Thursday, December 13, 2012

The KX3, the fridge and the hairdryer

In my post on Jul 7 2012 I noted that frequency stability of the KX3 was not good enough for digital modes even on 40m. It had been pointed out that the KX3 was designed as a portable mountain top rig for use on SSB and CW and it was never designed to be highly stable

Since that time Wayne N6KR has done some investigation and has come up with a procedure to generate a custom frequency offset versus temperature table for the KX3 that the KX3 can use to adjust the offset. The procedure is at http://www.elecraft.com/manual/KX3%20Custom%20VFO%20TC%20rev%20A8.pdf

To do the procedure,  a highly stable (the XG3 isnt good enough!) 50MHz signal source is needed. Elecraft will sell you a suitable XG50 for the purpose but there has to be something in the shack. The K3 runs off a GPS locked 10MHz source so the fifth harmonic could be used? Initial tests showed the 50MHz level was not high enough, I needed a harmonic generator. Searching the parts bin I found an LF minicircuits mixer with connectors (intended for my 477kHz TRUE transverter). These contain diodes so they must generate harmonics. I found that driving the IF port with 10MHz and connecting the RF port to my 6m antenna produced a healthy 50MHz signal. good enough for the procedure

After making sure that I understood the procedure the KX3 was put in the fridge for 30 minutes. When removed the KX3 starts to warm up and the KX3 records the audio frequency of the tone against temperature (actually temperature a to d value...you can see what is happening in the KX3 utility programme as well as on the KX3 LCD) and stores it. To calibrate the upper temperature range (above KX3 ambient temperature) a hairdryer is used to heat it up. The procedure suggests until 52C but, being winter mine stopped at 43C which is adequate for the UK so I stopped calibration there.

After applying the calibration table It was great to hear the note of the 50MHz signal stay at around 550Hz all the way back down to 25C. The process was speeded up by putting the KX3 on a bag of frozen peas!

Next to see what the KX3 looks like on LF QRSS


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