JPS6192007A - level compensation circuit - Google Patents

Abstract

PURPOSE:To compensate an operating level of an AC amplifier by synthesizing a negative amplitude component equal to a positive amplitude component of a signal waveform, for example, when a signal is at 0 level in the signal waveform. CONSTITUTION:Taking a reflected wave amplification and observing device OTDR as an example, when a photodetector receives a backward scattered light, it is amplified by a preamplifier 11 and an output is obtained at the output terminal 20. A level control circuit 47 and a switch 49 in a level compensation circuit synthesize again an output signal waveform outputted at an output terminal 19 of an AC amplifier 12 so as to use all optical pulses. That is, an inverting signal (g) having a waveform corresponding to a waveform (d) is obtained by the level control circuit 47 and the signal 0 level is compensated, then a waveform (h) is obtained by using the switch 49 as a signal synthesis means, thus all waveforms can be objects of the observation. Even when the 0 level of the waveform is changed due to output load or temperature, it is stabilized by using the level control circuit 47.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention converts weak electrical signals, such as weak optical signals, into optical signals.
The present invention relates to an operating level compensation circuit including an amplifier that amplifies an electrically converted weak signal with high gain. Furthermore, the present invention amplifies such a weak signal with an amplifier including an AC amplifier, and keeps the level at which the signal waveform becomes 0 (0 level) always constant despite changes in the amplified signal waveform and amplitude. The present invention relates to an operating level compensation circuit for compensating the operating level of an amplifier, such as a y-flow amplifier, which maintains the y-flow amplifier, improves the linearity of the amplifier, and minimizes fluctuations in the dynamic range.

[Conventional technology]

Recently, cutting-edge technologies such as optical communication technology have made remarkable progress supported by high-speed semiconductor technology.In these fields, there is an increasing need for broadband, low-noise, and high-gain amplifiers. High performance has become an important issue.

Optical signals are amplified after optical-to-electrical conversion, but since the signals are weak, a high gain is required for amplification, which is better than connecting multiple DC amplifiers, which have the inevitable drift problem. Using an AC amplifier with a sufficiently low low cutoff frequency is advantageous not only in terms of performance but also in terms of cost.

In optical communications and the like, since there is no need to use a DC amplifier, an AC amplifier is used, which does not have a drift problem, operates from a single power supply, is compact, and has low power consumption.

However, when measuring optical signals, for example, measuring fiber characteristics or detecting break points, there is a device that sends out a light pulse from one end of the fiber, and measures and observes the reflected waves that return from each point on the fiber. In the case of 0TDR (hereinafter referred to as 0TDR), it is necessary to accurately know the O level of the weak signal generated by the reflected wave, and it is necessary to use a DC amplifier.

However, in reality, DC amplifiers are not necessarily suitable for amplifying minute signals due to problems with drift, difficulty in multi-stage connections, and the need for multiple power supplies.

In particular, in the case of 0TDR, the amplitude of the reflected wave from near the measuring end is extremely large, and the amplitude of the reflected wave from far away is minute, so an amplifier that amplifies both extreme signals simultaneously, that is, a wide dynamic range Obtaining an amplifier was a major challenge.

In 0TDR, the backscattered light due to Rayleigh scattering of the fiber and the reflected wave due to Fresnel reflection at discontinuous parts such as fiber connection points and fracture points are separated, and this is used to widen the dynamic range and to observe the attenuation characteristics of the fiber. A logarithmic 1" 8-width instrument is used.

In this logarithmic amplifier, an AC amplifier is generally used for reasons such as avoiding drift and obtaining high gain.

However, in 0TDR, due to the above reasons, broadband,
High-gain, wide-dynamic-range amplifiers are made of artificial wood, but conventional ones are not only expensive, but also difficult to use when measuring input signal waveforms due to the characteristics of AC amplifiers, such as pulse waveforms. Since the operating level of the amplifier fluctuates due to variations in the duty ratio, etc., there is a drawback that the dynamic range and accuracy are seriously affected.

