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IF transformers loosely coupled? - RF Cafe Forums
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Below are the old forum threads, including responses to the original posts.
|-- Amateur Radio
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-- CAE, CAD, &
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Post subject: IF transformers loosely coupled? Posted: Fri
May 19, 2006 5:29 am
Joined: Fri 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
Why would they want such loose coupling? I would
think that you want maximum amplification in the IF stages.
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,
subject: Posted: Fri May 19, 2006 2:27 pm
Joined: Fri May 19, 2006 5:01 am
Looks like I answered my own question.. this
site was helpful:
http://www.tpub.com/content/neets/14180 ... 180_74.htm
I'm 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
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.
I 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
This worked great, and the device became incorporated
into the military Mallard C50/R222 transceiver system.
Post subject: Posted: Thu May 25, 2006
Joined: Fri May 19, 2006 5:01 am
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).
have a good working IF transformer I'll post some more pictures!
N.B. "koppeling" means "coupling", it's Dutch.