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Currently designing a circuit consisting of 50 Ohm rated ICs. Between two ICs (switches), I will place a BP-filter made out of discrete components (high-Q inductors and capacitors). The filter will probably be 7th order.

My question: Is it necessary to match the impedance of the filter to the ICs, or to the characteristic impedance of the circuit? If so, how can it be done?

Oh, and the frequencies will be in the 100-300 MHz area.

Thanks.

paw
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  • Matching is one way of doing it. Another way is simply to design what you need: a BP filter with 50 Ohm input and output impedance. It might need a different number of stages for the response you need, but that might introduce less problems than the matching networks would. Finally, you might wish to see if the filter you need isn't available as a predesigned module. – Kuba hasn't forgotten Monica Mar 12 '15 at 21:44

1 Answers1

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You WILL have to consider the output impedance of the driving chip AND the input impedance of the receiving chip when you design your filter or you will likely get disappointing filter performance. All your good work using high Q inductors could be ruined with an output impedance as high as 50 ohms - this resistor is in series with the output effectively and, if you are using series inductors in your design, the first stage will be massively affected if not taken into account.

On your last filter stage, the effective parallel resistance of the receiving chip being 50 ohms may also ruin that stage's abilty to be high Q.

Regards matching the nominal pass band impedance of the filter to the two IC's 50 ohm impedance I'd be less worried. 300MHz has a wavelength of 1m and you are not going to get standing wave problems on a few inches of circuit or bread board. Clearly don't make the filter's input impedance too low and don't make its output impedance too high or you'll have significant insertion losses (irrespective of whether the chips "ruin" the filter characteristics).

Andy aka
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  • I used the program Elsie to design the filters, with 50 Ohm input and output impedances. Now if I wanted the filter to have high input impedance, could I simply place a resistor with high value in front of the filter? Would this inclusion force me to change the rest of the components in the filter? No clue how to do this. – paw Mar 13 '15 at 07:59
  • No, because, as per my answer, that will likely affect the filter. If Elsie can use seperate input and output impefances, you could design a higher impedance at the input maybe. – Andy aka Mar 13 '15 at 08:30
  • Yeah, I tried that, but it seems like it can only have the same input and output impedance. Appreciate the help. Thank you so much. – paw Mar 13 '15 at 14:08
  • Design it with high impedances (1kohm) then use a buffer amp with high impedance after the filter maybe? – Andy aka Mar 13 '15 at 14:09
  • What about using a (very fast) op-amp as a buffer? That would provide a high-impedance input, wouldn't it? – paw Mar 13 '15 at 14:11
  • It might work but you have to watch out for the input capacitance of the op-amp. – Andy aka Mar 13 '15 at 14:16
  • Looks like we were writing at the same time. I'll look into both ideas. Once again, thank you so much. – paw Mar 13 '15 at 14:20