In particular, in the case of a logarithmic amplifier, fluctuations in the operating level due to its logarithmic characteristics cause a significant deterioration in measurement accuracy, so sufficient performance cannot be achieved without compensation to stabilize the operating level. It was.

This problem will be specifically explained below using FIGS. 7(a) to 7(d).

In FIG. 7, (a) shows an example of a conventional broadband light receiving circuit. 11 is a preamplifier which is a DC amplifier, and 12 is an AC amplifier, such as a logarithmic amplifier IC (integrated circuit).
12a, 12b and coupling capacitors 17, 1
8, and a signal is applied from the output terminal 20 of the DC amplifier 11 via the coupling capacitor 16. 14 is APD
(avalanche photodiode), etc., and Va is its bias power supply, and the output of the light receiving element 14 is
Through the input terminal 15 of the J\1 amplifier 11, the preamplifier 11
and is amplified by the AC amplifier 12 and the AC amplifier 12
The output can be obtained at the output terminal 19 of.

Here, the number of stages such as 12a+12b, which are logarithmic amplifier ICs, can be increased as necessary, and even higher gain ICs are possible.

When the light-receiving element 14 receives the backscattered light in the above-mentioned 0TDR, it is first amplified by the amplifier 11, and the output terminal 20K has a waveform as shown in FIG. 7(C). In (c), the dotted line is the level (O level) at which the backscattered light becomes zero. However, when this signal is passed through a circuit (b) in which the coupling capacitor C between each stage of the AC amplifier and the input impedance R are constrained to be equal, the result is as shown in (d), and the shape of (c) is formed. On the other hand, it shifts by Δ. The amount of this shift Δ varies depending on the amount of backscattered light, the magnitude of the 7-Renel reflected light, the repetition of optical pulses, that is, changes in the duty cycle, etc.

If the 0 level fluctuates in this way, it is impossible to expect an accurate range when the gain changes depending on the signal level, such as in a logarithmic amplifier.

In particular, as logarithmic amplifiers are connected in multiple stages, their amplification errors are amplified synergistically, and the usable dynamic range becomes significantly narrower.

Furthermore, even when a general AC amplifier is used, fluctuations in the θ level cause deterioration in precision in applications such as analog-to-digital converters that lose precision.

[Problem that the invention seeks to solve]

When a signal is applied to an AC amplifier, the operating level of the amplifier fluctuates depending on the waveform, resulting in an error in amplification degree and a fluctuation in the dynamic range. The present invention solves this problem.

[Means for solving problems]

When a signal waveform is applied to an AC amplifier, one reason for the zero level fluctuation is the positive amplitude component of the signal waveform (
This is because the area of the waveform in the positive direction) and the amplitude component in the negative direction (area of the waveform in the negative direction) are not equal.If these two amplitude components are not equal, for example, as shown in FIG. 7(c) In the case of such a waveform, the fluctuation in the operating level of the AC amplifier 111aii is removed by combining a negative amplitude component equal to the positive amplitude component, compensating the 0 level, and applying it to the AC amplifier, It is an object of the present invention to provide a level compensation circuit for the operation of such an AC amplifier.

[For production]

The present invention functions to compensate the operating level of an AC amplifier by combining, for example, a negative amplitude component equal to the positive amplitude component of the signal waveform during the period when the signal is at O level. It is something.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. child\
Components corresponding to those in FIG. 7 are given the same reference numerals.

In FIG. 1, 22 and 23 are both operational amplifiers, and 23 is a high input impedance FET input type with a large open loop gain and an offset vt
This is an amplifier with small pressure drift. 28 and 30 are resistors for setting the gain of the operational amplifier 22, and 31 is a capacitor for limiting the high frequency range of the operational amplifier 22 and stabilizing its operation.The operational amplifier 22 connected in this way is inverted. The output terminal 33 of the 0 operational amplifier 23 constituting the amplifier and the inverting input terminal (terminal with a minus sign) of the operational amplifier 220 are coupled via a resistor 32. 34 is a capacitor for limiting the high frequency range of the operational amplifier 23 and stabilizing its operation; 35 and 36 are respectively coupled to the inverting and non-inverting input terminals of the operational amplifier 230 (marked with - and 10 signs, respectively); This is a capacitor for storing the sampled signal.

