Here is the Propeller Output without a Low Pass Filter. The tall center signal is the Spectrum Analyser's LO, the Propeller 10.140MHz signal is to the right of the LO, and the negative images are seen on the left side. The third and fifth harmonics can be easily seen, odd harmonics are typical of a square wave output.
|Without LP Filter|
|With LP Filter|
Here is the Propeller and Low Pass Filter test configuration.
|Prop and Low Pass Filter|