[time-nuts] Homebrew frequency counter, need help
Tom Van Baak
tvb at LeapSecond.com
Thu Nov 27 13:03:21 EST 2014
A copy of the plots is at:
Your project sounds wonderful. The TDC-GP22 has been mentioned only a few times over the years and I keep waiting for someone to post actual results from this chip, or better yet -- schematics, photos, and source code.
You counter looks more like a 10 digits/second, not 11. Attached are ADEV/TDEV plots of your 71 10s data points.
Do not necessarily assume that 10 s measurements are more precise than 1 s measurements. It depends on the REF and DUT. When you are at the flicker floor of an oscillator additional averaging makes no difference. And if there is drift, additional averaging actually makes the precision worse.
One suggestion is not to output discrete gated frequency measurements, but instead output continuous phase difference measurements. That way the host software (for example PC running John's TimeLab program) can manage the data stream, simultaneously delivering phase, frequency, and ADEV plots.
----- Original Message -----
From: "lllaaa" <lllaaa at gmail.com>
To: <time-nuts at febo.com>
Sent: Thursday, November 27, 2014 6:08 AM
Subject: [time-nuts] Homebrew frequency counter, need help
> Hi guys,
> I've just get my homebrew counter working. And the resolution seems 10x
> better than my RACAL DANA 1992.
> This counter is heavily inspired by the idea from Kasper Pedersen.
> STM32F051RB & EMP240T100C5 do the control and counting job. TDC-GP22 as
> the interpolator. Linear regression is done by CPU.
> There are no fancy analog front for both signal path and refclk path.
> I'm using two SN75ALS176 and the schmitt input of CPLD to do the job.
> I've noticed that the 10s gate does not get more meaningful
> digits(looks worse than 1s gate). So here are the questions:
> 1) I'm wondering if I could say this is an 11 digits/s counter?
> 2) How can I improve that? Is it limited by the 485 transceiver? I can
> switch to a faster MCU, that gets more measures per second, but I think
> that only gets no more than 2 stable bits.
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