Spectrum Analyzer

[Eaglepride82]

I have an old Spectrum Analyzer (HP8591EM - EMC ANALYZER) and I would like to Measure the Amplitude and Frequency (Sound Level) of audiable noise that is present from coil whine of a common mode inductor.  Basically, is there an easy way to connect a microphone to a spectrum analyzer? Is there a simple preamp circuit than can be used with the microphone?  The purpose for these measurements are to demonstrate different dampening techniques and record sound level reductions.

[bob91343]

That spectrum analyzer probably isn't suitable.  It doesn't have the required frequency resolution to analyze audio signals.

I have a similar model 8594L and its best resolution is 300 Hz and not too good there at that.

[Eaglepride82]

OK. THanks.  This analyzer will scan 9kHz to 1.8GHz.  I was hoping I could put it in continuous sweep mode and scan the range from 15kHz to 20kHz.

[TimFox]

For audio purposes, the 9 kHz lower bound on typical SAs is a nuisance.  Within the range of your unit, you could use a preamplifier after a reasonable microphone.  In my collection, I have an -hp- 465A and two PARs:  113 and 117/114.
The 465A has a fixed bandwidth (covering audio up to about 1 MHz), a high-Z single-ended input, and can drive 50 ohms.
The two PARs have high-Z differential inputs and selectable hp and lp  filters.  The 113 has a 600 ohm output (needs a pad to drive 50 ohms neatly), while the 117/114 combination can drive 50 ohms.  I don’t offhand know the modern equivalents.

[daqq]

Unless there's something weird going on you should be able to pull it off with an oscilloscope + simple preamplifier.

[Eaglepride82]

Thanks Tim.  I have access to a few older preamps, all HP - 465A, 8447C, and 8447A.  I will have to look up the operating ranges for them.

[Eaglepride82]

Thanks Daqq.  I found a schematic to connect a Condenser mic with a LF353N circuit to couple to a scope with a 10uF cap.

[fourfathom]

You can do this type of analysis on a PC.  For example, take a look at Spectrum Lab, and Audacity.  Both of these have FFT and other audio analysis capabilities.