[ntp:questions] Very large jitter and offsets on GPS ref clock after upgrade to "p5"

David J Taylor david-taylor at blueyonder.co.uk.invalid
Wed Feb 8 17:45:04 UTC 2012

> Nearly that, but I'm splitting the power and planning on
> having +9V and -Ve on two of the lines and a 5V regulator
> since some of the GPS modules with interfaces take  over
> 200mA so common OV with the -Ve at the far end should
> remove any effect from voltage drop over that length of
> cable.

OK - usually for low-frequency stuff the more ground lines the better!  If 
you're having +9V then I would be inclined to common-up the four ground 
lines, and use a single line as the +9V.

> I might try rs422 for the PPS (and TxD lines) but they
> are 16-pin chips and have unused sections. The 8-pin
> versions with single Tx + Rx are much too expensive.
> David

It would be an interesting experiment in view of the recent discussions, 
and allow you to terminate the lines correctly at both sending and 
receiving ends.  Having said that, 18m will be about 54 ns divided by 
velocity factor, say 70 ns, so hardly critical to a receiver chip that 
likely has a couple of microseconds rise time input filter in any case. 
Please measure waveforms if possible, both on the line and at the chip 
input pin.  See what filtering may be included internal to the chip.

BTW: my own extensions over a ~4 m length have been with unscreened, 
three-way or four-way cable (TXD PPS ground), but that has been within one 
room.  I've used that to parallel the GPS 18x LVC output between two PCs.


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