GPS Locked Variable Frequency Reference

This page refers to my GPS referenced variable frequency RF oscillator published in the Sep/Oct 2018 issue of QEX. All updates since publication are here.

This project can generate any frequency between 500 kHz and 40 MHz with an accuracy approaching one part in 10⁹, for example 0.01 Hz at 10 MHz. Total parts cost is about $150.00.

Exterior

Interior

WWV

• Original proof

  GPSReference-QEX-proof.pdf

• Errata
  • Correct Figure 4
• Tips
  • Solder jumpers IM1, IM2 and IM3 on the display.
  • All connections to the display are made on the SPI end.
  • Some users report better reliability using real level shifters in the GPS and CLK1 lines such as this one.
• Source code and version history

  Current version: GPSReference-1.06.zip

  Version 1.06:

  • Add 1 sec startup delay for some nano clones
  • Shut off output when changing synths
  • Small increase in wild reading threshold

  Version 1.05:

  • Tap encoder switch to toggle 2 Hz modulation
  • Push encoder and rotate to cycle through four power settings

  Version 1.04:

  • Perform explicit GPS cold restart upon power up for increased stability
  • Tapping encoder switch selects next of four output power levels

  Version 1.03:

  • Display min and max signal qualities of sats used for PPS
  • Retire power-on time

  Version 1.02:

  • posted to QEX
• Sweep version

  Some folks have inquired about a sweep version. I made some tests and discovered the time for the DDS to settle to a new frequency requires about 10 ms average but with some pathological frequencies requiring as much as 250 ms. This makes a smooth sweep problematic at best. Furthermore, the GPS correction can only be performed with a constant frequency lasting at least 10 seconds so a repetitive sweep mode would gradually drift off frequency. For these reasons, I have abandoned any serious effort to create a sweep version of this project.