[time-nuts] Spec An for phase noise measurements
David I. Emery
die at dieconsulting.com
Tue Jan 22 23:04:00 EST 2008
On Mon, Jan 21, 2008 at 05:31:14PM -0500, David I. Emery wrote:
> Spectrum analyzer front ends often either have a blocking
> capacitor (to protect the mixer from DC) or don't. The kind that
> don't usually start to roll off pretty significantly below 10 KHz, and
> are typically spec'd only to 9 KHz. I suppose if one wants to live
> REALLY dangerously the cap might be removable in a few cases (NOT
I apologize to Matt - I was looking at SA specs this evening
and the 8596E has the capacitor out performance spec'd only down to 9
KHz (100 KHz with the capacitor in)... no real idea of what might
actually limit performance in the DC coupled case except the LO quality
of the analyzer itself...
It is quite surprising that the 859xE series is spec'd about
10-15 db worse on phase noise than the comparable 856xE instruments. I
wonder if this difference exists in the performance or just marketing
domains and what the actual differences are in the implementation that
accounts for it.
> But would I be too simple minded to suggest that maybe some form
> of A/D PC/workstation input device with high dynamic range and decent
> sample rate (certainly available in high end audio stuff to 192 KHz)
> would be the logical vehicle for close in measurement in a quadrature
> locked PLL type phase noise setup ? Otherwise why would you care about
> performance below 9 KHz ?
This was poorly phrased - I perfectly well know why one is
interested in phase noise on LOs and other signals closer to the carrier
than 9 KHz (no question there for me at least) but what I meant is why
one would want to use a general purpose wideband SA for making
measurements between 0-9 KHz or so... when FFT based technology is
available and cheap and very high dynamic range with very tight filters.
(Obviously this means high end soundcard type devices on a PC platform
I might also add that many of the cheap 24 bit 96/192 Kbs USB
sound input devices actually use codec chips which are DC coupled
internally all the way to the input pin though that is usually
referenced to half the analog supply voltage (3.3V/2 typically) rather
than 0. So going all the way to DC with one of these chips is not
impossible with external non-audio signal processing feeding the analog
input pin, thus removing the 20 Hz high pass from the audio preamps
ahead of the codec chip.
Dave Emery N1PRE/AE, die at dieconsulting.com DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."
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