[Elecraft] Drifting Away...Does it matter?

Thu Jul 24 11:08:01 2003

Ron,

IMHO, it's all relative... I'm happy with the stability of my K2. I'm still
able to keep a signal in the passband using the tuning knob which, thanks to
the PLL mod, I hardly *ever* have to do.

The problem will only be solved when the problem of increasing temperature
as solid state devices warm up (W=I*I*R) causing changes in conductance,
resistance, etc., in the devices themselves, has been resolved (Conservation
of Energy ??)

I submit that this stage of affairs is approaching with the advent of SDR's
(Software Defined Radios).

Send all flames to me direct - the list doesn't need to be swamped  <G>

Thanks for the bandwidth

Dave KK7SS
K2 #3003
----- Original Message ----- 
From: "Ron D'Eau Claire" <[email protected]>
To: "'Elecraft Reflector'" <[email protected]>
Sent: Thursday, July 24, 2003 7:41 AM
Subject: RE: [Elecraft] Drifting Away...Does it matter?

I love your comments Lee!  It's great to keep things in perspective.

The problem today is that most ops expect the signal to stay in the passband
of their 100 Hz I.F. filter. Back in the 50's we expected signals to stay in
the passband too - but it was often 1 or 2 kHz wide rather than 100 or 200
Hz wide <G>.

The K2 drift is caused by some normal variation of both the BFO and L.O. On
some bands they add and can be significant, even for casual SSB/CW
operation. Those bands are usually the higher frequency bands because of the
way the K2's mixing scheme works. Since the drift is caused by temperature
changes it showed up much more after ops started adding the KPA100 modules
to their rigs. With the heat added to the K2 by the KPA100, It is possible
to see more than 200 Hz of drift in an hour's operating on some bands.

Working contests, you probably would never notice it. Working a long SSB/CW
rag chew on some bands it would be noticed.

As I recall, the original concerns about the drift came from PSK ops. When
you're working in a mode where 31 Hz is as w-i-d-e as the sigs get, a little
drift can take you right out of the QSO.

So, in typical and laudable Ham fashion, these ops asked why the drift
couldn't be reduced. It could and it was.

It's important to realize that the drift is really an artifact of one of the
K2's greatest strengths - its very quiet PLL system used for the receiver
local oscillator. Fully-synthesized PLLs like many other rigs use can be
made very stable by using a highly stable reference oscillator. But the
tradeoff is that the rigs tune in discrete "steps". If you make the steps
small enough that you don't notice them when you tune across the bands,
there'll be some inherent phase jitter that shows up as "noise" in the
receiver. The operator can't tell it from "band" noise, but it's
locally-generated noise that hurts the weak signal performance of almost all
rigs.

Rather than do that, the K2 uses a tunable reference oscillator in the PLL.
The PLL changes frequency in much larger steps and so can be designed to be
electrically very quiet. The K2's reference oscillator frequency is shifted
to provide continuous tuning within the PLL steps. That makes for a superbly
quiet and high-performance receiver, but means that the K2 cannot use one of
the extremely stable fixed-tuned reference oscillators popular in some other
rigs.

A good rule of thumb concerning oscillators that was true in the 50's and is
still true today is this: Any time you design an oscillator to be able to
change frequency when you want it to, it will also try very hard to change
frequency on its own when you don't want it to. The K2's tunable PLL
reference oscillator is no exception.

It's a good trade off, I think. And, thanks to the work John Grebenkemper
did in coming up with a good temperature-compensation circuit to offset the
residual drift, the K2 is a very hard receiver to beat.

Ron AC7AC
K2 # 1289

-----Original Message-----
...Now, I've never worried about the drift.  I am never on one frequency
long enough to make it matter.  I am a contester and do a lot of search and
pounce, hunt and peck, whatever.  There are times when I sit on a frequency
and run the dickens out the rig, but I am using the RIT and am constantly
changing the RIT to suit the signal to my old ears.  So, I've never had to
really work about drift.

I guess when you are on the digital modes or on SSTV (do people run SSTV
with a K2?) I would thing that drift would be a factor.  But, I am always
fiddling with the rig or the software to see what it can do.

So, I what is all the fuss about the drift issue?  It would be nice to have
a stable rig, but the "normal" K2 is pretty good in my humble estimation.
For some, maybe we just need to warm up the rig and let it drift until it
stops.

Now, if you want to talk about drift...Heathkit's VF-1 VFO.  I remember when
I was 16 years old working with that old thing trying to get the drift out
and the chirping stopped.  That is when I learned about OA2 tubes and NPO
caps.  Well, I've digressed from my point....

Heck, what was my point?  Oh yea...I guess for some of us the drift is not a
big thing.

Comments to the contrary are welcome.

Lee -  Drifiting Away with My K2 (Where is Jimmy Buffet when you need him)
K0WA

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