JPH0810809B2 - Automatic gain control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] The present invention relates to an automatic gain control circuit suitable for use in a digital demodulator for satellite communication, which can converge an input signal level to a required level in a short time when a burst signal is input. For the purpose of doing so, at least the burst signal having an unmodulated wave portion at the beginning is frequency-converted to baseband and analog / digital converted, and deviation information between the obtained digital conversion signal and the target value is obtained. Through a loop filter, and controlling the amplitude of the burst signal based on the output of the loop filter, in an automatic gain control circuit for automatically performing gain control. The loop filter is operated with an operation clock faster than the operation clock of the loop filter in the part. Be configured so that.

TECHNICAL FIELD The present invention relates to an automatic gain control circuit suitable for use in a digital demodulator for satellite communication.

In satellite communication, an unmodulated wave portion is inserted at the beginning of a burst signal for carrier reproduction, communication is performed, and this signal is demodulated by a digital demodulator.

In such a demodulator for satellite communication, there is a strong demand for higher transmission efficiency, and therefore the demodulator is required to operate stably in a short time. For this reason, it is necessary to set the received signal level to a target level that is short.

[Prior Art] Conventionally, a digital demodulator for satellite communication has a fourth
An automatic gain control circuit as shown in the figure is provided. In this automatic gain control circuit, a burst signal having an unmodulated wave portion at its head portion is supplied to the frequency conversion circuit 2
(This frequency conversion circuit 2 includes a mixer circuit 21 and a local oscillator 22.
, And frequency conversion to baseband,
A / D converter with analog / digital converter (A / D converter) 3.
After the conversion, the deviation calculator 4 obtains the deviation information between the digital conversion signal from the A / D converter 3 and the target value, the deviation information is passed through the loop filter 5, and the output of the loop filter 5 ( By supplying the control signal) to the variable attenuator 1 via the digital / analog converter (D / A converter) 6, the amplitude of the verse signal is controlled and the gain control is automatically performed.

The output of the A / D converter 3 is sent to a carrier reproducing circuit (not shown).

The operation clock of the A / D converter 3 is a clock with a frequency of 2fs, and the operation clock of the loop filter 5 is the above.
The one having a frequency fs lower than the operation clock of the A / D converter 3 is used.

Furthermore, the coefficient input to the loop filter 5 is
It means the coefficient that sets the pass band.

[Problems to be Solved by the Invention] However, in such a conventional automatic gain control circuit, all the parts of the burst signal (the part of the unmodulated wave at the beginning and the part of the data after that) are set at the symbol rate. Since the automatic gain control circuit is operated, there is a problem that it takes time to converge to a required level in a portion where the signal level when a burst signal is input suddenly changes.

The present invention is intended to solve such a problem, and when the burst signal is input, the input signal level can be converged to a required level in a short time.
An object is to provide an automatic gain control circuit.

[Means for Solving the Problems] FIG. 1 is a block diagram showing the principle of the present invention.

In FIG. 1, reference numeral 1 denotes a variable attenuator, which variably adjusts the reception level of a burst signal having at least a non-modulated wave portion at its head.
The reception level can be adjusted according to a signal from a loop filter 5 described later.

Reference numeral 2 is a frequency conversion circuit. The frequency conversion circuit 2 frequency-converts the output from the variable attenuator 1 into a baseband signal.

Reference numeral 3 denotes an A / D converter, which converts the analog baseband signal from the frequency conversion circuit 2 into a digital signal. The operation clock of the A / D converter 3 is a clock whose frequency is n fs.

Reference numeral 4 denotes a deviation calculator which compares the digital output, which requires the amplitude information from the A / D converter 3, with the target value and outputs the deviation information between the two.

Reference numeral 5 denotes a loop filter, and this loop filter 5 is a digital type filter, which uses the deviation information from the deviation calculator 4 as feedback information and outputs a variable attenuator control signal based on this deviation information.

A D / A converter 6 converts the digital signal from the loop filter 5 into an analog signal.

Reference numeral 7 is a clock switching circuit.
Is to selectively switch the operation clock of the loop filter 5 between two types.
And m fs (n ≧ m> 1) are used. Then, the clock switching circuit 7 loops the burst signal as the received signal at the beginning with the operation clock (frequency m fs) faster than the operation clock (frequency fs) of the loop filter 5 in the other parts. 5 is switched to operate.

