After the not-so-great bandpass filter I built last week, I built another one this week using a capacitor-coupled resonator topology. Happily, it works pretty much as designed! However, I did discover that the "real world" method of Dishal tuning isn't quite so clean as the literature would have you believe.
Articles such as this one -->
As you could imagine, this problem tends to compound itself as you move from resonator to resonator. So, I'm curious:
1) Presumably the assymetries in one section arise due to the additional parasitics present when you un-short the next section? Or perhaps they're due to a little through-the-air coupling between sections? I'm making another board that shields each resonator section to find out... 2) I eventually was able to tune up the entire filter reasonably well by deciding that I was only going to tweak the "current" section and the immediately prior sections, getting things as centered and symmetrical as possible (a somewhat iterative process). Trying to go through multiple prior sections became as exercise in futility, and I rationalized that the further away a section was, the less impacts the parasitics of the current section had on it -- hence the idea of only considering the current section and the one immediately prior. How's that sound for a tuning procedure?I believe that all the plots in the Johanson paper were computer-generated. Anyone have some experience with tuning real LC filters?
Thanks,
---Joel