RF Matching Networks Project: Week 2 (13/2/2015): Pi Networks, T Networks and LCC Networks

We started our second week of the project by designing and testing some Pi networks. Unlike the previous L networks, where the bandwidth is fixed, with Pi networks you can design the circuit to have the bandwidth you want, within reason of course (some choices of bandwidth will result in impossible component values). At first we were a bit stuck about what bandwidth to choose. So we asked our lab supervisor, Dr Bowden, for advice and he suggested we aim for a bandwidth of around one tenth of the chosen centre frequency. So with a centre frequency of 1 MHz, we went for a bandwidth of 100kHz.

We had one other problem too. Our research has turned up formulas we could use to select component values for a low-pass circuits, but we had no information on high-pass circuits. At first we thought we could simply swap the placement of the inductors and capacitors, keeping the values the same, but this didn't work. The frequency response was completely wrong, with weird peaks outside the main pass band, and too much attenuation at 1 MHz.

After thinking about the problem a bit more, we realised that we had to keep the reactance between each node of the circuit the same, but change capacitive reactance to inductive reactance and vise versa, as we swapped capacitors with inductors. This meant working out new capacitor and inductor values to keep the reactance the same. Once this was figured out our circuits worked properly, and we were able to build and test both high pass and low pass circuits.

High-pass Pi network on breadboard

Testing the high-pass Pi network

Low-pass Pi network on breadboard

Testing the low-pass Pi network

In the afternoon, we moved on to T and LCC networks. The LCC network is a variation of the T network, but instead of being either high-pass or low-pass like the other circuits, the LCC network is bandpass. We designed the LCC network to have a pass band around 1 MHz, then we built and tested both T and LCC networks.