[Elecraft] K2 freq set - why is WWV off-freq?

[Steven Gibbs]

In the recent (and seemingly never-ending) discussion about
setting the K2 Control board 4MHz oscillator, there is something
may have been overlooked, namely that when you THINK you have re-
performed a CAL-FIL, it may be that NOTHING has been accomplished!

This may explain why your K2 shows a frequency error (e.g. a
reported 80Hz or more) when SPOTting on WWV instead of the +/-20Hz
to be expected after careful 4MHz alignment.

The K2 manual (rev D, p.85) says 'Whenever you switch modes or
filters, the K2 will first record your new settings, IF THEY HAVE
BEEN CHANGED' [my caps].  This  means that to properly calibrate
the VFO to the new MCU crystal frequency, the VFO control para-
meter MUST be changed, a new mode or filter selected, then the
original mode or filter re-selected and the control parameter
changed back to its original setting.  (If you notice a change
in pitch of a received carrier when switching filters, this is
a good indication that CAL-FIL has not been properly done).
If the 4MHz MCU has been changed by more than a few Hertz, the
original control parameter setting may no longer be the optimum
BFO frequency for that filter, so that's something else that
complicates things, and which must be taken into consideration.

N4SO posted a file I sent to him last December; I have added the
above remarks to paragraphs A5 & B4; the new text is below, so
you may consider it worthwhile to SAVE it as <K2-4MHz-set.txt>;
you can pass it to other poor souls with problems.

These methods allow DIRECT comparison of the K2 MCU oscillator
with WWV or a frequency standard WITHOUT using WWV tones,
signal generators, counters, other receivers, or measurement by
oscillators of unknown accuracy in your soundcard (such as when
Digipan or Spectrogram is used to measure a tone's frequency).
By touching the MCU crystal you can be sure you are looking
at the K2 frequency alongside all the crud which surrounds
WWV.

However, the need to properly perform CAL-FIL applies to
whatever method you use to set the 4MHz MCU, no matter how
hare-brained or ill-considered.

73, Steve GU3MBS    K2 #1595, 2317, 2723 (all sold), 3204.
______________________________________________________________

Elecraft K2 Control Board 4MHz Oscillator Adjustment
K2-4MHz-set.txt            27 Jul 03
by Steve Gibbs GU3MBS      mailto:sgibbs@guernsey.net

Here are two methods of correctly setting the K2 4MHz clock
frequency with greater confidence than the methods suggested
in the K2 manual, 'Alignment and Test, Part II'.  Neither
method needs a 'calibrated external frequency counter' or a
'calibrated short-wave or ham-band receiver'.

Method A needs a PC running the Spectrogram program, and
requires that assembly of the K2 is complete.

Method B needs an oscilloscope and a 1MHz frequency standard.
It can be performed at the 'Alignment and Test, Part II'
stage, or when assembly of the K2 is complete.

________

METHOD A

A1.  Set the K2 to USB.  Identify and tune (approximately
zero-beat the carrier) a standard frequency (WWV) transmission
on 10MHz, 15MHZ or 20MHz.  Tune the K2 dial about 1kHz lower.
On Spectrogram you should see the WWV carrier as a tone of
about 1000Hz.  Disregard any modulation tones that WWV may
also be transmitting.

A2.  Disconnect the antenna; you should see a weak signal
somewhere on Spectrogram - this is the 10th (or 15th, or 20th)
harmonic of a 1MHz signal derived by the MCU from its 4MHz
clock (you will recall that a similarly-derived 7MHz signal
is used in initial 40M alignment).  You can verify that you
are looking at the right signal by touching the Control Board
MCU crystal X2 or its associated C22 trimmer and noting that
the frequency on the Spectrogram display changes slightly.
If you don't see the MCU signal you can tune the K2 dial up or
down a couple of hundred Hz until you locate it.

A3.  If WWV is coming through strongly there may be enough
breakthrough to show WWV's signal also.  Use an insulated
tuning tool (sorry, but the Elecraft tool is rather poor for
this job) to adjust Control Board C22 so that the two signals
coincide - final proximity may show a beat which can be
reduced to zero.  If WWV isn't very strong you won't see its
breakthrough signal so you'll have to alternately connect and
disconnect the antenna until you have the two signals giving
the same frequency on the Spectrogram display.

