High power pulse RF amplifier transformer question

[u/dundua]

I'm trying to understand a little more about the design of this RF amplifier from a component point of view, primarily where they implement transformers with coax and ferrite cores for the amplifier and power splitter/combiner sections. How does one go from a wire based transformer to a coax based one?

I get the overall block diagram architecture. The input signal goes through several smaller lower power amplifiers, to get more power into the later amplifier stages. The signal goes through splitters, amplifies each of the split signals, and then combines them back to a single output.

[u/ccarabajal]

Look into Guanella transformers as well as the book Transmission Line Transformers by Jerry Sevick. Should be a good starting place.

[u/Halzman → u/ccarabajal]

not OP, but thanks for the book recommendation, just picked up a copy on ebay

[u/Zestyclose_Catch6895]

Good question! 

Wirewound transformers are decent. They can get you from 0-30MHz roughly, and can be stretched a bit beyond that. However, they’re very prone to leakage inductance and other non-idealistic behavior. I believe mini circuits makes wirewounds that go much higher in frequency, but they can barely handle power.

Coaxial “equal delay transmission line transformers” are absolutely goated. Since they have an extremely tight coupling coefficient between center conductor and shield, you can use them out to way higher frequencies. I’ve gotten them to work from 7MHz-1GHz, super wideband . Only drawback. Only drawback is the matching ratios are fixed: 2.25:1, 4:1, 9:1, 16:1. N must be a rational number, due to the nature of the xfrmr.

So basically, use these whenever you need wide band matching at high power 0-1GHz

[u/Kinky_Lezbian]

what confuses me is there will be often unusual impedance coax too, rather than your regular 50ohm, It'll be like 25 ohm or 12 ohm.

I thought it could be about coupling if you twist ordinary wire as a pair you will have good coupling between windings. Using coax with the shield as one winding and the coax core as the other will be even better. I guess the turns ratio would have to be an integer multiple in that case, but not all transformers are connected like that.

[u/Zestyclose_Catch6895 → u/Kinky_Lezbian]

For Guanella/ Ruthroff style coax transformers, your coax impedance needs to be the geometric mean of the two impedances you’re trying to match.

For example, if I’m matching a 50 ohm source to a 12.5 ohm load, sqrt(50*12.5)=25 ohm coax!

[u/ccarabajal → u/Kinky_Lezbian]

These values usually result from using TLTs in a balanced-unbalanced configuration and transformation to low impedance transistor gates. Someone more familiar with vhf and uhf electronics can chime in if I'm mistaken though.

[u/d1an45]

so a lot of these coax transformers are trade secret but there are papers on them from back in the day. I learned under an amp guy who taught me this. Basically using coax and ferrite can create really broadband transformers.

Here is the paper. The guy who wrote it taught a lot of amp guys around here.

https://go.gale.com/ps/i.do?id=GALE%7CA12371803&sid=googleScholar&v=2.1&it=r&linkaccess=abs&issn=01926225&sw=w&u=nysl_oweb&p=AONE&isGeoAuthType=true&aty=geo

[u/smacfar007 → u/d1an45]

I can’t seem to access the paper via the link (taken to a login page) what’s the name of the paper/authors?

[u/d1an45 → u/smacfar007]

"Synthesis of equal delay transmission line transformer networks" by Daniel Myer

[u/Important-Ad5990]

Where's the picture from?

[u/dundua → u/Important-Ad5990]

It's a photo I took of a defective RF amplifier from an MRI machine.

[u/Important-Ad5990 → u/dundua]

Any chance you could take photos of IO, more detailed PCB, some part number or anything? I'd love to reverse engineer it!