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

Simon Marsh subscriptions at burble.com
Fri Oct 10 15:46:29 EDT 2014


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:




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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