[Elecraft] K3 Frequency Calibration

[Fred Jensen]

I find that trying to match two beat notes when I can only hear one at a 
time to be hard, best I can do is maybe 10 - 15 Hz Hz depending on the 
frequency.

FWIW:  The Burdick Calibration Method [BCM] for the K3 is actually much 
easier to perform than has been suggested in this thread,it is 
incredibly accurate, and it works for me every time. A couple of 
possibly helpful hints:

1.  The WWV signal is somewhat complex.  In addition to the carrier and 
AM voice announcements, it transmits tones, 500 Hz and 600 Hz on some 
but not all alternate minutes, and 440 Hz as an hour marker.  
Additionally, a continuous time code in a slightly modified IRIG-H 
format is transmitted on a 100 Hz sub-carrier using both PWM and PM 
modulation.  The 500 and 600 Hz tones produce sidebands and it is very 
easy to attempt to zero beat one of those instead of the carrier, so 
choose the period :43 to :52 in each hour when there are no tones.

2.  Put the K3 into USB [or LSB], either will work fine.  Choose a time 
when at least one of the WWV frequencies is strong at your QTH.  Any WWV 
freq will work ... if you are OCD, 20 MHz may result in tiny 
improvements in calibration accuracy which you'll never notice in normal 
operation.

3.  Generally, wider DSP bandwidths will make the true zero beat more 
discernible so long as other adjacent signals are not present but it 
does depend on your hearing.

4.  Not all WWV frequencies are created equal.  If you have a P3, you 
can look at the spectrum of each of the signals.  For the 5, 10, and 15 
frequencies:  It appears these use 10 KW plate-modulated, transmitters, 
likely with Class C PA's and there is more distortion present than I 
expected from NIST/WWV. You can see the 100 Hz sub-carrier and the very 
close in sidebands from the IRIG-H time code, and the first sidebands of 
the 500/600 Hz tones.  Because of the distortion, the two 1st sidebands 
will be modulated by the 100 Hz sub-carrier and its sidebands.  You will 
also see the two sidebands from the 2nd harmonic of the tones at reduced 
level too, also modulated by the 100 Hz sub-carrier.  On 10 MHz, I can 
also see the 3rd harmonics of the tones clearly, and can sometimes make 
out the 4th.  It's a great demo of how to annoy your fellow hams with 
distortion in your transmitter. 😉

The 2.5 and 20 MHz signals use 2.5 KW transmitters that are apparently 
much more linear.  They exhibit the carrier, sub-carrier and its 
sidebands, and only the fundamental sideband of the tones.  This will 
help a little in finding exact zero-beat, however a strong overall WWV 
signal with minimal QSB is way more important.

5. Set your K3 for 1 Hz display resolution.  You may want to experiment 
between high and low counts/revolution of the knob, I find lower 
counts/rev works better but YMMV.  Don your headphones, AF gain up 
almost to the point of uncomfortable, set the freq display to exactly 
the WWV freq.  Then, tap MENU->CONFIG and tune VFO B to select the REF 
CAL menu entry.  The display will show a long number. Mine is 49.379682

6.  Now the fun part.  Carefully adjust VFO A to achieve zero beat.  V e 
r y  s l o w l y rock VFO A through zero beat. The tone will get too low 
to hear, and as you go thru ZB, the background noise will begin to 
"pulse", fast at first and then slower, until you can count the 
amplitude pulses, and then begin to speed back up as you move to the 
other side of ZB.  You want the point where the pulses are the furthest 
apart.  In my K3, I can get down to about 1 pulse/2 sec and then 
continuing one encoder step, it goes up to 1 pulse/sec.  True ZB is 
between those two encoder steps, I set mine to 1 pulse every 2 sec. Tap 
MENU, and call CQ on exactly the frequency of your choice.

You can switch between USB and LSB after you've found your ZB spot, it 
just inverts the process.  The whole BCM takes way longer to describe 
than to do.  I've run it a few times over the years and my K3 [S/N 642] 
has never varied more than about 4 Hz in that long Ref Cal number.

Ray wrote on 12/16/2021 11:04 PM:
> Gents
> Go to a WWV signal, (5.000000)  on a Good Strong Frequency.
> Place the Mode To Upper SSB, Listen to the Beat note Tone Freq,
> Now Move the Mode to Lower SSB. Adjust the  Ref. Cal. Osc. until the Beat Note is the Exact Same Freq
> On Lower SSB and Upper SSB.   Done
> Ray WA6VAB  K3
>



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