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From the Q&A

With TJ Byers


Scope Calibrator

Question:

I have an older oscilloscope that lacks a scope calibrator. I would like to build a square wave generator of about 1 MHz with a peak-to-peak voltage of exactly 5 volts. I can use this waveform to both calibrate the vertical scale and adjust the compensation of the scope probe. What do you have?

Mark Farrall
via Internet


Answer:

Your best bet would be to start with a 1-MHz crystal oscillator, then buffer its output so that it's isolated from the load (your test probe), like the circuit shown below.

The 78L05 regulator holds the output voltage stable within five percent of 5 volts, which is plenty adequate for all but the most demanding work. Although I specified the input voltage at 12 volts, it's only because 12 volts is a common source in most test instruments. Any voltage between 7 and 35 volts will work. Because the oscillator is crystal controlled, the output waveform can be used as a reference to calibrate the time base (each pulse is 0.5 µS wide, one cycle is 1 µS); this time base is stable to better than .001%; use the 40 pF trimmer capacitor to pull the oscillator frequency one way or the other.


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