[time-nuts] Digital Mixing with a BeagleBone Black and D Flip Flop

[Robert Darby]

Simon,

Welcome to the tangential world.

I'm sure the clean edge I saw was an aberration, perhaps analogous to 
phase locking in oscillators; I don't think it's desirable because 
common sense tells you that with imperfect clocks and small phase 
differences there are bound to be some number of glitches at each 
transition.  I did nothing specific to eliminate the glitches, it just 
happened that the positive going transition was very clean but there's 
no reason I am aware of to suggest that one transition should be better 
in this respect than another. Perhaps the flip flop I was using had a 
shorter set-up time on negative to positive transitions than vice versa; 
the smaller the set-up time the more likely one is to capture borderline 
events?

I seem to recall that Didier Juges and Bruce Griffiths had some 
discussions re DDMTD's (although I can't find it in the archives) but in 
any event you could do far worse than dropping them a note directly to 
ask them about their thoughts on the matter. I'm sorry I can't provide 
any analysis of your data; just not in my skill set.
Perhaps Marcus or TVB could comment.

Bob Darby

On 10/10/2014 3:46 PM, Simon Marsh wrote:
> Bob,
>
> It's good to know someone else is trying this and it's not just me 
> going off on a tangent somewhere. I'd be very interested in 
> understanding how you'd set this up and how you'd got a nice clean 
> rising edge.
>
> My understanding is that the 'glitches' occur because the clocks are 
> being sampled at a higher resolution than the cycle to cycle noise 
> inherent in both the clocks and the setup. Certainly, I don't expect 
> any of the oscillators I have available to be perfectly stable at 
> ~1E-12 resolution, I'm sure they are all over the place The clock 
> phase noise shows up as fast transitions near the actual beat edge as 
> the clocks wander backwards and forwards over a few cycles. I'm sure 
> analysis of the glitches themselves would probably say quite a lot 
> about the cycle to cycle noise.
>
> I've attached an example of the transitions near an edge for a random 
> TCXO. The edge goes from 0 at the start to 1 at the end and shows 
> noise over about 180 samples (@10mhz). This corresponds to about ± 
> 5E-11. The crossing line of the zero & one counts is where the edge is 
> measured from the software point of view.  ± 50ps sounds high to me, 
> but I'm open to views as to whether that seems reasonable or just 
> shows my shoddy setup ?
>
> For fun, also attached is plot of the transitions for a UBLOX8 GPS 
> module outputing 10mhz. Compared to the TCXO that has about 10k 
> transitions in a second's worth of data, the UBLOX module has over 
> 1.3M (this is with a beat frequency of ~60hz). I think this is down to 
> how the gps module is inserting/removing cycles to get 10mhz from its 
> internal clock frequency (as has been discussed on here recently).
>
> Unfortunately, I don't have any expensive counters, that's part of my 
> motivation for doing this, so I'm interested in ways that I can 
> understand the noise floor.
>
> I tried passing one clock through a 74AC hex inverter and then 
> measuring the phase between the inverted/non-inverted signals on the 
> basis that this should be more or less constant and what I'd be 
> measuring was noise. It's certainly a good way of measuring how long 
> the wire was that I used to make the connection   This seems to yield 
> an ADEV of 5.92E-11 @ 1 sec, plots also attached.
>
> Interestingly the phase seems to drift over the measurement interval, 
> I'm open to suggestions on this, but guess this may be temperature 
> related ? (open on bench, non-airconditioned etc)
>
> If the plots don't come through as attached, they are also on google 
> drive here:
>
> https://drive.google.com/open?id=0BzvFGRfj4aFkSEdYV3lXcmZIVTA&authuser=0
>
> Cheers
>
>
> Simon
>
> On 10/10/2014 02:01, Robert Darby wrote:
>> Simon,
>>
>> I breadboaded a set-up in March using 74AC74's and two 10 MHz Micro 
>> Crystal oscillators (5V square wave), one as the coherent source and 
>> one as the 10Hz offset clock. I had no glitch filtering as described 
>> in the article you cite (CERN's White Rabbit Project, sub nanosecond 
>> timing over ethernet) but found the positive zero crossing was very 
>> clean.  The negative crossing not so much; no idea why one edge was 
>> clean and the other not. To ensure I only measured the rising clock 
>> edge and not the noise on the falling clock, I programmed ATiny's 
>> (digital 555?) to arm the D-flops only after a period of continuous 
>> low states.
>>
>> In any event, the lash up, as measure by a 5370, produced a clean 
>> linear noise floor of 8e-12 at 1s. I regret to note that's very 
>> slightly better than my results from the Bill Riley DMTD device. 
>> That's an indictment of my analog building skills, not his design.  
>> It's also nicely below a 5370 on it's own and needs only a simple 10 
>> MHz counter for output. The zero crossing detectors for sine wave 
>> oscillator input will perhaps be more critical.
>>
>> This was encouraging enough that I thought I'd try to build an FPGA 
>> version of the same. The DDMTD is temporarily on back burner while I 
>> try to get a four channel 1ns resolution time tagger running on the 
>> FPGA to use with the DMTD.  Almost there. I look forward to hearing 
>> your results with the BBB; keep us posted.
>>
>> Bob Darby
>
>
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