JPH0156575B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic gain control circuit for maintaining a received signal level at a constant amplitude in a data transmission system or the like.

In data transmission using voice band telephone lines, the signal attenuation due to the subscriber line changes with each line connection, so automatic gain control (AGC) is used in the first stage of the data receiving device to keep the received signal level constant.
It is necessary to install a circuit. Recently, voice-band telephone line modems (modems) have tended to be constructed of digital LSIs, microprocessors, etc. due to advances in semiconductor technology, and digital signal processing technology has also begun to be applied to AGC circuits. However, compared to analog AGC circuits, digital AGC circuits provide stable operation without adjustment, but gain settings are discrete or step-like, so the received signal level can be determined using a single reference level. When the gain is determined and the gain is controlled based on the determination, the gain oscillates at the minimum step of the gain with respect to the reference level, generating noise. For this reason, digitized AGC circuits have had the disadvantage of lower performance than analog AGC circuits.

The present invention has been devised in view of these conventional problems, and an object of the present invention is to provide a high-performance digitized AGC circuit in which gain oscillation does not occur.

The automatic gain control circuit according to the present invention converts the output signal of a variable gain circuit that changes the amplitude of an input analog signal into a digital signal using an A/D conversion circuit, and then converts the DC component contained in the output signal into a digital signal using a DC removal filter. and further determine the average level of the output signal by comparing it with at least two reference levels, and based on the determination result, when the average level is greater than the larger of the at least two reference levels. The gain of the variable gain circuit is controlled so that the gain is lowered, the gain is increased when it is lower than the smaller reference level, and the gain is kept constant when it is between these two reference levels. be.

Therefore, according to the present invention, the gain becomes constant when the average level converges between two arbitrary reference levels, so that no gain fluctuation occurs and stable AGC operation can be achieved. can.

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, and an input terminal 1 receives an analog signal such as a received signal of a data receiving device. This input analog signal is subjected to amplitude control by a variable gain circuit 2 whose gain is changed by external control, and then converted to a digital signal by an A/D conversion circuit 3 and provided to a digital arithmetic circuit 4. That is, the output signal of the A/D conversion circuit 3 is removed from the DC component generated in the A/D conversion circuit 3 by the DC removal filter 5 constituted by a digital filter, and then the output signal is placed at the subsequent stage of the AGC circuit. The signal is output to a digital signal processing circuit such as a demodulation circuit via an output terminal 6, and the average level is detected by an average level detection means consisting of an absolute value circuit 7 and a low-pass filter 8 constituted by a digital filter. be done. The output of the low-pass filter 8 is passed through two level comparison circuits 9 ₁ , which are provided with different reference levels l ₁ , l ₂ .
The level is judged by a level judgment circuit 10 consisting of ₉₂ , and the judgment result is given to a control signal generation circuit 11 which is a gain control means. Control signal generation circuit 1
1 generates a control signal for controlling the gain of the variable gain circuit 2 in a stepwise manner based on the above determination result.

The operation of this AGC circuit will be explained using FIG. 2. FIG. 2a shows the operation of a conventional digitizing AGC circuit having only one reference level, and FIG. 2b shows the operation of a conventional digitizing AGC circuit having _{only one} reference level, _and FIG.
This shows the operation of the AGC circuit.

In the case of FIG. 2a, since the detected average level 21 is above the reference level l at time t ₀ ,
A control signal that lowers the gain of the variable gain circuit is output, and the average level detected at time _t1 becomes smaller than the level at time _t0 . Since the average level at time t ₁ is still above the reference level l, the average level further decreases at time t ₂ to become a level lower than the reference level l. Therefore, a control signal for increasing the gain of the variable gain circuit is output, and at time _t3 , the average level becomes higher than the reference level. In the conventional digitized AGC circuit that has only one reference level, the average level oscillates stepwise at the minimum gain control step around the reference level, and this level change is reflected in the digitized AGC circuit. This will be output as newly generated noise.

On the other hand, in the operation of the AGC circuit shown in FIG. 1, the average level 22 is set at times t ₀ and t ₁ as shown in FIG.
The reference level L ₁ is compared with the reference level L ₁ above.
approaches and enters between reference levels l ₁ and l ₂ at time t ₂ . Now, the average level 22 is the reference level for the gain control of the variable gain circuit 2 by the control signal generation circuit 11.
If we reduce the gain when it is above l ₁ , increase the gain when it is below l ₂ , and keep the gain unchanged when it is between the reference levels l ₁ and l ₂ , we get
As shown in FIG. 2b, the average signal level of the received signal becomes a constant value after time _t2 , and the step-like oscillations seen in the conventional digitized AGC circuit no longer occur.

According to the present invention, after the DC component included in the output signal after A/D conversion is removed by the DC removal filter, the average level is detected and AGC is performed, so there is no error at the 0 level during A/D conversion. are all removed by the DC removal filter, and finally a signal based on an accurate 0 level is obtained. Moreover, according to the present invention, since the DC component generated in the digital signal during A/D conversion is removed by the DC removal filter, it is possible to accurately detect the average level that reflects the amplitude value. This has the extraordinary effect of making extremely stable AGC possible.

Note that in the embodiment shown in FIG.
Although the present invention is constructed with one level comparison circuit, it is not necessarily necessary to install two level comparison circuits, and multiple comparisons between a plurality of reference levels and an average level may be performed using one level comparison circuit in a time-sharing manner. In addition, although the above embodiment has been explained with two reference levels, the number of reference levels is further increased and the amount of gain control (step width of gain control) by the control signal generation circuit is increased.
By varying the average level according to the judgment result of the level judgment circuit, that is, depending on where the average level is in the level area set as the reference level,
It is also possible to shorten the time required until gain control becomes stable.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a gain control circuit according to an embodiment of the present invention, and FIG. 2 is a waveform diagram for explaining its operation. 1...Input terminal, 2...Variable gain circuit, 3...
A/D conversion circuit, 4...Digital calculation circuit, 5
...DC removal filter, 6...Output terminal, 7...
Absolute value circuit, 8...Low pass filter, 9 ₁ , 9 ₂
... Level comparison circuit, 10 ... Level judgment circuit,
11...Control signal generation circuit.

Claims (1)

[Claims] 1. A variable gain circuit that changes the amplitude of an input analog signal by controlling the gain; an A/D conversion circuit that converts the output signal of the variable gain circuit into a digital signal; a DC removal filter that removes a DC component included in the output signal of the D conversion circuit; an average level detection means that detects the average level of the output signal of the DC removal filter; and at least two Level determining means for determining the level by comparing it with a reference level; and gain controlling means for controlling the gain of the variable gain circuit in a stepwise manner based on the determination result of the level determining means, the gain controlling means determining the average level. When the average level detected by the detection means is larger than the larger one of the at least two reference levels, the gain is lowered, and when it is smaller than the smaller one of the at least two reference levels, the gain is increased;
An automatic gain control circuit characterized in that the gain is kept constant when the gain is between two reference levels. 2. The level determining means compares the average level detected by the average level detecting means with at least two reference levels using one level comparison circuit in a time-sharing manner. Automatic gain control circuit according to range 1.