[time-nuts] Homebrew frequency counter, need help

Li Ang lllaaa at gmail.com
Fri Nov 28 07:26:15 EST 2014


I did a little calculation and it's a 10 digits counter.
log(10,000,000,005) = 10.
There is still a big gap between this one and 53132A :(

For 53132A, the time resolution is 150ps, which I think is 10digits/s with
interpolator. According to the schematics the only difference between
53132A & 53131A is the ADC of interpolator. It is the reason why 53131A
only has time resolution of 500ps (also 10digits/s). However, 53132A is a
12 digits/s counter. I guess the 2 more digits come from software. Linear
regression maybe ?

2014-11-28 11:15 GMT+08:00 Bob Camp <kb8tq at n1k.org>:

> Hi
>
> 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.
>
> Bob
>
> >
> >
> > TDC-GP22 has it problem, I will post some data/schematic/source code
> > about it later.
> >
> > Thanks
> >
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