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Old 19th May 2004, 08:39 AM   #1
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Default Why am I picking up so much noise on my scope?

Here are some "measurements" I made with my scope, O think there's much noise on it, is this normal?

1st picture, nothing connected to the scope (no probe)
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Old 19th May 2004, 08:43 AM   #2
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2nd picture, only a 10:1 probe connected on the scope, but not connected to any circuit.

The probe's tip is directly connected to the ground wire of the probe
You can see some 50Hz and HF

Note1: the amplitude of the 50Hz varries when I place the probe elsewhere

Note2: all the measurements have been made in storage mode, otherwise the camera would only take the spot and not the full screen
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Old 19th May 2004, 08:45 AM   #3
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3rd picture, the same configuration as above (10:1 probe, with the tip connected to ground) but touching a metal ruler (20cm)

The HF is much more important
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Old 19th May 2004, 08:48 AM   #4
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4th picture: the probe, and the way it was "connected to ground" during the tests
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Old 19th May 2004, 08:51 AM   #5
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RFI?

Do you have any high pass filters that can be set?
It looks like the scope's sensitivity is set to maximum?

I have a 100 Mhz Hewlett Packard 'scope that does
have a visible internal noise floor with the input set
to the lowest range. I think that's normal.
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Old 19th May 2004, 08:58 AM   #6
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What do you mean by RFI?

Yes the scope is set to its maximum sensitivity.
No I don't have any HP here, but I could build one it needed.

I didn't manage to measure the noise's frequency, since I can't get my scope to trigger on the signal
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Old 19th May 2004, 09:12 AM   #7
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First of all: You won't hear this noise anyway, so why do you care ?

2nd (and serious) try:

1.) The loop that you form with the ground wire will be able to pick up some noise and RF. If you a curious enough you might connect a shorted BNC to the input and see if there is an improvement.

2.) This small amount of noise isn't that bad at all with the scope set to its maximum sensitivity.

Regards

Charles
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Old 19th May 2004, 09:39 AM   #8
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Quote:
Originally posted by phase_accurate

This small amount of noise isn't that bad at all with the scope set to its maximum sensitivity

It makes my scope totally uneffective to measure power supply noise
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Old 19th May 2004, 09:47 AM   #9
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Compared to the 150MHz Tek probe, my selfmade Kwak Probe (I hope elso won't mint if I call it this way ) that is made of a 1m RG58U coax cable with a 50R at the scope side, is dead quiet
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Old 19th May 2004, 09:57 AM   #10
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Quote:
Originally posted by Bricolo

It makes my scope totally uneffective to measure power supply noise
You must probably make yourself a preamp with gain of 100 or 1000.

AD797, LT1028 etc with batteries. Don't forget to a LP-filter so you only measure < 20 kHz.

Your GND cable is an antenna! You can twist it around the probe in order to get a smaller loop (=antenna).
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