A4.  Note that the exact setting of the K2 dial and the exact
frequency on the Spectrogram display doesn't matter.  The
sidetone frequency which you have set doesn't matter either.
These are the important features of this method.  If you have
a KAT2 or a KAT100 you can switch to an unconnected ANT2
instead of disconnecting your antenna.

A5.  Re-perform CAL-FIL.  (Remember that the K2 manual says
'Whenever you switch modes or filters, the K2 will first
record your new settings, IF THEY HAVE BEEN CHANGED'.  This
means that to properly calibrate the VFO to the new MCU
crystal frequency, the VFO control parameter must be changed,
a new mode or filter selected, then the original mode or
filter re-selected and the control parameter changed back to
its original setting.  If you notice a change in the pitch
of a received carrier when switching filters, this is an
indication that CAL-FIL has not been properly done).  If the
4MHz MCU has been changed by more than a few Hertz, the
original control parameter setting may no longer be the optimum
BFO frequency for that filter, so that's something else that
complicates things, and which must be taken into consideration.
Re-perform CAL-PLL.

A6.  SPOT the WWV carrier in CW-nor and again in CW-rev; in
each case the K2 dial should show 10000.00kHz (or 15000.00kHz
or 20000.00kHz) probably plus or minus about 20Hz.

________

METHOD B

If you have access to a 1MHz frequency standard, here's a way
of setting the K2's 4MHz Control Board oscillator with greater
certainty than trying to zero-beat with WWV.  I used an
off-the-air standard derived from the high-accuracy carrier
frequency of the BBC's 198kHz LW transmission.

You will need a 4MHz bandwidth oscilloscope having a high-
sensitivity y-amp (5mV/div worked for me), with an independent
trigger input derived from your frequency standard.

B1.  Temporarily reduce the y-amp sensitivity, connect to the
trigger source, and adjust the 'scope's timebase and trigger
level to stably display one or two cycles of the 1MHz standard
frequency.

B2.  Connect the probe's ground connection to the Control
Board ground point.  Move the 'scope probe to the vicinity of
X2-C21-C22 on the Control Board, but do not touch anything,
just rely on loose capacitive coupling.  Switch the 'scope to
maximum y-sensitivity, and look for a trace showing pick-up
from the 4MHz clock oscillator.

B3.  Use an insulated trimming tool (sorry, but the Elecraft
tool is rather poor here) to carefully adjust C22 so that the
scope trace is resolved into a sine wave moving slowly or
rapidly either to the left or to the right.  Note that four
cycles occupy the same time as one cycle of the 1MHz
reference oscillator.  Careful tuning will result in a near-
stationary display when the trimming tool is removed, and the
'scope probe is gradually drawn away.  This indicates very
close matching of the K2's 4MHz oscillator to four times the
standard frequency.  (A left or right shift of 4 cycles in
one second means the frequencies are matched to one part in a
million.  Temperature effects will degrade frequency accuracy,
so it's probably not worthwhile trying for better than this).

B4.  If you are still building your K2, continue with
'Alignment and Test, Part II, PLL Reference Oscillator Range
Test'.  Otherwise, re-perform CAL-FIL.  (Remember that the K2
manual says 'Whenever you switch modes or filters, the K2 will
first record your new settings, IF THEY HAVE BEEN CHANGED'.
This means that to properly calibrate the VFO to the new MCU
crystal frequency, the VFO control parameter must be changed,
a new mode or filter selected, then the original mode or
filter re-selected and the control parameter changed back to
its original setting.  If you notice a change in the pitch
of a received carrier when switching filters, this is an
indication that CAL-FIL has not been properly done).  If the
4MHz MCU has been changed by more than a few Hertz, the
original control parameter setting may no longer be the optimum
BFO frequency for that filter, so that's something else that
complicates things, and which must be taken into consideration.
Re-perform CAL-PLL.