So it’s worth taking a look at the process here, and examining the effect of tweaking component values in this way. The software calculates ideal inductances and capacitances for the desired cut-off and impedance, and in selecting the closest preferred values we modify the characteristics of the result and possibly even ruin our final filter. The results are good, and anyone can become a filter designer, but as is so often the case there remains a snag. Happily as with so many other fields, in recent decades the advent of affordable high-power computing has brought with it the ability to take the hard work out of filter design, Simply tell some software what the characteristics of your desired filter are, and it will do the rest. The 30MHz low-pass filter, as QUCS delivered it.
There are tables and formulae, but even after impressive feats of calculation the result can often not match the expectation. It’s a network of capacitors and inductors usually referred to as a pi-network after the rough resemblance of the schematic to a capital Greek letter Pi, and getting them right has traditionally been something of a Black Art. If you are in any way connected with radio, you will have encountered the low pass filter as a means to remove unwanted harmonics from the output of your transmitters.
