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I've been reading through the LTC1264-High Speed Quad Universal Filter Building Block datasheet but I'm having some trouble deciphering the meaning of some of the variables used in the equations for the different modes of operation. For example, in the image below I have highlighted the variables fi, f0, and fn in the equations that govern the Mode 1b topology. Unfortunately, Linear Technology does not include a "Definition of Terms" anywhere in the datasheet (seems kind of silly).

enter image description here

Can anyone tell me what these three variables are? My assumption is that f0 is the center frequency of the filter but I don't have a clue about the other two values. To make things more confusing, later in the datasheet in the "Guide to Bandpass Design" on page 11 these equations are shown:

enter image description here

These equations are for the same circuit in the 1b mode. What then is fcenter and why does it not appear int the equations in the previous image?

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If you look at the equations right below the schematic you will see that all of the terms are defined: fi = fCLK/20 where fCLK is the input clock frequency. f0 = center frequency of the filter (fo is defined earlier in the data sheet as being the center frequency of the filter). fn = fo. That is enough information to be able to compute all of the required values. It is true that they don't explicitly define the meanings of fi and fn but their values are easily computed from the given equations.

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  • \$\begingroup\$ I understand that they explain how to get these values in the formulas, but other than the center frequency, I don't know what they mean or why they exist. Also why would they make a distinction between f0 and fcenter in the second image if f0 IS the center frequency? \$\endgroup\$ – Andrew Schroeder Oct 6 '18 at 0:49

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