Some BITX40 radios -- including mine -- built as designed exhibit tuning clicks. That is as the DDS frequency changes there is an audible click in the speaker or headphone output. There is some debate as to the cause. Some believe it is the result of how the DDS circuit is handled in software and that it can be fixed in software. Pavel Costa has written a library that has a number of benefits, including eliminating the tuning clicks. That link has some interesting reading, and the first thing I did was to compile and install his software and his modification of the BITX40 sketch. I still had tuning clicks, so not entirely a software problem.
The other solution proposed on the internet was that the Raduino voltage regulator was insufficiently decoupled from the 12VDC supply. Heavy computing and circuit use surrounding a change in frequency would cause an increase in current draw, which was bleeding back across the regulator to the main power circuits and appearing in the amplified audio. There are a number of ugly and somewhat dubious solutions adding components to the back of the Raduino board. None of these appealed to me so I took a different route.
If the problem is decoupling of the voltage regulator I could de-solder the regulator from the board, replace it with a header, then I could experiment with the required circuit. So that's what I did.
|Raduino with regulator replaced by header.|
With the header in place I could put the regulator on a prototyping board and build up the required circuit. I found a 66 ohm resistor in series with the input 12VDC line and a 333uF capacitor between the 12VDC line and ground eliminated the tuning clicks.
|Prototype decoupling circuit.|
|Final decoupling circuit.|