JPS60214128A - Automatic threshold value adjusting circuit - Google Patents

Abstract

PURPOSE:To adjust a threshold value with high accuracy by providing a reference voltage lower than a non-signal level of an input signal and a reference voltage higher than the signal level and holding the peak of an intermediate level between the low reference voltage and the input signal to process the result. CONSTITUTION:The 1st reference voltage outputted from a reference voltage source 11 is lowered than the non-signal level of a point A applied and the 2nd reference voltage at a point C is set while being increased for the share. The intermediate level between voltages between points A, B is subjected to peak hold by a peak hold circuit 12, a voltage at an output point D and a voltage at the point C are applied to an inverting input of a comparator 14 and the input signal is applied to a noninverting input of the comparator 14. The comparator 14 is provided with transistors (TRs) 15, 16, 17, the TRs 16, 17 are connected in parallel and the voltage at the points D, C is inputted individually. Thus, even if an error of fluctuation exists in the non-signal input level and the reference voltage, since the effective input level at the inverting input of the comparator 14 is not so much spaced away from an ideal value, the threshold value with high accuracy is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of the Invention The present invention relates to an automatic threshold adjustment (ATC) circuit used in an optical transmission receiving circuit or the like.

(B) Background of the Invention In general, in optical transmission receiving circuits, in order to obtain a clean output waveform of received data from input signals of various levels, a threshold value is changed according to the level of the input signal.
A TC circuit is used.

A conventionally used ATC circuit is shown in FIG. This ATC circuit divides the difference voltage between the voltage level of input signal i (at point A) and the voltage output from point B of reference voltage source 1 into two parts using resistors R1 and R2, and holds the peak using peak hold circuit 2. This output is applied to the (=) input terminal of the comparator 4 via the OR circuit 3, while the point C of the reference voltage source 1 is applied to the (=) input terminal of the comparator 4.
Therefore, a reference voltage greater than point B is output, and this reference voltage is applied to the (-) input terminal of the comparator 4 via the OR circuit 3, thereby providing a threshold voltage to the comparator 4. . In this ATC circuit, the reference voltage at point B is set to be the same value as the level at point A when there is no signal, and while the input signal is smaller than a certain value, the output of peak hold circuit 2, that is, point D Since the level of is lower than the reference voltage at the 0 point, during this period, the constant level reference voltage at the 0 point is applied to the (-) input terminal of the comparator 4 via the OR circuit 3, and the input signal exceeds a certain level. Since the output of the peak bold circuit 2 exceeds the reference voltage at the 0 point, this peak hold value is passed through the OR circuit 3 to the comparator 4 as the threshold voltage.
is input to the (-) input terminal of. Since the output of the peak hold circuit 2 increases as the input signal level increases, when the input signal exceeds a certain level, a threshold value corresponding to the input signal is input to the comparator 4.

However, this conventional ATC circuit has the drawback that it is very difficult to OR the peak hold circuit output and the reference voltage.In order to overcome this drawback, the inventors of this application developed an OR circuit. A separate application was filed for a receiving circuit that includes an unused ATC circuit.

The ATC circuit according to this separate application is shown in FIG.

The ATC circuit shown in FIG. 2 includes a reference voltage source 11 that outputs a first reference voltage (point B) and a second reference voltage source (point 0);
It is similar to the circuit shown in FIG. 1 in that it includes voltage dividing resistors R1l and R12 for voltage division, a peak hold circuit 12, and a comparator 14, but the OR circuit is omitted and the comparator 14 and 3
transistors 15, 16, and 17, of which 2
The transistors 16 and 17 are connected in parallel, and the output of the peak hold circuit 12 and the reference voltage source 11 are connected to both bases.
This method is characterized in that the second reference voltages are input individually.

In this ATC circuit, the voltage levels of various parts of the circuit with respect to changes in the input signal are shown in FIG. As shown in the figure, the level of the output of the peak hold circuit 12 (point D) is equal to the level of point B when there is no signal when viewed from below, and appears to rise linearly with the rise of the input signal.
Two transistors 16 and 17 are connected in parallel to the (-) input end of the comparator 14, and a larger current flows therethrough than that of the transistor 15, so that the effective (-) ) The input of the input terminal example is shown by the broken line D.

