What is inside a Balum ?

In a past life, while performing Phase Noise on the RF output of a DAC , which was embedded in a Statix-10 FPGA, I had the thought to pull the cover and take a look inside. So, what did I find ?

First of all, what makes it a Balum and not just a two-way power divider ? Well, in a two way power divider all ports are in-phase with each other. For a Balum, we need the labeled Port 2 to be 180 degrees out of phase with the labeled Port 3. Why ? This is to interface with a 100 ohm balanced output of the DAC, to the single ended RF measurement device such as a spectrum analyzer, or in this case a signal analyzer if you're doing Phase Noise. ...... and one may ask just how the magic works ?

You notice the Labeled Port 1 is divided inside in half, the Port 2 on the left and the Port 3 on the right. So how does it do the phase inversion ? Here is how it works. On the right side, the Port 3, which is the 0 degree input/output, is just a normal cable run from the Port 1 to the Port 3. However, on the left side, which is the 180 degree input/output, the center conductor is connected to case ground, and the shield of the cable is connected to the center pin on the port connector. The phase is reversed from Port 2 to Port 3, every time. Now, suppose one would ask, if the center connector is shorted to ground, then why does this not ground the signal ?

The length of the transmission line is about's a 1/2 wave length of the operating frequency , and we know on an RF transmission line a 1/2 wave length is an open to a radio frequency signal, with respect to the signal source. Also, how do we get the electrical isolation work between the two output ports ? The length of the transmission line between the Port 2 and the Port 3 is one full wave length, which is also an open at the operating frequency. As I look at some of the vendors data sheet(s), I see Port to Port Isolation in the range of 8 to 15 dB. With this trick, that's about a good as it gets. If you want any better port-to-port isolation, then you will need an active gain stage configured as a phase splitter.

Side note : Those three little black beads on the left transmission line, are ferrite beads. These help with the port-to-port isolation.