Hi ,
I've built a 2254 without the limit function and want to setup and test the noise floor.
I have a laptop and REW (linux)
Is there a specific setup to get good results to test noise floor so I can see if I'm in the ballpark please?
I think REW is not adequate for noise measurement.
First you have to be aware that measured noise depends on the measurement bandwidth. Dedicated noise mesurements apply filters, usually at 22Hz and 22kHz; AFAIK, REW does not. A workaround is to do measurements at 44.1k Sampling Rate.
For some reason, the value that is displayed in the measurement window seems to correlate poorly measurements done with dedicated harware; it measures sinewaves correctly, though.
I use REW for frequency response measurements, mainly because its generator is practical, but never had confidence in noise measurements.
I would recommend you use RMAA, which has an option for measuring noise in the 20Hz-20kHz BW.
Very often, I see people posting noise spectra (an FFT of the noise), claiming they measure -120dBu noise floor. This is wrong! Here is not the place to explain why. It is a good investigation tool to pinpoint interference sources, such as transformer and PSU- induced, though.
Finally, before attempting any noise measurement, your system must be calibrated, by injecting a known signal (you can measure accurately enough a 0dBu 1kHz tone with a basic DMM).