[GPS_Standard] strange exponents, GPS tips

[SP2IQW]

Hi Bert and all,

Few times I have also noticed irrationally small numbers as 10-27. I'm 
enclosing few printscreens but unfortunately log was not switched on 
during any of these issues.

Soon I will assemble next version of your GPSDO now with Oscilloquartz 
8663 DOCXO. I have added a clean PWM voltage reference with CMOS buffer 
so it is no suffered from voltage fluctuation coming from rest of 
digital ciruits. Other modifications added by Rafal SQ4AVS are 5/10MHz 
doubler and different output buffering with transistor voltage 
followers. Our schematic is enclosed too.
I would recommend Trimble Lassen iQ receiver which has really good 
±50ppm PPS specification. This number is even better in static mode. Low 
and stable current consumption (below 30mA at 3.3V) is a big benefit 
especially in portable operation. Many GPS draws over 100-150mA during 
acquisition. It concerns most of SiRF based as well latest uBlox Antaris 
5 (Atmel chipset) modules.
I doubt if expensive timing GPS receivers will give any significant 
improvement to VE2ZAZ design compared to iQ receiver. Counter is feeded 
with 100ns (10MHz) period so PPS with 15-25ns insted of 50ns accuracy 
should give every time the same count.

Here are few GPS configuration tips to achive best timing performance:

- use static navigation mode
- set elevation mask >5°
- set signal level mask C/No >30 dBHz
- use fix-based PPS (generated only on position fix)

Be sure that GPS antenna will see rather unobstructed sky. Window 
operation with half of sky shadowed don't warranty that GPS will have 
all the time enough good located sattelites to make a fix.

By the way, interesting information which is often missed.
Within NMEA frames only ZDA is strictly related with PPS pulse it 
follows! The time information in RMC or GGA frames may be 1s late in 
some cases.


73, Michal SP2IQW

PS - I'm not in any relation with Trimble, just using proffesionally 
from years.