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

[David A. Belsley]

Wayne:
  I'm kinda confused.  L33 in my K2 is not a toroid, but a 39uH solenoidal, 
shielded inductor. I believe there may have been a mod to replace this with 
a toroid, but I never did.  Does the solenoidal inductor have the same 
problem relative to drift?  Would it be better to switch this out with your 
modification kit when it becomes available?

thanks,

dave belsley, w1euy

--On Sunday, November 3, 2002 4:30 PM -0800 Wayne Burdick 
<n6kr@elecraft.com> wrote:

> 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
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----------------------------------
David A. Belsley
Professor of Economics