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Oscilloscope calibrator

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2005-2006 » Archive through 07 December, 2005 » Oscilloscope calibrator « Previous Next »

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Petelobus
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Username: Petelobus

Post Number: 38
Registered: 06-2005

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Posted on Saturday, 03 December, 2005 - 10:31 am:   Edit Post Delete Post Print Post

Having now discovered that the main fault with my D63 scope is the calibaration output value altering. I have calibrated it at DC, but not sure how good the amp specs are at DC, therefore I thought that I should build a square wave calibrator. Trouble is that the only thing that I can measure the output with is the scope which is out of cal!

Thought that I should be able to design something that can be set up using only a DVM and some precision resistors. Closest I've got so far is a fixed voltage source, and a changeover analogue switch, driven by a crystal controlled (or mains derived) square wave, with a unity gain op amp buffer following, to reduce the impedance effects of the switch. Seems a bit inelegant though.

Any thoughts?

BTW, spec I guess is P-P voltage about 1v +/- 2%. fast edges on the square wave nice but not essential, frquency ideally about 1khz

Ian
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Dean_huster
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Username: Dean_huster

Post Number: 110
Registered: 05-2005

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Posted on Sunday, 04 December, 2005 - 01:42 am:   Edit Post Delete Post Print Post

Don't be consumed by the calibration signal having to be a square wave, although it is the easiest to use. Sine waves are more prevalent and more easily measurable. Figure the RMS voltage needed to generate a peak-to-peak voltage that you need. For instance, 1 volt p-p is 0.5v peak which is 0.3535 v RMS. Use a variac into a transformer to generate this voltage as measured with a half-way decent DMM which will most likely have an accuracy good enough for this use (the scope is no better than ±3% and most decent DMMs are at least ±0.5% on ACV. Calibrate/check the scope from this voltage and generate the other necessary voltages in the same way.

Vrms = 0.707(Vp-p)/2


You can use a common function generator or HF signal generator and frequency counter to generate the proper timing signals to check timebase accuracy.

Dean
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Ron_h
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Username: Ron_h

Post Number: 15
Registered: 07-2005

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Posted on Tuesday, 06 December, 2005 - 04:12 am:   Edit Post Delete Post Print Post

Build a CMOS square wave oscillator (Google). When lightly loaded (scope input), its output will swing rail-to-rail. Set VCC with a DVM, and you have an accurate voltage calibration square wave. If you need an adjustable VCC, use an LM317 or LM317L. The datasheet should show you how to do it.
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Petelobus
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Username: Petelobus

Post Number: 40
Registered: 06-2005

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Posted on Wednesday, 07 December, 2005 - 10:09 am:   Edit Post Delete Post Print Post

Thanks Ron, thats a good idea. Can check that the output is rail to rail by swapping scope input from osc. output to supply rail (if traces line up it will tell me that all is ok, but if they dont it could be scope amp gain is dependant on frequency, or osc output not rail to rail).

Ian

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