Hi If you are looking at the low frequency beat note out of a mixer and seeing multiple transitions on an edge - you filtering or your limiter are not up to the task. In most cases it’s the filter, but it can be either.
Bob On Oct 11, 2014, at 9:10 AM, Robert Darby <bobda...@triad.rr.com> wrote: > 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 >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
