Sampling Oscilloscope Working and Block Diagram


The general cathode ray oscilloscope generally does not have a very wide bandwidth of operation. Hence, the direct tracing of waveforms for a very high-frequency signal may not be possible. Since the brightness and sharpness of the output image on the screen decrease at a very high frequency of operation.  So, we require some different technique for the proper tracing of an ultra high-frequency signal. Hence, the sampling technique comes into the picture. And, the sampling oscilloscope is a device which applies the sampling technique to trace the waveform.

Sampling Technique of Tracing

In the sampling technique, a large number of dots assemble a complete waveform. Each of the dots comes from one successive cycle of the wave one by one. In other words, one drop comes from one small portion of a cycle of the wave. Then the next successive dot comes from next small portion of the next successive cycle of the wave.

sampling tracing of waveform

Working of a Sampling Oscilloscope

The input signal passes through a diode sampling gate circuit before it enters the vertical amplifier.  A chain of sampling pulses controls the diode sampling gate. Each time the sampling pulse triggers on the diode sampling gate, the input signal reaches to the vertical input. As a result, a very tiny part of the wave reaches the vertical amplifier and creates just a dot on the screen. Once one dot is created, the next sampling pulse actuates to the diode sampling gate during the next cycle of the input signal wave. But the thing happens for the tiny portion of the wave just after the same of the previous cycle.  The horizontal sweep ramp signal maintains the horizontal position of every next dot on the screen.

waveform of a sampling oscilloscope

Block Diagram of a Sampling Oscilloscope

There is a trigger circuit which actuates a blocking oscillator. This blocking oscillator feeds a sweep ramp generator circuit. The output of the sweep generator is fed to a voltage comparator circuit.  The voltage output of the voltage comparator circuit runs a staircase waveform generator.

block diagram of a sampling oscilloscope

The staircase waveform generator directly feeds the horizontal amplifier and also feedbacks the voltage comparator through an attenuator as shown. By comparing the input voltage from the sweep ramp circuit and the feedback signal coming from the staircase waveform generator, the voltage comparator generates sampling pulse signals. Each time these two comparing voltages become equal, a pulse is created by the voltage comparator circuit. These pulses then actuate the diode sampling gate for samplings of the input signal. Also, the sampling pulses control the staircase waveform generator.

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