[Elecraft] Frequency / Temperature:

Thu Mar 13 23:33:00 2003

Hello Martin:

The issue is that the K2 uses a VFO, not a highly stable crystal
oscillator, for the PLL reference oscillator. The use of a VFO has
significant advantages in the overall performance of the receiver and
plays a large part in the outstanding price-performance combination of
the K2, but it comes at the cost of there not being a simple and easy
way to make the K2 extremely stable over wide temperature excursions.

Some of the drift occurs in the BFO as well as the VFO used for the PLL
reference oscillator, but the bulk of it is in the PLL Reference
Oscillator. Since both oscillators are involved, the drift varies from
band to band as the drifts of the two oscillators add on some bands and
subtract on others. 

To be precise in terminology, I should point out that the K2 uses VXO's
or "variable crystal oscillators" for the BFO and PLL Reference
Oscillator, not typical VFO's whose frequencies are controlled by
inductors and capacitors. A VXO is much more stable than a typical VFO,
but still does not approach what a good high stability, single-frequency
crystal reference oscillator can provide. 

Even so, Elecraft is working on a way to make the rig more stable. John,
KI6WX, has developed a mod that significantly reduces the drift in the
PLL Reference Oscillator. That  mod is being studied and more
engineering work is being done by John and Elecraft to make it work even
better before they recommend it to everyone. I, and a battery of others,
have done some careful testing of John's design. 

With John's mod, my K2 shows a typical drift of less than 40 Hz on any
band even when the internal temperature is driven high as 50C (about
120F). I heated my K2 by transmitting into a dummy load, and limited
myself to about that temperature because the KAT100 heat sink was
getting too hot to "handle", Hi!  That, in spite of Eric's assurance
that the amplifier can take a lot of punishment! 

You have a much newer K2 than my 3-year old "veteran". First check to
see if the currently-available "Stability mods" are applicable to yours.
They help a lot and are a required first step. These mods remove one of
the two crystals used in the PLL Reference Oscillator in the original
design. That does a LOT to stabilize the oscillator. They also make a
change to the BFO. Check with Elecraft before you order those mods
because they may be a part of your basic K2 since it has a serial number
> 3000. I don't recall exactly when they were released. 

Then, if you'd like to do more, check on John's mod or keep an eye open
for an announcement from Elecraft one of these days. I'm not sure if
John's posted his mod anywhere. He was concerned that it may NOT work on
ALL K2's, so he was keeping track of the people who were trying it, but
I am not aware of anyone who did not get a big improvement out of it.
John's mod involves adding a thermister to the oscillator that adjusts
the frequency to compensate for a rise in temperature. It is not too
complicated for an experienced builder, but it does involve involving
adding a number of resistors and the thermister to the bottom of the RF
board. 

In any case, in time you will see a package from Elecraft that addresses
this issue in even greater detail with a very thoroughly tested and
well-documented mod. 

That's one of the "hidden" features in the Elecraft rigs - with the
radio we get a couple of Engineers in Aptos who never seem to be able to
say "when" and they just keep on keeping on making it better. 

Hummmm.. sort of the "Energizer Bunnies" of the Ham business, Hi!

Ron AC7AC
K2 # 1289

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Martin AC6RM
Sent: Thursday, March 13, 2003 4:38 PM
To: [email protected]
Subject: [Elecraft] Frequency / Temperature:

Hello,

Over the last two days I operated from Joshua Tree.  No cell phones. 
Wanted email so subscribed to http://www.winlink.org/ to remain in touch
with the YL, etc.  This service is worthy.  I was reasonably successful
with getting the K2/100, my KAM+ and laptop to talk to various nodes on
20 and 40.

However, the VFO dial was up to 100 hz off (high) in some cases, and
dead on in other cases.  The 40m nodes required significantly more power
and the heatsink got quite hot.  For these nodes I resolved to wait
until someone else logged in, zero-beat their signal and then used that
frequency.  It was usually about 100hz off, plus or minus, but it
worked.

I'm looking for a solution.  A temperature-controlled reference
oscillator addition to the kit, perhaps?  I've heard that separating the
KPA100 seems to work, but I'd rather not -- I love the way my rig looks,
all compact and square -- and I'd rather not have to carry another box.
Any ideas?

Thanks and 73,

Martin AC6RM (k2/100 #3021)