[time-nuts] No State Of The Art Counter
tijddingen at yahoo.com
Fri Jan 7 22:45:25 UTC 2011
--- On Thu, 1/6/11, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> From: Magnus Danielson <magnus at rubidium.dyndns.org>
> Subject: Re: [time-nuts] No State Of The Art Counter
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Date: Thursday, January 6, 2011, 10:05 PM
> On 01/06/2011 08:02 PM, Tijd Dingen
> > To whom it may concerns,
> > Currently I am building a DIY frequency counter. Since
> this is my first serious counter project I am trying to keep
> things simple, hence It Will Not Be State Of The Art. Maybe
> a not-too-difficult hobby level counter will be of interest
> to some, so I'd thought I'd post here...
> > The architecture in a couple of bulletpoints:
> > - fpga based as much as possible to keep the parts
> count down
> > - coarse counters running at max 200 MHz for now
> > - interpolation is done using TDC's. The TDC's look
> suspiciously much like tapped delay lines and are
> implemented inside the fpga, using mainly the carry chains.
> > - 10000 continuous time stamps per second
> > - 500 ps timestamp resolution. And with resolution I
> mean the smallest resolvable thingy (related to bin size),
> not precision nor accuracy.
> 500 ps single-shot resolution is what you probably want to
Something like that yes. :)
> How will the input side work? How will you handle input
> signals of various kinds? In particular sine of various
> amplitudes and frequencies. Slew-rate can be a limiting
> factor as white noise will convert into jitter if hitting a
> straight comparator. Choice of trigger point can be done to
> achieve lowest jitter, so just AC-blocking and trigger on
> the 0V may not be the best solution if shape is not well
> known. Noise of input stage comes into play.
I am glad you asked. :) This is still one big To Be Determined at this point.
As you say, the choice of trigger point has a large impact on jitter. Do you have any suggestions on how to achieve this? Either for the situation where the signal shape is well known or not so well known.
> > The timestamps are transmitted over usb to the pc for
> > number crunching. The idea is to do some curve fitting to
> > get a frequency estimate, computate Allan Deviation, and do
> > the obligatory plots. With regard to Allan Deviation, as
> > long as I make sure the measurements have zero dead time, I
> > can compute Allan Deviation using the raw time stamps,
> > right?
> Yes. Make sure time-stamps has a format such that software
> can do time-wrapping extension.
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