Next I had to find out how the OLED noise was getting into the rest of the receiver. It could have been through the SCL SDA or even the ground lines. It could have been just through capacitive or inductive coupling from the display board itself. A big clue came when I tried powering the display from a completely separate power supply: BINGO! The noise disappeared. So I knew the noise was going into the rest of the receiver through the Vc line that powered the OLED.
I had been powering the OLED from the 5V regulator on the Arduino Uno. In an effort to isolate the noise, I put a separate 5V regulator in the circuit for the OLED. No joy -- noise still there. I then tried putting an RC low pass filter between the OLED and the 5V regulator. Still had the noise. Finally I remembered something from the AF AMP circuits of Roy Lewallen, Rick Campbell and Roger Hayward. ( I think Roy was the pioneer on this one.) They all used an "active decoupler" between the first AF amp and the power supply line. I confirmed that it was my first AF amp that was picking up the OLED noise. I built the active decoupler (just three parts!) and the noise disappeared. GONE!
There are only three parts, but the way this circuit works is kind of complicated and not very intuitive. There is a good discussion of how it works here:
www.facstaff.bucknell.edu/dkelley/elec351/Lab/elec351lab5_sp04.doc
Roy, Rick and Roger were using this circuit to knock down 60 Hz AC hum, but I found that my OLED noise was at around 200 Hz -- I figured (correctly) that the active decoupler would take care of this as well. I think this little circuit can be useful in dealing with the kind of noise generated by the digi displays that many of us are now using.
David Rowe has a really interesting analysis of this circuit here:
http://www.rowetel.com/?p=4781
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