[time-nuts] Close-in phase noise measurements

[Shane]

Wenzel has a setup you can purchase at low cost.  

http://www.wenzel.com/pdffiles1/PNTS%201000/BP-1000-SC.pdf

Phase noise test sets can be pricey... $200K

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bruce Griffiths
Sent: Tuesday, March 25, 2008 5:43 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Close-in phase noise measurements

John Miles wrote:
>> 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.
>
>   
The range can be extended with a couple of external mixers (plus low 
pass IF filters) and a low noise offset source.
The offset source is mixed with each of the signals to be compared to 
produce a pair of IF frequencies within the 0-30MHz range.
> 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.
>
>   
Most, but not all, sound cards have a low frequency cutoff of 20Hz or so.
Some (but not all) sound card ADCs can dc coupled.
A high resolution dc coupled ADC may be more effective for frequencies 
below 20Hz.
> 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.
>
>   
Sound card support appears to be something of a minefield, baudline 
thinks my 16 bit 48kHz motherboard sound system is a 24 bit 192kHz system.
This probably means that the frequency scale and consequently FFT filter 
noise bandwidths are unreliable.
However with a low frequency noise calibration source and set of marker 
harmonics derived from a crystal these calibration issues can be resolved.
Windows software fares little better and some crashes when set to sample 
at 192kHz (the windows machine has a sound system with a 192kHz 20 bit 
ADC system).
> 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
>   
A finite beat frequency is required when using a counter.
With low beat frequencies a lower noise zero crossing detector than the 
counter input trigger circuitry is required to minimise system noise.

A dual mixer time difference system can have a lower noise floor than a 
single mixer system.

Bruce

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