JPH02222343A - Afc system - Google Patents

Abstract

PURPOSE:To prevent an excessive control of a local signal and lock-off of carrier tracking by always executing a level detection of a receiving signal, varying a time constant for averaging a frequency error in accordance with the result of its output and controlling the output frequency of a local signal generating part. CONSTITUTION:For instance, a level detector 33 inputs a signal of the same timing as that of a signal inputted to a phase controller 34, detects a receiving signal level at the present time point, and thereafter, adds its output to a loop filter 36. In the loop filter 36, a relation of a C/N level obtained in advance and a time constant is provided as a table, and in accordance with an output of the level detector 33, the time constant for averaging an output of a phase error detector 35 is changed. Subsequently, by controlling an output frequency of a local signal generating part 4, the fluctuation quantity of the output frequency at the time of low C/N, etc., of a frequency converting part 1 is suppressed in a prescribed band. In such a way, an excessive control to the local signal generating part 4 can be prevented, or the frequency of lock-off of carrier tracking in a demodulating part 3 can be decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to AFC in digital demodulation of PSK waves.

(Prior art) Regarding AFC in conventional PSK wave demodulation, after pulling in the -degree carrier, the frequency fluctuation of the received signal is caused by phase error etc. that occurs when phase control is applied to the filter output signal in the demodulation section. The output frequency of the local signal is controlled based solely on the detected amount. BPS
During demodulation of the K wave, a phase error detector 35 detects the amount of frequency fluctuation from the two outputs 39a and 39b, and outputs it as an error voltage.

(Problem to be solved by the invention) In such a control system, in a carrierless state during guard time due to deterioration of line quality such as fading and blocking or burst signals, the reliability of information for the local signal generation unit is reduced. is not enough,
This will cause excessive control of the output frequency of the local signal.

(Means for Solving the Problems) In order to solve the above problems, the present invention demodulates the frequency-converted PSK wave, detects the frequency difference between the received signal and the local signal, and constantly detects the level of the received signal. AFC that controls the output frequency of the local signal generator by varying the time constant for frequency error averaging according to the output result.
It is a method.

(Function) The output of the phase error detector, which detects the amount of frequency fluctuation from the received signal in which the noise signal and the desired signal are added, generally does not have sufficient reliability depending on the C/N level. , the current C/N level is detected by a level detector, the time constant of the loop filter is varied according to the output, and sometimes the output of the loop filter is held. Preventing overcontrol,
Alternatively, the frequency of carrier tracking loss of lock in the demodulator is reduced. Examples will be described in detail below with reference to the drawings.

(Embodiment) FIG. 1 is a basic block diagram showing an embodiment of the present invention. In the figure, reference numeral 1 designates a frequency conversion section 2, a filter 3, a demodulation section 4, and a local signal generation section, which constitute an AFC loop. Further, a demodulated output signal 6 of the received PSK wave 5 is extracted from the demodulator 3.

FIG. 2 is a block diagram showing the inside of the demodulator 3 when the present invention is applied to BPSK.

Raw signal 37) is input to the LPF 32 for removal of unnecessary frequency components. Thereafter, the output signal 38 of the LPF 32 is input to a level detector 33 and a phase controller 34. Phase controller 3
4, the phase of the received signal is controlled, and the demodulated output signal 6 and the cosine component 39a and sine component 39b of the demodulated output signal 6 are
Output. The phase error detector 35 detects the amount of frequency fluctuation of the received signal from the two outputs 39a and 39b of the phase controller 34, and provides the output to the loop filter 36.

On the other hand, the level detector 33 inputs a signal with the same timing as the manually input signal to the phase controller 34, detects the current received signal level (C/N level and presence or absence of carrier), and then sends the output to the loop filter 34. Add to. The loop filter 36 has a table containing the relationship between the C/N level and the time constant obtained in advance, and changes the time constant for averaging the output of the phase error detector 35 according to the output of the level detector 33 ( (including holding), and by controlling the output frequency of the local signal generator 4, the amount of variation in the output frequency of the frequency converter 1 at low C/N, etc. is suppressed within a certain band.

Note that in the level detector 33 in this embodiment, L
A method such as amplitude squaring of the output of the PF 32 can be applied, and the detection location can also be applied to, for example, the output of the phase controller 34 other than immediately after the LPF 32.
An example in this case is shown in FIG.

In addition, although BPSK was described in this embodiment, QPSK
It is also applicable to other PSKs such as.

(Effects of the Invention) As described above, according to the present invention, it is possible to prevent excessive control of local signals and loss of lock of carrier tracking in situations such as level fluctuation of a received signal or lack of carrier during guard time of a burst signal, and also, C/
It is also possible to shorten the re-retraction time when returning to the N level.

[Brief explanation of the drawing]

Fig. 1 is a basic block diagram of an AFC loop showing an embodiment of the present invention, Fig. 2 is a block diagram of a demodulation section when the present invention is applied to BPSK, and Fig. 3 is a change in the level detection location in the present invention. A block diagram showing an example, Fig. 4 is a conventional AF
FIG. 2 is a block diagram of a demodulator in a C loop. DESCRIPTION OF SYMBOLS 1... Frequency converter, 2... Filter, 3... Demodulator, 4... Local signal generator, 5... Received signal, 6... Demodulated output signal, 31... A /D converter, 3
2...LPF, 33...Level detector, 34...
Phase controller, 35... Phase error detector, 36... Loop filter, 37° 38... Complex signal, 39a...
・Demodulated output signal cos component, b...Demodulated output signal si
n component.

Claims (1)

[Claims] A frequency conversion section that converts the frequency of the received PSK wave, a filter that removes unnecessary frequency components from the output of the frequency conversion section, and a demodulation section that demodulates the received PSK wave and detects the frequency difference between the received signal and the local signal from the filter output. , in an AFC loop consisting of a local signal generation section that varies the output frequency in order to convert the frequency of the received signal to a predetermined frequency by the output of the demodulation section, the demodulation section performs the above processing, detects the level of the received signal, and detects the level of the received signal; An AFC method characterized by providing a loop filter that changes the time constant for frequency error averaging according to the output.