Various information about using ClockTamer with GPS.
ClockTamer has ability to lock its clock to GPS, providing much higher stability then you can get from any TCXO or VCTCXO. To lock to GPS it uses 1pps signal, generated by many GPS chips. ClockTamer micro-controller then counts number of clock pulses on one of generator outputs between consecutive 1pps pulses and applies math filtering to get real (VC)TCXO frequency. Knowing real (VC)TCXO frequency we're able to compensate it in software.
There are two options of getting 1pps signal:
With GPS locking it's not critical to have very high precision XO, so it's possible to use 2.5ppm VCTCXO option instead of standard 0.28ppm TCXO. And that's the configuration we use in our own experiments.
Note: When GPS-locking option is installed, you can use only 5 of 6 ClockTamer outputs. One output is routed to micro-controller in this case. Exactly, output number 2 is not available TODO: Picture.
Current implementation first reduces measurement jitter by averaging measurements for 17sec. Then calculated values are used as input for exponential filter to get fine resolution frequency measurement. Calculations are done with 32-bit precision.
See WishList for the way to improve this.
You could also use ClockTamer as a GPS positioning device. To switch from SPI command mode to GPS positioning mode use '%%%' command. To leave GPS positioning mode and switch back to SPI command mode use '%' command.
When in the GPS positioning mode, you could use ClockTamer with any application which understands NMEA protocol. Below is a picture of xgps application, reading data from a ClockTamer: