JPH0478213B2 - - Google Patents

Description

[Detailed Description of the Invention] [Summary] In an automatic gain control circuit, when the input multilevel amplitude modulation signal is higher than the maximum value or lower than the minimum value of the normal signal level of the analog/digital converter, the gain of the automatic gain control amplifier is increases the gain if it is between normal signal levels above and below the center threshold voltage, otherwise it retains the control voltage if it was previously selected, otherwise the control voltage By setting the value to zero, correct control can always be performed regardless of the magnitude of the input signal.

[Industrial application field]

The present invention relates to an improvement in an automatic gain control circuit for multilevel amplitude modulation signals or multilevel quadrature amplitude modulation signals used in digital communications.

FIG. 3 shows a block diagram of a multilevel amplitude modulation signal transmission system.

In the figure, on the modulation unit side, the input m-bit binary signal is converted into a baseband n (= 2 ^m ) value amplitude modulation signal (hereinafter abbreviated as n-value AM signal) by digital/analog converter 1. After that, it is applied to a modulator 3 through a low-pass filter 2.

Here, the n-value AM signal in the baseband band is converted into the n-value AM signal in the carrier band by the output of the oscillator 4.

On the demodulator side, the n value AM of the input carrier band is
The signal is converted into a baseband n-value AM signal by the demodulator 5 using the output of the carrier generation circuit 6.
After passing through the low-pass filter 7, a part of the clock is extracted by the clock recovery circuit 10, and the rest is converted to a normal signal level by the automatic gain control amplifier (hereinafter abbreviated as AGC amplifier) 8, and then sent to the analog/digital converter ( The signal is converted into an m-bit binary signal by an A/D converter (hereinafter abbreviated as A/D converter) 9.

At this time, it includes an A/D converter and an AGC amplifier.
The AGC circuit needs to be controlled correctly regardless of the amplitude of the input signal.

[Conventional technology]

FIG. 4 is a block diagram of a conventional example of an AGC circuit, and FIG. 5 is an explanatory diagram of the operation of FIG. 4. Please enter
The AM signal is 8-value.

In Figure 4, the input 8-level AM signal is
The signal is applied to an A/D converter 9 through an AGC amplifier 8.

Here, as shown in FIG.
The normal signal level is a voltage at a value of d/2 above and below the threshold voltage (d is the value between the threshold voltages). For example, in the case of level ◎, a signal of 110 is output from a1, a2, and a3, and another threshold voltage (equal to the normal level) is used to differentiate this level from the normal level (black circle level). Detects the deviation (referred to as an error signal, indicated by ε), controls the gain of the AGC amplifier 8 using ε and a1,
Compensate for this deviation.

For example, 8 values slightly lower than the normal signal level.
When an AM signal is input, a1 is 1 and ε is 0 for a signal above V4, and a1 is 0 and ε is 1 for a signal below V4, so 1 from EX-OR circuit 11 is an integrator. Added to 12. Integrator 1
2 integrates this to create a control signal, and uses this to control, for example, to increase the gain of the AGC amplifier and generate 8 values.
Set the AM signal level to normal. If the input signal level is higher than the normal level, the opposite is true; if it is above V ₄ , a ₁ = 1, ε = 1; if it is below V ₄ , a ₁ = 0, ε = 0, and the EX-OR circuit The output becomes 0 and the gain of the AGC amplifier is controlled to be lowered.

Note that when the output signal of EX-OR applied to the integrator 12 is 1 or 0, the 8-level AM signal becomes the normal level when the level corresponds to 0.5 (1 and 0 are input with the same probability).

It is also assumed that the input signal applied to the A/D converter 9 has no offset, and the positive and negative parts move symmetrically.

[Problem that the invention seeks to solve]

As explained above, when the level of the input 8-level AM signal is slightly lower than the normal level, ε is
It becomes 0 in the part above V4 and 1 in the part below.

On the other hand, each amplitude of the input multilevel amplitude modulation signal is output so as to be averaged and distributed to each normal signal level of the analog/digital converter within a predetermined transmission time, so the amplitude is equal to the normal signal level. The gain of the amplifier is controlled so that the normal signal level is obtained when the signal level is different from the normal signal level. However, as shown in Figure 5, the input signal level changes rapidly due to disturbances on the transmission path, etc.
Assume that the level of the value AM signal (indicated by an x in the figure) becomes very small and a signal with a level difference αd between adjacent signal points is input (α<1).

At this time, the output of the EX-OR circuit 11 has the same number of 1s and 0s as shown in Figure 5, so even if this is integrated by the integrator 12, the level of the input 8-level AM signal is not normal. As a result, the gain of the AGC amplifier is not controlled.

That is, depending on the state of α, the number of 0's is greater than 1. The number of 1's is greater than 0.
There are three possible situations in which the number is the same,
In the case of , the control sensitivity decreases because the number of 0s decreases, and in the case of , control is performed in the opposite direction, and in the case of , there is no control.

Another problem is that if the difference between the input signal level and the normal level is too large, control will not be performed properly.

