How to for isolation transformer?

[k9fv]

I asked this question on the end of an existing thread. Since there was no response, perhaps I should have created a new thread for this related subject?

Do I understand this article correctly? cp.literature.agilent.com/litweb/pdf/5091-4480E.pdf
On page 1 they show an isolation transformer for balanced line testing. This transformer looks like it has dual winding, a few turns on the input side, and a few turns on the output side of the transformer so there is NO direct contact between input and output of transformer. I would expect an equal number of turns on both sides so it would be a 1:1 transformer?

Is this not what's required for an isolation transformer to testing balanced line with the AIM4170?

Thanks to any and all for the help in understanding.

73 de Ken H>

edit (12-01-10): Surely someone knows the answer to this question. Does isolation not require the input to be DC isolated from the output as shown in the above referenced file? If not, then it's easy and the MFJ-917 balun should work.

The file states a "balun transformer" - isn't that a voltage transformer that has DC isolation from input to output?

The current balun (MFJ-917) does not have DC isolation from input to output while a transformer balun (voltage) would have DC isolation from input to output as the Agilent AN 346-2 PDF file states is a requirement?

As I read - and write - I am now thinking that is the answer, per Agilent AN 346-2 PDF file, the MFJ-917 balun will NOT work for what they are selling it for - as a current balun it would not provide required isolation?

Big problem with this AIM4170 - it shows me just how little I know about this stuff, but I'm sure having fun!

73 de Ken H>

[Bob]

Hi Ken,

The AIM makes an excellent tool to evaluate baluns. Calibrate the AIM with the calibration loads connected to the RF connector, as usual. Then connect the balun and attach various resistors to the output of the balun. Do a scan over the freq range of interest and see the effect. In this way you can evaluate different core materials and turns ratios.

Note that a balun wound with coax will have a "built in" transmission line corresponding to the length of the coax. This will act as a transformer too.

Later when you decide on a good balun for your application, you can calibrate with the cal loads at the output of the balun to cancel the small imperfections.

-- 73/Bob

[k9fv]

Thanks for the input Bob - I know you are the man with knowledge about these things. I have been playing with baluns in the manner you mention and the AIM4170 sure works good for testing baluns.

My question was about working with balanced loads... ladder line, etc. The Agilent AN 346-2 PDF file that has been referenced in the past on this forum as a good study guide says "isolation balun" and shows a drawing of the balun as having windings on input to test equip (4170 in this case) with seperate windings on output to load under test. These windings are shown wrapped around a core - this gives DC isolation between input and output and indicates this DC isolation is required to isolate load from test equip, much like using a notebook AND 4170 on battery power to be sure there is no ground loops involved.

Sure looks like if a current balun was used with no DC isolation between 4170 and load, it wouldn't matter so much about isolation of power source.

Still confused.

73 de Ken H>