Lowpass Filters

 
 
 

Transmitter Lowpass Filter Board

After the power amplifier, we have a set of fifth-order LC Chebychev lowpass filters. These are pretty ordinary designs. My filter design tool is a very expensive commercial one that I have access to at work: Filter Solutions. For this application, it’s generally good enough to use the tables in the ARRL Handbook, or just copy some else’s design.


I fabricated my filter board on double-sided copper clad using the Sharpie method. Toroids are the 68 size to handle >100 W and all capacitors are 500 V dipped mica. Relays are Axicom V23079D1003B301, available from Mouser for about $2.50 each. They are among the few small, affordable, relays that I’ve seen that have real RF specifications to 900 MHz and 1 kV/2A contact rating. Certainly good enough for my purposes.

Here are the transmission loss measurements and input return loss measurements. I like to do a wideband test, way out into the VHF range, just to see how good my construction techniques and components are. As you can see, these simple filters provide >50 dB rejection all the way out to 200 MHz. That’s the ground plane at work, and also the fact that these filters are all-pole designs as opposed to elliptic topologies with transmission zeros (capacitors in parallel with the inductors). In those filters, you often see poor attenuation at high frequencies due to non-ideal component behavior.

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