[ntp:questions] GPS_NEMA, but high offset and jitter?
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 ---+
>> pps --- R1 ---+--- b
>> | e
>> R2 |
>> | |
>> 0V ----------+------+
> 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.
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