Sweep generator for an oscilloscope. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Sweep generator for an oscilloscope. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In some designs of home-made oscilloscopes (and sometimes in industrial designs), when the level of the signal under study and its frequency change to a large extent, synchronization is disturbed, and in its absence (in standby mode), the sweep does not start. When operating such oscilloscopes, you often have to use the "SYNC LEVEL" knob, which, of course, is inconvenient.

The proposed sweep generator is free from these shortcomings. It provides a linearly rising voltage (LVR) generation time from 1 µs to 100 ms. The amplitude of the synchronization signals can vary from 50 mV to 5 V, and their frequency - in the range up to 20 MHz. In the absence of the studied signal, the generator automatically switches to self-oscillating mode. The generator can also work in pure standby mode.

The generator circuit is shown in the figure.

(click to enlarge)

LNN is formed on capacitors C1 and C2 High linearity is ensured by the fact that the capacitors are charged from a current generator made on the transistor VT1, which is powered by stabilized sources.

The amount of current through the transistor VT1 is determined by the resistance of one of the resistors Rl-R3 and the circuit of its emitter (selected by switch SA1).

The LLT period (in seconds) can be calculated using the formula:

T=CUm/fk,

where C is the capacitance of capacitors C1 + C2, F;

Um - LNN amplitude, V;

fk - collector current VT1, A;

In this design of the generator, the sweep period is set discretely by switches SA1 and SB1.1 (it changes the capacitance of the time-setting capacitor). Switch SA1 sweep period changes 10 and 100 times, and SB1 - 1000 times (for each of the positions of the switch SA1). Thus, a set of three resistors (R1-R3) and two capacitors (C1-C2) allows you to have six values of the sweep period. Their number and discretization can be changed by an appropriate choice of elements.

LNN through the buffer cascade (VT2, VT4) is fed to a single vibrator made on the elements VT5, DD1.1. The single vibrator response threshold and, consequently, the LNN amplitude depend on the divider R7R8. For the resistors R7 and R8 indicated in the resistance diagram, the LNN amplitude is approximately 3,5 V. At the end of the LNN formation, the single vibrator generates a pulse that is not fed to transistors VT3, VT6. Transistor VT3 opens and discharges capacitors C1 and C2 almost to zero, and transistor VT6 generates a reverse beam damping pulse. The amplitude of this pulse is about 15 V. If a large amplitude is required, then it is necessary to increase the supply voltage of the cascade and select the appropriate type of transistor. At the end of the action of the single vibrator pulse, the process is repeated.

If there is a signal under investigation at the input of the oscilloscope, it enters the Schmitt trigger, made on the elements DD1.3, DD1.4 and the transistor VT7. The Schmitt trigger generates pulses with steep fronts. These pulses are rectified by diodes VD2, VD4 and charge the capacitor C9. The voltage on the capacitor C9 opens the transistor VT8, and the voltage level of the logical unit is applied to the input 10 of the element DD1.2. Elements DD1.1 and DD1.2 make up the RS flip-flop. At the end of the one-shot pulse RS - the trigger remains in a state in which the transistor VT3 remains open. In this case, the charge of the capacitor C2 is impossible. From this state, the RS flip-flop outputs a differentiated Schmitt trigger pulse, after which the charge of the capacitor C2 begins again. The role of the differentiating chain is performed by the elements C7, R16.

In self-oscillatory mode (when there is no signal at the input of the clock pulses), the capacitor C9 is discharged and the transistor VT8 is closed. The level of logic zero at the input of element 10 DD1.2 and logic one at its output does not affect the operation of the LNN generator.

To transfer the generator to standby mode, +4 V voltage must be applied to the additional input of the device.

Transistor VT1 must be selected with a minimum value of the collector reverse current. Capacitors C1 and C2 must be film or metal film, C5 - type K15-5-H70-1.6 kV - 4700 pF, C9 - K50-6. The remaining capacitors are of the KM-5 or KM-6 type. Switch SA1 can be a nut or push-button with the required number of positions, SB1 - type P2K.

Establishing the generator is reduced to the selection of resistors R1-R3 according to the required sweep scale in each position of the switch SA1. Capacitor C2 is selected so that the sweep scale changes a thousand times when switch SB1 is turned on (μs - ms). For a more accurate selection, C2 can be made up of two capacitors.

Author: V. Greshnov, Ulyanovsk

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