The N6KR Method for setting the 4 MHz reference oscillator
(This is the unedited text from Wayne's original post to the Elecraft Reflector on 8/20/03)
All of the methods that have been described for calibrating the K2's 4-MHz oscillator will work. But I use a different technique that allows C22 on the Control board to be set to precisely the correct position, with no guesswork.
This method relies on the following simple observation. If you tune in an on-air signal at a known frequency, the difference between the *measured* VCO and BFO (using CAL FCTR) *must* equal that frequency, or C22 is not set correctly. (Actually, this holds for 160-17 m; on 15-10 m it's the sum, or VCO + BFO, that must equal the signal's frequency. But it's easier to do the adjustment of C22 on 17 m or lower because you don't have to do any math at all, as I'll explain below.)
Here's the procedure. It requires revision 2.XX firmware, and assumes you have already done Alignment and Test, Part II, at some point. The K2 should also be allowed to come up to room temperature.
1. Tune in a signal at a known frequency. Use one that's at an *exact* kHz boundary, so you can easily see when the VCO and BFO readings match in step 2. (I use WWV at 10, 15, or 20 MHz.) Use USB or LSB mode rather than CW, so that there will be no CW receive offset. In the case of a K2 I was calibrating, the VFO read 10000.17 when the signal was tuned in perfectly. If it had read 10000.00, no further improvement would have been possible.
TIP: Zero-beat the carrier precisely, or listen to a voice signal and adjust the VFO for the best quality. The more accurately you tune in the signal, the more accurately you'll be able to set C22, below.
2. Run CAL FCTR. Now alternately move the K2's internal counter probe between TP1 (VCO) and TP2 (BFO), adjusting C22 in small increments until the kHz and Hz digits at the two test points match as closely as possible. In my case, the two readings matched at 14913.60 and 4913.60. The difference is exactly 10000.00--the frequency of the on-air signal.
3. Put the counter probe on TP1 (VCO), switch to 40 meters, and run CAL PLL.
4. Put the probe on TP2 (BFO) and run CAL FIL. For each operating mode, vary each filter (or BFO) setting up 1 count, then back down, to force the K2 to take a new BFO measurement for each and store it in EEPROM.
The VFO dial should now be very well calibrated.
If we get a lot of positive feedback on this method, we'll post it as an application note.
73,
Wayne
N6KR
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