Balun Transformer vs. 180 Degree Hybrid Coupler?

[Helmholtzerton]

Is there a big difference between these two devices? It appears a signal can be applied to one port of a balun, which will generate two signals 180 degrees out of phase with respect to each other on two of the other ports of the device.

The results can be achieved with a 180 hybrid coupler. With the hybrid coupler I'm using, I see the same results as when I use 2:1 balun transformer. I'm able to get the difference or sum with respect to two signals being applied to two different ports.

When looking into this I came across an explanation from highfrequencyelectronics.com which I found somewhat confusing.

"Balanced Port Isolation: Usually referred to simply as isolation, this has the same meaning as in other power dividers and couplers, namely the insertion loss from one balanced port to the other in dB. Most baluns do not offer high isolation because the even mode is reflected instead of being properly terminated with a resistive load. The exception is 180° hybrid circuits, where the even mode is output to a port that can be resistively terminated."

Can someone break this down for me?

Thank you.

 

[davenn]

Is there a big difference between these two devices? It appears a signal can be applied to one port of a balun, which will generate two signals 180 degrees out of phase with respect to each other on two of the other ports of the device.

The results can be achieved with a 180 hybrid coupler. With the hybrid coupler I'm using, I see the same results as when I use 2:1 balun transformer. I'm able to get the difference or sum with respect to two signals being applied to two different ports.

The differences are in the names and what those names mean

The BALUN provides matching between a BALanced and an UNbalanced transmission line, coax (unbalanced) to antenna (balanced)
300 Ohm ribbon line (balanced) to coax ( unbalanced). It is often used as an impedance transformer.
Where as a hybrid coupler does neither of those things
Unless specified, a hybrid couple will be used with an unbalanced line in and out, eg. coax and as in your linked example, there is no impedance transformation occurringDave

 

[Helmholtzerton]

The differences are in the names and what those names mean

The BALUN provides matching between a BALanced and an UNbalanced transmission line, coax (unbalanced) to antenna (balanced)
300 Ohm ribbon line (balanced) to coax ( unbalanced). It is often used as an impedance transformer.
Where as a hybrid coupler does neither of those things
Unless specified, a hybrid couple will be used with an unbalanced line in and out, eg. coax and as in your linked example, there is no impedance transformation occurringDave

Thanks for the reply Dave.

Then I think the specific 2:1 balun transformer I have, and the 180 hybrid coupler - they are functionally the same thing, as all of the ports are at 50ohm. Considering when going from a 50ohm coax to 100ohm twisted pair on the balun transformer, each line of the twisted pair is still 50ohm individually.

But I'm still a little confused by the explanation,

"The exception is 180° hybrid circuits, where the even mode is output to a port that can be resistively terminated."

What is the benefit of this?

 

[davenn]

Then I think the specific 2:1 balun transformer I have, and the 180 hybrid coupler - they are functionally the same thing, as all of the ports are at 50ohm. Considering when going from a 50ohm coax to 100ohm twisted pair on the balun transformer, each line of the twisted pair is still 50ohm individually.

that doesn't sound correct

please link to the specific BALUN you are talking about

 

[Helmholtzerton]

that doesn't sound correct

please link to the specific BALU you are talking about

Yes, it is incorrect.

It is a 1:2 BALUN: https://www.minicircuits.com/pdfs/ADT2-1T.pdf

Also, with that Balun, the primaries and secondaries are electrically isolated from one another.

While on the 180 Hybrid, each of the 6 ports I tested are not electrically isolated from one another.

 

[Baluncore]

"The exception is 180° hybrid circuits, where the even mode is output to a port that can be resistively terminated."

That is possible if it is a transmission line transformer operated as an inverter. Unlike a typical power transformer, there is no voltage isolation between the primary and secondary. Consider a 1:1 ratio transformer where the input relative to ground is applied to one end of the primary, while the opposite end of the secondary is grounded. The other end of the pri and of the sec will give you the differential signals with 180° shift. More complex arrangements give better balance and isolation.
Notice the bandwidth limitations and power handling capacity of the different networks.
https://en.wikipedia.org/wiki/Power_dividers_and_directional_couplers

 

[tech99]

I think a balun can be represented by a perfect isolating transformer connecting an unbalanced source to a balanced load. Let's assume the primary has one side grounded. Now we have the choice of leaving the secondary floating, giving total isolation, or we can ground the centre point of the secondary. I think the hybrid described by Helmholzerton is similar to this, where the outputs are unbalanced (one side grounded) but in antiphase.

 

[Baluncore]

Here is a simple transmission line phase-splitter that generates 0° and 180° phase signals. In effect coupled lines are being operated to extract odd and even modes. You can use twisted pairs for the transmission lines at high frequencies, or wind those pairs onto a toroid to make transmission line transformers for use at longer wavelengths.

 

[Helmholtzerton]

That is possible if it is a transmission line transformer operated as an inverter. Unlike a typical power transformer, there is no voltage isolation between the primary and secondary. Consider a 1:1 ratio transformer where the input relative to ground is applied to one end of the primary, while the opposite end of the secondary is grounded. The other end of the pri and of the sec will give you the differential signals with 180° shift. More complex arrangements give better balance and isolation.
Notice the bandwidth limitations and power handling capacity of the different networks.
https://en.wikipedia.org/wiki/Power_dividers_and_directional_couplers

Thank you for taking the time to reply.

Ahh. I don't have mine with me right now, but if I'm understanding you correctly, you're saying this device can just be used to shift the phase 180 degrees when terminating the other output?

I'm a little confused by the spice model. I thought I should still lose 3dB of signal relative to the input when using a hybrid coupler. I believe stated you are modeling a transmission line phase-splitter, so is this a bit different than hybrid coupler where I would use either the diff or sum port? In this spice model I see Vp and Vn having the same amplitude as Vin, I'm trying to wrap my head around that.

Also, when speaking about even and odd modes - does this mean 0 degrees is the even and 180 degrees is the odd?

 

[Baluncore]

you're saying this device can just be used to shift the phase 180 degrees when terminating the other output?

Yes. The signals inside the transmission line are independent of the outside. Who is to say what and where is the output reference ground.

In this spice model I see Vp and Vn having the same amplitude as Vin, I'm trying to wrap my head around that.

I produced a quick ideal model with a zero impedance voltage source. A more realistic model would need series matching resistors where the two lines are driven in parallel. Including matching networks will give you the amplitude reduction expected. Also, the ideal numerical output impedance matching prevents all reflections and so the model could get away without series termination at the input.

 

[Baluncore]

Also, when speaking about even and odd modes - does this mean 0 degrees is the even and 180 degrees is the odd?

In a twisted pair, each conductor will couple to the other, but they will also both couple indirectly via the screen or environment. Since magnetic coupling can be seen to reverse the direction of the coupled wave, the count of coupling events in the coupling path will decide the mode as being odd or even, with implications to forward or backward, summing or subtracting, inverting or non-inverting.
https://www.microwaves101.com/encyclopedias/coupled-line-couplers
https://www.microwaves101.com/encyclopedias/even-and-odd-mode-impedances