Order of components in filters - Page 1
Forum section: electronics / beginners

author: dfox416
author_rank: Contributor
timestamp: November 25, 2013, 07:01:58 pm
content: I was wondering if it depends what order you put the components in when building a filter. For example, all diagrams of low pass filters show a series resistor at the input and a parallel capacitor at the output. Does it make a difference if you put a parallel capacitor at the input and a series resistor at the output?

author: KJDS
author_rank: Super Contributor
timestamp: November 25, 2013, 07:42:01 pm
content: Yes, it does make a difference.Sometime, when I've got a spare day i'll put a tutorial together on passive circuit design.

author: senso
author_rank: Frequent Contributor
timestamp: November 25, 2013, 07:49:58 pm
content: The cap acts like a short for the high frequencies and a open for low frequencies, so if you put the cap in the place of the resistor you will block all dc and low frequency.When the cap is in its supposed place, it lets all the dc and low freq pass, and as frequency goes up it start shunting the signal to ground/the other wire.

author: dfox416
author_rank: Contributor
timestamp: November 25, 2013, 10:23:58 pm
content: I don't mean swapping the places of the resistor and capacitor, I mean changing the order in which they come (ie, moving the capacitor closer to the input and the resistor closer to the output)

author: KJDS
author_rank: Super Contributor
timestamp: November 25, 2013, 10:49:49 pm
content: Quote from: dfox416 on November 25, 2013, 10:23:58 pmThink what happens to the input impedance if you do that. Hint: the cap is a short circuit at high frequencies.

author: JackOfVA
author_rank: Unknown
timestamp: November 25, 2013, 11:02:36 pm
content: I believe your question is "is the filter symmetrical" so that the input and output can be interchanged. The "resistor" you describe is the filter impedance or at least that's the convention most filter design software uses to illustrate the filter input impedance and the filter output impedance.If the filter is designed for input impedance = output impedance then in most cases which end you call the "input" and which end you call the "output" isn't material.However, there are cases where it is important. One example that comes to mind is paralleling filters to split the input into two frequency ranges, each of which goes to, for example, different circuits. (Think of an AM and FM broadcast band receivers fed from a common antenna.) In this case you want each of the filters to look like a high impedance to the other so they can be paralleled at the common antenna. If the AM receiver is fed from a low pass filter, you would like the input element to be a series inductor instead of a shunt capacitor. Likewise with the FM high pass filter, you would like the input element to be a series capacitor not a shunt inductor.If the high pass and low pass filters in this example are symmetrical then either end can be used in this application. But if they are not symmetrical, then not.I've taken some liberties with the parallel filter concept in this discussion, but this approach can be used and does work. (Been there and done that.)

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 26, 2013, 12:14:30 am
content: Quote from: dfox416 on November 25, 2013, 07:01:58 pmYes, it makes a difference, in general (there can be symmetric filters, but they're a special case).One way to analyze the classic passive two-component RC filter is as you've stated, to call it a series resistor followed by a parallel capacitor. Another way to analyze it, without changing the location of any components, is to call it a voltage divider. But instead of two resistors, you've got a resistor and capacitor, each with complex impedance instead of pure resistance.You might traditionally draw that voltage divider with the resistor vertical instead of horizontal, but that's a trivial detail in the way you draw the schematic.Thinking of it as a voltage divider, consider the voltage output, the input impedance, and the output impedance. Are they the same if you move the resistor to the output side?

author: PA4TIM
author_rank: Super Contributor
timestamp: November 26, 2013, 08:41:30 am
content: In filter design the order is the number of poles after each other, so a 2nd order filter uses less components as a 6th order filter. Rearanging the parts of a "n-th " order filter does not change the order but it can change the function. For instance a Ls-Cp filter does not the same thing a a Cp-Ls filter and a Cs-Lp is not the same as Ls-Cp. Download RFfilter99 or cuqs and simulate some combinations. That will shiow you the results. You can use simetrix or spice to, then choose AC analyse and carthesian view in dB or use S21 (transmission gain/loss) and S11 (reflections thatr tell you about impedance matching).Besides the change in function (low pass versus high pass, notch, bandfilter etc) there is also the factor impedance matching.Like AG6QR wrote see simple one pole RC filters as voltage dividers.

author: Psi
author_rank: Super Contributor
timestamp: November 26, 2013, 10:19:51 am
content: Think about how a typical low pass RC filter works and it should become clear why it only works in one direction.The input signal charges and discharges the capacitor through a resistor.The resistor is limiting the current that can flow and therefor limits how fasts the capacitor can charge/discharge.So the input signal can be changing rapidly but voltage across the capacitor changes at a slower rate. ie, Rapid changes are over too quick to charge/discharge the capacitor by much where as slow changes can fully charge/discharge the capacitor.You can see the importance of the input signal going through the resistor first then capacitor.