If I switch from one profile register to another will the phase of the RF output be continuous? For example if Profile 1 has a sine wave of 5 MHz, phase = 0 and Profile 2 has a sine wave of 10 MHz, phase = 0, and I switch the output from Profile 1 to Profile 2 will the phase of the RF output be continuous? Or will there be a discontinuity in the output at the time of the switch?
I've measured it and found a discontinuity. What do I have to do to change the frequency of the output and keep the phase continuous?
Also, I've found that is I keep Profile 1 active and just write a new frequency into it while it's running there doesn't seem to be a discontinuity in the phase. Assuming this works is it ok to just keep the same profile active and write different frequency into it while the output is on?
The DDS works by accumulating phase based on the frequency tuning word (FTW). The phase increases in discreet steps given by 2π(FTW/2^N) radians, where N is the tuning resolution in bits (N=32 for the AD9910). Given two profiles with two different FTWs you have two different phase step sizes. When you switch profiles, the DDS uses the last accumulated phase value and starts accumulating the new phase steps. So, by definition, the DDS output is phase continuous.
Sometimes you may see what appears to be a discontinuity in the output sinusoidal waveform. This is normal behavior. It actually is not a phase discontinuity -- it is an amplitude discontinuity. The behavior typically shows up when the FTW values differ significantly. For example, suppose FTW1 yields phase steps of 0.01 radian, while FTW2 yields phase steps of 1 radian (~60°). While FTW1 is in effect you see tiny increments in amplitude because the phase steps are small. Then, when you switch to FTW2, the next system clock cycle causes a 1 radian step resulting in a significant amplitude change. However, the phase is exactly where it is supposed to be!
Sorry, I forgot to answer your other questions...
Yes, you can use a single profile and just keep rewriting new FTWs. However, this should not be any different than switching between profiles (assuming the only difference between the profiles is the FTW). If there is indeed a different behavior between writing FTWs and changing profiles, then check carefully to make sure that the profiles are IDENTICAL except for the FTW.