Reference numeral 8 denotes a coefficient setting circuit, which can set a plurality of coefficients so that a plurality of band characteristics of the loop filter 5 can be set. The coefficient setting circuit 8 is a head portion of the burst signal. In the above, the coefficient can be changed so that the band characteristic of the loop filter 5 can be changed so as to be wider than the pass band of the loop filter 5 in the other portions.

[Operation] In the above-described automatic gain control circuit of the present invention, at least for the burst signal having the unmodulated wave portion at the beginning,
This is frequency-converted into a baseband by the frequency conversion circuit 2, A / D-converted by the A / D converter 3, and the deviation information between the obtained digital conversion signal and the target value is passed through the loop filter 5,
By supplying the output of the loop filter 5 to the variable attenuator 1 to control the amplitude of the burst signal based on the output of the loop filter 5, the gain control is automatically performed. Switches the clock switching circuit 7 to operate the loop filter 5 with an operation clock faster than the operation clock of the loop filter 5 in other portions.

Further, in the head portion of the burst signal, the coefficient from the coefficient setting circuit 8 is changed, and the band characteristic of the loop filter 5 is changed so as to have a wider band than the pass band of the loop filter 5 in other portions.

[Examples] Examples of the present invention will be described below with reference to the drawings.

FIG. 2 is a block diagram showing an embodiment of the present invention. The automatic gain control circuit shown in FIG. 2 is a burst modem automatic gain control circuit used in a small-to-medium-capacity satellite communication digital demodulator. However, this automatic gain control circuit includes a variable attenuator 1, a frequency conversion circuit 2, an A / D converter 3, a digital filter 9, a deviation calculator 4, a loop filter 5, a D / A converter 6, and a clock switching circuit 7. The coefficient setting circuit 8 is provided.

Here, the variable attenuator 1 varies the reception level of a burst signal (in this case, this burst signal is input as an IF signal) having a non-modulated wave portion (CR preamble) for reproducing a carrier at the head portion. The reception level can be adjusted according to a control signal from the loop filter 5 described later.

The frequency conversion circuit 2 frequency-converts the output from the variable attenuator 1 into a baseband signal, and includes a mixer circuit 21 and a local oscillator 22.

The A / D converter 3 converts the analog baseband signal from the frequency conversion circuit 2 into a digital signal,
Operates with a clock frequency of 2fs.

The digital filter 9 is a noise removing filter,
For example, a roll-off rate of 32% is used. This digital filter 9 also operates with a clock having a frequency of 2 fs. The output from the digital filter 9 is sent to a carrier regenerating circuit (not shown) and used as feedback information for attenuation amount control.

The deviation calculator 4 compares the digital output having the amplitude information from the digital filter 9 with the target value and outputs the deviation information between them.

The loop filter 5 is a digital type filter, and uses the deviation information from the deviation calculator 4 as feedback information and outputs a variable attenuator control signal based on this deviation information.

The D / A converter 6 converts the digital signal from the loop filter 5 into an analog signal.

The clock switching circuit 7 selectively switches two types of operation clocks of the loop filter 5, and two types of clocks, one having a frequency of fs and one having a frequency of 2fs are used. Then, the clock switching circuit 7 receives the CR preamble detection signal, and in the head portion (CR preamble) of the burst signal, the operation clock (frequency) of the loop filter 5 in the other portion (data and the like).
The loop filter 5 is switched to operate with an operation clock (frequency 2 fs) faster than fs).

The coefficient setting circuit 8 can set a plurality of coefficients so that a plurality of band characteristics (2 to 4 types) of the loop filter 5 can be set. In the preamble, the band characteristic of the loop filter 5 can be changed by changing the coefficient so that the band is wider than the pass band of the loop filter 5 in the other portion (the portion such as data).

That is, the coefficient setting circuit 8 immediately pulls in the lock immediately after receiving the CR preamble detection signal, that is, in the initial stage when the high speed clock (2fs) is selected by the clock switching circuit 7. The coefficient is set so as to widen the characteristic, and thereafter, the coefficient is changed so that the band characteristic of the loop filter 5 is gradually narrowed.

In the case of a satellite communication system that employs TDMA or the like, the transmitting and receiving sides have a common clock, so that the CR preamble detection signal is detected based on this clock.

