Re: Calibrators vs Signal/Function Generators


Chuck Harris
 

If you use a function generator, at the very minimum, you will have
to use it in conjunction with an accurate frequency counter, and an
accurate level meter of some sort... say an AC True RMS voltmeter.

Tektronix made their calibration equipment easy to calibrate to
national standards. For instance, their voltage calibrator PG506,
uses very, very low impedance transistor switches to switch a zener
derived DC reference voltage on and off to make a square wave.

Because of this topology, the PG506 can be calibrated by setting
its low impedance switch to ON, and measuring the DC voltage output
of each output voltage position.

The switch's resistance is a tiny, tiny fraction of the 1M input
impedance of the scope, so as to be a part per million, or so, which
is insignificant.

Your function generator is built in an entirely different direction,
and cannot be calibrated to this sort of accuracy... so it will need
to be measured. Most AC meters aren't anywhere near this accurate,
either... or, if they are, it is only at one frequency... likely 60Hz.

You can do what you want, but it will be slow, tedious, and error prone,
and when you are done, it won't be as accurate as your AC voltmeter..

For timing measurements, amplitude isn't important, but a very fast
rise time pulse is, so small errors in where you line up the graticule
on the pulse will be insignificant. The square wave output of most
function generators is not even close to fast enough... And can in
no way make high enough frequency signals to align the higher time base
speeds. An RF synthesizer can, but it makes a sine wave, which is
hard to line up on the graticule, and to make adjustments for linearity
of the sweep...

Your time ought to be worth something. If you have a "pile of scopes"
that you will be looking to calibrate, then you ought to either hire it
out, or get the proper equipment to do the job effectively.

-Chuck Harris

David Berlind wrote:

Question for the hive:

I have a pile of scopes here that I'll be looking to calibrate at some point as a part of their resurrection. I see that the primary function of some Tek calibration fixtures is to generate square and sine waves. How is this significantly different from what a well-calibrated signal/function generator can do?

Thank you.

David



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