[time-nuts] Homebrew frequency counter, need help

Li Ang lllaaa at gmail.com
Fri Nov 28 11:14:20 EST 2014

> 1) What frequency is the crystal at? (can you drive the chip from an OCXO?)

I'm clking this chip with the refclk/2=5Mhz, which is done by cpld.
something like always @(posedge refclk) tdc_clk <= ~tdc_clk; Not sure if
it's noisy.

2) Is there more bypassing on the circuit than shown? (If not, add some
> more).

Since there is only few power supply pins, I just put 100uF + 100nF there.
I can try put more to see if helps. And I'm going to use dedicated LDO for
digital part and analog part next time to make a new pcb.

3) How confident are you of your input signal? (can you check it with a
> “known good” counter?)

The best counter I get is the RACAL DANA 1992. That is not good enough.  I
think the accurarcy is not a problem, as if the reference is good enough,
it's only the matter of math. I prefer to enhance the resolution and reduce
noise. I'm using Trimble GPSDO(NTPX26AB) as the signal source and FE5650 Rb
as the ref. I've tried Rb+MV89A and Rb + SMY01 signal generator, same
performance. The 2nd-hand 53132A,PM6690,SR620 about 600~800$ here in China.
I'm trying not to get one unless necessary. If I have get one to compare
the performance, what's your suggested model?

> 4) Have you tried jumping the 10 ohm resistor on the regulator output? (it
> may not be helping things …)

I'll try to remove that tomorrow

All of which is *plenty* good enough to make a decent counter.  That
> assumes that they are talking about accuracy (even 1 sigma) rather than
> just the resolution of the LSB. Specs are often confusing on parts like
> this.

I guess the accuracy is not important in the interpolator scenario.  It has
a feature to output the result of delta_time / ref_cycle_time(it will
measure the 2cycle_time - 1cycle_time after the delta_time measurement and
do the float calculation).  All I need is this part(it's the fraction part
of refcnt). So if I can reach 90ps resolution they claim, the counter can
tell 1/1000 of one reference cycle. That's 3 digits.


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