[GPS_Standard] VE2ZAZ

[Dave Platt]

> I’m not using that circuit on this application, but it could be tuned for
> it. Yes, 200nS is quite a long time.
> 
> The PPS output can deliver up to 5mA, so we could lower the 10K resistor
> down to drive the gate capacitance harder, and bring the rise time to
> shorter times (note we actually need to sink current in this application)
> 
> Note that the rise time from the Resolution-T directly is “typically less
> than 20 nanoseconds”, so it may not be very important to have a very fast
> pulse as long as the trigger level of the Schmitt Trigger in the PIC remains
> constant over time (which should if we keep temperature and supply voltage “
> constant”)

I had a somewhat similar issue in my board, as I wanted to
drive both the controller's PPS input, and a Maxim MAX238
TTL-to-RS-232 level shifter (for DCD output to the PC) from
an Oncore's PPS output.  The Oncore PPS driver isn't terribly
strong, and I was afraid that it couldn't drive both devices
without slowing down too much.

I chose to use a classic two-stage common-emitter driver,
which is (I think) what Bert was alluding to.  Two
transistors (I had a whole bag of 2N2369 fast NPN switches
in my junk box) and three resistors.  This really isn't
all that much more than the one-MOSFET, two-resistor level
shifter you alluded to, and I think it's a lot faster (the
2N2369 has turn-on and turn-off times of under 20 ns).

With fast-switch bipolar transistors being such "jellybean"
parts these days, why risk degrading the PPS rise-time?
You can even get "digital transistors" which have the
necessary base resistors build right into the package
(NEC sells 'em)... two of these daisy-chained together,
and maybe one external resistor would do the trick.