JPS6128207A - Level compensating circuit - Google Patents

Abstract

PURPOSE:To eliminate fluctuation of the operating level of an AC amplifier and to prevent fluctuation of 0 level of a signal by synthesizing a negative amplitude component equal to a positive amplitude component of, e.g., a signal waveform while the signal is at 0 level and applying the signal to the AC amplifier while the 0 level is compensated. CONSTITUTION:A signal in synchronizing with the 1st clock a1 is amplified by a preamplifier 11 and a waveform (b) is obtained at an output terminal 20. After the gain and level are set and adjusted by a non-inverting amplifier 47, the result is amplified by an operational amplifier 22 and a waveform (c) of opposite polarity with the waveform (b) is obtained. On the other hand, a switch control circuit 26 is applied with the 1st clock a1 through a line 43, analog switches 24, 25 are controlled, a 0 level of the signal of the waveform (b) is stored in a capacitor 35 and a level when the signal of the waveform (c) is sufficiently smaller is stored in a capacitor 36. The level difference of the both is fed to an inverting input of the operational amplifier 22 via an operational amplifier 23, fed back negatively to the capacitor 36 and the level difference is zeroed. The switch 25 is operated as a signal synthesis circuit and the area above and under the 0 level is equal in the waveform (d) at an output terminal 27.

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 always maintains the zero level (0 level) of the signal waveform despite changes in the amplified signal waveform and amplitude. The present invention relates to an operating level compensation circuit that compensates for the operating level of an amplifier, including an AC amplifier, which compensates to keep it constant, improves the linearity of the amplifier, and minimizes fluctuations in the dynamic range.

[Conventional technology]

Recently, cutting-edge technologies such as optical communication technology are making remarkable progress supported by high-speed semiconductor technology, but in these fields there is a high need for broadband, low-noise, and high-gain amplifiers. , improving its performance has become an important issue.

Optical signals are amplified after optical-to-electrical conversion, but since the signals are weak, high gain is required for amplification, which requires multi-stage connection of DC amplifiers with built-in drift problems that are unavoidable. Rather, it is advantageous not only in terms of performance but also in terms of cost, to use an AC amplifier with a sufficiently low low cutoff frequency.

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, in measuring optical signals, for example, measuring fiber characteristics and detecting break points, devices (hereinafter referred to as In the case of 0TDR), it is necessary to accurately know the O level of the weak signal that is 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 an important issue.

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 break points are amplified, which is useful for widening the dynamic range and observing the attenuation characteristics of the fiber. An upper logarithmic amplifier is used.

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

However, for 0TDR, an amplifier with a wide band, high gain, and wide dynamic range is required for the reasons mentioned above, but conventional ones are either expensive9 or have problems with the input signal due to the characteristics of AC amplifiers. When the waveform is a pulse waveform, for example, the operating level of the amplifier fluctuates due to fluctuations in its duty ratio, etc., which has the drawback of seriously affecting the dynamic range and accuracy.

In particular, in the case of a logarithmic amplifier, due to its logarithmic characteristics, fluctuations in the operating level cause a significant deterioration in measurement accuracy, so sufficient performance could not be achieved without compensation for stabilizing the operating level. It is.

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 a coupling capacitor 17,
18, and a signal is applied from the output terminal 2o of the DC amplifier 11 via the coupling capacitor 16. 14 is APD
(avalanche photodiode) etc.
is its bias power supply, and the output of the light receiving element 14 passes through the input terminal 5 of the preamplifier 11, is amplified by the preamplifier 11 and the AC amplifier 12, and obtains its output at the output terminal 19 of the AC amplifier 12. be able to.

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

When the light receiving element 14 receives backscattered light in the 0TDR described above, it is amplified by the preamplifier 11, and a waveform as shown in FIG. 7(c) is obtained at its output terminal 20. In (c), the dotted line is the level (
0 level). However, when this signal is passed through a circuit (b) that shows isometrically the coupling capacitor C between each stage of the AC amplifier and the input impedance R, it becomes as shown in (d), and the waveform in (c) is obtained. On the other hand, if you shift by Δ, it becomes ＼. The amount of shift Δ varies depending on the amount of backscattered light, the magnitude of Fresnel reflected light, the repetition of optical pulses, that is, changes in the duty cycle, etc.

If the O level fluctuates in this way, accurate amplification cannot be expected in a case where the gain changes depending on the signal level, such as in a logarithmic amplifier.

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

Even when using a general AC amplifier, there is no 0 level fluctuation, so precision is required.

In applications such as digital converters, this has caused accuracy deterioration.

