I've been having some fun with an Analog Devices' AD8307 log. amp, using it for general RF monitoring. I've now got the desire to increase its selectivity (specifically to center it @ 216mhz). I'm thinking the easiest way is to put together a passive bandpass filter and put it inline between the physical antenna and the antenna input on the AD8307.
The design for the bandpass filter I'm studying at the moment is detailed here and here.
I can't seem to wrap my head around how to go about selecting the capacitors for the circuit - It took some playing around to find capacitor and resistor values that are actually available commercially, what trips me up are the tolerances.
For example: a 100 Ohm resistor and 7.4 pF capacitor would result in the lowpass side of my filter having a cutoff frequency of 215mhz. Perfect, because I want the highpass side to cut off at 217 (so my center freq. equals sqrt(215*217) = approx. 216). However, the tightest tolerance I can find for a 7.4pF ceramic cap. is +/- .1pF - So unless I bought a bunch and binned them myself to find one that's dead nuts on @ 7.4pF, my lowpass side is actually going to have a cutoff at anywhere from 212MHz (7.3pF) to 218MHz (7.5pF). Best case I skew the center frequency off its mark, worse case I end up with a bandpass filter without a passband.
How do I deal with this? Is this design more of a theoretical construct, and is there perhaps a similarly simple design that can use components with less stringent tolerances?
The design for the bandpass filter I'm studying at the moment is detailed here and here.
I can't seem to wrap my head around how to go about selecting the capacitors for the circuit - It took some playing around to find capacitor and resistor values that are actually available commercially, what trips me up are the tolerances.
For example: a 100 Ohm resistor and 7.4 pF capacitor would result in the lowpass side of my filter having a cutoff frequency of 215mhz. Perfect, because I want the highpass side to cut off at 217 (so my center freq. equals sqrt(215*217) = approx. 216). However, the tightest tolerance I can find for a 7.4pF ceramic cap. is +/- .1pF - So unless I bought a bunch and binned them myself to find one that's dead nuts on @ 7.4pF, my lowpass side is actually going to have a cutoff at anywhere from 212MHz (7.3pF) to 218MHz (7.5pF). Best case I skew the center frequency off its mark, worse case I end up with a bandpass filter without a passband.
How do I deal with this? Is this design more of a theoretical construct, and is there perhaps a similarly simple design that can use components with less stringent tolerances?