The resistor 37 has an impedance that does not affect the high frequency components of the signal output to the output terminal 20a of the preamplifier 11, and is used to extract the low frequency component. Resistor 38 is also used for a similar purpose.

24, 25 and 49 are all switches, which are semi-integrated analog switches 24a, 24b and 2 that operate at high speed.
5a, 25b and 49a, 49b.

2ψ1 power control circuit with switch 24.25°4
9 and level control circuit 47, and control these switches via lines 39-42. Railway track 39.
Signal A is applied to line 44, and signal B is applied to line 40.45. 27 is the switch 25 and the coupling capacitor 1
6 and is connected to an output terminal connected to one side of the switch 25.

A level control circuit 46 includes operational amplifiers 22, 23, a switch 24, and the like.

47 is a level control circuit similar to 46, and each includes an operational amplifier, a switch, etc. included in 46.

21 is an output terminal of a level compensation circuit according to the present invention coupled to one side of switch 49.

Figure 2 shows the waveforms of each part of the circuit shown in Figure 1.
The circuit operation will be explained using this.

In FIG. 2, (al) is a first clock for timing, which is the basis of circuit operation. In synchronization with this first clock, for example, a laser diode (not shown) is driven, and the backscattered light of the fiber is driven. The optical-to-electrical converted signal is amplified by the preamplifier 11, and the waveform shown in (b) is obtained at the output terminal 20a. The output terminal 20b has (b
) is observed with a signal waveform of opposite polarity to the waveform shown in (). The signal at the output terminal 20a is input to the inverting input terminal of the operational amplifier 22 via the resistor 28, and after being amplified, the output of the operational amplifier 22 has a waveform as shown in (c), which is different from the waveform in (b). Obtain a waveform with opposite polarity.

On the other hand, the switch control circuit 26 is connected to (al) by the line 43.
A first clock shown in is applied, and the switch control circuit 26
In other words, the second clock shown in (a2) is obtained using a bistable circuit, the monostable multivibrator is triggered by this second clock to obtain the waveform shown in (e), and after this waveform, At the falling edge, a separate monostable multivibrator is further trigged to obtain the waveform shown in (f).

Similarly, by using two separate monostable multivibrators, the waveforms shown in (j) and (b) are obtained from the first clock shown in (al). If every other good pulse shape is extracted from the waveform shown in (k), the waveforms shown in (c) and (c) are obtained.

These waveforms have (al) on the line 43, respectively.
The line 39.44 has <1>, the line 40.45 has (c), the line 42 has (f), and the line 41 has the inverted signal of the waveform shown in (f), that is, (f). If is a Q output signal of a monostable multi-by-break, the Q output signal is applied.

At this time, the analog switch 24a is turned on at the low level of the waveform shown in (1), and the storage capacitor 35 is connected to (b).
) The level (0 level) when the waveform signal shown in ) becomes sufficiently small is stored. Similarly, the analog switch 24b is turned on at the low level of the waveform shown in (c).
The storage capacitor 36 stores the level at which the signal having the waveform shown in (C) becomes sufficiently small. At this time,
When the difference in the levels accumulated in the storage capacitors 35 and 36 is taken into account, the error voltage is amplified and appears at the output terminal 33 of the operational amplifier 23, which operates as a differential amplifier for error detection, and is output via the resistor 32. Since the voltage is applied to the inverting input terminal of the trench width filter 22 and is negatively fed back to the capacitor 36 via the resistor 38 and the analog switch 24b, the capacitor 3
The level of the signal stored in capacitor 6 is equal to the signal level stored in capacitor 35 (0 level). That is,
A circuit including this negative feedback loop operates as a level control circuit.

