JP2689714B2 - Threshold control circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a threshold control circuit in an analog-digital converter (hereinafter referred to as an AD converter), and in particular, it automatically corrects a threshold. Threshold control circuit.

Here, the threshold value is a threshold level (usually the zero level of the central value of the quantization step) that serves as a reference for a predetermined quantization step when quantizing an analog signal in the AD conversion circuit. This is an improvement in the means for correcting this threshold error.

[Conventional technology]

As shown in the block diagram of FIG. 3, the conventional threshold control circuit includes an AD converter 4, an integrator 5, a reference level generator 6,
It is composed of a subtracter 7 and an adder 8. Next, the conventional operation of correcting the threshold level will be described with reference to FIGS.
This will be described with reference to the explanatory views of (c) and (d) and FIG.
4 (a), (b) and FIG. 4 (d), in the enlarged view and the corresponding explanatory diagram of the quantization level and the output code of the AD converter, the AD conversion is performed. The encoded output bits of the device 4 are quantized into 16 steps (= 2 ⁴ ) when 4 bits are output as an encoding condition. That is, the fourth
As shown in FIG. 7D, the output of the AD converter is converted to the quantization level and is 0 level (the output of the AD converter is "000".
From 0 ") to 16 levels (" 1111 "in the same). Here, the center level is 8 levels (" 1000 "at the output of the AD converter).

Here, the input signal consisting of a pulse train of "1,0" input to the A / D converter 4 becomes a waveform dulled by a filter or the like like a signal obtained by demodulating a digitally modulated signal, and at the sampling point, the center level ( (8 levels) The signals are distributed around two points equidistant from each other. Here, as shown in FIG. 4 (d), the signal is dispersed centered on either "0100" (corresponding to level 4) or "1100" (corresponding to level 12).

Next, the operation of each unit in the state where no offset occurs in the signal will be described. Now, the waveform of the analog input signal A1 as shown in FIG. 4 (a) is quantized in the A / D converter 4 from, for example, "level 0" to "level 15" (however, the center is "8"), The digital signal corresponding to the step is converted into a binary code. The quantization level at the sampling point is 2
It is input as a digital signal A2 in the form of a binary code to an integrator 5 which calculates the average of the input signals. In the integrator 5, after the binary-coded signals at a plurality of sampling points are integrated (the original "0100" level and "1100" level are averaged and input, the integrator output is "1000", that is, the center. Becomes)
In the subtractor 7, the output C1 from the reference level generator
(“1000” = threshold value) and the threshold value error signal B3 is output. That is, FIG. 4 (C) is a schematic analog display of the threshold value error signal B3 which is the difference. The adder 8 inputs the detected threshold value error signal B3 and the original digital signal A2, corrects the error of the digital signal A2, and outputs a correction signal A3.

Next, the signal flow and operation of each circuit shown in FIG. 3 will be described. Now, the analog signal A1 input from the input terminal 1 is converted into a digital signal A2 by the AD converter 2, and then branched into two systems. One of the digital signals is input to the integrator 5, and its output signal B2 is subtracted from the signal C1 from the reference level generator 6 by the subtractor 7, and AD
It becomes the threshold error signal B3 of the converter input A1. D-
The two input signals of the other digital signal A2 from the A converter 4 and the threshold value error signal B3 are added by the adder 8 to become the signal A3 in which the deviation of the threshold value is corrected and output from the output terminal 9. To be done. Assuming that the change of the amplitude of the input analog signal A1 of the AD converter 4 with respect to time is f (t), the amplitude of integration (average level of a certain time interval) over a certain time interval is shown in FIG. 4 (a). As shown in (1), it does not always become almost constant, and when expressed by the equation, the condition of the equation (1) is not satisfied.

∫f (t) dt = constant (1) Therefore, as shown in FIG. 4 (c), the output level B2 of the integrator 5 is not balanced between the amplitudes + and − due to the time interval. However, the condition of the expression (1) is not satisfied, and the threshold error signal B tends to expand.

[Problems to be solved by the invention]

The above-described conventional threshold control circuit integrates the threshold control circuit when the input signal f (t) of the AD converter described above does not always satisfy the condition shown in the equation (1). Vessels have the disadvantage that they may perform incorrect control. For example, even in an example of soft decision discrimination such as a digital demodulator using this conventional example, the conventional threshold value control circuit cannot be used when a fixed bit of "0" or "1" level continues. Get nervous.

