Active probe for oscilloscope. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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The oscilloscope occupies a special place in the measuring laboratory of a radio amateur, because it allows you to "see" the processes occurring in the cascades of electronic devices. But sometimes the input circuit of an oscilloscope, which has a certain resistance and capacitance, can introduce distortions into the observed signals. That's when an active probe is connected to the oscilloscope - a kind of matching device, the input circuit of which has a significantly higher resistance and lower capacitance compared to the input circuit of the oscilloscope. Such a probe is described in the proposed article.

In [1], a description of a low-capacity active probe made on an insulated-gate field-effect transistor was published. Although the probe is designed to work with signals of relatively large amplitude, such as CMOS chip levels, it can also be used to examine small signals, because modern oscilloscopes have high sensitivity.

The probe made on the KP305I field-effect transistor has good frequency characteristics. At the same time, the use of such a transistor causes certain limitations both in the manufacture of the probe and when working with it. It is known that the insulated gates of transistors are easily broken by static electricity or mains voltage pickups. In addition, the electrical characteristics of the probe are largely determined by the parameters of the used transistor.

The range of commercially available insulated gate transistors is small, and only group I of the KP305 series is suitable, from which it is possible to select specimens with suitable parameters. It is almost impossible to make several probes with different characteristics. The use of a field-effect transistor with a gate in the form of a p-n junction in the probe makes it possible to eliminate the noted limitations. The possibility of such a replacement is based on the publication in [2]. A large range of produced transistors with such a gate makes it easy to select specimens with the necessary parameters in order to make probes with different characteristics.

The electrical circuit of the proposed probe is shown in fig. 1 - it is similar to the scheme given in [1]. The probe is the simplest source follower, the load of which is the resistor R3. Resistors R1, R2 form an input voltage divider.

In practice, it is necessary to carry out various measurements; it is impossible to make one probe "for all occasions". Therefore, it is advisable to have several probes made on the widespread KP302AM transistors and the KPZ0Z series and powered by a constant voltage of 9 V. The frequency characteristics of these transistors are slightly worse than those of KP305, therefore, the probes assembled on them are inferior in characteristics. The input capacitance and drain follower is practically determined by the pass capacitance of the transistor, and for KP302, KPZ0Z it is greater than that of KP305. In addition, with large input signals, the transistor may be in forward bias mode, when the pn junction of the gate opens and current begins to flow through it. For a transistor, this mode is not dangerous, since the current is limited by resistor R1, but the input resistance of the probe decreases and becomes equal to the resistance of resistor R1. The table shows the main characteristics of several probes and the parameters of the transistors on which they are assembled. Here Uzi.otc is the cutoff voltage of the transistor; Is.nach - initial drain current; Uo - constant voltage at the output of the probe in the absence of an input signal; Io - current consumption of the probe at voltage Uo; +Umax and -Umax - the maximum and minimum input voltage at which the transfer coefficient (Kper) of the probe is reduced to a level of 0,7 from the nominal value.

The main factor determining the operating range in the region of negative input voltages is the value of the cutoff voltage of the transistor. In the region of positive input voltages, the operating range can be extended by increasing the probe supply voltage.

On fig. Figure 2 shows the transfer characteristics of two probes at a supply voltage of 9 and 15 V. Increasing the supply voltage is more effective for a probe made on a transistor with a large lo value (Fig. 2b) than in the case of using a transistor with a small lo (Fig. 2 ,A).

Resistor R3 is chosen such a resistance to ensure the dynamic characteristics of the probe. With a large resistance of the resistor, the effect of "pulling" the decays of the pulses begins to appear. The power supply of any probe can be autonomous, for example, from a battery of the Korund, 7D-0,125 types, but in most cases it is convenient to power it from the device under study.

Installation of the probe is volumetric - the conclusions of the radio elements are connected directly to each other (Fig. 3). If the probe is intended to work with both large and small amplitude signals, it is advisable to place its parts in a screen to protect against interference, which can be made of a metal braid of a cable of the appropriate diameter. The probe is connected to the oscilloscope using a coaxial cable or a shielded wire of the minimum allowable (according to the operating conditions of the probe) length.

Resistors MLT-0,125 are used in the probe. The 22 MΩ resistor (in some instances) is small-sized, similar to those used in electronic wristwatches. Capacitor C1 - the same miniature or homemade, made directly on the resistor R1. To do this, the resistor is wrapped with a layer of dielectric film (preferably fluoroplastic), and a piece of shielding braid from the coaxial cable is put on top, which is then soldered to the right output of the resistor R1 according to the scheme. The end of the PEV wire 0,15 ... 0,35 is soldered to the left terminal of this resistor and the wire is wound onto the screen located above the resistor.

The capacitance of the capacitor is adjusted by changing the number of turns of the wire - the probe setting is practically reduced to this operation. You will need a rectangular pulse generator that provides an output signal amplitude of 2 ... 5 V at a repetition rate of 1 ... 10 kHz. The calibration pulses that are applied to the input of the probe must have steep edges. By changing the capacitance of the capacitor, the presence of steep fronts and decays of pulses on the oscilloscope screen is achieved. In this case, the amplitude of surges at the fronts should not exceed 10% of the pulse amplitude.

Literature

1. Grishin A. Active probe for an oscilloscope. - Radio, 1988, No. 12, p. 45.
2. Mezhlumyan A. Unusual operating mode of the field-effect transistor. - Radio, 1991, No. 3, p. 58-61.

Author: D.Turchinsky, Moscow

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