Products > Test Equipment
Fluke 79 179,... wrong readings
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temperance:
The setup:
-A  1 Vrms 1 KHz sine wave, transformer balanced signal with 600 Ohm impedance, completely floating. (The source is powered from a 9V battery.)
-ESD mat on the table and both the signal source and the Fluke are on the ESD mat.

I was expecting to see 1 Vrms at the output but instead I got 0.56 Vrms. Expecting the Fluke to be broken I take the same measurement with an other Fluke. This one seems to show 0.8 Vrms. But after changing the polarity, this one also measures 0.56 Vrms instead of 1 Vrms.

Confused I measure the signal with an oscilloscope and the signal is indeed 1 KHz, 1 Vrms as should be.

I then put the Fluke which was still connected to the signal source aside, of the ESD mat. But to my surprise, the reading is now 1 Vrms.

The Fluke manual states nothing about the source impedance for AC signals. What's wrong here?
bdunham7:
Tell us about the test leads.  Perhaps a picture?  I don't think the source impedance is an issue here.  Does the oscilloscope measure 1V when the polarity is reversed as well?  How about if both are connected simultaneously?
temperance:

--- Quote ---Tell us about the test leads.  Perhaps a picture?  I don't think the source impedance is an issue here.  Does the oscilloscope measure 1V when the polarity is reversed as well?  How about if both are connected simultaneously?
--- End quote ---

-The test leads are the regular Fluke supplied test leads.
-The output is floating and the unit is battery powered. The oscilloscope measures correctly 1 V rms in both "directions".
-I didn't try connecting them both simultaneously.

The Fluke only measures 1 V rms as it should when not placed onto a conductive earthed surface.
bdunham7:

--- Quote from: temperance on October 31, 2023, 01:47:33 am ----I didn't try connecting them both simultaneously.

The Fluke only measures 1 V rms as it should when not placed onto a conductive earthed surface.

--- End quote ---

So try connecting them simultaneously--that will answer a lot of questions.  This is surprising behavior at 1kHz, so I'm curious as to the cause.  I don't have either of the meters you mention, so I can't do any experiments.  The most obvious idea is some sort of capacitive coupling, but 1nF would have an impedance of 159k at 1kHz.  You'd need many times that to affect your 600R output impedance.  Hmmmm.
temperance:
This happened at a service company which I sometimes visit. In a few weeks I might visit them again. Pictures will follow.
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