[ntp:questions] Raspberry Pi error in PPM offset

Hans Jørgen Jakobsen hjj at wheel.dk
Sun Aug 25 18:38:02 UTC 2013


On Fri, 23 Aug 2013 08:28:31 -0700 (PDT), james.peroulas at gmail.com wrote:
> Answering both unruh and David Taylor here!
>
>
>
> To measure the RPI's actual clock frequency, I'm using the gpioclk program from https://github.com/JamesP6000/WsprryPi. Basically, the 19.2MHz crystal on the Pi is being used to generate the internal PLLD clock running at 500MHz which I then divide down by a factor of 50 to produce a 10 MHz clock on the RPI's GPIO4 pin.
>
Loking at the link I see that they try to do some magic at line 54 of 
https://github.com/JamesP6000/WsprryPi/blob/master/wspr.cpp
" #define F_PLLD_CLK   (500000000.0*(1-2.500e-6)) "

https://github.com/JamesP6000/WsprryPi/blob/master/BUILD
taks about:
 "Make sure you are using the latest kernel by updating your system. The latest
kernel includes fixes wich improve NTP ppm measurement accuracy"

> I measure the actual frequency on GPIO4 using an HP5328 counter which has been recently GPS calibrated and has a frequency error of better than 0.010 PPM. From this, I calculate the actual PPM error of the crystal to be around -43 PPM. It varies a bit with room temperature, but rarely differing by more than 0.5 PPM from -43 PPM.
>
Until the next calibration you dont know how valid your curent measurements are. :-)

/hjj



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