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.

dishalfilter\

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

filterploit-1

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

filterploit-0

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

filterplot

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.

 

filterplot

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.

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