TinyPFA is firmware from Erik Kaashoek for NanoVNA-H4 hardware for accurate measurement of oscillator and clock source stability.
This article documents a first experience with it performing a quite simple operation, calibrating the internal 10MHz reference oscillator of two devices:
- Motorola R2009D service monitor; and
- Rhode & Schwarz SML01 signal generator.
The calibration reference used was a Trimble Thunderbolt GPSDO which is locked.
Above, the R2009D before calibration had 15Hz error, it has not been calibrated for 15 years, way too long. It should hold say 1 part in 10e7 medium term. The oscillator has been running for more than 12h, and ambient temperature is around 20°, so conditions are good for calibration.
The pic is a bit poor, it is taken with a mobile phone and Android is trying to sharpen the picture on a display that is very actively changing.
Above is a pic of the error after calibration, frequency error is just less than 10mHz, but it will not hold that accuracy in the medium term.
Above, the SML01 internal reference is adjusted from the front panel.
Above, after calibration the error is less than 10mHz, but again it will not hold that in the medium term. The increments of the oscillator adjustment are just over 200mHz, so it is a bit of luck that this is so low.
This was an excellent device for calibrating both sources, it was quick to set up and direct reading.
More to come, the next evaluation was to produce Allan Deviation plots for several source combinations.