# How do you find the order of a passband filter??

Discussion in 'Homework Help' started by zero_coke, Nov 21, 2011.

1. ### zero_coke Thread Starter Active Member

Apr 22, 2009
294
1
We have to design a passband filter and I have a bunch of filters now but I don't know how to determine what order they are.

2. ### crutschow Expert

Mar 14, 2008
12,540
3,073
You can measure the frequency response of the filters. The rolloff outside the passband determines the filter order with each order adding 6dB/octave, i.e. 1st order is 6dB/octave, 2nd order is 12dB/octave. etc.

3. ### zero_coke Thread Starter Active Member

Apr 22, 2009
294
1
Do you have those #'s in decade? I know 1st order is 20 db / decade, what is 2nd and Nth order??

Mar 14, 2008
12,540
3,073

5. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718

For switching between decade and octaves. 3.32 octaves in a decade. from: 3.32 = log2(10)

Do you have homework questions you haven't asked here?

Was the CPU one correct?

Last edited: Nov 23, 2011
6. ### zero_coke Thread Starter Active Member

Apr 22, 2009
294
1
Haha! The CPU one is messed up because his wording of the question was so vague...I mean, apparently, I also found out that LOAD/POP also add to the AC, not replace its contents. So if AC contained say, "5", and we POP'ed "3" from the stack, the AC is now supposed to contain "8", not "3". Yeah and same thing with LOAD.

But anyways, we'll have to wait until he releases answer key for the homework so I can check...

Oh and about the filter, how can I tell what the order is by looking at the schematic? Is each pair of opamps = 1 order ?

7. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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I'm curious as to what CPU would respond that way. Model # please, so I can stay away from it.

Filter: For passive filters, roughly each RC or RL pair adds an order (pole/zeros of functions if you do the math).
With active filters (op-amps), that isn't a rule, an extra capacitor can change the behavior entirely. There are entire books on op-amp filters and layouts, so I don't want to give you a "rule of thumb", unless you have several thumbs.

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8. ### justtrying Active Member

Mar 9, 2011
329
324
the very crude way is to look at the number of RC combinations - this will give you the number of poles

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Mar 14, 2008
12,540
3,073

etc.

10. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
No clue what I was thinking there. Thanks for the fix.

Figured out where it came from. Above first order was stated as 6dB per octave or 20dB/decade. I was thinking "3dB down" rather than filter.

3.32 = log2(10) For switching between decade and octaves. 3.32 octaves in a decade.

Last edited: Nov 23, 2011