This code turns a NEO-6M GPS module (or better) into a accurate frequency reference source.
This can be used to calibrate freqeuncy counters, SDRs, and so on.
The accuracy is minimal 2e-8 it seems - which the mind fails to comprehend ;)
http://pirshared.org/sdrplay/ has a video of this system in action!
Connect GPS' TX pin to pin 12 (GP9) of Pi Pico. Connect GPS' RX pin to pin 11 (GP8) of Pi Pico.
The code will configure the GPS module to generate a 12 MHz frequency reference.
Connect the PPS output pin, and GND of the GPS module to Pin 15 and GND (respectively) of Raspberry Pi Pico for frequency measurement.
https://github.com/kholia/pico_ft8_xcvr/tree/main/PicoFrequencyCounter has more details on this topic.
This GPS frequency reference source can be used to calibrate a SDR device as
well!
Attach a loop-of-wire (antenna) to the SDR, and keep it very very near to the PPS pin of the GPS module.
The Gqrx software provides native support to change the ppm value on the fly.
I strongly recommend using the Airspy HF+ Discovery SDR for lab work.
This project uses https://github.com/earlephilhower/arduino-pico core. Install
this core into your Arduino IDE, and then Compile this project using Arduino IDE.
This project is based on https://github.com/MxFxM/uBlox7_TimePulse work.
Note: Keep frequency in the form of 48 / N. Keep it at 12 MHz or 8 MHz for
minimum jitter.