[R-390] VFO squirreliness solved FOR GOOD

[Tim Shoppa]

"Dan Merz" <mdmerz at verizon.net> wrote:
> Tim,  thank you for taking the time to relay your findings.  I never messed
> with the linearization on my Cosmos because it looked messy.  And I don't
> recall really cleaning mine like you did when I had it open to take a turn
> off,  which might be worth the effort.

I doubt that the grease-monkeys at Raytheon put so much oily goo inside
your PTO's, Dan :-).

> I wonder how much the linearization scheme improves the linearity.  How
> non-linear are the other pto's that don't have this feature? Or how much
> correction are you making as you tweak these?

One of my two units has a nonlinearity of about +/- 1.2kHz without
linearity correction, and the other one (the one Raytheon gooped
all up inside) is about +/- 2.0kHz without
linearity correction. (By "without linearity correction" I mean
that all the little screws have been adjusted to midrange and
I've already optimized the endpoints.)

The maximum range of linearity correction by turning the little screws
is about 4kHz at the 3.55MHz end (and 2.something kHz at the 2.455
MHz end), so even the extreme one is still within correction. That's
about two turns of the little screw. To align at the 200Hz level the
screws need to be adjusted to the tenth of a turn, and this is
probably about as good as can be done. (I'm sure somebody's gonna
chime in here about how they linearized theirs to the 10Hz level...!)

I don't think my PTO's are "cream of the crop" with respect to
accuracy before correction.

Getting them much better than 200Hz is probably not feasible because
the linearity screws aren't flat to that level on the back.

Hypothetically, I think the correction range could be extended somewhat
by taking turns (there aren't that many to start with!) off the endpoint
inductor and moving them to the linearity inductor.

Just moving some of the flying wires inside the PTO can will change linearity
at the 1-kHz level.

As to "overanalyzing" this, I've got a few dozen tables of frequencies
vs dial positions as I tweak the endpoint, sometimes at the 25kHz points
but usually at the 100kHz points. It's interesting to an extent (but
also realize that I spent many many years in grad school graphing
data and fitting curves so I'm not actually averse to the subject.)

Tim.