My previous entries discussed the when/where/why to use a DDS based approach in place of a PLL-based one. One reason folks lean towards PLLs though, is familiarity. So let’s get more familiar with DDS. (Note this does not require reaching out to the American Dental Association)
Before I launch into a brief review of a state-of-the-art DDS, let me once again direct you to the tutorial we put together here if in an effort to avoid rehashing something that’s already published.
If you have any questions after running through the tutorial, please ask them, then come back and keep reading...
So let us jump ahead to exploring the elements of a more advanced DDS architecture with more powerful capabilities.
The blocks in white were explained in the tutorial, they are fundamental necessities for any DDS system. I’m going to speak on the green blocks. They are critical to many of the advantages I wrote about before. A valuable addition not depicted above is the existence of alternate FTW registers which can be switched between for frequency hopping/FSK applications.
The Phase Offset Register (POR)
On its own, the addition of this block provides some valuable functions.
- The ability to digitally reposition the phase of the output in very small steps (as small as 0.0055 degrees in some of our products). The digital nature of the function makes it both perfectly repeatable and reliable.
- The ability to implement Phase Shift Keying (PSK) by toggling between multiple Phase Offset Word (POW) options.
- The ability to establish perfect quadrature for two DDS channels relying on a common system clock source AND the ability to compensate for static phase offsets which might degrade the perfect quadrature.
The Amplitude Scale Factor (ASF)
Provides a similar level of control to Amplitude as the POR provides to the phase of the signal.
The Auxiliary Accumulator
By far the most powerful of these additions, though some of that power results from combining it with the POR or ASF capability. To date, these are the most meaningful uses for that auxiliary accumulator
- Originally this was introduced to enable frequency sweeps. Using a second accumulator enables the establishment of a steadily increasing variable. Advanced DDS's may be configured to add that variable to the base Frequency Tuning Word. The net result is a signal with a steadily changing frequency.
- Alternatively, you can combine it with the POW to create a phase sweep.
- Alternatively, you can combine it with the ASF to create an amplitude sweep.
- Programmable Modulus – I wrote about this already in prior Blogs, and you can read more about it here.
- Phase coherent switching. By its nature, a DDS switches phase in a phase continuous manner. Some applications (radar and others) want switch phase coherently instead.
Phase coherent switching is a topic with enough depth that it warrants separate treatment, if you want to know more, before I get back to blogging again, respond to this email or post a question to the DDS forum. As always, any questions raised in the minds of readers will gladly be fielded as well.