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The Cigar Box Pico-W Noise Suppressor for the Project TouCans Ham Radio

 I spent a lot of time this week working on getting radio frequency interference from the rig itself and the Pico-W that controls Project TouCans out of the audio output. Some of this work was not fruitful. Turns out the audio wiring does not belong in Project TouCans no matter how much the Pico-W may induce helicopter blade audio overtones in the rig's headphone routing. Some of the work, however, not only removed the helicopter noise, it was a simple patch, like scissors and tape and cardstock simple.

Please allow me to introduce the cigar box Pico-W Noise Suppressor.




At present, the Pico-W rides atop the heavy duty opto-isolated relays that it uses to control Project TouCans internal keyer. As it turns out, the helicopter noise was being coupled into the audio output through these relays. Bear with me, there are a combination of audio issues that I'm optimizing for here. It's kind of like the old lady that ate a fly story. Audio ground needs to be routed through the same set of relay contacts that are used to connect the key down line to ground. The reason is purely empirical. When audio ground is routed directly to the USB-C power adapter seen on the left of the picture above, power supply noise is emitted through the speakers. When the audio ground is instead attached to power supply ground through a relay contact, the noise disappears. 

wiring diagram showing the USB-C power output marked + for positive and with a ground symbol on the other power lead. The ground lead travels through a normally closed relay used to interrupt power, then to the normally open contact on the relay used to key the CW ham radio. The normally open contact is where the audio ground wire is connected to eliminate power supply noise
Wiring diagram that kinda says it all: power noise gone = Pico-W noise introduction


OK, so that gets rid of power supply noise, but I was talking about the Pico-W, right? The key to eliminating the Pico-W noise is simply to space it off of the relays a bit. That's what the wedge made from a cigar box shown above does. It props the Pico-W power pins, as well as the I2C/SPI pins off of the relays. Doing that eliminates the noise. 

Copied from the Pico-W documentation

How did I guess that the power inputs and the I2C/SPI pins were the culprits? Grasping either set of those pins between thumb and index finger increased the helicopter noise. Propping them away from the relay contacts almost completely eliminated it. The final fix that took the noise away was to pull away the tape that was holding the top of the Pico-W to the relay boards. The Pico-W is now held in place only by the orange and yellow control wires as well as the black and read power wires pictured above. That and a piece of double sided sticky foam tape between the Pico-W's USB A connector and the cardboard of the cigar box.

Project TouCans: Bringing you ridiculous old-school solutions to modern engineering problems every week :)



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