HF Transmitter #10 - Low Pass Filter

In the prior episode I glossed over the low pass filter (LPF). So today I'm taking a deeper look at the purpose of it being in the transmitter, what performance it needs to have, how I chose the design parameters, and finish with some test results.
No endorsement is made nor implied to any commercial products referenced in this episode.

[@R2AUK]

Thanks for the update! I never tried ultraspherical filters before. Should give it a try in one of my future projects! 🙏

[@sidbhattacharjee1872]

Very nice and educative. I have seen commercial LPF quoting 0.1dB insertion loss for 5 pole or 7 pole filters, is this figure achievable or it's a false claim?

[@amirb715]

the transmission zero (notch) ALWAYS occurs because of some cross coupling between two resonators or between the input and output. So it is either a magnetic coupling bween the two toroids that is causing it or most likely your board layout is causing a (weak) coupling between the input and output.

[@levelupeelab]

Interesting. I didn't apply any thought to coupling when I laid out the board.

[@amirb715]

@@levelupeelab I think in this filter it is more likely due to a weak coupling between the toroidal inductors rather than between the input and output because in the layout nothing stands out to me as being the culprit. You can put a metal shield around the coils or something to separate them and see if that eliminates the notch. Simulation should be able to show that too

[@Paul_VK3HN]

Interesting how the attenuation turns up, I've seen something similar and wondered why, your explanation seems reasonable, I wonder if other explanations will surface.

[@TheGmr140]

S21, and s11 and s22 should be measured too in filters

[@t1d100]

Interesting.