JPS60200656A - Automatic frequency control system - Google Patents

Abstract

PURPOSE:To execute an automatic frequency control which requires no pilot signal by eliminating a high frequency component from an output obtained by frequency-discriminating a PSK modulated wave which brought to frequency conversion, by a frequency discriminator, and thereafter, controlling an oscillation frequency of the receiving local oscillator concerned by the discriminator concerned whose high frequency component is eliminated. CONSTITUTION:For instance, a phase shift keying PSK modulated wave having a spectrum as shown in a figure, which is applied to a frequency converter 20 from a terminal 24 is mixed with an output wave from a voltage control oscillator 21, and brought to frequency conversion, for instance, to an intermediate frequency. A part of the PSK modulated wave brought to frequency conversion is applied to a frequency discriminator 23, and how much a symmetrical property of a power spectrum which this PSK modulated wave has is shifted from the center of this frequency discriminator 23 is detected. That is to say, a voltage corresponding to a difference of the center frequency of the frequency discriminator 23 and a received input frequency is fetched from this frequency discriminator 23. As for this output voltage, its noise component is eliminated by a low-pass filter 22, and thereafter, said voltage is applied to the voltage control oscillator 21, and the oscillation frequency is controlled so that the output voltage becomes ''0''. Therefore, the frequency of the PSK modulated wave brought to frequency conversion by the frequency conveter 20 always coincides with the center frequency of the frequency discriminator 23.

Description

[Detailed Description of the Invention] fa+ Technical Field of the Invention The present invention relates to an automatic frequency control method, and particularly relates to an automatic frequency control method used in a digital wireless communication reception method using a PSK (phase shift keying) modulated wave. It is something.

(bl Conventional technology and problems) In a satellite communication line, a carrier wave from a ground station is modulated with data, such as PSK, and transmitted to other ground stations via a repeater on the satellite. Other ground stations receive this radio wave. and retrieve the original data.

In this case, the receiving frequency at the ground station receiving section may fluctuate by about ±40 KHz, for example, due to fluctuations in the oscillation frequency of the crystal oscillators used in the ground station transmitting section, receiving section, and repeaters on the satellite, as well as the Tompler shift. do.

On the other hand, since the reception level of radio waves arriving from about 36,000 km away is generally weak, a bandpass filter is inserted in the receiving section of the ground receiving station to improve the carrier-to-noise ratio.

The characteristics required of this bandpass filter are selected so that code amount interference and thermal noise are minimized.

If the transmission path including the satellite repeater is linear, the best method is to limit the band so that the spectrum of the output wave of the bandpass filter becomes a raised cosine type, and is commonly used.

However, even if the spectrum of the output wave of the bandpass filter is a raised cosine type at a certain receiving frequency, if the receiving frequency fluctuates, it will deviate from the best point of the bandpass filter's characteristics, resulting in code amount interference and polyphase ps. In the case of a modulated wave, the orthogonal component (1
-CI+ and Q-C11) Mutual interference occurs and the error rate worsens.

Furthermore, when demodulating PSK waves, a synchronous detection method is generally used, and for this purpose, regeneration of the Kishou carrier wave is essential. It is difficult to configure this reference carrier regeneration circuit to track large frequency fluctuations under a low signal-to-noise ratio.

Due to this reason, the above-mentioned ground station receiving section controls the oscillation frequency of the receiving local oscillator so that the frequency of the received wave inputted to the bandpass filter is always at the best point. That is, automatic frequency control (hereinafter abbreviated as AFC) is performed.

Figure 1 shows an AF that performs AFC control using pilot signals.
A conventional example of a C circuit is shown.

In the figure, 1 is a frequency converter, 2 is a voltage controlled oscillator, and 3 is a frequency converter.
5 represents a low-pass filter, 5 represents a phase comparator, 4 represents a pilot signal extraction bandpass filter, 6 represents a reference pilot frequency oscillator, and 7 to 8 represent terminals, respectively.

The operation of the AFC circuit connected in this way is as follows.

The received wave input from the terminal 7 is mixed with the output wave of the voltage controlled oscillator 2 by a frequency converter and converted into, for example, an intermediate frequency. A pilot signal is extracted from the frequency-converted received wave by a pilot signal extraction bandpass filter 4. This biloft signal is multiplied by the output of the reference pilot frequency oscillator 6 by the phase comparator 5, and a component corresponding to the phase difference between the two is extracted via the low-pass filter 3.

The output of the low-pass filter 3 is applied to the voltage controlled oscillator 2, and the oscillation frequency of the voltage controlled oscillator is controlled so that the component corresponding to the above phase difference becomes zero.

Therefore, the frequency of the received pilot signal matches the frequency of the reference pilot signal, and the frequency of the PSK modulated wave, which is a certain frequency away from the received pilot frequency, is also kept constant.

