I'm working my way through Bowick's RF Circuit Design. In the resonant circuit chapter he does a great job of describing how component Q affects circuit Q, and how loaded Q works. There, you set the bandwidth of a band pass filter by deliberately changing its quality.
But in the filter chapter describing filter prototypes, the design procedure is quite different. First you select a normalized prototype based on the attenuation and passband ripple needed. Then you transform if needed, then scale. But the component Q criteria are left at this table:
The lower the component Q, the wider the bandwidth and the shallower the rolloff attenuation. However, it was not explained by exactly how much. To calculate total Q, I can presume that I could take the Q of the last shunt segment and combine it with the Q of the second last series segment using Qtot = 1/(1/Q1+1/Q2). But I'm not sure how to then combine that with the next shunt Q. Or I could try to transform all resistance out of the filter into one series resistor. Not sure which approach to take. So that's my first question.
My other questions are:
- Where do these figures in the table above for minimum Q come from?
- How does component Q affect the actual Butterworth parameters?
- How do I adjust the Butterworth parameters to account for component Q, rather than just having to follow the general guidelines above?
- If an ideal Butterworth has no resistance, how is it possible to have a wide bandwidth?