[time-nuts] DIY TimePod

[Bob Camp]

Hi

What sort of phase noise levels are you trying to measure? If you are after -80 dbc/Hz sort of numbers, there are a lot of ways
to do the job. For low noise stuff (-160 dbc/ Hz) the quickest  way to do it is a mixer / two oscillators in quadrature and a sound 
card. The “pure digital” equivalent of that is an XOR gate running into a sound card. The XOR is not as quiet as a proper mixer. 
The “detector” (ADC / mixer / XOR) is what sets the noise floor and limits measurement capabilities of the device. 

Bob



> On Jun 12, 2016, at 11:50 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?
> 
> Thanks,
> 
> John S.
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