The Mooshimeters have been at the testing lab getting their regulatory certifications. We hit a bump but are hopefully past it.
The Good News:
The Mooshimeter passed emissions testing without a hitch!
The Bad News:
The Mooshimeter demonstrated susceptibility to radio interference that messed with the current reading. In the lab, when blasted with radio waves in the neighborhood of 200MHz, the meter would erroneously display a large current (up to 2A). That’s not acceptable.
The test lead is acting as a large antenna and blasting our poor current sense amplifier with RF noise. Instrumentation amplifiers have a well known but poorly characterized ability to rectify common mode RF noise. Analog Devices have a nice paper about it here.
The designer must protect amplifiers from RF noise through careful filtering. In this case there was a gap in the filters that subjected the reference input of the instrumentation amplifier to too much RF noise. This noise is rectified and appears as an erroneous voltage offset within the measurement chain.
We recreated the issue in our own lab using a HackRF Jawbreaker – ProtoDave, thanks for the loaner! This allowed us to quickly test the changes to the RFI filters. With the changes in place we are unaffected (error is below measurement threshold) by interference sources between 20MHz and 6GHz.
The production design has been updated to include new filter components, which were added by hand on the boards going through regulatory. They’ll be back in the compliance test lab this week to undergo radio immunity testing again.