Wednesday, February 4, 2015

MIDI2VC+, demo with low noise

Last post I wrote about the noise problems when running the MIDI2VC+ with a mock up synthesizer. I suspected that the I2C bus picked up noise and crosstalk from the USB bus, as they where routed very close each other. I also looked at emitted noise on the board and saw that there where lots of emissions at the USB connector. To see the emissions I used a looped semi-rigid coax with a slit. The coax was connected to a scope with a 50Ohm standard coax:
Electromagnetic emissions sniffer made from semi rigid cable
I captured  image on the scope showing background levels and levels from USB connector:
Background emissions vs USB connector emissions
I patched the board by cutting the I2C traces that passed the USB connector, soldered wires instead. This made the noise go away completely, as can be heard in the demo.
MIDI2VC+ board with I2C bus patched with white cables.

 The only problem left right now is that, from no known reason, the I2C transmission process stops prematurely, before it has completed. If I don't find the cause, I may add a time out and resend feature that may solve the problem.

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