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IF transformers loosely coupled? - RF Cafe Forums

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Charl
Post subject: IF transformers loosely coupled? Posted: Fri May 19, 2006 5:29 am

Colonel


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Hello folks,

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 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,
Charl


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Charl
Post subject: Posted: Fri May 19, 2006 2:27 pm

Colonel


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Hello again,

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!

Kind regards,
Charl


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nubbage
Post subject: Posted: Mon May 22, 2006 7:32 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Charl
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 cans.
This worked great, and the device became incorporated into the military Mallard C50/R222 transceiver system.

G3OAD


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Charl
Post subject: Posted: Thu May 25, 2006 12:14 pm

Colonel


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Hello folks,

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!

Kind regards,
Charl

[img]http://www.turingbirds.com/content/radio/if.jpg[/img]
N.B. "koppeling" means "coupling", it's Dutch.



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