Scoposcopy: Measuring Frequency With The Analog Oscilloscope
Quantitative frequency measurement with an analog scope? Without numbers in boxes? (by request… thanks shubus) Well, we did use one number from a box. You need some kind of external frequency standard so that you can properly set your scope’s vernier timebase control. I used a digital instrument, the Philips PM6676 oven-stabilized frequency counter and the Interstate F43 function generator to provide an accurate frequency reference, then I spun the dial on the F43 and measured the unknown frequency from the screen of the RM503. Compared to the reading of the unknown on the Philips, I was a little over one percent off, and I’m sure I could get even closer if I had both hands and both eyes to use. But your frequency standard could be almost anything. Use your guitar tuner to provide a known audio frequency and feed it to a microphone to your scope. Use the AC line frequency (50 or either 60 Hz quite accurately, usually) as a low frequency reference ! When displaying a waveform to measure its voltage or other characteristics in detail, we normally would display only 3 or 4 complete cycles at most. But to determine frequency we want as fine a measurement as our weak old eyeballs will allow, so set the timebase to give as many cycles as you can comfortably and accurately count, across the middle 8 graticule divisions on the screen. Then the time interval is simply 8 x the timebase setting, in seconds. So count the peaks carefully and simply divide. Watch out for the decimal …
