On Wed, 23 Feb 2011 16:43:10 -0800, Joel Koltner wrote:
I graduated from college back in the 1994, and even then we were
admonished to avoid magnetics whenever possible. Of course, these days
I know better, but as a result my academic coverage of IF transformers
was non-existant.
I'm now trying to make up for that transgression. :-)
I've done a fair amount of reading and have a good understanding on how
IF transformers work, how they should be modeled, how to build them,
etc. (Most of the books that address this in detail are from the 1970s
or older, it seems...) I still have a few questions, though, that I'm
hoping a few of the older reads could help me out on. They a
1) The really big 450kHz IF transformers you see in tube sets... why did
they wind the coils in the form of "pancakes" rather than "the usual
way" (single-layer coils)? Is it just a consequence of needing lots of
turns (to get enough magnetizing inductance) but, for the coupling
coefficient desired, finding that you'd end up with, e.g,. a foot-long
tranformer if you only used a single layer?
Googling "Pi-wound coil" may help. It was to reduce shunt capacitance,
as mentioned.
2) I can readily see why you'd want a center-tapped primary, or a
primary with, say, a tap 10% "up" as a small feedback winding, but why
do you get such things as an IF transformer with 103 and 50 turns on the
primary (on either side of the tap) and then 27 turns on the secondary?
(E.g., http://www.mouser.com/catalog/specsheets/XC-600014.pdf ). None
of my books address this, and the only thing that looks close on the web
is this article: http://hem.passagen.se/communication/ifcan.html . Is
his conclusion, "by tapping the transformer the Q value increases" the
main reason?
3) Sticking a parallel capacitor on the primary to resonate
out the magnetizing inductance makes sense to me. I'm a little less
clear on parallel capacitors on both the primary and secondary -- a
double-tuned arrangement. Hagen's "Radio Frequency Electronics" assigns
leakage inductance to the secondary and then converts the resonating
capacitor in parallel with your load resistance back into a series
circuit and, voila!, you now have a series RLC circuit so clearly
bandpass behavior... but this approach implies that you could just use a
*series* resonating capacitor on the secondary instead. Is that
correct? (I am aware that there are a handful of commonly used
transformer equivalent circuit models, you can transform magnetizing or
leakage inductances and losses from primary to secondary or vice versa
at will, etc.)
Are you talking transistor IF transformers, with one slug each, or tube-
type, with two? Two-slug IF transformers, with lightly coupled and
independently tuned coils, gave you two filter sections in one can.
4) Anyone have pointers to good books or articles that ideally discuss
some actual design examples of the more complicated cases (weird primary
turns ratios, double-tuned circuits, etc.)? -- The ones I've found so
far as the simpler single-tuned case, just center-tapped, etc.
Old radio texts. If you still get up to Portland from time to time, dig
through Powell's Technical books.
What are you trying to do? There's good reason for not wanting to have a
circuit with a bazillion tweaks that all have to be right for the thing
to be in tune.
--
http://www.wescottdesign.com