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• Version 1.1 (prototype) has been fabricated

  Gerriko IO • 12/31/2019 at 15:52 • 0 comments

  Now that I've digested and summarised the design documentation, here is what I have come up with. A new iteration (version 1.1)...

  For now, I left in the component prefixs on the PCB silkscreen but I have made improvements on the initial design, which I published earlier. Removing these prefixs will provide room for improvement. Here's a comparison with the OM5578 component layout:

  I also decided to modify the antenna slightly so that I could measure the differences in performance.  After all, the OM5578 antenna design can very readily be cloned.

  So the trace antenna still consists of 4 turns but I now used a 14MIL trace instead of 20MIL as per OM5578. The external length / width is 1564 MIL instead of 1555 MIL. The spacing between traces is 24MIL, which results in a 10 MIL gap. The OM5578 had horizonal trace spacing of 35MIL, which creates a 15MIL gap. The vertical trace is more tightly spaced at 30MIL, creating a 10MIL gap.

  I used an online Inductance calculator as way of comparing designs. 

  https://www.translatorscafe.com/unit-converter/EN/calculator/planar-coil-inductance/

  The OM5578 antenna inductance was calculated as 1.4uH:

  The new FeatherWing PCB trace antenna design is not precisely a rectangle and thus I viewed it as somewhere between a rectangle and an octagon. The rectangle inductance calculation gave me 1.52uH and the octagonal calculation was 1.41uH. 

  So, I felt I was in the right ballpark and have some wriggle room to make my antenna more octagonal should I need to reduce the actual measured inductance to match the OM5578 matching circuit (or come up with my own capacitance values for my own custom matching circuit). For now I am happy with what I have come up with.

  Anyhow, the "proof is in the pudding" and the antenna works (and no white smoke) when attaching it to the OM5578 shield (I disabled the existing antenna by removing the 0 ohm resistors).

  Let me now describe the design detail.

  As mentioned in the original write up, the antenna part can be removed from the controller part. I did not make this snappable as I wanted something more robust. Nevertheless, with a bit of filing and cutting, the antenna does snap off.

  When the antenna is removed there are two pads on the antenna side to allow you to solder a Twisted Pair cable instead of a u.FL cable (as per original diagram). I took this decision based on some antennae I had seen before, as shown here (Taoglas antenna):

  With my first prototype board, I found that the solder pads were too small for my liking (as shown below), so I have redesign and increased the size slightly.

  On the controller side there is a hybrid footprint for a 2-pin JST-SH or JST-ACH connector. Some copper has also been exposed on the trace to allow you to cut the unwanted trace to the antenna (in case this is really needed for say measurements etc).

  Then on the reverse side there are a few configuration options.

  The first is antenna voltage. 

  I have it set up so that the antenna voltage is powered from USB (i.e. at 5V) as this provides better antenna performance (as per PN7150 data sheet). I have provided 2 other power selection options. You can power the antenna via battery or via 3v3. This is all done here (left is USB, right is battery and top is 3v3):

  Then you have 4 I2C address options. The default is set with both ADR0 and ADR1 pulled to ground. This is done here:

  Finally there is another solder joint (SJIC). I simply included this because the OM5578 has the IC pin on the PN7150 connected to ground even though the datasheet shows this as not connected (NC).

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