[Elecraft] VFO calibration

Sun Dec 29 12:36:00 2002

At 12:20 29/12/02 -0500, Helmut Usbeck wrote:
>I had mine sent to a lab where it was compared to a rubicon
>standard.

= rubidium ??  ('rubicon' has an altogether different meaning)

Helmut, I don't know why there should still be such confusion
about setting the 4MHz control board oscillator.  Here's a
couple of methods which have previously been posted to this
reflector.

73, Steve GU3MBS

==============================================================

Elecraft K2 Control Board 4MHz Oscillator Adjustment

by Steve Gibbs GU3MBS      [email protected]       18 Dec 02

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 & 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 & CAL PLL.