Miniature oscilloscope probe. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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When repairing and adjusting electronic equipment, there is often a need for a miniature self-powered oscilloscope probe, with which one could monitor the presence of a signal and at least roughly estimate its parameters.

The oscilloscope probe presented to the attention of readers largely meets these requirements. The use of a low-voltage multi-digit vacuum luminescent indicator and digital microcircuits of the K176 series made it possible to design an economical device the size of a pocket calculator and powered by a 9 V battery. The current consumed by the probe does not exceed 15 mA, and the main consumer is the direct-heated cathode of the indicator. The probe can control signals with an amplitude of 1 ... 320 V with a frequency of up to 50 kHz with a duty cycle of 1,14 to 8, as well as single pulses. Input resistance at the limit "1 ... 32 V" -220 kOhm, at the limit "10 ... 320 V" - 2,2 MΩ. There are three modes of operation: automatic, positive edge-triggered standby, and negative edge-triggered standby.

Fig.1 (click to enlarge)

The schematic diagram of the probe is shown in fig. 1, time diagrams at its characteristic points - in fig. 2 (automatic sweep mode) and 3 (standby sweep mode). The device consists of a sweep generator, a device for vertical deviation of the "beam" and a multi-digit sign indicator HG1. The generator, in turn, contains a multivibrator on the elements DD1.1-DD1.3 and a counter-decoder DD2, a device for vertical deflection of the "beam" - comparators of positive (op-amp DA1) and negative (op-amp DA2) levels and a coincidence element DD1.4. The multivibrator generates a sequence of pulses (Fig. 2, g), the counter-decoder alternately generates high-level pulses at its outputs (Fig. 2, c-p), which, sequentially entering the grids of the HG1 indicator, create a horizontal scan of the image.

Ris.2

The controlled signal is fed to the inputs of the comparators through a voltage divider consisting of resistors R3, R5 and R6. The potential of the common wire, necessary for the normal operation of the op-amp DA1, DA2 when powered from a unipolar source GB1, is artificially created by the voltage divider R8-R11. The same divider also sets the threshold voltages at the inverting input of the op-amp DA1 and the non-inverting input of the op-amp DA2, which differ from the potential of the common wire by +100 and -100 mV, respectively; elements R3, R5, VDI, VD2 protect the inputs of the op-amp from overloads. The proportion of the input signal at which the comparators are triggered is set by the switch SA1 and the variable resistor R6 (if necessary, the signal amplitude is judged by the positions of the switch and the resistor slider).

Ris.3

The HG1 indicator uses horizontal anode segments a, g and d (in reference books they are sometimes denoted by Russian letters a. g, g), indicating, respectively, the positive, zero and negative levels of the controlled signal. If the signal voltage exceeds (in absolute value) a positive or negative threshold level, a high-level voltage appears at the output of the op-amp DA1 or DA2 and the anode segments a or d are lit. If both comparators (DA1 and DA2) are in the zero state (their outputs are low-level voltages), a high level is present at the output of the DD1.4 element and the anodes-segments g are lit, displaying the zero level of the input signal (Fig. 3, p ).

The repetition rate of the multivibrator pulses, and hence the scanning speed of the image on the indicator, is set by resistors R2, R4 and one of the capacitors C1-C8, selected by switch SA2. Smoothly the pulse repetition rate is regulated by a variable resistor R4. Resistor R1 limits the input current through the microcircuit, its resistance is chosen within 3 ... 10 kOhm. If you need other than indicated in the diagram, the duration of the sweep, then this can be done by recalculating (according to the formula T \u1,4d 1RC, where T is the period of oscillation) the values ​​\u8b\u2bof capacitors C4-CXNUMX and resistors RXNUMX, RXNUMX.

