[ntp:questions] GPS_NEMA, but high offset and jitter?

David J Taylor david-taylor at blueyonder.co.uk.invalid
Fri Feb 26 11:10:29 UTC 2010

"David Lord" <snews at lordynet.org> wrote in message 
news:7upjodF9s4U1 at mid.individual.net...
> I checked pps from MSF rx with digital meter and that showed a
> signal but not very well. That has a 50ms pulse so 100ms from
> Garmin should be as easy to spot. Analogue meters I have around
> would give a much better indication.
> Another alternative is a transistor + three resistors and
> LED. A pot could be used for R2 to confirm voltage swing
> of PPS output.
>     +5V -- R3 --- LED ---+
>                          |
>                          c
>     pps --- R1 ---+--- b
>                   |      e
>                   R2     |
>                   |      |
>      0V ----------+------+
> David

On the GPS-18 devices here, I found that an LED and 1K5 resistor alone 
were adequate to indicate the PPS signal, and it pulled down the voltage 
hardly at all.  You could even hand hold the resistor and LED across the 
relevant RS232 pins (PPS/DCD and ground).  I used a low-current LED for a 
brighter display.

Yours is the better way, as it loads the GPS a little less, and I know we 
have had discussions about exact signal levels before.


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