[Elecraft] Re: BFO Drift (and a solution for it)

[Wayne Burdick]

Earl,

Removing one crystal tends to cut VFO drift by 30-40%, but this will not work for the BFO.
(The amount of "pull" possible with one crystal is, unfortunately, not proportional to
carrier frequency. It gets harder as the frequency goes down.)

The good news is that we recently found a way to substantially reduce BFO drift, and had
planned to announce it next week. Now is as good a time as any. (By the way, your past
observations about drift were very helpful when we started looking into the problem in depth.)

We have determined that nearly all of the BFO drift is due to L33. As you know, we had to
use a toroid at L33 in order to minimize BFO leakage into adjacent circuitry. In order to
keep the number of turns on this toroid reasonable, we originally used a core with a high
permeability and a moderately-good temperature coefficient (about 250 ppm/degree C--a lot
better than ferrite). 

But since this turned out to be the source of most of the drift, we have decided to begin
supplying a new, pre-wound, highly stable inductor. This new inductor uses a T44-7 core.
It's the same size as the old T44-8 but has a temperature coefficient a factor of 8
smaller (30 ppm/degree C). In order to get enough inductance on the T44-7, we had to use
over 90 turns of #34 wire. This is not a winding we could ask customers to do, so we are
having them professionally wound.

We've tested the new inductors on about half a dozen K2s. BFO drift was reduced by as much
as 80%. The average was about 75% (a factor of 4 improvement), and the worst case was
around 70%. It may be possible to further reduce BFO drift by using N750 capacitors in the
BFO, but we haven't received our test samples of these capacitors yet.

In actual operation the difference is dramatic: the BFOs remain much more closely aligned
with the selected filter's center frequency.

Within a week or two we will begin offering a mod kit that includes the new toroid, as
well as a single, wide-range PLL reference oscillator crystal. (We're already including
these parts in new K2s.) If the N750 capacitors help, too, we'll also add these to the mod kit.

I appreciate your patience while we've tried to resolve this. I'll be sending you one of
these kits the moment it becomes available, at no charge. Please try it on your K2 and let
us know if the degree of improvement is satisfactory.

73,
Wayne
N6KR