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Since there are two basic types of LC circuits that can be used for passing/blocking signals, I'd like to know the differences when using series or parallel LC circuits for noise at a known frequency.

Of course, the parallel LC (tank) circuit is a stop band filter:

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and the series LC circuit is a pass band filter:

enter image description here

However, if one wanted to remove a signal with frequency f, one could either use the parallel LC circuit tuned to that frequency to resonantly absorb the signal, or the series LC circuit tuned to that frequency but shorted to ground to preferentially pass that signal to ground.

Which is better to use and why?

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    \$\begingroup\$ Is this a real world question? As far as I know L&C are ideally noiseless, but in reality especially L picks up EMI noise, therefore an active filter without L might be preferable. Anyway it makes sense to pass the signal rather than block noise at a specific frequency, since often you can't know that the noise characteristics won't change in the future. \$\endgroup\$ – HKOB Feb 4 '15 at 5:40
  • \$\begingroup\$ @HKOB I've seen both filters used for the same and different purposes in the literature but could not find any analysis as to which is more efficient in which situation, and that is the crux of the question. \$\endgroup\$ – iwantmyphd Feb 4 '15 at 13:08
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If your receiving circuit were high input impedance, then using a parallel LC would not work as well as a series LC to ground. If your input impedance were low then the parallel LC would likely work more effectively.

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  • \$\begingroup\$ Isn't impedance for the 0 at resonance? \$\endgroup\$ – iwantmyphd Oct 24 '14 at 21:46
  • \$\begingroup\$ I'm referring to the circuitry that follows the LC circuit. Think about the parallel circuit. In reality it's impedance may rise to 100kohms when blocking but if the output went into a 10 Mohm input you'd barely see any blocking behaviour at all. It would be nice to think a parallel LC produced infinite blocking impedance but inductor ESR highly restricts this in practise. \$\endgroup\$ – Andy aka Oct 24 '14 at 21:59
  • \$\begingroup\$ How does one determine what to do then? If the available LC circuit has inductance L and capacitance C, and the input impedance is R, what is the condition for when to use each circuit? Is there some sort of mathematical relation to use here? \$\endgroup\$ – iwantmyphd Oct 24 '14 at 22:09
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    \$\begingroup\$ When wanting to block a frequency, use the right one for the impedance of the circuit that follows. Parallel works best into a low impedance and series works best into a Hugh impedance. \$\endgroup\$ – Andy aka Oct 24 '14 at 22:16
  • \$\begingroup\$ Use the "right" one? If you mean correct one, then I repeat, what determines which is correct? What is the dividing line between high and low impedance to make this decision? \$\endgroup\$ – iwantmyphd Oct 24 '14 at 22:32

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