Also, Figure 3 shows a case where there is no error in the reference voltage and input voltage, but in reality there is usually an error in the no-signal output signal level and the reference voltage, and there are errors in these. FIG. 4 shows the voltage levels of various parts of the circuit of FIG. 2 in this case. In the same figure, Amax and Am1n each have an input voltage (level at point A) of + with respect to the reference voltage.
This is the peak value when the value shifts to the negative side. Dmaχ,D
min is the output voltage of the peak hold circuit 12 corresponding to Amax and Am1n, respectively. Also Emax
, Emin is the input of the (-) input end of the effective comparator, and D'may and D'min are the axes ax and D'min, respectively.
, Am1n is the ideal peak hold circuit output voltage. Emax, Emin is D' max,
It is desirable to be closer to D'min, but here Emax is relatively close to D'max, but l
Emin is quite far from D'min.

Generally, the above error can be ignored for the amplitude of human power, but if the error between the no-signal input level and the reference voltage is large for the input signal, Emin for Am1n should be closer to D'min. is requested.

(c) Purpose of the Invention In view of the above, the purpose of the present invention is to eliminate the need to OR the peak bold circuit output and the reference voltage outside the comparator.
Moreover, even if there is an error between the no-signal input level and the reference voltage,
The objective is to provide an ATC circuit with adjustable thresholds.

(d) Structure and Effect of the Invention In order to achieve the above object, the automatic threshold adjustment circuit of the present invention sets the reference voltage source connected to the dividing resistor for obtaining the peak hold voltage to be lower than the no-signal input level. I try to do that. That is, the automatic threshold adjustment circuit of the present invention includes a reference voltage source that outputs a first reference voltage that is lower than the no-signal level of an input signal and a second reference voltage that is higher than the first reference voltage. , a voltage dividing means for deriving an intermediate level between the input signal and the first reference voltage, a peak hold circuit for holding the peak value of the voltage from the voltage dividing means, and receiving the input signal at one input terminal. , the output of the peak hold circuit and the second
According to the automatic threshold adjustment circuit of the present invention, there is no need to provide an OR circuit outside the comparator, and it is possible to adjust the no-signal input level and the reference voltage. Even if a fluctuation error occurs, the dynamic input level at the input end of the comparator does not deviate much from the ideal value, so a highly accurate threshold can be obtained.

(e) Description of examples Next, embodiments of the invention will be described.

The circuit configuration of an ATC circuit according to one embodiment of the present invention is substantially the same as that shown in FIG.

This embodiment circuit differs in that the voltage level of each part of the circuit is different from that of the circuit according to the prior application shown in FIG. The second reference voltage, that is, the voltage at point 0, is raised by that amount.

By setting the reference voltage of the reference voltage source 11 in this way, the voltage level of each part of the circuit with respect to the input voltage becomes as shown in FIG.

As is clear from this figure, by lowering the level at point B, Emin approaches D'min compared to FIG. 4. On the contrary, Emax and D'may
The difference between the two becomes large on the large input side, but as the input becomes large, the influence of the ATC level becomes smaller, so there is no particular B problem.

In the example circuit shown in Fig. 2, an NPN type transistor is used for the comparator transistor, and it is assumed that the input signal swings to the positive side. However, a PNP type transistor may be used, and the input signal It goes without saying that the present invention is applicable even when the signal swings to the negative side.

Further, although an example is shown in which a bipolar transistor is used as the transistor of the comparator, an FET may be used instead. When using FETs, the channel width can be changed to increase the effective potential difference.

[Brief explanation of drawings]

FIG. 1 is a circuit configuration diagram showing a conventional ATC circuit, and FIG. 2 is an AT circuit diagram showing an invention prior to this invention and an AT in which this invention is implemented.
FIG. 3 is a diagram showing the voltage level of each part of the circuit when the input signal of the circuit shown in FIG. 2 is changed. FIG. A diagram showing the voltage level of each part of the circuit when the input signal is changed, taking into account the error between the signal input level and the reference voltage.
FIG. 5 is a diagram showing the voltage levels of various parts of the circuit when the input signal is changed in consideration of the error between the no-signal input level and the reference voltage in the ATC circuit according to the embodiment of the present invention. 11: Reference voltage source, 12: Peak hold circuit, 14:
Comparator, R11/R12: voltage dividing resistor, 15/16/17
:Transistor. Patent Applicant Tateishi Electric Co., Ltd. Agent Patent Attorney Shigeru Nakamura Figure 1

Claims (1)

[Claims] (1) a reference voltage source that outputs a first reference voltage lower than the no-signal level of an input signal and a second reference voltage higher than the first reference voltage; a voltage dividing means for deriving an intermediate level of the reference voltage; a peak hold circuit for holding the peak value of the voltage from the voltage dividing means; one input terminal receiving the input signal and the other input terminal receiving the peak hold circuit; An automatic threshold adjustment circuit comprising a comparator that individually receives the output of the circuit and the second reference voltage.