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IF transformer isolation (casing) - RF Cafe Forums

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Charl
Post subject: IF transformer isolation (casing) Posted: Fri Sep 15, 2006 5:25 am

Colonel


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

My 10.7MHz IF stages are oscillating. I was hoping they wouldn't, but it's not much of a surprise considering they have no casing at all. Are there any guidelines for making such a casing? Can I just use a piece of aluminium tubing and seal off both ends?

Thanks in advance,
Charl

PS. Out of curiosity, do the two (very) loosely coupled IF windings (the stage's input winding and the same stage's output winding) provide 180 degrees of phase shift?


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nubbage
Post subject: IF transformer isolation (casing)Posted: Fri Sep 15, 2006 10:18 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Charl
I had the same problem with a 60dB IF amp at 28MHz.
We found that putting a screening can over the entire amp only made the situation worse. The problem was traced to earth/ground loops that coupled low energy from the output to the input.

Two things that helped solve the problem were
1) very careful dc supply decoupling by multiple types of capacitor
2) breaking the dc ground between the first stages and the final stages, and connecting the dc earth through an RF choke, than taking all dc earths to a common point.

Look on a spectrum analyser at the spectrum of the oscillation: you may be able to see the main frquency and track it down to a resonance within the circuit components.

I hope this helps.


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nubbage
Post subject: IF transformer isolation (casing)Posted: Fri Sep 15, 2006 10:21 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Charl
As an afterthought to your last point, the phase shift of the transformer at resonance will either be 180 degrees or 0 degrees depending how you connect the windings. But very small offset in resonant frequency of the two windings will dramatically change the phase due to the normally hi Q of these circuits.


nubbage


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Charl
Post subject: Posted: Fri Sep 15, 2006 3:21 pm

Colonel


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

Thanks for your reply. I actually tracked it down to a single stage oscillating with itself at the IF frequency.

[img]http://www.turingbirds.com/personal/random/if.jpg[/img]

Adding ceramic and polyester caps in strategic places didn't help. When I short the input to ground the oscillation stops. It has all the looks of being an Armstrong oscillator...

What I actually meant to ask in the first post was whether some aluminium around the transformers themselves would suffice.

Kind regards and thanks again,
Charl


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Charl
Post subject: Posted: Fri Sep 15, 2006 3:22 pm

Colonel


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Here we go..




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Charl
Post subject: Posted: Sun Sep 17, 2006 10:07 am

Colonel


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Well, it looks like it has been resolved. I stopped putting it off and made an aluminium casing. I took a piece of aluminium pipe, cut four holes in it for the connections, and then cut it in half lengthwise. Two tie-wraps hold the two halves together. The oscillations are gone. Still have to do some testing to see how this impacts stage gain, but I don't think it'll be that much of a problem.

Thanks for your help nubbage.





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Charl
Post subject: Posted: Fri Sep 29, 2006 10:03 am

Colonel


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

Just thought I'd post the final version of the homemade transformers. To secure the windings and bobbin in place, I used a small bit of very cheap (40c) foam. This was then cut to size and squeezed (very tightly!) into the two aluminium halves.



Final result:



Hope this helps all you homebrewers out there!

Kind regards,
Charl



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