FM notch filter

I'm not sure why I never shared this, but I completed the notch filter.  I had made it with the same board outline as the TINYLNA Rev A, so I had to cut out the tabs on either side to fit it down into the housing.  But I turned on sharing of the project on OSH Park.  Also, I have a schematic and layout I will share below.  OSH Park

Sharing on OSH Park

I've been very busy with work lately, so I haven't had enough time to work on my RF projects.  So I decided to at least share the board layout.  With the other information on this blog, it would be easy for anyone to build the board themselves.  So check out the OSH Park layout and order a few boards.  They're only like 80 cents each since they're so small.  Anyone who gets them going drop a comment.

https://oshpark.com/shared_projects/0jyERLQD

Rev B boards are in

Just received the next revision TINYLNA boards with the slots for the housing pins.  This will allow me to drop the board straight down into the housing and solder it from the top.  The boards look good, but it appears the metal pads flake off a bit when the board outline is cut.  That's to be expected since they request the metal doesn't come to the edge of the board.  The board sits well into the housing, so the dimensions are all good.  I can use this board outline in the future for other functions (like the FM notch filter board).  Next is soldering components and soldering into the housing.

New installation

I had an idea for a new installation method.  I dremmeled slots where the housing pins are located and dropped the board straight down into the housing so it lies flat.

The board is flush to the bottom of the housing.  The board is held in place by the tab like a through hole component.  It's just the right height.  It's possible this is how the housing is intended to be used it works so well.

I will update my layout and do another board spin.  I'll also shrink the board dimensions just slightly to help it fit better.  In the meantime I'll try to come up with a quantitative way to measure its performance with the tools on hand.

Components on board

Got the components soldered onto the TinyLNA.  It's tough work with the tools at hand (no microscope, no solder paste).  I included the option to have the DC voltage shoot through towards the antenna, but did not solder it on this version.  I also did not solder on the ESD diodes for this version.


And then installing the board into the housing was even more difficult.  Getting the board to stay put while soldering was pretty tough.  Electric tape wouldn't hold it in just the right spot so I had to rely on fingers way too close to the iron tip.  I may have to come up with a different installation method.

PCBs are here!

PCBs have arrived!  As expected, the boards have mouse bites on them which will definitely prevent the board from sitting inside the housing.  I expected this, so if figured I would have to sand down the edges once the boards arrived.  But once the mouse bites were sanded, the boards were just a hair too long still.  So I just kept sanding!  Maybe another 100 microns or so and the boards fit just fine. Ran out of time today, so tomorrow I will check the continuity of the traces and perhaps even solder components

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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.