
BackgroundSome time after I put the first version of the "CWfilter" into the FT225 I began looking on the original BPfilter. First test with an extra RClinkSince I had made the calculations for a 3pole filter according to the already used components I first tried to add the "missing" RClink. I got the values 5K1 for the resistor and 10nF for the capacitor. (Calculation example further down this page, to clarify any doubts  schematics with the later used labels is found here.)ResultsSubjectively, I really think this extra RClink really gives effect! The simple "CWfilter" is not loaded in the same way so this extra filter also works much better!I didn't have any audioinstrument to measure the filterbandwidth. The easiest way to get a hint about the filterbandwidth is to record received noise with a soundcard. I used software, made by OK1FIG, to graphically show the intensity on the frequencies of interest. Calculation of a new 3pole UGfilterSpecificationIt is always important to have a goal, so before we can calculate any filter components we must know the wanted specifications.
Number of polesThe specification "Butterworth" and "3* f_{g}" inform us that we have to use a filter with 3 poles.The UGfilterSchematics on the UGfilter is found here and the labels later used are the same as in this schematics. There are three capacitors and three resistors to be calculated. This calculation is made in a standard procedure.The angular frequency is calculated as; w=2*pi*f with f=2500Hz, we get w=15708 rad/s. This angular frequency is used to denormalize some different factors, I will not point out the function of these factors and you could find this described in several different books. The factors might be called something else, but the result will be the same. ButterworthSince I have chosen a Butterworhfilter the normalized factors S_{p} and w_{op} will be for the first, and the only single pole;S_{p}=1 and w_{op}=not defined For the other two poles, the normalized factors are; S_{p}=0.5 and w_{op}=1 Calculation of the RClinkThe RClink, or the first pole, is calculated by denormalizing the S_{p};S_{p} = 1*15708 = 15708 = 1/(R*C) This gives us, with the same R as before, 3900ohms; C = 1/(R*S_{p}) = 1/(3900*15708) = 16.3nF This is not a standard value, we therefore change the Cvalue to a standard value, ie C=10nF. The R is then; R = 1/(C*S_{p}) = 1/(10^{9}*15708) = 6366 ohms The closest standard value is 6340 ohms. Calculation of the other componentsThe resistors n and m are chosen to 3900 ohms, since this value already has been used. This gives us the e and f; e = 1/(m* S_{p}) = 1/(3900*0.5*7854) = 33nFf = e/4 = 33/4 = 8.2nF Measurements and resultsNext stepIf you get tired of these "simple" filtersolutions then you might be ready for a lineoutput were it's possible to have more sophisticated filterarrangements. 