multimeter question

pucho812 · Mar 10, 2005

I got a fluke multimeter that reads DBM. For the first time I have had to calibrate the I/O of a DAW without a console. Which as per factory standard they are supposed to be +4DBM/u. Anyway I am interfacing my fluke with a XLR to banna. I have the standard balanced wiring on the XLR and on the banna side I am combining the cold and the ground together. Yet my meter is showing a reading of -2 DBM and not +4.
Am I lossing 6 DB in the cable from unbalancing it? Or is my meter calibration off?

DrFrankencopter · Mar 10, 2005

Connect the cold to the meter ground and the hot to the meter hot.

what is a dBM? I know dBV and dBu (1 volt = 0 dB and 0.775V = 0 dB respectively)...

Kris

alk509 · Mar 10, 2005

[quote author="DrFrankencopter"]what is a dBM?[/quote]

A dB scale referenced to the drop through a 600 Ohm load dissipating 1 milliwatt (hence the "m") of power. The drop just happens to be .775V, so for all intents and purposes, 1dBm=1dBu.

Peace,
Al.

tmbg · Mar 10, 2005

debalancing takes the two signals, inverts one, and sums them, considering that the two signals are identical but polarity opposite, you end up with one signal with twice the amplitude of the original signals. It just so happens that twice the voltage is 6dB, so yes, when you measure between ground and hot or ground and cold, you get a value that is 6dB lower than when you measure between hot and cold.

PRR · Mar 10, 2005

> I am combining the cold and the ground together.

Why?

And it isn't "cold". Balanced and Floating outputs have two "hot" pins. Unfortunately nobody ever gave them good names.

NewYorkDave · Mar 10, 2005

Short answer: measure between "hot" and "cold." Don't connect the meter to ground. Do not short either side of the signal to ground.

Shorting one side of the signal to ground can even damage your equipment, depending on the kind of output stage it uses.