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WWVB Receiver Shield Enclosure

Kapton tape over traces

Kapton tape over traces

The little C-Max CMMR-6P-60 WWVB receiver board is somewhat sensitive to its surroundings: putting it too close to fast-switching digital signals is a Bad Idea. Of course, when there’s an antenna connected to the thing, it’s hard to separate the effects, but I’ve been testing reception with the antenna at the end of a two-foot twisted pair: far enough away to eliminate most problems.

Just to see what happens, I built a little shield enclosure around the receiver board. The clock board has a pair of solid planes isolated from everything else, with a header matching the receiver’s pinout, for this very purpose. The receiver has a fairly solid ground plane on the bottom, but it’s quite sensitive being snuggled up against other objects; the header holds it about 5 mm above the circuit board.

The dark amber square is Kapton tape across the traces. If I ever do this again, I’ll put the traces on the bottom so the board is entirely shielded and the tape isn’t needed.

Shield soldered to base

Shield soldered to base

Some 1-inch copper tape with adhesive on one side serves as the shield enclosure, with a layer of Kapton tape covering all but about 2 mm of the adhesive near the bottom to insulate the copper from the receiver. Bent those 2 mm strips outward, with the adhesive on the bottom, stuck it to the shield plane, and soldered it in place at the corners.

The antenna leads poke out through one side; it’s not very elegant, but I think it’s about as good as is needed for this sort of thing.

I cut the tape at the corners and folded it down to make a little box, stuck a square of copper tape over the top flaps, soldered the corners, and it’s cute. Admittedly, it doesn’t have perfect conduction around the joints; the next time it’s on the bench I’ll add a few solder dots at the midpoints.

Completed shield enclosure

Completed shield enclosure

The immediate effect was to raise the receiver’s Glitchiness score by a factor of about four. However, that’s not entirely a bad thing; it turns out that the reciever is much less Glitchy when it’s subject to high noise levels: the receiver AGC cranks the gain down so low that only heroic pulses get through and the number of glitches drops dramatically.

As nearly as I can tell, when there’s no WWVB signal, as during the day, a low Glitchiness count means there’s extremely high noise. Thus, a higher count means less noise and better sensitivity.

More data collection is in order, but the receiver’s LED showing data pulses now tracks the Alpha Geek Clock‘s display almost perfectly.

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