillator 37 can be controlled using the discrimination output signal.

However, if the bandwidth is limited as described above, PSK
The instantaneous angular frequency of the signal constantly changes, and the change is particularly large near the code conversion point. This corresponds to modulation pattern jitter, and appears as noise in the discrimination output.

The influence of such a modulation pattern can be absorbed by the switch circuit 35.
The switch circuit 35 is controlled on and off by the clock signal regenerated by the clock regeneration circuit 38, and the output signal of the frequency discriminator 34 near the code conversion point is excluded. The discrimination output signal is extracted and passed through a low-pass filter 36 to be used as a control voltage for a voltage controlled oscillator 37. Furthermore, instead of regenerating the clock signal by the lock regeneration circuit 38, the clock signal is regenerated in the demodulator, so it is also possible to utilize the regenerated clock signal.

As mentioned above, the frequency of P converted by the frequency converter 32 is
The frequency of the SK signal is discriminated by a frequency discriminator 34, and the signals, excluding those corresponding to the vicinity of the code conversion point, are used as the control voltage of the voltage controlled oscillator 37, and the frequency is kept constant at the output terminal 33. This makes it possible to output a PSK signal.

FIG. 4 is a block diagram of an example of a frequency discriminator used in the present invention, where 41 is an input terminal, 42 is a hybrid circuit, 43 and 44 are balanced modulators, 45 is a reference oscillator, and 46 is a phase shifter. 47 is a low-pass filter, 48 is a high-pass filter, 49 is a multiplier, and 50 is an output terminal. The reference oscillator 45 outputs a signal of the same frequency as the desired frequency of the frequency-converted PSK signal applied to the input terminal 41, and the phase shifter 46 outputs a signal of 0 degrees to the balanced modulator 43 and 0 degree to the balanced modulator 744, for example. The phase shift is performed so that the addition is made at a phase of 90 degrees. PSK added to input terminal 41
The signal is branched by the hybrid circuit 42 and applied to balanced modulators 43, 44, and from the balanced modulator 43 As1
nΔωt, and a signal of A cosΔωt is output from the balanced modulator 44. Note that Δω indicates the frequency difference between the oscillation frequency of the reference oscillator 45 and the frequency of the PSK signal.

The low-pass filter 47 and the high-pass filter 48 are set to have the same cutoff frequency and the same order,
Since the high-pass filter 48 corresponds to a differentiating circuit, by adding the output signal of the balanced modulator 44, the output signal becomes
AΔω sin Δωt. Furthermore, the low-pass filter 47 corresponds to an integrating circuit, and in this case, harmonic components of the output signal of the balanced modulator 43 are removed, and the above-mentioned signal As1nΔωt is output. The multiplier 49 multiplies the output signals of the low-pass filter 47 and the high-pass filter 48, and therefore, the output signal is A2Δω sin Δωt. Therefore,
A DC component proportional to the frequency difference Δω appears at the output terminal 50.

FIG. 5 is an explanatory diagram of an example of the spectrum of the received PSK signal, where F is the carrier frequency and FM is the Nyquist frequency. For such a spectrum, the frequency discriminator 34 is configured to have linear detection sensitivity over the occupied band of the PSK signal, as shown in FIG. Thereby, as described above, when frequency discrimination is performed on a PSK signal that does not have a line spectrum at the carrier frequency, it is possible to obtain a discrimination output signal corresponding to the carrier frequency.

From the point of view of stability of the center frequency, a frequency discriminator using a crystal etc. can be considered, but such a frequency discriminator is
A frequency discriminator that has a narrow eJI range and uses a tuning circuit consisting of a coil or a capacitor to widen the linear range has the disadvantage of poor center frequency stability. However, in the frequency discriminator having the configuration shown in FIG. 4, the stability of the center frequency can be easily set to a desired value by using a highly stable crystal oscillator as the reference oscillator 45.
Furthermore, since the linear region uses modulation using a 90 degree phase difference in the balanced modulators 43 and 44, it is possible to make the linear region as wide as desired.

Alternatively, the switch circuit 35 may be configured to simply turn on and off the frequency discrimination output signal, and the signal may be smoothed by the low-pass filter 36 and used as the control voltage for the voltage controlled oscillator 37.
It is also possible to provide a sampling and holding circuit to sample and hold the frequency discrimination output signal at the identification timing.

DETAILED DESCRIPTION OF THE INVENTION As described above, the present invention provides frequency-converted PSK
a frequency discriminator 34 to which a signal is input and which has linear detection sensitivity over the occupied bandwidth of the PSK signal;
A switch circuit 35 connects the output signal of the frequency discriminator 34 on and off according to the reproduced clock signal, and converts the low frequency component of the output signal of the switch circuit 35 into a control voltage that controls the oscillation frequency of the receiving local oscillator (voltage controlled oscillator 37). The receiving local oscillator is controlled using a frequency discrimination output signal at the identification timing of the PSK signal, and frequency error information of the PSK signal can be extracted. , and a control voltage that is not affected by the modulation pattern can be applied to the receiving local oscillator (voltage controlled oscillator), so automatic frequency control can be performed without the need for a pilot signal and carrier regeneration circuit. Therefore, there is an advantage that it is possible to configure an economical communication system by applying it to satellite communication or the like.

[Brief explanation of the drawing]

Figure 1 is a block diagram of the main part of a conventional automatic control system using a pilot signal, Figure 2 is a block diagram of a main part of a conventional automatic frequency control system using a carrier regeneration circuit, and Figure 3 is a block diagram of the main part of an automatic frequency control system using a conventional carrier regeneration circuit. FIG. 4 is a block diagram of the main part of the embodiment, FIG. 4 is a block diagram of the main part of the frequency discriminator, FIG. 5 is an explanatory diagram of the spectrum of the PSK signal, and FIG. 6 is an explanatory diagram of the detection sensitivity of the frequency discriminator. 31 is an input terminal for the received PSK signal, 32 is a frequency converter, 33 is an output terminal, 34 is a frequency discriminator, 35 is a switch circuit, 36 is a low-pass filter, 37 is a voltage controlled oscillator as a local oscillator, 38 is a This is a clock recovery circuit. Patent Applicant Fujitsu Ltd. Representative Patent Attorney Akira Aitani Representative Patent Attorney Hiroshi Watanabe - Figure 1 Figure 12 t Figure 3 Figure 6 Figure 4 Figure 5 > 44 48

Claims (1)

[Claims] In an automatic frequency control method that performs frequency conversion by mixing a received PSK signal and an output signal of a receiving local oscillator, and also controls the receiving local oscillator to make the frequency of the frequency-converted PSK signal constant, the frequency Converted PS
a frequency discriminator to which the K signal is input and has linear detection sensitivity over the occupied bandwidth of the PSK signal; a switch circuit that intermittents the output signal of the frequency discriminator in accordance with a regenerated clock signal; and an output signal of the switch circuit. and a low-pass filter that uses the low-frequency component of the signal as a control voltage for controlling the oscillation frequency of the receiving local oscillator, and using a frequency discrimination output signal at the identification timing of the PSK signal, the receiving local oscillator is controlled. Automatic frequency control method characterized by control.