# Trifilar Transformer Common-Mode Rejection Circuit (Signal Recovery, Hybrid Combiner, RF)

I'm trying to develop a circuit that allows me to effectively subtract two different sources of RF noise from a single signal at the same time. I am planning to use something called a "trifilar" transformer, which is effectively a transformer with 3 separate windings. My thinking is that, if I input my original noisy RF signal on one of the windings, one type of RF noise on the second winding, and the other type of RF noise on the third, that the signal can be recovered.

I've heard of this configuration called a "hybrid combiner" which allows you to get the sum and difference of two different signals using this three-winding transformer, using common-mode rejection, like a choke.

In effect, what I would like to do, is subtract two different separate signals from a third signal. I realize this is a poor way of describing what I want to do, so I've included a diagram below with equal turns for each of the three windings (note the dot convention):

Of course, the DC signals won't be transmitted from winding to another, so that isn't relevant, but I add them for generality.

My question is, when you input signals (or noise) to each of the windings of a transformer, how do they react? In my typical experience, a transformer has two windings, one of which is the "input" (or primary) and the other is the "output" (or secondary). What happens when all three windings (primary, secondary, tertiary[?]), have currents applied to them?

Is it possible to recover the original signal?

Thanks!

• Is the noise you are injecting an exact replica of the noise infesting the signal? Commented May 10, 2014 at 19:15
• @Andy aka - yes it is. Commented May 10, 2014 at 20:58
• My gut feel tells me that it will be difficult to implement in a easy to replicate manner. If you think of a Balun it is doing much the same thing and able to make a go of it. Commented Oct 27, 2014 at 16:51
• @KalleMP I agree, it does act pretty much like a balun, but since it is removing two different signals of separate windings, it's more like two baluns built into one. Actually, these trifilar transformers exist as a commercial of the shelf product, the issue is, I can't really model this (I couldn't find triflar transformers on SPICE/PSPICE, or even how to write out the circuit equations for it) to see if it would be effective before building it, and if I build it first, I don't know how to test to see if it is doing what it is supposed to do. Commented Nov 13, 2014 at 17:14
• @iwant, In LTspice you can model two inductors as coupled by a SPICE command, "k L1 L2 1" (1 indicates 100% coupling). Modeling three coupled inductors is easy: "k L1 L2 L3 1" ...
– Zulu
Commented Apr 19, 2015 at 19:48

The trifilar transformer you have drawn will couple all three of its windings together. In the ideal transformer case, that means the voltage on each set of windings will be identical.

It will therefore effectively parallel all the signals together. The fact that you have drawn load impedances of Z on the end of each winding means that the transformer will not actually short your two noise sources together, which without the Zs could have made them malfunction. The average noise source voltage, ie half the sum, will be added to your signal.

Nevertheless, effectively connecting your two noise sources in parallel like this is a rather 'dirty' way to do it.

You can design a three input combiner, that isolates each signal from the other. Alternatively cascade a pair of two-input combiners. The design equations for these things are readily available, or you can buy the finished part, see MiniCircuits for instance.

If the sources are low impedance, then it would be better to cascade a pair of simple transformers, adding one noise source in each. The transformers will put the noise voltage in series with the signal voltage, so simply adding them. As the two noise series are then in series, they won't load each other.

• Thanks for the info, are you saying that regardless of the signal being applied to each winding, that each will end up carrying the exact same signal? The MiniCircuits part you mention is something I was thinking about, although not called a trifilar xformer in their docs. What I really want to do is eliminate two separate sources of noise present on a line by dealing with each independently, one per winding. Is this possible? I've tried modeling this with little luck, mainly due to my ignorance on how multi-winding transformers work. What's the best way to approach this? Commented Sep 20, 2015 at 0:02