CW Filter Board

So the plan was to add some kind of filtering to the DC Receiver to give some selectivity. Easiest way to achieve that is to use audio filters, of which there are plenty of types that can be used, passive, scaff, active, etc. I settled initially on an fixed width active opamp filter. Further down the track i will change that to scaf capacitance filtering but, i wanted to have this down pat first..

cw-filter-graph

The filter parameters are shown above. 8th order Chebyshev, 500hz wide, 600hz centre and -40db down at 2k. A fairly aggressive filter which should take out a lot of the adjacent signals and noise.

cw-filter-schematic_1

The circuit was optimised for low noise, and one of the nice features of doing that makes all your cap values 100nf and a bulk of 5% 100nf is cheap as chips off ebay, leaving only many odd values of resistors to locate.

cw-audio-filter

Circuit is laid out with R13/14 making a virtual ground for the OP Amp negative voltage rail, a jumper runs from V- to V- to supply the VCC/2 to the negative voltage rail.

DSC_1575

Gerbers were created and converted to G-Code and the circuit mirrored and routed, drilled, cut out and given a coat of board lacquer ready for populating with parts. Board came up rather nice. .

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