"Joel Koltner" wrote in message
...
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


[snip]

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?

Short answer: yes.

Long answer:

That IF transformer is intended for driving a stage with an input impedance
of 5K. When they say the primary impedance is 20k, they mean between pins 2
and 3. 50:27 = ~ 2:1 turns ratio = ~ 4:1 impedance ratio. So you connect
pin 2 to the supply and pin 3 to the collector of your transistor and it
sees a load of ~ 20k, ignoring coil losses. The 180pF tuning capacitance is
connected between pins 1 and 3. You could try to resonate it by connecting
a much larger cap (1.7nF !) between 2 and 3 and not use the 1-2 section at
all; but the smaller cap has higher stability, tighter tolerance and lower
loss.

The 20k and 5k impedances are dictated by the nature of bipolar transistors.

The required bandwidth and IF centre frequency fix loaded Q.

Dynamic impedance and loaded Q in turn determine the reactance needed at
resonance.

You could design an untapped 4:1 impedance ratio transformer to do the same
job; but you would have difficulty achieving the required Q.