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Oscilloscope Bandwidth
A member of our forum asks:
Hi people,
Can anyone help me with a different method for establishing oscilloscope bandwidth. My problem lies in calibrating 1M ohm impedance scopes up to 250M ohm. We currently use a levelled sinewave source (SG503), set up 6 divisions on the scope at 50kHz, increase the frequency until display drops by 3dB and hey presto a bandwidth measurement. All this would be great if the impedance of the scope stayed anywhere near 1M ohm. However at frequencies of 150-250 MHz the 1M ohm actually drops below 50 ohm on most scopes (depending on input capacitance) 50 and 50 in parallel means the SG503 is driving into anywhere down to 25 ohms. I have found that almost 1dB can be lost due to mismatch, and the uncertainty of measurement is BAD!
I know a fast rise can be used but that throws up a whole new set of problems regarding the mismatch and the filter properties of the scope input.
Can anyone help. How do calibration labs achieve good uncertainties?
Find this post here.
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