[time-nuts] Homebrew frequency counter, need help
kb8tq at n1k.org
Thu Nov 27 22:15:23 EST 2014
One way of looking at resolution is at the one standard deviation point. Another way of looking at it is as a +/- 1 digit accuracy point. Each approach has it’s advantages. It’s more common to see single shot timing specified as one sigma and frequency specified as +/- 1 count. Often you need to read the fine print to see just what is being spec’d.
> On Nov 27, 2014, at 5:30 PM, LiAng <lllaaa at gmail.com> wrote:
> Thanks for the info.
> What's the standard to be 11 digits/s? For real 11 digits/s, the ADEV
> needs to reach the 1e-11 level? I'm not sure if my GPSDO & Rb is stable
> enough. Maybe 2 MV89A as the refclk and signal?
For the “easy” approach, first feed the counter’s reference back into the input. That will usually give you a “best result no matter what” sort of reading. It also will suppress a variety of problems coming from the reference signal.
A source with a <1x10^-12 ADEV at 1 second should be good enough for testing a 10 to 11 digit counter. It’s not going to do the trick for a 12 digit device. In the case of a 12 digit device, use a second copy of what ever you are using for the reference for the counter ….
Another approach, don’t measure frequency, measure period / time / phase. Generating a pulse that is 100 ns wide is fairly easy. Doing so with < 1 ps jitter is not impossible. If your signal source is good to a few ppm, your pulse generation accuracy will be “plenty good enough”. Things like rise and fall times through buffers will be a much bigger deal in the delivered result than the absolute accuracy of the clock feeding the circuit. If the counter measures the resulting pulse with a 10 ps one sigma error, you have a 10 ps counter. If it says that 100 ns is 102 ns, that’s to be expected with a simple pulse generation technique. Yes, you eventually do need to verify that 100 ns is 100 ns, but that can be done a different way.
> TDC-GP22 has it problem, I will post some data/schematic/source code
> about it later.
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