[Elecraft] L31 vs PLL range

[John, KI6WX]

A couple of comments on my comments yesterday.
#1 - The resistor values were picked to minimize the drift on my K2 using a
crystal that was in the middle of the drift range of 7 crystals that I
tested.  All 7 of the crystals showed a PLL tuning range of 12.9 kHz,
+/-200Hz.

#2 - The tuning range of 11 to 15 kHz was suggested since this centers on
the 12.9 kHz on my K2.  Gary is technically correct that the K2 will
function at 1.8 MHz with a 9 kHz range.  However, from what I know about the
drift in the PLL reference oscillator, I wouldn't recommend a tuning range
of less than 10 kHz.  So let me modify my statement to say that the PLL
tuning range should be between 10 to 15 kHz.

#3 - Do not try to tune your K2 so that the PLL tuning range is exactly 12.9
kHz.  The drift on the crystals does vary by crystal so getting it exactly
the same as mine won't necessarily minimize your drift.

#4 - If you want to minimize your drift, follow the procedure in the
installation guide.  Measure the frequency of your K2 using WWV with the K2
both cold and hot, and then adjust the one 1/4 watt resistor on the board
and repeat again.  We purposely made that resistor large so that it can be
easily changed.  It should only take a few iterations to minimize the drift
in your K2.

#5 - If you don't want to go to the extra work to minimize your drift, then
just change L31 to a value that will reduce your PLL tuning range to less
than 15 kHz.  This will get you most of the benefit from the drift reduction
modification.

As for Lee, which inductor gives you a PLL range less than 15 kHz.  Use that
one, and you should be fine.

I have yet to hear of anyone who has actually changed the resistor value to
minimize the drift.  If  you have tried doing that, send me a private email
and let me know how it went.
-John
 KI6WX

----- Original Message ----- 
From: "Lee Buller" <k0wa@swbell.net>


> So let me get this straight because I have the BFO mod and the tempurature
compesation mod on the bench right now.  I should leave the L31 at 10uh
instead of putting in the 12 ub L31?
>
> Lee - K0WA
>
>
> Ron ZL1TW <ron_w@clear.net.nz> wrote:
> Hi all,
> I was very interested in John's (WI6WX) very clear explanation of
> why L31 had been changed in value, as I did the "Temperature Compensated
> PLL mod" and changed L31 from the original 10 uH to 12 uH. After doing the
> change I really wasn't very impressed with the improvement in temperature
> stability (if any) and I am sure I know why now.
> When I built my K2 #2204 we were advised that a PLL range of 11.5 to
> 25kHz was good. I recorded 24.8kHz . So far so good.
> Then along came the BFO mod and Temperature Compensated PLL mod, where we
> were advised the range "must be greater than 9kHz". Mine was miles above
> that so I assumed I was still OK.
> Last night John's note comes along and suggests a range of 11 to 15 khz.
> Hells Bells......I am miles too high!!
> I then read up the latest K2 Construction Manual and it states a range of
> 9.8 to 13 kHz . A fourth set of figures, and they concur with John's.
> At this stage I took the bottom off my K2 and replaced L31 back to the
10uH
> inductor. Measurements now show a PLL range of 10.64 kHz. I then did a
> CAL PLL and left it at that.
> I have to say that the quick test I have done so-far, the temperature
> compensation appears to be much better, but I intend giving it "the works"
> when WWV comes in a bit better later this evening.
> Just as a safeguard I queried Gary at Elecraft about my PLL range and he
> replied " a 9 - 15 kHz swing is OK"
> So there we have it. I enclose this info FYI on the off chance someone
else
> might have gone along with the same mods and had similar results?
> Cheers.......Ron ZL1TW