in-circuit spectral noise characterization?

"BW" a écrit dans le message de news: snipped-for-privacy@x41g2000hsb.googlegroups.com...

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For signal or noise measurements below 20KHz a (good) (external) sound card connected to your PC gives surprisingly good qualitative results with their

Yours, Robert

I think the key here is that measuring power supply noise is different from a general purpose noise measurement, in that your souce impedance should be *extremely* low. So you may be able to improve the noise measurement by taking advantage of this. You can go all-out for very low voltage noise, without worrying about current noise (usually you have to trade-off).

You could make an ultra-low noise preamplifier, perhaps using one of the very low (voltage) noise opamps such as LT1028 or AD797. And/Or use a transformer to step up the noise voltage. I seem to recall a discussion here about using multiple parallel jfets to get really low voltage noise for power supply noise measurements - was one of the regulars here (but brain not working today unfortunately).

Anyway, then feed the output into your HP35670A and make measurements as usual, subtracting the gain of your preamp of course.

The soldered coax is very good. Pass it through one of those common-mode ferrite things to reduce ground loops.

We use scopes for this, but we care mostly about time-domain noise. For freq domain, use a spectrum analyzer. Oscilloscope FFTs aren't very good.

If you see a spectral line, switch the DUT on and off to be sure it's coming from your supply. Expect fake lines from ground loops, PCs and monitors, stray RF, etc.

John

I found the thread now; it was Fred Bartoli I was thinking of.

(This may be overkill for you).

So what exactly is wrong with an old HP DSA if it still works? Most DSOs have 8 bit sample heads, which is kind of coarse for noise measurements.

I never used one of their DSAs, but SRS makes products like the older HP DSAs.

Hi!

Agilent sells a replacement for the old HP, but that is also a very old piece of equipment by now. It probably works fine for this purpose, but the thing that bugs me is that there has to be a more modern way of measuring noise in bias-supplies and power-supplies in noise-critical circuits than this. Perhaps there isn't, I don't know. Sometimes an old "work-horse" sticks because the market changes so the modern instruments are optimized for other tasks (i.e. telecom).

Thank you for the link to SRS, I'll check that out as well. Also, yes, my feeling as well was that oscilloscopes are not really useable for this.

/Bjorn

Yes! SRS makes a nice (old) spectrum analyzer for DC to 100 kHz. (SRS

George

Tek has some nice FFT spectrum analyzers that do 40 MHz at a time, and work down to (really) DC. (The Agilent ones don't.) I have both HP3562 (true double channel) and HP35660A (fake double channel) DSAs, and am often frustrated by the 102.4 kHz maximum frequency.

Cheers,

Phil Hobbs

I have a HP 3562 also. While it doesn't do DC, it goes very low in frequency. I don't have the specs handy, but something like 60uHz resolution comes to mind. Yeah, the high end limit sucks.