9mhz IF Filter

What to do when bored? Experiment with some bits you have sitting about of course. Its been so long since i have experimented just for the sake of experimenting. So in many designs you see the 4 crystal ladder filter with various value capacitors of the same value. So i grabbed 4 9mhz crystals out the parts box and some 100pf capacitors and soldered them up in the ugly style. Connected up the signal generator and scope and began to plot out the shape of the filter.

The basic setup.


With 100pf caps a filter with about 900Hz -3Db width, not sure why the right hand side is not a smoother curve, i probably need to match impedance’s and use better quality caps.


Next i changed the capacitor values to 50pf, and plotted out the filter shape again, this time converting millivolts to Db to see what sort level of attenuation is being achieved, and after some stuffing about to get my math right, this is what i ended up with, well 3rd times a charm, to get the math right HIHI. Kind of looks like a filter now and not a Mr Squiggle drawing.


The test setup.


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