Wednesday, November 11, 2015

Tiny FM band-stop filter

I've known for a while that I will need some kind of FM filtering.  I figured I could use the same SMA housing for a passive band-stop filter.  The main requirement is that it must pass the DC voltage to power the TinyLNA.  That includes no inductors connected to ground and preferably no resistors which will only consume power.

After reading this post, I figure I would use the same approach.  A simple 3rd order filter, but with slightly different design parameters.  I used 98MHz center frequency and wanted the edges to be -6dB.  This will probably not be enough, but I didn't want to put so much filtering on it that I lose the nearby air-traffic band just above that.

I plan on receiving several of these filters, so it will be relatively easy to adjust them.  I may consider assembling a test rig so I don't have to solder these into a connector to test them out.  Or perhaps I will just make a larger sized board that I can solder individual SMA connectors on either end.

I put together a simple schematic with some extra components just in case:

And then copied over most of the layout of the TinyLNA like board outline, copper pours, solder mask around the outside, etc.

This came together pretty quickly since I had all of the design files from the TinyLNA.  Considering the cost will be the same (80 cents for 3 boards), I figure I can go ahead and order some.  Even if they are a disaster, it's trivial to fix.

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