little help on bode plots

[RobinM]

Hello there, was trying to find a tutorial but havent had much luck so I came here, does anybody know how to add the roll-off in the upper and lower stop bands ?

**broken link removed**

Ive been asked what the characteristics of the filter are and to estimate the filter order. I have no electronics knowledge at all, I only just figured out about logs, and the pass band so this is the best my ability at present.

The help will be much appreciated.
Thanks

 

[Sceadwian]

Band-pass filter - Wikipedia, the free encyclopedia
The order of filter is determined by the slope of the 3db cutoffs. If I'm not mistaken for each 3db per octave, the filter order increases by 1.
So a second order filter is 6db per octave at the cutoff point.

 

[MrAl]

Hi there,


Another way of looking at it is the asymptotic approximation for Bode plots is 20db per decade per order.
You also have to check for the break frequencies so you know were the asymptote begins.
For example, a very simple RC filter made up of a resistor of 1592 ohms and cap of 0.1uf has a break frequency of 1kHz, so it is considered 0db at 1kHz even though it isnt and that decreases by 20db per decade which brings it down to -20db at 10kHz, to -40db at 100kHz, etc.
You have to be a little careful though because sometimes the true curve does not follow the asymptotic approximation very well.

Maybe i'll post a Blog on this sometime.

 

[crutschow]

Band-pass filter - Wikipedia, the free encyclopedia
The order of filter is determined by the slope of the 3db cutoffs. If I'm not mistaken for each 3db per octave, the filter order increases by 1.
So a second order filter is 6db per octave at the cutoff point.

The power rolls off at 3dB/octave, but the filter output is usually plotted in dBV, which would give 6dBV/octave for each filter order.

 

[MrAl]

Hi there Carl,


Yes i agree. The approximation is 6db per octave or more conveniently, 20db per decade.
For the low pass filter example i gave earlier, if we call the 1kHz break frequency 0db, then 2kHz would be -6db, 4kHz -12db, 8kHz -18db and this last one is close to the decade 10kHz which is -20db.
20db per decade is easier to plot on most log paper