IF Transformers Loosely Coupled? - RF Cafe Forums
Post subject: IF transformers loosely coupled? Posted:
Fri May 19, 2006 5:29 am
May 19, 2006 5:01 am
I'm building a digital superheterodyne tube radio
(for kicks, I'm sure you'll understand). Looking at some IF transformers
from actual working tube radios, I find that they are very loosely
coupled. The ferrite cores are about a centimeter away from each
other (primary to secondary).
Why would they want such loose
coupling? I would think that you want maximum amplification in the
I was hoping to be able to wind my own 10.7MHz
IF transformers on some ferrite cores, which were used for EMI filtering.
Any idea whether this is the right core material for the job, or
rather - will it work?
Thanks in advance,
Post subject: Posted: Fri May 19,
2006 2:27 pm
Joined: Fri May 19, 2006
Looks like I answered my own question.. this site was helpful:
http://www.tpub.com/content/neets/14180 ... 180_74.htm
sure that using those ferrite transformers as I intended to would
have caused massive overcoupling.
I might be bugging y'all
with some more questions in the future!
Post subject: Posted: Mon
May 22, 2006 7:32 am
Joined: Fri Feb
17, 2006 12:07 pm
Location: London UK
I was going to post you a reply when I had found more info.
designed a 28MHz IF amp some years ago that had a very tight selectivity
spec, and I used loosely coupled LC discrete component resonators.
There is a critical coupling mutual inductance required between
the coils that ensures the Q is not significantly reduced by either
the input circuit resitance or the output resistance. Also the Q
of the individual resonators is quite critical. I set this by trial
and error using discrete resistors then realised I could achieve
the same result more elegantly and save cost and components by winding
the coils with nichrome resistance wire. The resonato LC parallel
circuits were coupled through length-wise slots cut in the screening
This worked great, and the device became incorporated into
the military Mallard C50/R222 transceiver system.
Post subject: Posted: Thu May 25,
2006 12:14 pm
Joined: Fri May 19, 2006
A little update. I'm doing some experiments right now with airwound
IF coils. First I'm just going to wind one IF trafo, tune the primary
and secondary, and then take it to my university where I will be
adjusting the coupling to get optimal bandwidth (they have a nice
and expensive spectrum analyser).
I'm moving one step at
a time. The (rather silly) setup works fine, but the bandwidth of
this IF transformer setup is probably way off (and only works because
it is corrected by other IF stages).
One I have a good working
IF transformer I'll post some more pictures!
N.B. "koppeling" means "coupling", it's Dutch.