Therefore, the analog switch 25a is turned on at the high level of the waveform shown in (f), and the analog switch 25b is turned on at the high level of the waveform shown in (f).
), when the output terminal 27 is turned on at the low level shown in (b
) and (c) are combined to obtain the waveform shown in (d). That is, the switch 25 now operates as a signal combining means.

The waveform shown in (d) obtained at the output terminal 27 is a composite signal obtained by combining the waveforms shown in (b) and (c) whose O levels are made equal, so the areas above and below the 0 level are equal, and this Even when the voltage is applied to the AC amplifier 12 via the coupling capacitor 16, highly accurate amplification is possible without causing level fluctuations.

If there is no negative feedback loop, the output terminal 27 will have a waveform with a zero-level shift caused by Δ, even if the positive side waveform and negative side area are the same, as shown in (n). You can only get it.

As an example, suppose that in the waveform shown in (d), only the portion corresponding to the waveform shown in (b) is to be observed. Therefore, it is also possible to use the signal shown in (e) instead of the signal shown in (f). In that case, it becomes difficult to observe the rising portion of the waveform shown in (b), that is, the portion of the backscattered light of the fiber near 0TDR. If the signal shown in (f) is used, 0T from the rising part of the waveform
It becomes possible to display on the tube surface of a DR observation cathode ray tube (not shown). The time required to display the rising portion of this waveform (delay time), that is, the time difference between the rising edge of the waveform shown in ff) and the rising edge of the waveform shown in (b) can be easily adjusted, so that the waveform is displayed on the CRT surface. By the way, in the waveform shown in (d), if only the part corresponding to the waveform shown in (b) is to be observed, only half of the repetition of the optical pulse will be used. This not only reduces the efficiency of laser diode usage, but also increases the signal processing time required for aperaging to suppress noise and improve the signal-to-noise ratio (S/N). . Among these, in the level compensation circuit according to the present invention, the output signal waveforms output to the output terminal 19 of the AC amplifier 12' are combined again by the level control circuit 47 and the switch 49, so that all optical pulses can be used. . In other words, the level control circuit 47 generates an inverted signal □ of the waveform corresponding to waveform (d).
□□) and after compensating the O level of the signal, the waveform (h) is obtained using the switch 49 as a signal synthesis means, so all waveforms can be observed. Also)'"
゛″″V0°V < Az l): l″′j′.″″
'-? tMt'JKxb'-? It will be clear that the tMt'JKxb change path 47 can be used for stabilization.

(Modification 1-1) In the level control circuit 46 of FIG.
When used in a wide band, a capacitor 31 is required. The same can be said of the level control circuit 47. Further, the level control circuit 47 may be replaced with an inverting amplifier shown in FIG. At this time, it is desirable that the offset voltage and drift of the amplifier be small. In FIG. 6, 50 is an operational amplifier, 54 is a bias resistor, 51 is a gain setting variable resistor, and 52.53 is a gain setting resistor.

Since the AC amplifier 12 is generally a single power source, its gain may differ in the positive direction of the waveform.
For such things, fine adjustment can be made using the variable resistor 51.

(Modification 1-2) In the level control circuit 46 of FIG.
It is desirable to process the output signals as accurately as possible without adversely affecting their waveforms, bands, etc., and combine them with the switch 25 to obtain the waveform shown in FIG. 2(d) at the terminal 27. However, in reality, it is limited by the characteristics of the operational amplifier 22. In addition, as mentioned above, the AC amplifier 12
Since it is generally a single power supply, its gain may differ between the positive and negative directions of the waveform.

In such a case, the output of the AC amplifier 12 is changed to the waveform shown in FIG.
When all waveforms are to be observed as shown in h), (h
), the portion corresponding to (d) and the portion corresponding to (g) are not equal, which is undesirable.

Therefore, the circuit shown in FIG. 3 focuses on only the positive waveform of the AC amplifier 12 and provides a circuit that faithfully amplifies the waveform of the preamplifier 11 in as wide a band as possible.

