On 01/06/2011 08:02 PM, Tijd Dingen wrote:
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 say.
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.
That 500 ps is conservative. I intend to do better than that, but just to give
some idea of the starting point... The main thing for now is to get a handle on
the DNL of the TDC.
I am currently playing around with some designs in a spartan-6 and a
spartan-3e. The spartan-6 design has more promise resolution wise, but it is
also not very hobby friendly (bga package). So if this is ever to be built
using hobby level soldering skills it will be spartan-3e. The 250k gates
devices are available in tqfp-100 package which is doable IMO.
BGAs solder with hot-air gun and correct use of fluss.
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.
Cheers,
Magnus
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