Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: Binocular choke extras
Date: Fri, 21 Mar 2025 00:30:29 +0100
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On 3/21/25 00:24, john larkin wrote:
> On Fri, 21 Mar 2025 00:08:53 +0100, Jeroen Belleman
> <jeroen@nospam.please> wrote:
> 
>> On 3/20/25 16:43, john larkin wrote:
>>> On Thu, 20 Mar 2025 10:49:08 -0400, Joe Gwinn <joegwinn@comcast.net>
>>> wrote:
>>>
>>>> On Thu, 20 Mar 2025 07:30:58 +0000, liz@poppyrecords.invalid.invalid
>>>> (Liz Tuddenham) wrote:
>>>>
>>>>> Jeroen Belleman <jeroen@nospam.please> wrote:
>>>>>
>>>>>> On 3/19/25 22:40, Liz Tuddenham wrote:
>>>>>>> I've just taken delivery of a couple of ferrite 'binocular' choke cores;
>>>>>>> each one came with two thinwalled metal tubes and some bits of printed
>>>>>>> circuit board.  The tubes appear to go through the holes in the choke
>>>>>>> and the holes in the boards fit over the ends of the tubes, with copper
>>>>>>> areas that could possibly be soldered to them.
>>>>>>>
>>>>>>> Does anyone know what purpose these serve?
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> This sounds very much like the transformers used in wideband RF
>>>>>> power amplifiers. See for example Helge Granberg's application
>>>>>> note 762. The tubes with a piece of circuit board form the single-
>>>>>> turn low impedance winding, and a number of turns of insulated
>>>>>> copper wire going through the tubes form the high-impedance
>>>>>> winding. This makes for a good coupling factor and consequently
>>>>>> good wideband operation.
>>>>>
>>>>> That's the sort of thing I suspected.  Each square pad surrounding the
>>>>> end of a tube is individually isolated but they could easily be joined
>>>>> to make a loop circuit with some wire straps.
>>>>>
>>>>> I intend using this as a 1:1 balun and was worried that the conventional
>>>>> way of twisting the primary and secondary conductors together before
>>>>> threading them through the core would create a capacitive imbalance.  If
>>>>> I use the tubes as a 1-turn secondary and thread the inner of the feed
>>>>> co-ax through them, this will give much lower capacitance imbalance.
>>>>
>>>> This also sounds like it could be a transmission-line transformer;
>>>> these are very wideband.  The ferrite cores serve as RF chokes,
>>>> ensuring the shield and center currents are exactly equal and
>>>> opposite.  It is _not_ an ordinary RF transformer, despite the name.
>>>>
>>>> "Transmission Line Transformers", Fourth Edition, Jerry Sevick, W2FMI,
>>>> 2001, 289 pages, ISBN 1-884932-18-5, TK6565.T7 S48 2001,
>>>> 621.384'11--dc21.
>>>>
>>>> Joe
>>>
>>> I have the Sevick book but it's not very useful.
>>>
>>> We make super wideband tline transformers from micro-coax and pot
>>> cores.
>>>
>>> https://www.dropbox.com/scl/fi/32s2rfcu4q4iq6l6v1eb4/Pot_Core_TXline.JPG?rlkey=6k7xusurck0jf1ky9n6ja2ebz&raw=1
>>>
>>> https://www.dropbox.com/scl/fi/adcocf1rb7lnanj7zo9xp/TX_1.jpg?rlkey=m7prsxj94fa57ynqoep0ydgnl&raw=1
>>>
>>> Or toroids, which are harder to make.
>>>
>>> https://www.dropbox.com/scl/fi/xuqjzt3h1oq7uexwiu6c8/T750_1.JPG?rlkey=si165mntuu0h40zgsbi0qzxj7&raw=1
>>>
>>>
>>
>> Those are 1:1 baluns. It's not too hard to get stupendous bandwidths
>> with those. Six decades of frequency should be quite easy. It gets
>> harder when you want different impedances at the ends.
>>
>> Jeroen Belleman
> 
> I call them transformers. We use them to isolate pulse generator
> outputs, and sometimes to get a voltage step-up.
> 

There are windings and a magnetic core, so I'm OK with calling
them  transformers. A balun is just a specific kind of transformer.

I suppose one end has the shield connected to ground and the other
end connects it to something that isn't ground?

Jeroen Belleman