Low pass filters: How one capacitor can make a big difference

[@kp4md]

Thanks for the tip, Peter. Adding one 270 pF disc capacitor across the 820 nH series inductor in the low pass filter of my 7 MHz Tuna Tin "S" adequately notched out the 10.6 MHz second harmonic of 5.3 MHz, allowing the transmitter's use on 5 MHz,

[@janetwinslow2039]

Thank you. I didn't realise it was so easy to increase the attenuation of the second harmonic without the extra capacitor significantly affecting the overall characteristics. But obvious I guess if you think about it!

[@robertvandersanden]

Adding a capacitor in parallel to the middle inductor is a good way to get a notch just outside the passband. I’ve done the same with good results in my homebrew rig. There is something to consider here. And for that I will simplify for a single band filter for - say - 7MHz. You have to start with reducing the inductance of the middle inductor to 75% of its value and then calculate what capacitance is needed to get a parallel resonance at 14 MHz. Put that cap in place. Now the filter has a notch at 14MHz without changing its passband behavior, its corner frequency or its impedance. Why? Well, at 14MHz the impedance of L and C are equal but opposite in phase. And so they cancel out and become infinite, hence the notch. At 7 MHz Zc is twice as high and ZL is half that of 14MHz. The parallel impedance of this gives a 25% larger effective inductance value then without the C. Therefore you need to start with L being 25% less so that with C in parallel you are effectively back at what the value was in the original filter.

[@paul_daniels]

Good work Peter, very interesting

[@IrishvintageTVRadio]

Very interesting, I didn't realise this could be done. Thanks Peter.

[@BoB4jjjjs]

Nice one Peter.

[@acestudioscouk-Ace-G0ACE]

Really useful video, thanks Peter! What capacitor values would your recommend for 70MHz operation please? 73s G6XIB

[@vk3ye]

Wouldn't know off the top of my head. Use the online calculators around to calculate resonance with the inductor in parallel with it.

[@acestudioscouk-Ace-G0ACE]

@@vk3ye I just wondered if you had any personal experience but that's a good call Peter, thanks I'll try it.

[@batica81]

Interesting, haven't seen that filter configuration before. What is the insertion loss?

[@flexairz]

17 dbm

[@karlD1963]

Could the toroids be replaced by axials for under 200mW use?

[@vk3ye]

Yes.

[@karlD1963]

@@vk3ye Thanks, Peter. I'll give it a go this weekend! ...update...it works beautifully. Many Thanks.

[@Paul_VK3HN]

Very useful, I've made lots of W3NQN LPFs but never tried this. The only data I couldn't see is insertion loss before and after. BTW only one of your filter's modes would result in a legal transmission in VK which (I think) is -50dB.

[@Steve-GM0HUU]

👍 although I knew this would work, I was still impressed at just how effective that single capacitor was. Wondering if additional capacitors across the other inductors would give even more 2nd harmonic attenuation.

[@robertvandersanden]

It does but above the notch frequency the cap takes over from the inductor as it becomes lower in impedance. This means that higher harmonics are less attenuated. You could add just one more parallel cap across another inductor to add a notch at the 3rd harmonic.

[@Ed.T]

Thank you, that's great!

[@AliReza-zx8km]

Nice.........