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

Additional notes and information.

Both Michael Masleid and I did extensive runs using the N6KR method and found several interesting factors.  If you want to get the dial calibration 'right on', you may wish to take advantage of our findings (My observations and results were consistent with Michael's, and he posted a more complete description than I generated).  The contents of Michael's analysis follows - note particularly his Third pass results and his 'Conclusions' and 'Things to notice' near the end.  Carefully Zero-beating the reference used is a big key to success, I recommend using Spectrogram as an aid.

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From: "Masleid, Michael A." <[email protected]>
To: <[email protected]>
Sent: Wednesday, August 27, 2003 6:25 PM
Subject: RE: [Elecraft] N6KR's 4 MHz oscillator cal method for the K2


Notes on CAL FCTR using N6KR's method, and other methods:

Equipment used for first pass: The K2 and a short wave radio.

First pass using N6KR's method:
  WWV at 10 MHz LSB, zero beating tones against   WWV at 10 MHz AM on a different receiver.

End result of the first pass:
  WWV at 10 MHz at 10,000.01 kHz LSB on K2 for near zero beat compared to the AM radio.
  WWV at 10 MHz at 10,000.02 kHz USB on K2 for near zero beat compared to the AM radio.

Second pass using N6KR's method:
  K2 set to 10,000.01 kHz LSB near zero beat vs. AM.
  CAL FCTR shows  4913.70 to  4913.71 on BFO.
  CAL FCTR shows 14913.70 to 14913.71 on VCO.
  Since the frequency is different by 10000.00 I did not adjust C22.
  Ran CAL PLL (with the cover off)
  Ran CAL FIL (with the cover off)

End result of the second pass (no difference):
  WWV at 10 MHz at 10,000.01 kHz LSB on K2 for near zero beat compared to the AM radio.
  WWV at 10 MHz at 10,000.02 kHz USB on K2 for near zero beat compared to the AM radio.

Additional results from the second pass:
  A signal generator known to be accurate to 4E-9 was used to generate signals at 4 MHz 28 MHz and 28.1 MHz.
  CAL FCTR shows 4000.01 kHz with 4 MHz in.
  CAL FCTR shows 28000.03 kHz with 28.0 MHz in.
  With 28.1 MHz AM modulated 600 Hz, LSB gives 28100.06 on display.
  With 28.1 MHz AM modulated 600 Hz, USB gives 28100.07 on display.
  Removing the top cover and closing the stand made the following change:
  With 28.1 MHz AM modulated 600 Hz, LSB gives 28100.07 on display.
  With 28.1 MHz AM modulated 600 Hz, USB gives 28100.08 on display.

Small adjustments were then made to C22 to improve overall calibration.  Good results were obtained with CAL FCTR set near 28099.96 with 28.1 MHz input to the probe.

First pass using 4 MHz zero beat method:
  A signal generator known to be accurate to 5E-9 was used to trigger a 'scope.  (The signal generator OCXO drifts slightly, it is checked
  against WWVB using a strip chart).  A 1X probe was placed near the MCU was used to pick up the 4 MHz MCU oscillator.  C22 was adjusted
  so that the 4 MHz clock was stationary to 1 cycle in 8 seconds.  With the 4 MHz clock set to 4,000,000 Hz, +/- 1/8 Hz, CAL PLL and CAL FIL
  were run with the covers on and the tilt stand down.  The clock was checked after CAL PLL and after CAL FIL, it was still +/- 1/8 Hz.
  Spectrogram   version 5 was used to measure beat notes and tones so calibration offset could be measured within 2 Hz.  The spot tone
  was measured at 607 Hz, and that was used for CW offsets.

End result for first pass, precision 4 MHz oscillator adjustment.
            Against 10 MHz WWV    Against 18.1 MHz reference
  CW offset       -34 Hz                 +109 Hz
  CWR offset      +44 Hz                  -92 Hz
  LSB offset      -41 Hz                 -105 Hz (600 AM modulation)
  USB offset      +37 Hz                  +93 Hz (600 AM modulation)
  CAL FCTR shows  3999.98 with 4 MHz in.
  CAL FCTR shows 28099.86 or .87 with 28.1 MHz in.
  CAL FCTR shows 31099.84 or .85 with 31.1 MHz in.

A note about offsets:  For CW mode at 10 MHz, -30 Hz offset will give zero beat with the display showing 9999.97 kHz.  For CW reverse mode at 10 MHz, +40 Hz offset will give zero beat with the display showing 9999.96 kHz.  Offsets are calculated by subtracting the measured frequency from the expected frequency as shown by spectrogram.  If the expected frequency is 607 Hz (spot tone) and the measured frequency is 641 Hz, then the offset is -34 Hz.