[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]

One reason for fluctuations in the O level when a signal waveform is applied to an AC amplifier is the positive amplitude component of the signal waveform (
The area of the waveform in the positive direction) and the amplitude component in the negative direction (the area of the waveform in the negative direction) must not be equal, and if these two amplitude components are not equal, for example, as shown in FIG. 7(c). In the case of such a waveform, by combining a negative amplitude component equal to the positive amplitude component and applying it to the AC amplifier after compensating for the 0 level, the fluctuations in the AC amplifier's operating level are removed and the signal is The present invention provides 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, 47 is a non-inverting amplifier 51, 52
.

53 is a resistor for setting the degree of amplification, 21 is an operational amplifier, 54
is the input bias resistance. This resistance 5], , 52
, 53, the amplification degree of the non-inverting amplifier is
Can be set arbitrarily. 22 and 23 are both operational amplifiers, and 23 is a high input impedance F"
E T input type with large open loop gain.
In addition, the amplifier has a small offset voltage 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. It constitutes an amplifier.

An output terminal 33 of the operational amplifier 23 and an inverting input terminal (the terminal marked with a minus sign) of the operational amplifier 22 are coupled via a resistor 32. 34 limits the high frequency range of the operational amplifier 23 and stabilizes its operation. 35 and 36 are the inverting and non-inverting input terminals of the operational amplifier 230 (marked with - and "-" respectively). )
A sampling signal storage capacitor coupled to the capacitor.

The resistor 37 has an impedance that does not affect the high frequency components of the signal output to the output terminal 2o of the preamplifier 11, and is used to extract the low frequency components. Resistor 38 is also used for a similar purpose.

Both 24 and 25 are switches, and include semiconductor analog switches 24a, 24b and 25a, 25b that operate at high speed.

26 is a switch control circuit, which controls switches 25 and 2.
Generate control signal for switch changeover of 6 12, line 3
These switches are controlled by 9-42. Reference numeral 27 is an output terminal of a level compensation circuit according to the present invention, which is arranged at the junction between the switch 25 and the coupling capacitor 16.

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

FIG. 2 t1 shows waveforms of various parts of the circuit shown in FIG. 1, and 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, and in synchronization with this first clock, for example, the illustrated laser diode is driven, and the backscatter of the fiber is The optical-to-electrical converted light signal is amplified by the preamplifier 11 and sent to the output terminal 20 (
The waveform shown in b) is obtained.

The gain and level of the signal at the output terminal 20 are set and adjusted by the non-inverting amplifier 47, and the signal is input to the inverting input terminal of the operational amplifier 220 via the resistor 28 and amplified. As shown in C), a waveform with a polarity opposite to that of (1)) is obtained.

On the other hand, the switch control circuit 261c and the path 43 (a
The first clock shown in (a2) is applied, the second clock shown in (a2) is obtained by the bistable circuit built in the switch control circuit 26, and the monostable multivibrator is triggered by this second clock (e ), a separate monostable multivibrator is triggered at the trailing edge of this waveform, which is the falling edge, to obtain the waveform shown in (f).

In the same way, two separate shore stabilizing multivibrators are used to clock (g>
And the waveform shown in (h) is obtained. Every other pulse waveform is taken out alternately from the waveform shown in (h) to obtain the waveforms shown in (j) and (k).

These waveforms have (al) on the line 43, respectively.
The line 39 has (j), the line 40 has (1()), the line 42 has (f), and the line 41 has the inverted version of the waveform shown in (f), that is, (f) is simple. If it is a Q output signal of a stable multivibrator, 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 (j), 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 (k),
The storage capacitor 36 stores the level at which the signal having the waveform shown in (c) becomes sufficiently small. At this time,
If there is a difference in the levels accumulated in the storage capacitors 35 and 36, 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 calculated via the resistor 32. Since the signal is applied to the inverting input terminal of the amplifier 22 and is negatively fed back to the capacitor 36 via the resistor 38 and the analog switch 24b, the level of the signal accumulated in the capacitor 36 is equal to the level of the signal accumulated in the capacitor 35 (0 level). ) is equal to That is, the 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 synthesis circuit.

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 0 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 of Δ, as shown in (1), even if the positive side waveform and negative side area are the same. You can only get it.

In this example, in the waveform shown in (cl), only the portion corresponding to the waveform shown in (b) is observed.

Therefore, it is also possible to use the signals shown in (,) instead of the signals shown in (f). In that case, it becomes difficult to observe the rising-L portion of the waveform shown in (b), that is, the portion of the backscattered light of the fiber in the vicinity of 0TDR. If the signal shown in (f) is used, it becomes possible to display the rising portion of the waveform on the screen of an 0TDR observation cathode ray tube (not shown). It is also possible to easily adjust the time required to display the rising edge of this waveform (delay time), that is, the time difference between the rising edge of the waveform shown in (f) and the rising edge of the waveform shown in (b).・It is possible to display the entire waveform from the rising part on the CRT surface.

