Have you ever been stuck without a hex wrench and resorted to trying to use a flat head screwdriver to fasten a hex bolt? It can work, but wouldn’t you rather have the right tool for the job? Well if you need to synthesize a sinusoidal signal, knowing a little about DDS will help you settle on the optimal solution.
Most engineers are exposed to PLLs during their schooling, but DDS is less universally taught. Seems like something worth blogging about. I wanted to get the ball rolling with a few comments on the comparative advantages – both have a place in your tool chest. The table below is a handy reference:
Finer resolution allows you to get closer to hitting your desired frequency precisely
Do you need a signal that changes (sweeps or hops) over time?
Phase Resolution & Agility
Same as above, but applied to the phase of the signal
Amplitude Resolution & Agility
Same as above, but applied to the amplitude of the signal
Broad Spectral Purity
Can you work with an output spectrum that has a lot of low level spurs?
How much external circuitry are you willing or able to add?
Are you trying to synthesize a frequency higher than the reference you are working with?
I expect a couple of these comparison (Price, Power) terms don’t need any clarification, but the rest are less common.
Frequency Resolution: You are trying to get a signal of a specific frequency out, how close to that exact frequency do you need to be? Greater resolution allows you to get closer to your desired frequency.
Frequency Agility: Many applications only need a constant, stable output frequency, but some need to jump between frequencies and others need to see a slowly changing frequency – these latter applications require an agile frequency synthesizer.
Phase/Amplitude Resolution & Agility: Same as Frequency Resolution and Agility above, just applied to phase or amplitude of your signal.
Broad Spectral Purity: Most easily explained with pictures (below). While not an apples to apples comparison, it shows what I am talking about – the DDS has more (and larger) frequency spurs in its output spectrum than the PLL. There are things that can be done to mitigate them (for more see ADI AN-823).
PLL based design example
(from the AD9510 datasheet)
DDS based design example
(Lab data taken using AD9911)
Ancillary circuitry: How many other components are needed to get to the complete solution?
Frequency Upconversion (That’s not a word): Both approaches require that you feed in a reference signal of a given frequency, but if the frequency you are trying to synthesize is higher than the frequency of this reference, you are upconverting (That’s not a word either!).
Now to be fair, these are NOT hard and fast rules, almost every line in this table has some nuance that I’ll discuss in my next post 2 posts.
If you’d like to learn more about the guts of a DDS engine, check out the tutorial we put together here; you’ll find some slides with audio content that you can go through at your own pace.
If anything above raises a question - ask it; I’ll try to include answers in my next post as well.