generating proper waveform and use of dBm?

hey folks, i start a new thread just to get some more infos on my issue and some other questions.

i am going to do some calibration for a tapecho and i am said to put 3,8mV square wav into the input. unfortunately i cannot read the 3,8mV rms (aprox. 9mVpp) on my scope, neither on my multimeter. it's simply to low to be measured, it stops at 9mV and stays there on the multimeter even with no signal (the scope gets very muddy from their on). why is that like this? is it due to my scope? is it due to my signalsource? do i need a proper waveform generator (did it so far always with my rme/or my synth sh101)?

and another question: some service manuals i have do ask for inputsignals in dBm which is usually not common in audio. as i remember it it's related to P=0dB=1millwatt which i can get together from the input impendance and the amount of db. so lets say for +2dBm squarewave on 47Kohm you'd have to feed the input with 8,6308Vrms which would make 12,21Vpp on my scope. Is that right? Why is milliwatt used as a reference here?

dBm is an archaic notation for audio and was used interchangeably with dBu back when everything used 600 ohm terminations, today not so much, but they are generally talking voltage into an implied 600 ohm termination, so substitute dBu and you should be OK.

To generate a precision mV level signal when you don't have the resolution to measure that directly consider making a precision pad, with some 1% resistors, so you can directly measure a signal say 20dB hotter accurately.

This is commonly done with 40dB pad for feeding mic pre inputs, etc.

JR

JohnRoberts said:

dBm is an archaic notation for audio and was used interchangeably with dBu back when everything used 600 ohm terminations, today not so much, but they are generally talking voltage into an implied 600 ohm termination, so substitute dBu and you should be OK.

Click to expand...

but the unit is not (as i understand it) a 600ohm termination. the input impendace from input jack-1 is said to be 47Kohm with switch set at 0dB (service manual), or am i getting it wrong?

thanks for the idea with the precision pad, but to make it more clear - do you think it is due to my old scope that i am not able to get the resolution? would i be able to measure if i would get a newer scope (i have an grundig g10/13z) or is it usually not easy to measure such a low square signal?

Aye.. there's the rub...  dBm technically means power wrt 1 mW, but audio pukes routinely "intend" it to mean a voltage wrt .7746V.  The (once) defacto standard termination of 600 ohms, is how this all came about.

Dealing with precise measurements of small signals routinely use pads or fixed gain stages (like inside your scope or analyser) to make small signals look bigger.

I used to have a little DIY box on my bench that was a battery powered fixed gain stage, with something like 40dB of voltage gain and an "A" noise weighting curve, for quick and easy spot noise measurements.

Do whatever you like,

JR

JohnRoberts said:

Aye.. there's the rub...  dBm technically means power wrt 1 mW, but audio pukes routinely "intend" it to mean a voltage wrt .7746V.  The (once) defacto standard termination of 600 ohms, is how this all came about.

Click to expand...

okay, so this actually is interchangable? so i stick to dBu and i'll be fine? good, thanks for that. i think i will also try using a pad, thanks!