With the above configuration, the burst signal having the unmodulated wave portion at the head portion is frequency-converted into the baseband by the frequency conversion circuit 2, A / D converted by the A / D converter 3, and further filtered by the digital filter 9. The deviation information between the obtained digital conversion signal and the target value is passed through the loop filter 5, and the output of this loop filter 5 is passed through the D / A converter 6 and the variable attenuator 1
By controlling the amplitude of the burst signal based on the output of the loop filter 5, the gain control is automatically performed.

At that time, in the leading portion of the burst signal, the clock switching circuit 7 is switched to operate the loop filter 5 with an operation clock faster than the operation clock of the loop filter 5 in other portions (clock faster than the symbol rate). At the same time, at the beginning of the burst signal, the coefficient from the coefficient setting circuit 8 is changed so that the band characteristic of the loop filter 5 becomes wider than the pass band of the loop filter 5 in other parts. change.

As a result, as shown in FIG. 3A, the input signal level can be converged to the required level within a short time when the burst signal is input.

Note that FIG. 3 (b) is a characteristic diagram when the loop filter 5 is operated with a low-speed operation clock (fs) even in the head portion of the burst signal. These FIG. 3 (a), (b)
The effect of the automatic gain control circuit is obvious by comparing the two figures.

In FIGS. 3A and 3B, the control code is a control code for the variable attenuator 1 and corresponds to the amount of attenuation, and the number of symbols represents the number sampled by the clock fs.

Further, even if the head portion of the burst signal is operated in a narrow band similar to the pass band of the loop filter 5 in other portions without switching the coefficient, the convergence is improved as compared with the conventional one. . However, preferably, in the beginning portion of the burst signal, the loop filter 5 is operated with a high-speed operation clock, and the loop filter 5 has a wider band than the pass band of the portion other than the beginning portion of the burst signal. It is also preferable to change the band characteristic of 5.

The variable attenuator 1 may be provided on the input side of the frequency conversion circuit 2 as shown in FIG. 2, but it may be further provided between the frequency conversion circuit 2 and the A / D converter 3 or the A / D converter 3 And the digital filter 9 may be provided on the output side of the digital filter 9.

[Effects of the Invention] As described in detail above, according to the automatic gain control circuit of the present invention as set forth in claim 1, at the head portion of the burst signal, the speed is higher than the operation clock of the loop filter in other portions. Since the loop filter is operated by the operation clock of (1), there is an advantage that the input signal level can be converged to the required level in a short time when the burst signal is input.

Further, in the automatic gain control circuit according to the present invention as set forth in claim 2, in the beginning portion of the burst signal, the loop filter is operated with an operation clock faster than the operation clock of the loop filter in the other portions,
Since the band characteristic of the loop filter is changed so that it is wider than the pass band of the loop filter in the part other than the head part of the burst signal, the input signal level is converged to the required level in a shorter time when the burst signal is input. There is an advantage that can be.

[Brief description of drawings]

 1 is a block diagram showing the principle of the present invention, FIG. 2 is a block diagram showing an embodiment of the present invention, FIG. 3 (a) is a characteristic diagram of the automatic gain control circuit, and FIG. FIG. 4 is a block diagram showing a conventional example. In the figure, 1 is a variable attenuator, 2 is a frequency conversion circuit, 3 is an A / D converter, 4 is a deviation calculator, 5 is a loop filter, 6 is a D / A converter, 7 is a clock switching circuit, and 8 is A coefficient setting circuit, 9 is a digital filter, 21 is a mixer circuit, and 22 is a local oscillator.

Claims (2)

[Claims] 1. A burst filter having a non-modulated wave portion at least at its head portion is frequency-converted into a base band and analog / digital converted, and deviation information between the obtained digital converted signal and a target value is loop filtered. In the automatic gain control circuit for automatically controlling the gain by controlling the amplitude of the burst signal on the basis of the output of the loop filter (5) through (5), An automatic gain control circuit, characterized in that the loop filter (5) is operated with an operation clock faster than the operation clock of the loop filter (5) in other portions. 2. The band characteristic of the loop filter (5) is changed so that the band width of the leading portion of the burst signal is wider than the pass band of the loop filter (5) in other portions. The automatic gain control circuit according to claim 1.