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

David Lord snews at lordynet.org
Fri Feb 26 11:48:09 UTC 2010


David J Taylor wrote:
> "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.

When I had very good results from MSF + PPS in June last
year I used an isolated power supply and had output GND
offset slightly positive to circuit 0V so the pulldown
was to below 0V. I can't remember how much offset, I
doubt any more than 1V. It's possible I used same for the
Garmin when that gave good results with pps to serial DCD.
Currently with output GND = GPS 0V, I've been unable to
get same useful low offsets from serial connected pps but
parallel connection gives 2 day average for offset 0.000ms
and jitter 0.002ms.

David




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