Hi all, I recently redid temperature compensation on my KX3 using a GPSDO that I purchased to see what kind of results I could get. (I last did it a while ago with the XG50 and didn't have any complaints about operation afterward, but I also pretty much only used 20 meters and below.) Below is what I did and what I observed.
*Device info:* KX3 purchased February 2013 (revision D RF board, S/N in the 3000 range), 2 meter, battery charger and roofing filter options installed VE7FMN heatsink installed *Steps performed:* 1. Warmed up GPSDO (~24h beforehand). 2. Warmed up KX3 until OSC temperature stopped climbing (~1h to reach 34C). 3. Performed reference calibration at 50.000MHz (5th harmonic of 10MHz square wave output) using the automatic method. 4. Placed KX3 in fridge for ~1-1.5 hours and set up heating equipment (100W incandescent bulb and makeshift box using baking sheets to keep heat in). 5. Removed KX3, turned it on and began data collection/storage. I also started logging using KX3 Utility. (OSC was ~14C at this point.) 6. After ~70min or so, stopped data collection at 52C and turned off bulb. 7. Reset reference calibration after allowing cooldown. 8. Started fldigi in analysis mode and recorded frequency data for ~15 minutes. *Discussion:* See attached. In short, frequency differences vs. temperature seem to be linear up until close to the end of calibration, where the frequency starts to go back up. The data collected from fldigi also seems to indicate an average frequency shift of 0.04ppm at 10MHz, which IMO is perfectly acceptable. (For reference, the specs for the Yaesu FT-817's TCXO claim 0.5ppm.) That said, I did notice--as with others in previous posts to the various Elecraft/KX3 lists--that the signal is significantly shifted on 6 meters when performing the second reference calibration based on a HF signal (and vice versa). However, there doesn't seem to be much of a difference regardless of where one calibrates on HF. For me, since I don't do much operating on 6 meters, I kept the 10MHz based reference calibration. It would be nice if one could set a separate reference frequency for 6m/2m vs. HF so that the radio can be right on frequency regardless of where it's being used. Also, I haven't tried transmitting with the new calibration yet, but given that I didn't have any problems with the digital modes I commonly use in the past, I don't expect any issues now. I did find http://www.bigskyspaces.com/w7gj/KX3HeatSinks.htm before doing this, which (a) indicated that there's a newer version of the RF board and (b) required board modifications to get the oscillator to be stable enough for all but JT65A on 6 meters. I'm not planning on modding my own KX3 in this manner, but out of curiosity, is there a RF board newer than revision D (as indicated in the schematics document on Elecraft's site)? Anyway, let me know if you have any questions or if there's anything with the methodology I could have done better. Offhand, calculating and setting ppm offsets in fldigi might improve the frequency analysis a little bit, though I don't know if it'd be by enough to matter much. Not to mention doing a temperature compensation with the XG50 again to see how much a GPSDO truly adds. Thanks, -Mooneer K6AQ
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