[Means for solving problems]

The above problems can be solved by the present invention, which includes an automatic gain control amplifier that amplifies the input multilevel amplitude modulation signal, an analog/digital converter that converts the output into a digital signal, and an automatic gain control amplifier that selects the converted output digital signal. The analog/digital converter includes a selection circuit that generates a control voltage for the gain control amplifier, and the analog/digital converter converts the multi-level amplitude modulation signal from the automatic gain control amplifier to the intermediate level of each of the normal signal levels divided into a plurality of equal parts. When the input multilevel amplitude modulation signal exceeds the maximum normal signal level in the analog/digital converter, or below the minimum normal signal level, the selection circuit converts it into a binary digital signal string at the threshold voltage of the point A control signal is generated to lower the gain of the automatic gain control amplifier based on the binary digital signal when It initially generates a control voltage that increases the gain of the automatic gain control amplifier, otherwise holds the control voltage if there is a previously selected control signal, and otherwise sets the control voltage to zero. This problem is solved by the characteristic automatic gain control circuit.

[Effect]

In the present invention, the gain of the amplifier is determined based on the binary digital signal output from the A/D converter when the level of the input multilevel AM signal becomes higher than the maximum value or lower than the minimum value of the normal signal level. the gain of the amplifier is increased when within normal signal levels above and below the center threshold voltage, and otherwise the previously selected control signal, if any, is maintained; In other cases, the control voltage is set to zero.

In other words, we set only two thresholds and did not look at intermediate levels, so the multi-value input at this time
When the level of the AM signal becomes small, the EX-OR output is only 1, and when it becomes large, only 0 is output, so there is no error.

Therefore, normal control can be performed regardless of the input level.

〔Example〕

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is an explanatory diagram of the operation of FIG. 1, and FIG.
-d show diagrams explaining the operation when the input signal level goes up and down a little. The same reference numerals indicate the same objects throughout the drawings, and the parts added in the embodiment of the present invention are indicated by dotted lines.

Therefore, the operation of the present invention will be explained with reference to FIG.

First, as shown in Figure 2a, input the 8-value AM
The signal level is the maximum normal signal level of 111 and
A control signal is formed by a ₁ and ε when the value is above and below 000 or between the minimum value 100 and 011 (shaded area).

Therefore, when an 8-level AM signal whose level is significantly lower than the normal signal level (the x mark in the figure indicates the level) is input to the A/D converter 9, as in the conventional example, the selection circuit 13 receives 1000. Since 1 is sent to the integrator 12 only when 0111 is input, the same as above
The AGC amplifier is controlled to increase its gain.
Incidentally, when there is no input from the selection circuit 13 to the integrator 12, the previous value is held, for example.

Here, the configuration of the selection circuit is, for example, configured with a read-only memory, and uses a 4-bit signal output from an A/D converter as an address to select 1000, 1000,
It is sufficient to output 1 when 0110 is input, and 0 when 1111,0000 is input.

Further, when there is no input from the selection circuit 13 to the integrator 12, the previous value is held, for example.

Next, an 8-level AM signal having the level shown in Fig. 2b is input, and when the level of this signal becomes small, 1 is output from the selection circuit 13 at points a, d, e, and f as shown in Fig. 2c. Since it is added to the integrator 12, the AGC
Amplifier 8 is controlled to increase the gain and the center
Stable at 0.5.

On the other hand, when the level in Fig. 2b becomes large, 0 is added to the integrator 12 as shown in Fig. 2d.
The AGC amplifier is controlled so that the gain is reduced.

It is assumed that the amplitude of the input signal takes a random value and that the probability that the amplitude of the signal takes only between the diagonal lines is very small, but this condition is satisfied in practice without any problem.

The circuit can also be used for carrier band amplitude modulated signals or quadrature amplitude modulated signals and tap coefficient transformers for transversal automatic equalizers. Furthermore, AGC
Even if the amplifier is placed in the carrier band, exactly the same effect can be obtained.

〔Effect of the invention〕

As described above in detail, the present invention has the advantage that control is always performed correctly even if the difference between the input level and the normal level is large or small.

This improves the deterioration of error rate and line quality.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is an explanatory diagram of the operation of Fig. 1, Fig. 3 is a block diagram of a multilevel AM signal transmission system, Fig. 4 is a block diagram of a conventional example, FIG. 5 shows an explanatory diagram of the operation of FIG. 4. In the figure, 8 is an AGC amplifier, 9 is an A/D converter, 12 is an integrator, and 13 is a selection circuit.

Claims (1)

[Claims] 1 An automatic gain control amplifier that amplifies the input multilevel amplitude modulation signal, an analog/digital converter that converts the output into a digital signal, and a control voltage for the automatic gain control amplifier by selecting the converted output digital signal. The analog/digital converter converts the multilevel amplitude modulation signal from the automatic gain control amplifier at the threshold voltage at the midpoint of each of the normal signal levels divided into a plurality of equal parts. Convert to a binary digital signal string,
The selection circuit selects whether the input multilevel amplitude modulation signal is analog/
A control signal is generated to lower the gain of the automatic gain control amplifier based on the binary digital signal when it exceeds the maximum normal signal level in the digital converter, or when it falls below the minimum normal signal level. Generates a control voltage that increases the gain of an automatic gain control amplifier based on a binary digital signal when it is within normal signal levels above and below a threshold voltage, and otherwise when a previously selected control signal is present. An automatic gain control circuit characterized in that the control voltage is maintained at one time, and the control voltage is set to zero in other cases.