[Means for solving the problem]

Comparing the threshold value which is the reference of the quantization step of the analog input signal consisting of the "1,0" pulse train in the analog-digital converter with the reference threshold signal of the reference threshold level generating circuit which is provided in advance. In a threshold value control circuit for correcting an error, the values of the first and second digital signals obtained by digitally converting the analog input signal at sampling points of a predetermined cycle are converted into digital signal values corresponding to the reference threshold value signal. It has a control means for detecting a point that crosses a value and using only the second digital signal as a signal for threshold value error correction.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of one embodiment of the present invention. First
The embodiment shown in the figure has the same configuration as the conventional example except that a zero-point detector 1 and a switch 2 are added to the conventional circuit shown in FIG.
In FIG. 1, an analog signal A1 input from an input terminal 3 is converted into a digital signal A2 by an AD converter 4 and then branched into three systems. The digital signal A2-1 of the first system is input to the zero point detector 1 together with the digital signal C1 from the reference level generator 6, detects a zero point described later, and outputs the control signal D1 of the switch 2. The digital signal A2-2 of the second system is input to the switch 2, and the control signal D1 causes, for example, the gate to be closed and only necessary information be selectively passed and input to the integrator 5. The digital signal B2 output from the integrator is subtracted from the digital signal C1 of the reference level by the subtractor 7, and becomes the threshold error signal B3. The digital signal A2 of the third system is the threshold error signal.
Addition is performed by the adder 8 of B3, and a signal A3 in which the deviation of the threshold value is corrected is output from the output terminal 9.

Next, the operation of this embodiment will be further described with reference to FIG.
Description will be given by comparing the explanatory views of (b) and (c) with FIG. The analog input signal A and sampling points shown in FIGS. 2 (a) and 2 (b) are the same as those in FIGS. 4 (a) and 4 (b). The zero-point detector 1 of this embodiment detects the point where the waveform of the analog input signal A1 crosses the threshold level, as shown in FIG. 2 (c). Specifically, the binary code that is the digital signal A2-1 input to the zero detector 1 and the binary code of the digital signal C1 of the reference level generator 6, that is, the reference threshold binary code are input. , Digital signal A2
The point where -1 crosses the digital signal C1 is detected. Now 4
Taking a binary code having a bit structure as an example, a reference threshold value 2
If the value code (corresponding to C1) is "1000" (corresponding to level 8), the input binary code (corresponding to A2-1) is "0111".
If "1001" (corresponding to 9) is input next to (corresponding to 7), it is determined that the input signal has crossed the threshold level, and the control signal D1 is detected. The switch 2 is controlled by the control signal D1 to immediately follow the sampling point, that is, FIG. 2 (c).
Only the signals of sampling Nos. 2, 3, 6, 8, 13, and 14 shown in are input to the integrator 5. Therefore, a sampling point where the levels are continuous as shown in FIG. 4 is not input, and as shown in FIG. 2 (d), a true threshold error signal B3 is generated to perform correct control. Be done.

〔The invention's effect〕

As described above, the present invention provides the conventional threshold control circuit with the zero-point detector and the switch controlled by the zero-point detector, whereby the threshold error can be corrected correctly. is there. Therefore, there is an effect that it can be used for the soft decision discrimination of the digital demodulator described above.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention, and FIG.
(D) is an explanatory view of this embodiment, FIG. 3 is a block diagram of a conventional threshold control circuit, and FIG. 4 is an explanatory view of a conventional example. 1 ... Zero detector, 2 ... Switch, 3 ... Input terminal, 4
... AD converter, 5 ... integrator, 6 ... reference level generator, 7 ... subtractor, 8 ... adder, 9 ... output terminal.

Claims (2)

(57) [Claims] 1. A "1,0" in an analog-digital converter.
In a threshold control circuit for correcting an error by comparing a threshold value serving as a reference of a quantization step of an analog input signal consisting of Detecting the point where the values of the first and second digital signals obtained by digitally converting the analog input signal at sampling points of a predetermined cycle cross the value of the digital signal corresponding to the reference threshold signal, 2. A threshold control circuit having control means for using only the digital signal of as a signal for threshold error correction. 2. The control means inputs the first branch signal in the digital signal output from the AD converter, compares it with the reference threshold signal of the reference threshold level generating circuit, and outputs a control signal. A zero point detector; and a switch that inputs the second branch signal in the digital signal output from the AD converter and passes the second branch signal only when the control signal is input. The threshold control circuit according to claim 1.