We bet when [devttyS0] made his latest video about RF filter design (YouTube, embedded below), he had the old saying in mind: in theory, there’s no difference between theory and practice, but in practice, there is. He starts out pointing how now modern tools will make designing and simulating any kind of filter easy, but the trick is to actually build it in real life and get the same performance. You can see the video below.
One of the culprits, of course, is we tend to design and simulate with perfect components. Wires have zero resistance, capacitance, and inductance. Inductors and capacitance have no parasitic elements in our rosy design world. Even the values of components will vary from their ideal values and may change over time.
Because of that, [devttyS0] uses trimmer capacitors in the resonant part of the circuit. That way you can tune to account for circuit variances. That’s not the only practical advice offered. Need a small capacitor? Twist some wire together (a trick we’ve heard called a “gimmick capacitor”).
If you’ve been enjoying [Elliot Williams’] posts about filters, you’ll see there is a lot of difference between active audio filters and passive RF filters, although some of the ideas are the same. Watching the effect different circuit tweaks have in real time on a spectrum analyzer is very instructive. You’ll even see the effect of using shielding to separate the filter sections. If you really want to dive into RF design, you could do worse than watch these videos from [Michael Ossmann], too.
http://hackaday.com/2017/03/30/real-world-rf-filter-design-and-construction/
