The value sets a 'turn-over frequency' we can define as A Normalised version of the circuit would assume that was one Ohm, and was one Farad. In reality, normalised values will be impractical, and we will usually want a specific turn-over frequency very different to the above value. To shift the turn-over frequency to we choose the and values so that expressions 3.7 is correct To obtain convenient component values we then 'trade off' the values of and against each other until we get something suitable. The choice of sensible values is a matter of practical experience so is hard to define on a webpage This is one of the areas of electronics where design is a skill, not a science which provides a uniquely correct answer. When using op-amps for the amplifiers in the active filer, we generally try to end up with resistor values in the range to , and capacitors in the range from down to .
Lets look at some examples of designs, then try scaling one for a specific result.
For a Normalised version we would assume that and . Let's now take as an example a real filter which we want to have a turn-over frequency of, say, . This means that in this case If we try choosing a resistance value of this means we would need to choose to set the required turn-over frequency. In the case of the above circuits the value just sets the gain for frequencies well within the Passband of the filter. So we can choose to amplify the output by whatever amount is suitable for our purpose.