75 : 300 Ω Balun for 50 Ω matching at 900 MHz

Question

I have a Murata Balun available DXP18BN7514T to match an unbalanced 50Ω source to a 300Ω differential load over about 50 - 60 MHz bandwidth. Unfortunately the DXP18BN7514T is specified as a 75 : 300 Ω Balun. I have some options available, some being more convenient than others, so :

1. I could use a suitable 50 : 75 Ω Balun / transformer and complete the match to 50 Ω this way. Problem is that most such transformers available from Mini-Circuits or Macom are wire-wound, large and fairly high profile components relative to the rest of the design. Otherwise have not been able to source an SMD solution that is readily available so far ...
2. I could do a discrete component match of the unbalanced 75Ω to the unbalanced 50Ω, but this will be more involved and more time consuming, and also require extra discrete components, calling for a broad-band match due to the 60MHz bandwidth. Extra difficulty is that typical S-parameters are not readily available for the DXP18BN7514T.

There may be a third or fourth option that I've not thought about ... What would one recommend, keeping in mind that simplicity and use of readily available components is prefered, over a more complicated solution (even though the later is not completely excluded...)

thanks in advance for any suggestions...

Answer 1

For these frequencies and your particular needs, a discrete LC balun with Z transformation would be the way to go, ditching the Murata part altogether. You really should not have any issues with standard SMD stuff here.

Here is such an example, it consists of two parallel LC filters:

The way this works is pretty intuitive, the LC networks are designed to operate at the resonant frequencies, acting as phase shift networks to produce the balanced phase at output. The impedance matching can be viewed as a standard LC section providing an impedance match between 50 ohms and 150 ohms. So this should tell you a bit about the bandwidth you can expect: $$Q=\sqrt{\frac{150}{50}-1}=\sqrt{2}$$ For a centre of 57MHz: $$BW = f_0/Q=40MHz$$

You can also use the calculator here.