# How to design a balun circuit for an RF application?

I am designing balun circuit for the first time.

Baluns are mainly used to convert an unbalanced signal to a balanced one or vice versa, 50 ohm to 100 ohm impedance matching.

Can somebody tell me how to design the pi-pad attenuator circuit for the 100 ohm impedance side and the selection of the AC coupling capacitor?

In my application, I am using the balun TCM1-63AX+ for the range of signals from 10 MHz to 4000 MHz to interface to a Xilinx RFSoC XCZU28DR-2E RF fabric.

Also attached reference circuit design of the evaluation platform.

• What you have enclosed in red boxes I'm assuming you want to know how to design yet, both red boxes are already "balanced" so that does not make sense if you want a BALUN. If you are looking for a resistive impedance matcher then that makes more sense but, you question is confusing. Jan 7, 2021 at 9:19
• @Andyaka I wanted to know how to design the PI PAD attenuator circuit (Four resistor), which is highlighted in red color.. Jan 7, 2021 at 9:33
• Input (left) impedance and output impedance (right) please Jan 7, 2021 at 9:35
• Input impedance is 50 Ohm Output impedance is 100 Ohm Jan 7, 2021 at 9:36

Input impedance is 50 Ohm Output impedance is 100 Ohm

For an unbalanced 50 Ω to 100 Ω impedance matcher you can use this online calculator: -

Rin is the input impedance and RL is the load impedance.

To make that circuit balanced, you need to split R1 into two resistors of equal value: -

• Obtained different value than that of the value chosen in Xilinx XM-500 evaluation platform. Also from the below link i got to know that this pi pad circuitry is mainly used to attenuate the signal/dB loss Link: electronics-tutorials.ws/attenuators/pi-pad-attenuator.html But How we can obtain the dB loss for this circuitry/my application? Jan 7, 2021 at 9:59
• @AnantheshAcharya yes, you would get a different value because a full pi-pad can also provide attenuation as a variable. The loss in the above circuit I provided is minimized compared to the pi-pad. The calculator indicates a value (Av) as being 0.58579. In other words, the voltage appearing across RL is 58.579% of that appearing across R2 i.e. 4.645 dB attenuation. The pi-pad is a simple derivation from the above L-pad circuit I have shown. Jan 7, 2021 at 10:04
• @AnantheshAcharya if you put two L-pads back-to-back with an "invented" middle impedance of (say) 150 ohms in between (aka a phantom resistance), you get a circuit that becomes a pi-pad but, with the ability to alter attenuation as a variable. Maybe I should think about adding that to my website. Jan 7, 2021 at 10:14
• Can you share your website link? Jan 7, 2021 at 10:16
• There is a link in my answer - first line below your quoted comment. Jan 7, 2021 at 10:16