Modification (No. 1) The error detection ability of the negative feedback circuit including the operational amplifier 23 of FIG. 1 needs to be sufficiently larger than the gain of the AC amplifier connected to the subsequent stage of the level compensation circuit of the present invention. . Therefore, the open loop gain of a general operational amplifier (for example, 105
106), the output terminal 33 of the operational amplifier 23 and the resistor 32 shown in FIG.
It is desirable to insert the amplifier shown in FIG. 3 between the two. Here, 55 is an operational amplifier, 56 and 57 are resistors for setting gain, 58 is a resistor for input bias, and the other components are the same as in FIG.

For amplification of weak signals, an AC amplifier with a large gain connected after the level compensation circuit of the present invention is used, so adding the amplifier shown in FIG. 3 is effective. In this case, it is important to use an operational amplifier 23 with a small offset voltage, or to perform an offset voltage main adjustment (not shown). this is,
In 0TDR, it is common to perform averaging, which is a method of sequential noise processing, in order to remove noise from weak signals, but in this case, when the attenuated waveform of backscattered light is logarithmically transformed and displayed. Second, if the offset voltage is not sufficiently small, the linearity of the waveform when expressed logarithmically will be impaired.

Modification (Part 2) In the operation of the switch control circuit 26 shown in FIG.
It is desirable to prevent j) and (k) from becoming low levels at the same time. This is because, if the two inputs of the operational amplifier 23 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 (No. 3) If the load driving capability of the preamplifier 20 shown in FIG. 1 is sufficient, the non-inverting amplifier 47 can be omitted. It will be clear from the description of FIG. 1 that the resistor 28 may be a variable resistor if necessary.

Modification (No. 4) It is obvious that the switch 25 shown in FIG. 1 may be changed to a transfer type switch shown in FIG. 4.

Modification Example (No. 5) When using an amplifier whose output has both positive and negative polarities as shown in FIG. 5 as the preamplifier 11 shown in FIG. 1, the output terminal 20a shown in FIG. It is connected to the resistor 37 and analog switch 25a, and the output terminal of 2ob is connected to a non-inverting amplifier 47, and an inverting amplifier as shown in the middle figure is connected to the output terminal side of 20a or the output terminal side of 20b.
Alternatively, it will be clear from the above description that by inserting it between the non-inverting amplifier 47 and the level control circuit 46, the same effect as in the case of FIG. 1 can be obtained.

Modification (No. 6) If the signal applied to the input terminal 15 in FIG. Alternatively, in the case of 0TDR, the 0 level can also be obtained by providing an optical shutter or optical switch in front of the light receiving element to set the backscattered light to 0.

〔Effect of the invention〕

As is clear from the above description, even if the input signal waveform changes, its 0 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, a function similar to that of a high-gain DC amplifier can be obtained, and on the contrary, a highly accurate amplifier without drift can be obtained.

According to the present invention, since the 0 level is sampled and level compensation is performed by negative feedback, stable operation can be expected even with temperature changes.

According to the present invention, a level compensation circuit that can stably operate even an AC amplifier with a gain of 80 dB or more can be obtained.

[Brief explanation of drawings]

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. 11... Preamplifier, 12... AC amplifier, 14...
... Light receiving element, 15 ... Input terminal, 16.17.18
...Coupling capacitor, 21.22.23.50.55
...Operation amplifier, 24.25...Switch, 26.
...Switch control circuit, 46...Level control circuit, 4
7...Non-inverting amplifier.

Claims (4)

[Claims] (1) A preamplifier that amplifies an input signal, a first signal obtained from a part of the output signal of the preamplifier, and the first signal obtained from another part have the same amplitude but opposite a second signal having polarity and a 0 level having the same value as the 0 level of the first signal;
level control means for obtaining a second signal having a level; and switch means for sequentially switching the first signal and the second signal to obtain a composite signal, A level compensation circuit characterized in that the voltage is applied to the level compensation circuit. (2) The level compensation circuit according to claim 1, wherein the switch means for obtaining the composite signal includes a transfer type switch. (3) The level control means includes a first switch and a first storage capacitor for detecting and storing the 0 level of the first signal, and the signal stored in the first storage capacitor is connected to one side. an operational amplifier applied to a terminal; an inverting amplifier to which signals from the output of the operational amplifier and the output of the preamplifier are applied to output the second signal; and a 0 level of the output signal of the inverting amplifier. a second switch and a second storage capacitor for detecting and storing the second
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 level compensation circuit according to claim 3, further comprising an amplifier for compensating for insufficient amplification of the operational amplifier in the level control means.