A 150 MHz Lowpass Filter

15th Dec 2024

For the lab I was in need of a lowpass filter to suppress harmonics above the VHF range. These are my notes on design and build of such a filter.

The Design

The design process is as simple as it gets. I used RFSim99 to come up with the following design for a 5th order Chebyshev with 0.1 dB passband ripple and a cutoff frequency of 150 MHz.

lpf150M_schematics

The Build

The idea I came up with was to use copper cladded FR4 material as capacitors and just solder patches of it to a enclosure made of tin. I cut the patches from 0.6 mm PCB base material and iteratively trimmed and measured them to the desired capacitance value.

For the inductors 3 resp. 4 windings with a diameter of around 8 mm should yield the desired inductance. Again the inductors were measured repeatedly while adjusting the turn spacing to yield the desired inductance value.

lpf_c_soldered lpf_completed

The picture above show one of the tin lids of the enclose with the soldered patches of FR4 base material. I used a SMD rework preheater to heat from below and some hot air from the top to reflow the patches on some solder that I had previously applied to the lid with the soldering iron.

The following plot shows the measured performance without any further adjustements. The measuements match the calculation quite closely.

lpf_comparison