In the case of the AFC method using the above-mentioned pilot signal, there are the following problems. That is, in satellite communications, the bandwidth and power capacity of repeaters on the satellite are limited, so the bandwidth allocated to data signals must be reduced by the frequency fluctuation of the pilot signal. The power for transmitting the PSK modulated wave must be reduced by the amount of power allocated to the transmission of the PSK modulated wave.

Furthermore, if the reference station that sends out the pilot signal is disconnected, the entire system will be disconnected, so a backup reference station is required and the system configuration becomes complicated.

fcl OBJECT OF THE INVENTION The present invention has been made in view of the problems of the prior art described above, and it is an object of the present invention to provide an AFC system that does not require a pilot signal.

tdl Structure of the Invention The object of the above invention is to control the oscillation frequency of the oscillator of the receiving local oscillator when frequency-converting the received PSK modulated wave with the output wave of the receiving local oscillator, thereby converting the frequency-converted PSK. In the automatic frequency control method that keeps the frequency of the modulated wave constant, after removing the high frequency component from the output obtained by frequency discriminating the frequency-converted PSK modulated wave with a frequency discriminator, the high-frequency component is removed. This is achieved by providing an AFC method characterized in that the oscillation frequency of the receiving local oscillator is controlled by the discriminator.

tel Embodiments of the Invention In general, a PSK modulated wave uses a carrier wave as a digital information 0,
■Available by switching the phase according to the pulse. For example, for a two-phase PSK modulated wave, the phase of the carrier wave is 0 degrees,
Obtained by switching to 180 degrees. , the 4-phase PSK modulated wave is 2
By orthogonally combining phase PSK modulated waves, 0.90.
It is obtained as a modulated wave that takes one phase out of four phases of 180.270 degrees.

Since a line spectrum cannot be obtained at the carrier frequency in such a PSK modulated wave, there is no energy in the carrier frequency component, and therefore, it is considered that frequency error information cannot be obtained even if the PSK modulated wave is directly frequency discriminated.

On the other hand, if a frequency discriminator having linear discrimination sensitivity over the signal bandwidth occupied by the PSK modulated wave is used, frequency error information can be obtained even if this carrier frequency does not have a line spectrum.

This principle can be thought of as follows.

For simplicity, consider a two-phase PSK modulated wave.

In the two-phase PSK modulated wave, 0 depending on the digital information.
．． Although a carrier wave with a phase state of 180 degrees is obtained, the carrier wave does not have a line spectrum because the two phases are opposite to each other and cancel each other out.

The frequency discriminator operates by differentiating the angle-modulated wave, converting it into an amplitude-modulated signal, and detecting its envelope to extract the original signal. That is, for example, when the input wave Eo=ASin (ωL+θ) is differentiated, Eo=AωCos ωL.

Here, A indicates amplitude, ω indicates angular frequency, and θ indicates phase.

That is, this equation shows that frequency discrimination characteristics can be obtained by differentiating the angle-modulated wave and then performing envelope detection.

Here, if we differentiate the PSK modulated wave without band limitation, an impulse will be generated at the sign conversion point, and a signal proportional to the angular frequency will be obtained in other parts, so PSK without band limitation
By frequency-discriminating the K-modulated wave, a signal proportional to the frequency can be extracted.

Furthermore, when the PSK modulated wave is band-limited, the phase change does not occur instantaneously, but instead occurs over a period of time.

This means that the instantaneous angular frequency changes, but this change depends on the modulation pattern and is generally averaged because it is spread by pseudorandom codes, etc., and the average DC voltage of the discriminator output is proportional to the input frequency. You can get something.

Therefore, this DC voltage may be used to control the voltage controlled oscillator.

The second is a diagram showing an example for implementing the present invention based on the above idea.

In the figure, 20 is a frequency converter, 21 is a voltage controlled oscillator, 22 is a low-pass filter, 23 is a frequency discriminator, 24-
27 indicate terminals, respectively.

FIG. 3 is a diagram for explaining the operation of FIG. 2. FIG. 3 shows the spectrum of the PSK modulated wave manually input to the terminal 24, and FIG. 3 shows the frequency difference of the frequency discriminator 23.

Note that No represents the carrier frequency, and +Pn and -Fn each represent the Nyquist frequency.

Now, the operation shown in FIG. 2 will be explained with reference to FIG.

For example, a third
A PSK modulated wave having a spectrum as shown in the figure is mixed with an output wave from the voltage controlled oscillator 21 and frequency-converted to, for example, an intermediate frequency.

Here, the shape of the spectrum shown in Figure 3 differs depending on the characteristics of the transmission path, but theoretically the transmission bandwidth is 2Fn.
It is said that it is good to have.

