[GPS_Standard] Problems PN2222A Inverter missing occasional 1PPS

[Dave Platt]

On 11/17/2013 04:47 PM, Bob Stewart wrote:
> Michal and all,
> 
> I'm still getting the occasional rare (maybe 5 or 10 a day?) glitch in my 1PPS pulse.  The symptom is a 2D00 instead of a 9680 in the oscillator count value.  This was puzzling until I realized that it's simply 2 times 0x9680, which means that from time to time I am not getting a 1PPS pulse.  As mentioned, the circuit is a simple inverting amp with 1800 ohms in the base lead to the 1PPS signal from the Adafruit, and 1800 ohms in the collector lead.  So, should I use a different transistor, or different values for the two resistors?  I want to keep this as simple as possible, so I would prefer not to have to add a chip, even if that would be the best solution.  I am not a hardware person, and just blindly designed the amp for low current.

On my GPSDO "shmoo" board (regulators, MAX232 serial interface,
and the "glue" connections between the GPS and the controller
board) I use a two-transistor inverter chain to buffer the
Oncore UT+ PPS signal and send it over to the controller.

It uses three 4.7k resistors - one in the base lead of
the first transistor (the drive from the UT+ PPS line)
and one in each collector lead.  The base of the second
transistor is fed directly from the collector of
the first.

I haven't had any reason to believe I'm missing PPS
signals... I don't think I've seen any of those
"modulus is twice what it should be" glitches you
have seen.

I did use fast switching transistors rather than
general-purpose one... I had a bag of nice
JAN-rated NPN fast switches sitting around and
grabbed a couple of those.