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cascaded filter design

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vinodquilon

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I have designed one LPF using the cut-off frequency equation 1/2*pi*RC.
I am planning to implement the designed filter as a cascaded form of three LPFs.

Then, how can I split-up R & C values into three values each for each section ?

I know, in a cascaded connection net cut-off frequency would be the product of
cut-off frequency of each sections.

So, can I divide net R & C values by 3 to get values of each section ?
 
Cascaded RC filters have very slow roll-off characteristics at the corner frequency. For multi-pole filters, active or LC type filters are much better.
 
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The short answer to your question is no, it's just not that simple.

There are a few filter design programs out there in cyberspace to shortcut the design time (doing all those calculations). The one I use/prefer is Filter Pro by T.I. It is a free download with no licensing strings attached. You can download it, play with it and design up to a 10 pole LPF if you like. Here is the download link:

Active Filter Design Application - FILTERPRO - TI Software Folder
 
Here I am dealing with
a simple passive RC filter only used for blocking carrier frequency at
demodulation stage.
 
I see. That information was not in your initial post. What is the carrier frequency, the type of modulation, the bandwidth of the modulation and the type of detection/discrimination employed? A schematic of the circuit would help, also.
 
Hi,


If you are looking for a three stage passive, then you can do this another way if your input and output impedance allows.
We start with an RC section of the right values, then add to that the second section where the cap value is divided by 10 and the resistor value is multipled by 10, then for the third section we do that again dividing the second stage cap value by 10 and multiplying the second stage resistor by 10. The output has a response that is very close to three stages of RC filtering where each stage has the same R and C values but the stages are isolated with a op amp with a gain of 1. The nice thing is, we dont need op amps for this design. The drawback is that the impedance of the input is relatively low and the output impedance higher so the application has to be ok with this.

For example, if we started with a 1k resistor and 0.1uf cap, the second stage would have a 10k resistor and 0.01uf cap, and the third stage would have a 100k resistor and 0.001uf cap.
Note that each stage is tuned to the same frequency, but the impedance grows so that the current stage does not load the previous stage.
 
For a specific 3dB filter rolloff corner, each RC filter stage would have to have a 1dB rolloff at this corner. The -1db point for each filter is the frequency at which the capacitor impedance equals 2.86 of the resistor value.
 
Hi,


I was assuming he wanted a third order cut. Of course i could be wrong :)

To get a response similar to an isolated amplifier third order (9 or 10 db down at the cutoff frequency), the three resistors and capacitors can be made all the same value, where the resistor value is calculated from:
R=0.0871855442553/(C*F)

although it will be a little less sharp.
 
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Here is my Pulse Amplitude Modulation (PAM) application....
 

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Here is my Pulse Amplitude Modulation (PAM) application....

Now how does that square with your post #4? There exists a plethora of information re: PAM on the web. Perhaps you should look at those offerings to see where the demodulation actually occurs along with other necessities to accomplish such.
 
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