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

David Lord snews at lordynet.org
Sat Feb 27 00:20:49 UTC 2010

unruh wrote:
> On 2010-02-26, David Lord <snews at lordynet.org> wrote:
>> David J Taylor wrote:
>>>> Is there any other way to watch the signal to see if it's reaching the 
>>>> serial port correctly?
>>> I would use my 'scope, but even an analogue voltmeter might work - see a 
>>> regular flick of the needle.  I built the equivalent of a simple 
>>> break-out box into my two connections, one in the RS232 DB-9 plug itself:
>> 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 ----------+------+
> No need for any of the resistors except R3. but you do not really  need
> the transistor, since the PPS line should source enough current to run
> the led. Ie, just
> PPS ------ R3 ---LED--
>                      |
>                      |
> Gnd-------------------
> should do it, with R3 being about 1K
> I would not advise this as a permanant setup but for testing it should
> be fine. 

In my universe ttl could sink about 4mA but only source
about 1.6mA vs 8mA/0.4mA for LS-ttl so more appropriate
circuit would be to have pps output sinking current from
the LED with resistor from +5V.

Thing is there appears to be a problem with the PPS so
I was suggesting a more sensitive circuit should a scope
or moving coil meter not be available.


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