[time-nuts] DIY TimePod

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Jun 13 03:51:02 EDT 2016

Adjusting the sampling clock frequency so that is neither a harmonic nor a harmonic of a subharmonic of either of the clock frequencies being compared ensures that each clock waveform isnt repeatedly sampled at a small set of  points.i.e. 
fsample != (m/n)*ftest 
andfsample != (p/q)*freference
where m,n,p,q are integers.
When using a dual reference the difference between the frequencies of the pair of reference sources should be significantly less than the lowest offset frequency of interest.

    On Monday, 13 June 2016 6:00 PM, John Swenson <johnswenson1 at comcast.net> wrote:

 Hi TimeNuts, this is my first post to this list, I've been reading it 
for years but haven't needed to post, now I'm starting a project and 
need some advice.

I need to do a bunch of phase noise measurements but can't afford the 
"big guys", the TimePod seems perfect and since the schematic has been 
published I decided I would try my hand at making my own version.

I'm just doing phase noise measurements of digital clocks (square waves) 
so it seems to me I don't need some of the circuitry in the TimePod, in 
particular the digitally controlled RF attenuators and the ADCs 
themselves. My idea is to use LVPECL flip-flops to sample the DUT and 
reference clocks, convert the differential outputs to CMOS and feed the 
FPGA inputs from that. Yes you loose AM noise riding on top of the 
square wave, but is that really necessary for just square wave phase 
noise measurements?

For a first pass cheap and dirty version of this I was planning on using 
the LVPECL version of the Crystek 575 for the sample clock, will this 
work? The TimePod schematic shows a VTUNE signal fed to the OCXO, if I 
don't use that is something going to break? In other words will timelab 
try and tweak the sample freaquency and get confused when nothing happens?

I plan on using the 2 reference clock measurement technique, but have a 
couple questions about this. In the TimePod ch 0 and 2 are the input, 
with separate jacks available. The "ref" input goes to ch 1 and 3. So it 
looks like the two references have to go to 0 and 2 and the DUT to 1 and 
3, even though that puts the references on the "input" and the DUT on 
the "reference". Do you need to do anything special in TimeLab to 
support this or does it automatically support it? Since I am doing my 
own hardware and have four independent inputs do I do the same thing 
(ref clocks on 0 and 2 and DUT on 1 and 3) or put the refs on 1 and 3 
and the DUT on 0 and 2?

Any thoughts?


John S.
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