Electronics > Beginners

Scope and Pk-Pk Values 1x vs 10x

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Dreded:
I am confused... I have never used a scope to check anything other than basic Blink type crap and mainly Serial communication(my usual job is to pass fail based on a serial output.. I dont fix)
so I have a 12v power supply signal that seems quite noisy it comes from an industrial VFD
if I use a 1x probe with scope set to 1x the Pk - Pk on the noise is 168mV
if I use a 10x probe with scope set to 10x the Pk - Pk on the noise is 1.60V

clearly this is 10x of difference.. however since the signal is sitting where its supposed to be shouldn't the noise be relative too?

the noisy signal repeats at ~ 8.8us


David Hess:
The x10 probe has much higher bandwidth than the x1 probe so more high frequency noise is included in the measurement.

schratterulrich:
In addition to the higher bandwidth of a x10 probe, there may be another effect.
A x10 probe does not attenuate the shield voltage effect caused by common mode currents.

Try to connect your scope ground and tip together... now touch the shorted probe to ground of your power supply. You should only see the shield voltage. And this voltage is not influenced by the x1 or x10 setting.

See "High-Speed digital design, A Handbook of Black Magic" from Howard Johnson for details.

dzseki:
Also make sure your head in 10x position is properly compensated using the scope's square wave generator.

Dreded:

--- Quote ---The x10 probe has much higher bandwidth than the x1 probe so more high frequency noise is included in the measurement.
--- End quote ---
still confused... so the noise is +/- 100mV or +/- 1V? with +/- 1V of noise how can I figure out if my PSU is dropping 1volt for real?

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