Third pass using N6KR's method.
  K2 set to 9999.96 kHz LSB for near tone match on spectrogram.
  Entered CAL FCTR.
  The reference tone changed 10 Hz.
  Exited CAL FCTR.
  The reference tone changed back.
  Well, that will mess things up.  Anyone spot this one?
  Set K2 to 9999.95 kHz LSB,
  Entered CALL FCTR.
  Now the reference tone is about matched on spectrogram.
  VCO measures 14913.62 or .63
  BFO measures  4913.67 or .68
  The difference is 50 Hz.
Note: The VCO measurement will change 3 times faster than the BFO measurement.  So, the difference will change 2 times faster than the BFO measurement.  Since the difference is 50 Hz, the BFO measurement must change 25 Hz, and the VCO measurement must change by 75 Hz.  So first adjust the VCO to 14913.70, then check the BFO to see that it is at 4913.70 - this really is a one pass procedure.
  Adjust C22 so that VCO - BFO = 10000.00
  VCO set to 14913.70 - BFO set to  4913.70 works.
  So does VCO changing 14913.69/70 - BFO changing 4913.69/70
  I used VCO changing back and forth 14913.69 14913.70
  CAL FCTR shows 28099.98/99 with 28.1 MHz input.
  Ran CAL PLL (with the cover on)
  Ran CAL FIL (with the cover on)

End result of the third pass, almost N6KR's method:
              Against 10 MHz
  CW offset       +12 Hz (Zero beat with display at 10000.01 kHz)
  CWR offset       +4 Hz
  LSB offset      +18 Hz
  USB offset      -17 Hz
  CAL FCTR shows 28099.98 or .99 with 28.1 MHz in.

Conclusions:

N6KR's method works well.  Calibration against WWV at 10 MHz tends to produce a +11 Hz error on K2 #3430, on average, at 10 MHz; the display will show 10000.01 kHz when carefully tuned to WWV.  The only thing better is fudging the setting on C22 to remove residual errors.  BTW, I get 11 Hz from (+12 -+4 +18 --17)/4.  Subtract for CWR and USB since they work backwards.

Comparison to other methods:

1 Setting the 4 MHz clock oscillator to exactly 4 MHz will produce an   average  error of -39 Hz at 10 MHz on K2 #3430 on average.  The   display will show 9999.96 when carefully tuned to WWV.  This requires access to a 'scope, WWVB frequency comparator, and  a frequency standard.  (A GPS disciplined clock would also work.)

2 Not shown here:  Setting CAL FCTR by counting a known reference signal, say 28.1 MHz, will offset the 4 MHz oscillator by about
  -15 parts in 28.1 million compared to the third pass N6KR method, or about 5 Hz at 10 MHz.  This will result in an average error of
  about +16 Hz at 10 MHz on K2 #3430.

3 Not shown here: The offset is -39 Hz with CAL FCTR at 28099.86/87 (precision 4 MHz method).  The offset is +11 Hz with CAL FCTR at
  28099.98/99 (N6KR method) both with CAL FCTR with 28.1 MHz input.  The difference in offset is 50 Hz at 10 MHz.  The difference
  in CAL FCTR is 120 Hz at 28.1 MHz.  To change the average offset, change CAL FCTR at 28.1 MHz by 2.4 Hz/Hz.  So, from the
  4 MHz method, change 39*2.4=93.6, so CAL FCTR should show 28099.96.  Or, from the N6KR method, change by -11*2.4=26.4, so CAL FCTR should show 28099.96.

Things to notice:

1 Run CAL PLL and CAL FIL with the K2 the way you use it, covers on, and tilt stand up or down as you wish.  It makes a difference.

2 The spot tone might not be exactly 600 Hz.

3 You have to run CAL PLL and CAL FIL at least once before you can use N6KR's method.  Works OK with C22 set mid range.

4 Since the 4 MHz method is off by 93.6 Hz at 28.1 MHz, then the correction to the 4 MHz crystal is 93.6 * 4 MHz/28.1MHz, or 13.3 Hz.  The 4 MHz clock oscillator itself should be set to about 3,999,986.7 Hz for best calibration, give or take a little.

Suggestions for piano tuners and clock makers:

1 Run CAL PLL and CAL FIL with C22 set mid way.

2 Use the K2 for a little while, figure out good settings for the BFO and filter bandwidths.

3 Get a stable 28 - 30 MHz oscillator that you can measure using CAL FCTR.  It doesn't matter what the frequency is, but it must not drift more than 5 Hz between calibration runs.

4 Warm up the K2 and the oscillator.  Use N6KR's method to set CAL FCTR, then measure the oscillator with CAL FCTR.
  Run CAL PLL and CAL FIL.

5 Figure what the average calibration offset is now using spectrogram.  Lets say it is +15 Hz at 15 MHz.  Then the oscillator CAL FCTR value is too high by about 30 Hz at 30 MHz.  Run CAL FCTR against the oscillator again, but change C22 to get 3 counts lower on the display.  Run CAL PLL and CAL FIL again.

6 If the display is within +/- 1 count of the actually frequency on all bands, all modes, stop worrying.  The actual frequency steps in the K2 aren't 10 Hz.  Sometimes they're 13 Hz, sometimes they're 5 Hz.

73, de Michael, AB9GV

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