Components in FIG. 3 that correspond to those in FIG. 1 are given the same reference numerals. Here, 48 is a level control circuit, 61 is a switch similar to 25, 62 is its output terminal, 63 is a coupling capacitor, 64 is an AC amplifier, 65 is a bias resistor, 66 and 67 are gain setting resistors,
60 is an operational amplifier constituting an inverted t, 7-width amplifier.

The operation of the circuit shown in FIG. 3 will be briefly explained below with reference to FIG.

The signal at the output terminal 20a of the preamplifier 11 is applied to the terminal 27 by the level control circuit 46 and the switch 25 as a waveform (d) shown in FIG. The signal is input to the control circuit 47.

On the other hand, the signal at the output terminal 20b is similarly applied as a waveform (g) to the terminal 62 by the control circuit 48 and switch 61, is amplified by the AC amplifier 64, and inverted and amplified by the operational amplifier 60, and is then applied to the level control circuit. 47. Furthermore, the waveforms (a) and (g) are combined by the switch 49 with the 0 level compensated for, and the waveform (h
) is obtained at the output terminal 21.

As explained above, the circuit of FIG. 3 has an increased number of circuit components compared to the circuit of FIG. 1, but eliminates the disadvantages of an AC amplifier,
Furthermore, it is possible to perform level compensation for a wideband signal while reducing waveform distortion.

(Modification No. 2) The error detection ability of the negative feedback circuit including the operational amplifier 23 shown in FIG. 1 needs to be sufficiently larger than the gain of the AC amplifier connected after the level control circuit 46. Therefore, the open loop gain of a general operational amplifier (for example, 10 to 1
(approximately 0), and if a large gain is required, the first
It is desirable to insert the amplifier shown in FIG. 4 between the output terminal 33 of the operational amplifier 23 shown in the figure and the resistor 32.

Here, 55 is an operational amplifier, 56 and 57 are resistors for setting the gain, 58 is a resistor for input bias, and the others are the same as in FIG.

It is obvious that the same thing can be said about the operational amplifiers related to level control circuits 47 and 48. For amplification of weak signals, the gain of the AC amplifier connected after the level compensation circuit of the present invention is used. is used, so the fourth
Adding the amplifier shown in the figure is effective, but in such cases, it is also possible to use an operational amplifier 23 with a small offset voltage, or to adjust the offset (not shown) to zero. be. This is 0TDR
In order to remove noise from weak signals, it is common to perform averaging, which is a method of statistical noise processing. If the offset voltage is not small enough,
This is because the linearity of the waveform when it is displayed logarithmically is impaired.

(Modification 3) In the operation of the switch control circuit 26 shown in FIG. 1, it is desirable to prevent (A') and (C) shown in FIG. 2 from becoming low levels at the same time. This is because if the two inputs of the operational amplifier 230 shown in FIG. 1 fluctuate simultaneously, undesirable problems such as oscillation may occur.

As is clear from the above explanation, the switch control circuit 26
The timing of signal generation is not limited to that explained in FIG.

(Modification 4) It is obvious that the switches 24, 25, 49, etc. shown in FIG. 1 may be changed to the transfer type switches shown in FIG. 5.

(Variation No. 5) If the signal applied to the input terminal 15 in FIG. 1 does not include the 0 level, the input terminal 15 or the output terminal 20a may be grounded for a certain period of time by a switch not shown. Therefore, in the case of 0TDR, the O level can also be obtained by providing an optical shutter or an optical switch in front of the light receiving element to set backscattered light to 0.

Regarding the embodiments shown above, to summarize, the present invention provides 0
A preamplifier (11) that amplifies a repetitive signal having an asymmetrical waveform with respect to the level, and a preamplifier (11) that makes the output (second figure) equal to the O level and inverts the first signal (see figure 2 (C)
)) and a first level control means (46) for obtaining a preamplifier (one output);
) in synchronization with the cycle of the repeated signal, and obtains an output with a waveform symmetrical with respect to the 0 level (Fig. 2 (d)). and,
An AC amplifier (12) that amplifies this output, makes this output equal to 07 bells, and generates an inverted second signal (Fig. 2 (g)
)) to obtain the output of the second level control means (47 in FIG. 1, 48 in FIG. 3, 60 to 67, and 47) (FIG. 2 (g)). Signal period 1/I: Amplified output that is synchronously and alternately taken out and has an asymmetrical waveform with respect to the θ level (Figure 2 (h))
and a second switching means (49) for obtaining.

