After further research, it looks like I am going to use the 50% duty cycle PWM with a 40us period to produce an approximate 25.5kHz square wave instead of a true sine wave.
I am using the output to drive an antenna. I additionally need to READ a responding wave at the same frequency and measure the phase shift.
Can the Netduino do this? The specification looks like the ND is 5000 times faster than this frequency... but, can I access and the analog or digital pins fast enough to measure phase shift somehow?