Thank you, some light - I hadn't seen that the pi filter is two L match filters pushed together. That's rather neat and helps me understand the design.
Quote:
Originally Posted by G0HZU_JMR
It looks like you are always using 3.3nF and 8.2nF caps in every version of pi filter and this is restricting what you can do.
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I have been using those values because they are, with the 10µH inductor, close E24 values to the numerically correct ones.
If I use the pi-match tool for 300Ω into 50Ω at 1MHz, and adjust the Q factor to 6.44 it gives me 10.011µ for the coil, 3.4nF and 7.8nF. Unfortunately, one of the mathematical tricks I have never understood is how to turn a formula around so that I can calculate any particular value form the given ones. So what I need to do is calculate the Q from using E24 capacitors, which would be 3n3 and 8n2.
I have been heavily relying on LTspice, using the circuit that we've discussed, and looking at the output waveform as a rough indicator of Q. Now I have seen that, using 3n3, 15µ and 3n3 there's a rather neat 4v p-p sine wave across the 560Ω output. So this morning I've learned that, to work properly, I need to understand the importance of the load resistor. Pi-match tells me that these values give me a Q of about 10.