On 8/11/2015 7:17 PM, Dave Platt wrote:
In article , John S wrote:

I think their minimums are not realistic. I would not know how to
achieve 0pF with a 3926pF variable cap, for example. Also, why 3926
rather than 3900 or even 3920? It isn't all that easy to measure 6pF out
of 3920pF. I'm thinking that somebody just threw some numbers at the
spec sheet.

BUT! It is better than nothing and may help to make some reasonable
judgements.

Typically, these tuners use banks of capacitors, switched in and out
(parallel for caps, series for inductors) using relays. The
components are often in a 1:2:4:8:... power series. "256 values"
implies 8 components, and 8 relays, giving you anywhere from 0 to 255
times the "fundamental" value.

I expect that "0" means "all caps open, or all inductors shorted",
and "3926" is the sum of the nominal values of the caps in the bank.
If their starting value is around 15 pF, 255 times that would be
3825; add in a few pF of stray capacitance per relay and you'd be
in the neighborhood of 3926.

(Having figures like that, in the face of component variations and
strays and measurement errors, reminds me a bit of some dialog from
Harry Harrison's "Star Smashers of the Galaxy Rangers."

"[certain ancient events] took place approximately 14 billion years
ago."

"That figure is exact?"

"Of course."

Haw!

Very good, Dave. It seems that I am a fossil. The world changed while I
slept.

Thanks for the education.