Re: in-circuit spectral noise characterization?

On Oct 21, 1:56 am, BW <bjorn.we...@xxxxxxxxx> wrote:
Hello!

When designing ultra low-noise PSU's and related circuits, what do you
use to characterize them with regards to noise ? One of my component
suppliers for example use an old HP 3571A Dynamic Signal Analyzer,
which is a DC-100 kHz spectrum analyzer, with a 50 ohm coax soldered
directly on top of the measuring-point at the PCB. Supposedly this
allows them to find (spectral) noise on the critical supply-rails on
the order of 10-100 microvolts.

Is this realistic ? I do need to measure noise at an absolute level of
<< 1 mV but it is OK to solder the measurement cable directly to the
design, I don't need to probe around.

Even if that setup works, the problem is that HP discontinued this
device long ago and the Agilent successor the HP35670A is also a very
old technology part. I'm sure there MUST be a modern way of measuring
this, but all spectrum analyzers I find now are geared towards RF
measurements, 3G/GSM/RFID demodulating etc., with frequency responses
of 9 kHz - 3 GHz but I need to go down to almost DC. I guess telecom
is a much bigger market...

Perhaps there are options to buy for oscilloscopes adding a low-signal
FFT mode and I'm sure the sales-guy or FAE would tell me just that,
but input from someone who has actually used it is more helpful of
course :)

I should add that the reason I'm looking for a spectrum-analyzer and
not just a very sensitive pre-amp for an oscilloscope is because my
parts that need power have different sensitivities to noise in
different frequency ranges.

Any recommendations would be received gratefully!

Brgds
/Bjorn

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.
http://www.thinksrs.com

.

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