Oscilloscope calibrator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The main and most widely used device for studying the voltage waveform is an electronic oscilloscope. In order not only to visually observe electrical signals, but also to measure their parameters, oscilloscopes are calibrated using calibrators. The amplitude calibrator is designed to calibrate or check the accuracy of calibrating the vertical axis of the oscilloscope screen in units of voltage, and the duration calibrator, respectively, for the horizontal axis in units of time.

Many radio amateurs have in operation a lot of oscilloscopes produced back in the USSR, and they have not been trusted for a long time. Some of them do not have a built-in reference signal generator. Other models have it, but decades later, it can only be trusted with great care. For example, the S1-5 (SI-1) oscilloscope I have at my disposal has a built-in amplitude calibrator. But, firstly, it generates a sinusoidal signal with a frequency of 50 Hz, and secondly, even in the days of its "childhood" the error in measuring the amplitude of signals in the 0,2 ... 1,2 V scale section was ± 10%, which is too much for today's standards.

To the attention of radio amateurs who have such devices, a calibrator for an oscilloscope with a measurement error is offered, determined only by the capabilities of the measuring instruments available to radio amateurs, in my case, the digital multimeter M890G, the main measurement error of which is the error of the measure. The device generates a rectangular signal (meander) with a swing of 2 V, a frequency of 1 and 20 kHz. This allows you to use the calibrator, for example, when setting up the compensation of the high-frequency probe of an oscilloscope or to check the dynamic parameters of audio power amplifiers.

As mentioned above, the digital multimeter M890G is used to establish (and then for periodic verification) of the calibrator. The relative error of measuring DC voltage with the M890G multimeter, according to the passport data, is ±0,5% of the measured value plus/minus the LSB unit, and the frequency measurement is ±1% of the measured value plus/minus the LSB unit with a resolution of 10 Hz. When measuring the maximum voltage at the limit of 2 V, the absolute error is ± 11 mV with a resolution of 1 mV, measuring the frequency of 10 Hz - ± 20 Hz, and the frequency of 20 kHz - ± 210 Hz. Unfortunately, the indicator of the M890G multimeter, like most others, allows you to display only 3,5 digits. Therefore, only the following specifications of the calibrator can be guaranteed: output amplitude 1,999 V ±11 mV, output frequency 1 kHz ±20 Hz and 19,99 kHz ±210 Hz.

Rice. 1. Scheme of the calibrator

The scheme of the calibrator is shown in fig. 1. The 1,999 V precision voltage source (amplitude calibrator) is assembled on an adjustable voltage regulator LM317T (DA1). This IC maintains a stable reference voltage of 1,25 V with high accuracy between the output and the control pin. Since the control pin draws very little current, the output voltage U_O=1,25(1±R3/R4). Typically, the resistance of the resistor R4 is chosen equal to 240 ohms. But in our case, in order to ignore the current through the control output and make it independent of changes at the input and in the load, a current equal to the initial load current must be taken from the output of the stabilizer through resistors R3, R4 (it must be more than 10 mA, since the DA2 timer at a supply voltage of 2 V consumes a current of no more than 60 μA). If the load is insufficient, the output voltage will increase [1].

The duration calibrator is assembled on the integral timer ICM7555IN (DA2). It is made using CMOS technology, so the voltage at its output (pin 3) can vary from zero to the supply voltage. In addition, this microcircuit also works at a supply voltage of 2 V. The timer is connected according to a typical generator circuit. The timing circuits R1C1 and R2C1 are connected to the timer output. This ensures high accuracy of the meander formation, since the charging and discharging of the capacitor C1 occur through the same resistor (either R1 or R2). The frequency of the generated pulses can be calculated using the formula f=0,7215/(R1•C1) [2]. Resistor R6 prevents the timer from shorting the output.

Considering that the vast majority of oscilloscopes have an input impedance of at least 1 MΩ, this practically does not affect the accuracy of the calibration. Resistor R5, together with the internal discharge transistor of the timer, forms an additional high-resistance square wave output. Capacitors C2 and C3 smooth out surges in the output voltage of the stabilizer DA1 at the time of switching the timer DA2.

Rice. 2. Calibrator board drawing

The calibrator is assembled on a printed circuit board made of fiberglass foiled on both sides with a thickness of 2 mm, the drawing of which is shown in fig. 2. When repeating the design, there are no special requirements for the elements. The main thing is that the resistor R3 is multi-turn (in the author's version - SP5-2). Instead of imported, you can use the domestic timer KR1441VI1. Capacitor C1 - SGME-A with a tolerance of ± 1%, but it is possible to use other capacitors with other ratings and with a minimum TKE, especially since the calibrated frequency of the output pulses is set by a selection of resistors R1 and R2. In the author's version, each is made up of two MLT-0,25 resistors with a tolerance of ± 5%, connected in series. A place on the printed circuit board is provided for this. Capacitor C2 - any ceramic, C3 - K53-1A or imported, suitable in size. Jumper S1 is used from the SVP device of the ZUSCT TV.

Set up the device like this. The supply voltage is applied and a voltage of 3 V is set at the output of the voltage regulator with a tuned resistor R1,999, controlling it with a multimeter M890G at the limit of 2 V. This operation is very painstaking. The resistance of the tuned resistor should be slowly increased from the minimum until the required voltage is obtained. Then the multimeter is switched to measure the frequency and by selecting resistors R1 and R2 set the output frequency to 1 and 19,99 kHz. When setting up, it is convenient to use a multi-turn resistor SP5-1VA with a resistance of 10 kOhm, connected in series with a constant resistor of 5,1 kOhm, for a frequency of 20 kHz and a multi-turn resistor SP3-36 with a resistance of 100 kOhm (from the SVP TV 3USTST) with a series-connected constant resistor of 180 kOhm for frequency 1 kHz.

The performance of the calibrator is maintained when the voltage of the GB1 (G6F22) battery drops to 5 V. Considering that the current consumed by the load is slightly more than 10 mA, and the calibrator is used only periodically, its capacity lasts for a long time.

Literature

1. Horowitz P., Hill W. The Art of Circuitry, v. 3. - M .: Mir, 1993.
2. Aleksenko A. G., Kolombet E. A., Starodub G. I. Application of precision analog microcircuits. - M.: Radio and communication, 1985.

Author: S. Semikhatsky

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