Measuring Industrial Control Noise with a Spectrum Analyzer

[connor_v]

I have a 0-10V signal from a sensor in a industrial environment (~25 meters of shielded cabling) with unacceptable noise (>10mV).  I want to see what frequencies the noise is coming from. I have access to a spectrum analyzer, an oscilloscope, and a 6.5V benchtop DMM. I measured the noise with a DMM but it has a built in 200Hz filter so I can't determine frequency. I then measured the noise with an oscilloscope on AC coupling and did a FFT but it can not measure such low of a signal. I am considering getting a LNA10 to amplify the signal and determine it's frequency. Finally I have access to a spectrum analyzer. I was planning to get a DC blocking inline cabling but have a few questions. 1) Am I correct in needing a DC blocking adapter do I need to do any thing else to protect the signal analyzer? 2) Do I need to do a 50 Ohm termination somewhere? 3)What figure in the data sheet of the spectrum analyzer tells me how low of a signal it can measure? 4) any other suggestions for determine where the noise is coming from?
Thanks!

[srb1954]

I have a 0-10V signal from a sensor in a industrial environment (~25 meters of shielded cabling) with unacceptable noise (>10mV).  I want to see what frequencies the noise is coming from. I have access to a spectrum analyzer, an oscilloscope, and a 6.5V benchtop DMM. I measured the noise with a DMM but it has a built in 200Hz filter so I can't determine frequency. I then measured the noise with an oscilloscope on AC coupling and did a FFT but it can not measure such low of a signal. I am considering getting a LNA10 to amplify the signal and determine it's frequency. Finally I have access to a spectrum analyzer. I was planning to get a DC blocking inline cabling but have a few questions. 1) Am I correct in needing a DC blocking adapter do I need to do any thing else to protect the signal analyzer? 2) Do I need to do a 50 Ohm termination somewhere? 3)What figure in the data sheet of the spectrum analyzer tells me how low of a signal it can measure? 4) any other suggestions for determine where the noise is coming from?
Thanks!

The spectrum analyser data sheet should have a figure for the amplitude measurement range. This is usually specified in dBm so you need to convert that into the equivalent mV or uV figure. There are plenty of on-line calculators that will do that calculation for you.

If using a spectrum analyser in an industrial environment you should take precautions to protect the spectrum analyser input against transients. Spectrum analysers are very sensitive and the inputs are easily damaged by transients.

I would recommend that you at least use a limiter on the input. With the signal levels you are seeing you could also afford to use an attenuator, say 10-20dB, on the input to provide additional protection. Both the limiter and the attenuator should go after the DC block capacitor - this will produce a large spike when initially charging which needs to be clamped by the limiter/attenuator.

[nick liu]

The spectrum analyser data sheet should have a figure for the amplitude measurement range. This is usually specified in dBm so you need to convert that into the equivalent mV or uV figure. There are plenty of on-line calculators that will do that calculation for you.