455Khz IF Filter Part 2

So in moving this forward some, i decided to knock up a 4th order filter based on the Dishal parameters, and then plotting out the filter to see how well theory and reality collide.


These were the settings i used in dishal based on previous experiments.


This is the filter realized, 2nf were used for the 1984pf and parallel 1nf and 580pf for the 1708pf capacitors.


Test rig was scope on output and function gen on the input, i did not worry about impedance matching 50ohm to 150ohm.


Measurements were taken every 200hz and plotted in a spreadsheet. A couple of things, firstly there are 2 peaks not sure why, second major one is the filter is 2K wide not 400hz wide, might almost work as a narrow SSB filter HIHI and the third major issue is the center freq is even lower than predicted at about 442 -> 443 Khz, no where near the 447 predicted and still miles away from 455Khz where the radio’s IF is.

This has been a very useful experiment, I think i can improve on this first effort by a long margin and I intend to go back the the drawing board and see if i can get more accurate motional parameters and also if i can pull the frequency UP somewhat with added capacitance to get this thing closer to 455Khz.



Ok, so as an addendum to this, i terminated the filter at 150 ohm, turned the power on the function gen right down to its lowest setting and this is how the filter now looks.