In the automatic sweep mode, a cycle consisting of eight cycles is formed, the counter-decoder DD2 is transferred to the zero state by the front of the ninth pulse (Fig. 2, f). In standby mode, the sweep generator is triggered by the controlled signal itself. In this mode, it can be started both by a positive input voltage drop (switch SA3 in the middle - according to the diagram - position), and negative (switch in the lower position). When a positive level difference appears at the output of the comparator, to which the differentiating circuit R12C9 is connected, a short reset pulse is formed at the input R of the counter-decoder DD2 (Fig. 3, e). As a result, a low-level voltage appears at output 8 and the multivibrator begins to generate pulses. When a high-level counter-decoder appears at this output, the generation stops. In other words, the sweep runs for one cycle. With a periodic input signal, a stable image is observed on the HQ1 indicator. The direct glow cathode of the indicator is connected to the GB1 battery through a current-limiting resistor R13 (output I connected to the conductive coating of the inner surface of the cylinder must be connected to its negative terminal).

Construction and details. The probe uses fixed resistors MLT, variable resistors SPO-0,15, capacitors KM-5. Instead of OU K140UD6, you can use OU K140UD7, K140UD8 (with any letter index), K140UD12, K140UD14, instead of K176 series microcircuits - their counterparts from the K561 series. Socket XS1, switches SA1-SA3 and switch QI can be of any type, it is only important that they are small.

On the front wall of the probe housing there is an XS1 socket with elements of the input voltage divider R3, R5, R6 and a switch SA1, switches SA2 (with capacitors C1-C8 soldered to its contacts) and SA3 (with capacitor C9), a power switch Q1, a variable resistor R4 and indicator HG1. Variable resistors R4 and R6 are equipped with scales, an approximate view of which is shown in fig. four.

Ris.4

The mark "X 1" of the scale of the resistor R4 ("Time / div.") Corresponds to the extreme left (according to the diagram) position of the engine, and the mark "1V" of the scale of the resistor R6 ("Level") corresponds to the extreme top (also according to the scheme). The remaining parts of the probe are placed on the printed circuit board (Fig. 5), made of foil fiberglass with a thickness of 1,5 mm. A design option is possible, in which the elements of the input voltage divider, together with the SA1 switch, are mounted in a remote probe (it will be more convenient to work with such a probe).

Establishing the device consists in setting (by selecting resistors R8 and R11) voltages of +100 mV at pin 2 of the op-amp DA1 and -100 mV at pin 3 of the op-amp DA2 relative to the midpoint of the divider R8-R11. . You can increase the brightness of the indicator segments by increasing the supply voltage to 9 V (in this case, the resistance of the resistor R12 must be increased to 13 Ohms).

Working with a probe requires some skill. If it is necessary to determine only the presence of pulses and their duration, then the variable resistor R6 ("Level") sets the sensitivity equal to 1 V, the switch SA2 ("Time / div.") Selects such a sweep duration at which one or two periods are displayed on the indicator signal, and a variable resistor R4 ("Time / div.") A stable image is achieved. If it is not possible to synchronize the image in this way, the device is switched to standby sweep mode with triggering by a positive or negative input voltage drop. The period of controlled oscillations or the duration of the pulse is determined by the position of the switch SA2 and the knob of the variable resistor R4.

If it is required to measure the signal amplitude, the knob of the variable resistor R6 and the switch SA1 are set to the positions corresponding to the ignition of the segments of a positive or negative (depending on the signal polarity) level. The amplitude (in the range of values ​​set by switch SA1) is measured on the scale of the resistor.

The shape of oscillations is determined by the nature of the change in the image on the indicator when installed with a variable resistor. R6 different sensitivity values. As an example, in fig. 6 shows the information displayed by the indicator when a triangular-shaped signal is applied to the input and various positions of the variable resistor R6 slider (the dashed lines show segment anodes that glow in full).

Ris.6

As practice has shown, it is not always necessary to achieve complete synchronization of the sweep - in some cases, the image of the controlled signal is perceived better if it slowly moves in one direction or another.

Authors: I. Sinelnikov, V. Ravich, Kaliningrad; Publication: N. Bolshakov, rf.atnn.ru

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