[time-nuts] Next step up from basic GPS/PPS timekeeping

[Jim Harman]

On Wed, Feb 24, 2016 at 2:53 PM, Nick Sayer via time-nuts <
time-nuts at febo.com> wrote:

> I can measure 1 ppb, but it takes 1000 seconds to do it ... My
> understanding is that better GPSDOs are able to provide for more granular
> phase detection.


It doesn't take a lot of hardware to get about 1000x better than that. The
Arduino-based design Lars Walenius posted a while ago can take a reading
every second with a resolution of about 1 nsec. It uses an HC390 divider to
reduce a 10 MHz OCXO to 1 MHz. The 1 MHz and pps pulse from the GPS feed an
HC4046 phase comparator, producing a 1 pps pulse 0-1 usec wide, with the
width proportional to the phase difference. This feeds one diode and an RC
network to make a ramp with its peak height (0 - 2.5V) proportional to the
phase difference. This feeds the processor's 10-bit A/D converter, which is
set for 2.5V full scale.The 1 pps signal triggers the A/D, capturing the
height of the pulse. One more resistor discharges the capacitor between
pulses. Because the discharge time constant is much longer than the
interrupt latency and A/D conversion time, the A/D captures the peak value.

I have been experimenting with this design for some time, with a lot of
learning along the way. Of course there are always potential improvements,
but the current design works surprisingly well. If someone can provide
access to a good set of test gear (I am in CT), I could better quantify the
performance.

My latest schematic is attached.


-- 

--Jim Harman
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