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?
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.)
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.

Thanks a lot... I appreciate the help here!

---Joel

"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.

Thanks Andrew, that helps a lot!

(...I do think in the example I linked to --
http://hem.passagen.se/communication/ifcan.html -- his circuit's Q as drawn
actually increases 1/(0.67^2) times rather than 1/(0.33^2) times, though; I've
e-mailed the guy to ask.)

---Joel

On 02/23/2011 06:43 PM, Joel Koltner wrote:

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?

They do this to try to keep the self-resonance of the winding up above
the operating frequency. By spreading the winding out into series
connected pancakes the stray capacitances have a harder time shunting
large inductances. You see this kind of construction on RF chokes too.

Paul Probert

"Paul Probert" wrote in message
news
On 02/23/2011 06:43 PM, Joel Koltner wrote:

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"?
They do this to try to keep the self-resonance of the winding up above the
operating frequency. By spreading the winding out into series connected
pancakes the stray capacitances have a harder time shunting large
inductances. You see this kind of construction on RF chokes too.

Ah, clever!

One I was looking at the other day has some fancy weaving involved as well; it
must have been quite an interesting machine that made them.

Poking around You Tube some I see that winding single-layer (no fancy waving)
coils is quite doable at home (the Tesla coil guys are quite into it). It's
pretty slick: http://www.youtube.com/watch?v=N3N-tw6OEXw .

Thanks for the help,
---Joel

On Feb 25, 1:44*pm, "Joel Koltner"
wrote:
"Paul Probert" wrote in message

news
On 02/23/2011 06:43 PM, Joel Koltner wrote:

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"?
They do this to try to keep the self-resonance of the winding up above the
operating frequency. By spreading the winding out into series connected
pancakes the stray capacitances have a harder time shunting large
inductances. You see this kind of construction on RF chokes too.

Ah, clever!

One I was looking at the other day has some fancy weaving involved as well; it
must have been quite an interesting machine that made them.

Poking around You Tube some I see that winding single-layer (no fancy waving)
coils is quite doable at home (the Tesla coil guys are quite into it). * It's
pretty slick:http://www.youtube.com/watch?v=N3N-tw6OEXw.

Thanks for the help,
---Joel

Back as far as the 70's hand operated machines could be purchase for
winding coils. I think I remember seeing them in my old Allied
Electronics catalog.

Jimmie

"Joel Koltner" wrote


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?


See

http://www.crystal-radio.eu/enkoppelfactor.htm

"John J" wrote in message
...
See
http://www.crystal-radio.eu/enkoppelfactor.htm

Thanks John.

What stymies me about the double-tuned transformer is this: If you look at
Hagen's exaplanation for how they can be modeled -- I've stuck a scan he
http://koltner.com/Hagen.png -- he's modeling it as the primary's parallel
capacitor is resonating with the magnetizing inductance of the IFT and the
secondary's parallel capacitor is resonating with the leakage inductance...
and this is then made more obvious if you use the high-Q approximation and
transform a parallel RC circuit into a series RC circuit. But if that's the
case... doesn't it seem as though the most straightforward way to use an IFT
would be to have a parallel capacitor on the primary and a *series* capacitor
on the secondary?

I realize that you can move impedances from one side of an IFT to another and
change the equivalent circuit model and so on and present numerous different
"views" that all end up with the same correct mathematical behavior to model
what are really just two coupled inductors, but still... does anyone using a
series resonating capacitor on their secondaries?

---Joel

Answering myself he

"Joel Koltner" wrote in message
...
does anyone using a series resonating capacitor on their secondaries?

Googol books provides some pages from the "Electromagnetic Compatibility
Handbook" by Kenneth Kaiser
(http://books.google.com/books?id=nZz...9&lpg=RA1-PA19) and
states:

"There are four different configurations for the double-tuned transformer
based on the positions of the capacitors on the primary and secondary sides.
For example, one capacitor can be in series with the primary winding, whereas
another capacitor can be in parallel with the secondary winding. A few
reasons why one configuration is selected over another will be discussed after
a few circuits are analyzed."

....and so on...

This is Good To Know.

---Joel

"Joel Koltner" wrote

What stymies me about the double-tuned transformer is this: If you look at
Hagen's exaplanation for how they can be modeled -- I've stuck a scan
he http://koltner.com/Hagen.png -- he's modeling it as the primary's
parallel capacitor is resonating with the magnetizing inductance of the
IFT and the secondary's parallel capacitor is resonating with the leakage
inductance... and this is then made more obvious if you use the high-Q
approximation and transform a parallel RC circuit into a series RC
circuit. But if that's the case... doesn't it seem as though the most
straightforward way to use an IFT would be to have a parallel capacitor on
the primary and a *series* capacitor on the secondary?

I realize that you can move impedances from one side of an IFT to another
and change the equivalent circuit model and so on and present numerous
different "views" that all end up with the same correct mathematical
behavior to model what are really just two coupled inductors, but still...
does anyone using a series resonating capacitor on their secondaries?

If you use a capacitor in series with the winding to create series resonant
circuit, then the impedance at resonance will be low.

For a tube IF, you really want high impedances at resonance.

In the plate circuit this is to minimise current draw.

The grid input of the preceeding stage is high impedance and is voltage
driven, therefore a parallel tuned circuit is ideal.

So hence the use of dual parallel tuned circuits.