A part of the frequency-converted PSK modulated wave is sent to the frequency discriminator 2.
3, and it is detected how far the symmetry of the power spectrum of this PSK modulated wave deviates from the center of this wave number discriminator 23. That is, as described above, a voltage corresponding to the difference between the center frequency of the frequency discriminator 23 and the receiving human power frequency is extracted from the frequency discriminator 23.

After noise components are removed from this output voltage by a low-pass filter 22, it is applied to a voltage controlled oscillator 21 to reduce the output voltage to 0.
The oscillation frequency is controlled so that

Therefore, the frequency of the PSK modulated wave frequency-converted by the frequency converter 20 always matches the center frequency of the frequency discriminator 23.

Incidentally, the bandwidth of this low-pass filtering needs to be such that it can track the frequency fluctuation of the input PSK modulated wave.

FIG. 4 is a diagram showing an example of the configuration of the frequency discriminator shown in FIG. 2, and was filed by the present inventor on September 30, 1957.

In the figure, 30 is a high print circuit, 3I and 33 are balanced modulators, 32 is a 90 degree phase shifter, 34 is a low pass filter, 35 is a crystal oscillator, and 36 is a high pass filter. , 37 indicate multipliers, respectively.

As described above, in order to realize the present invention, a frequency discriminator with a stable center frequency and good linearity over the entire spectrum band of the PSK modulated wave is essential.

If we focus on stabilizing the center frequency, we can consider a frequency discriminator using a crystal, but this has a narrow linear range. On the other hand, if a tuning circuit consisting of a wire ring and a capacitor is used to widen the linear region, the linearity will be widened, but the stability of the center frequency will deteriorate.

This frequency discriminator satisfies both requirements and operates as follows.

The output of the crystal oscillator 35, which has an oscillation frequency that matches the expected center frequency of the received PSK modulated wave, is branched by a 90-degree phase shifter 32 into waves with a 90-degree phase difference and applied to balanced modulators 31 and 33, respectively. .

On the other hand, the received PSK modulated wave is also branched into two and applied to the balanced modulators 31 and 33, so for example, U1=ASinΔωt is on the output side of the balanced modulator 31, and E2= ACosΔωt are obtained respectively.

Here, Δω is the frequency difference between the oscillation frequency of the crystal oscillator 35 and the expected center frequency of the psK modulated wave.

The outputs E1 and E2 of each balanced modulator are passed through a low-pass filter or a high-pass filter having the same cutoff frequency (frequency at which the amplitude becomes %) and order, and then multiplied by a multiplier 37. When combined, direct current proportional to Δω can be divided.

FIG. 5 is a diagram for explaining the frequency characteristics of the low-pass filter 34 and the high-pass filter 36.

In the figure, the amplitude is shown normalized as l and 0.5. At frequency 0, the low-pass filter 34 approaches 1, and the high-pass filter 36 approaches O; conversely, as the frequency increases, the former approaches O and the latter approaches 1. These two filters have the same cutoff frequency p.

The sixth time is a diagram showing the frequency characteristics of the frequency discriminator shown in FIG. 4.

In the figure, since the cutoff frequency p of the low-pass filter and high-pass filter is the same, changing the cutoff frequency from p to p' changes the discrimination sensitivity, but ± The discrimination sensitivity between the two is constant. Therefore, by selecting this cutoff frequency, it is possible to construct a frequency discriminator that has the Nyquist frequency dissipation linearity and has a stable center frequency.

ff) As described in detail, according to the present invention, AFC operation can be performed without using a viroft signal, so it can be applied to a line with a low signal-to-noise ratio, such as a satellite communication line, for example. can.

Furthermore, an inexpensive and highly reliable satellite communication system can be realized.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a conventional AFC circuit, FIG. 2 is a diagram showing an example for implementing the present invention, FIG. 3 is a diagram for explaining the operation of FIG. 2, and FIG. 4 is a diagram showing an example of the frequency discriminator shown in FIG. 2, and FIGS. 5 and 6 are diagrams for explaining the characteristics of FIG. 4, respectively. In the figure, 20 is a frequency discriminator, 21 is a voltage controlled oscillator, 22 is a low-pass filter, 23 is a frequency discriminator, 24-
27 indicate terminals, respectively. Fig. 1 2nd ear 2θ 3rd U''A

Claims (1)

[Claims] (1) When frequency-converting the received PSK modulated wave by mixing it with the output wave of the receiving local oscillator, the frequency of the frequency-converted PSK modulated wave is controlled by controlling the oscillation frequency of the receiving local oscillator. In the automatic frequency control method to keep the frequency constant, after removing high frequency components from the output obtained by frequency discriminating the frequency of the frequency-converted PSK modulated wave with a frequency discriminator, the output of the discriminator from which the high frequency components have been removed is An automatic frequency control method characterized in that the oscillation frequency of the receiving local oscillator is controlled by. 2. The automatic frequency control system according to claim 1, wherein the frequency discriminator has linear detection sensitivity over the signal band occupied by the PSK modulated wave.