[time-nuts] Close-in phase noise measurements

[John Miles]

> I am continuing my phase noise measurement quest. I gathered
> equipment (HP 8662a/11729C/8568B/multipliers) to measure
> 100Hz+ from the carrier. I now need to get a grip on the
> 0.1-100Hz range, which is where most of my applications are.
>
> What is the suggested measurement methodology for this range?
>
> My first idea would be to squeeze out the most of the above
> equipment...maybe add a dynamic signal analyzer like the
> 35660/3561/3562? What is a good HP LF analyzer?
> This combined with multiplication (to 1GHz) and EFC
> locking, could take care of the 1-100HZ range?
> Anythings else needed?
> No ideas for  0.1-1... maybe by counting with the 5370B?
>
> I am sure this has been answered before but the archives are
> difficult to search...

So, I've started to look into the sub-100 Hz regime lately, but haven't had
time to get very serious about it.  Here's my take on the question:

1) The TSC-51xx analyzers are sure nifty if you have the budget and can live
with being limited to HF measurements.  They offer high performance with
less fiddling around than any other PN-measurement platform I've seen.

2) The next best thing would be a sound-card FFT option that works in
conjunction with an HF analyzer.  PN.EXE will do this at some point, using
the 11729's front-panel LF analyzer output.  It's really just a matter of
writing the UI code to support it.

3) The next best thing, after that, is a dedicated FFT analyzer, perhaps in
conjunction with an HP 3047A or 3048A system (i.e., an FFT analyzer plus a
35601A or 11848A interface).  Boxes like the HP 3561A and 3562A are neat,
but they are complete technological relics.  They are limited to about 80 dB
of dynamic range in a 100-kHz bandwidth, they take up quite a bit of space,
and they require either a lot of custom coding or some obsolete HP software.
The 3582A is in the same boat, only more so.  An 11729B/C plus a simple
sound-card interface will be the clear winner once the software support
issue is resolved.

Disclaimer: I do have a 3561A and a 3562A.  They are really cool boxes, but
if I ever write any code to support them in PN.EXE, it'll be because I think
they're cool, not because I think they're the right answer for any
FFT-analysis applications today.  Technology has left them behind in a
drastic way that hasn't happened with RF spectrum analyzers.

An FFT analyzer can still be used at offsets below 1 Hz, but at that point
people usually want to see Allan-deviation graphs rather than PN graphs.  It
becomes a different problem, since you most likely want to use a counter
rather than a spectrum analyzer.

-- john, KE5FX