〔Effect of the invention〕

As is clear from the above description, even if the input signal waveform changes, its QV level is fixed, enabling highly accurate amplification and ensuring a wide dynamic range.

According to the embodiment of the present invention, the effect is remarkable when a high gain logarithmic amplifier is used as the AC amplifier 12.

When a linear amplifier is used as the AC amplifier 12, it is possible to obtain a function similar to that of a high-gain DC amplifier, and on the contrary, an amplifier with a high Im[°] without drift can be obtained. Since level compensation is performed by sampling and negative feedback, stable operation can be expected even with temperature changes, etc. According to the present invention, the level can be stabilized to operate even AC amplifiers with gains of 80 dB or more. A compensation circuit is obtained.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing one embodiment of the present invention, and Fig. 2 is a circuit diagram showing an embodiment of the present invention.
3 to 6 are circuit diagrams for explaining another embodiment, and FIG. 7 is a diagram for explaining a conventional example.

Claims (6)

[Claims] (1) A preamplifier that amplifies a repetitive signal having an asymmetrical waveform with respect to the 0 level, and a first level that makes the output of the preamplifier equal to the 0 level and obtains an inverted first signal. A control means, the output of the preamplifier, and the output of the first level control means are taken out alternately in synchronization with the cycle of the repetition signal, and outputs having a symmetrical waveform with respect to the 0 level are obtained. a first switching means for obtaining;
an AC amplifier for amplifying the output of the first switching means; a second level control means for making the output of the AC amplifier equal to the 0 level and obtaining an inverted second signal; and an output of the AC amplifier. and the output of the second level control means are taken out alternately in synchronization with the cycle of the repetition signal,
and second switching means for obtaining an amplified output having an asymmetric waveform with respect to the 0 level. (2) The first level control means includes a switch and a storage capacitor for detecting and storing the 0 level of the output of the preamplifier, and the signal stored in the storage capacitor is applied to one terminal. an operational amplifier; an inverting amplifier to which the output of the operational amplifier and the signal from the output of the preamplifier are applied to output the first signal; and detecting and accumulating a 0 level of the output signal of the inverting amplifier. A negative feedback path is formed by applying a signal accumulated in the storage capacitor to the other terminal of the operational amplifier, thereby forming a negative feedback path.
The level compensation circuit described in section. (3) The second level control means includes a switch and a storage capacitor for detecting and storing the zero level of the output of the AC amplifier, and an operational amplifier to which the signal stored in the storage capacitor is applied to one terminal. an inverting amplifier to which the output of the operational amplifier and the output of the AC amplifier are applied to output the second signal; a switch and a storage capacitor for detecting and accumulating the 0 level of the output signal of the inverting amplifier; 2. The level compensation circuit according to claim 1, wherein a negative feedback path is formed by applying the signal accumulated in the storage capacitor to the other terminal of the operational amplifier. (4) The second level control means includes a switch and a storage capacitor for detecting and storing the zero level of the output of the AC amplifier, and an operational amplifier to which the signal stored in the storage capacitor is applied to one terminal. and an inverting amplifier to which the output of the operational amplifier and a signal having the same polarity as the output of the AC amplifier are applied to output the second signal, and for detecting and accumulating the 0 level of the output signal of the inverting amplifier. Claim 1 comprising a switch, a storage capacitor, and a negative feedback path formed by applying the signal stored in the storage capacitor to the other terminal of the operational amplifier.
The level compensation circuit described in section. (5) Claim 1, wherein the first switching means and the second switching means include a transfer type switch.
The level compensation circuit described in section. (6) The level compensation circuit according to claim 2, 3, or 4, further comprising an amplifier for compensating for the lack of amplification of the operational amplifier.