IF transformers loosely coupled? - RF Cafe Forums
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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
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 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,
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
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!
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.
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 screening
This worked great, and the device became incorporated into the military Mallard C50/R222 transceiver
Post subject: Posted: Thu May 25, 2006 12:14 pm
Joined: Fri May 19, 2006 5:01 am
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
N.B. "koppeling" means "coupling", it's Dutch.