DDS wisdom

Rickman just wants to argue and insult people. Better to ignore him.

I don't mind arguing, but arguing without plugging in the numbers to try to understand the example is, *ahem*, unproductive.

Cheers

Phil Hobbs

So why don't you want to discuss a real example then? You picked three numbers that were not possible, they don't add up... or divide up actually. Do you wish to pick some real numbers? I would prefer to use numbers I can work with on my calculator so a modulus of less than 2^32 would be better.

In all cases Fclock/Fout = Modulus/StepSize. When you talk about "relative prime" modulus and step size while the clock and output frequencies are in a ratio of 40, you have picked impossible numbers.

I find it interesting that Larkin won't discuss this either but he is happy to jump in and insult people.

Get a better calculator. ;)

c:\>mc2 2**48/7036874417767

That will make 10 MHz with a 48-bit, 400 MHz DDS such as an AD9956.

All I need for the argument is that the increment is relatively prime to

What part don't you like?

Cheers

Phil Hobbs

Ok, let's ignore that this does *not* produce 10 MHz. I guess it is close enough. 10.000000000000900E+00 according to my spread sheet.

DAC irregularities are not something that the DDS is about really. That is an issue of the DAC in question, no? I'm not sure the time scale would need to be days or whatever. I expect they will be more related to the input clock period. Regardless this comes from the fact that it is producing an analog signal. You have the same problem in using amplifiers or any other analog component.

Is that what you wish to discuss, the DAC caused spurs? I didn't get this from the referenced discussions, but then they were all over the map with a lot of imprecise language, so hard to tell.

I'll summarize this again for clarity. There are three sources of errors (spurs) in a DDS. Errors from phase truncation, limited amplitude resolution and if you are using a DAC to produce an analog signal, DAC related errors.

The phase truncation errors are easily avoided by not truncating the phase which limits the producible frequencies. However, there are still many ways to produce the frequency of interest. For all DDS, or digital circuits for that matter, the frequencies must be related by integer ratios - "rational" ratios. But if you limit your design to say, modulus of 2^N, you further limit the possible output frequencies.

Limited amplitude resolution is also an inherent feature of digital designs. However, the spurs produced can be made arbitrarily small by using larger word sizes. No design, analog or digital, is without spurs. In digital solutions the spurs are easily controlled.

DAC related errors are not really digital issues since they are from the analog portion of the DAC and depend on the details of the design and implementation. If you need an analog output they are necessary. But they are analog components and the issues involved are largely analog. Just as is true with any analog component there are methods of design that can minimize these errors but they are indeed inherent and can not be made arbitrarily small.

In the end - you can eliminate phase generated errors altogether, amplitude related errors can be reduced to any extent you need while DAC errors can be minimized, but not to arbitrarily small levels.

Ricky made the argument that as long as M and S are integers, the ratio is rational (gosh!) so the DDS has no spurs. It follows that no DDS can ever have spurs.

The DAC is on-chip. We're talking reality here. You might want to go read a couple of datasheets, e.g. that very nice AD9956 under discussion.

I just calculated that it could be up to a week, with very plausible and typical values. You could make it as long as months if you picked some odd increment closer to 2**47 (200 MHz).

You're weaselling. As I said, it's an analogue issue, because the ADC waveform won't repeat exactly (even in the spherical cow universe) for more than a week, so you can get spurs that are only a few microhertz from the carrier.

No, because there isn't the periodicity issue.

As I said above, >>>>>>>> The DAC values may repeat more often than this, or >>>>>>>> very nearly repeat (which is what Gerhard was talking >>>>>>>> about on time-nuts) but there is the potential for DAC >>>>>>>> nonlinearities and slewing effects to produce phase >>>>>>>> and amplitude perturbations on time scales of hours to >>>>>>>> days. >>>>>>>> >>>>>>>> Since time and frequency can be measured to absurd >>>>>>>> accuracy, it's quite possible to get easily >>>>>>>> measureable phase errors at surprisingly long time >>>>>>>> scales.

I'm not sure what you find unclear about that.

If you throw away almost everything that makes the DDS attractive in the first place, and reduce it to a divide-by-N counter, you can get rid of all the spurs that aren't harmonics of the output frequency, sure. But then why not use a frequency divider and save $20?

Cheers

Phil Hobbs

Notice how Larkin won't discuss this with me directly. I never said what he claims I did say. But when it comes to phase calculation related spurs, I have stated clearly that this is caused by phase truncation. At one point I was confusing some of the issues and was saying the modulus had to be an integer multiple of the step size. I corrected this in later posts.

If you use the full phase accumulator in calculating the sine from the phase, there will be no resulting spurs from the phase.

Yes, in a DDS chip the DAC is on most DDS chips, but the fact remains that it is not an inherent part of a DDS function. There are digital DDS functions which are all digital such as the one discussed previously in the Timing Solutions design. Their patent even says it included a DDS, but it does not feed a DAC, it feeds a multiplier to do the mixing.

If you want to discuss imperfections of DACs then we can do that, but don't blame them on the DDS circuit that feeds it even if it is on the same chip. I don't think this is semantics. I don't see any value in mixing the three causes of spurs.

You are talking about the cycle time of the digital pattern. But this pattern does not create spurs. All of these numbers are exact.

First, why not dispense with the personal comments, ok? No need to call names. If you don't want to discuss this, then let's stop.

When you say the waveform won't repeat "exactly" how does that cause spurs? You mean the digital words won't repeat exactly, but that is irrelevant. All that matters is that they don't have error.

Any spurs you see at the output of the DAC are due to aliasing from the sampling or imperfections from the DAC. A good enough anti-alias filter should reduce the anti-alias problem as needed. Of course an anti-alias filter can cause other issues you may not like.

Irrelevant. Amplifiers have distortion just as any analog part does. Periodicity is not important to that. Distortion in the amplifiers will be related to the signal frequency. Distortion in the DAC will be related to the signal frequency as well as the clock frequency since the clock shows up in the signal until it passes through the anti-alias filter.

Please separate the DAC distortion effects from the digital issues. How would the DAC values (I assume you mean the sine values) repeat more often than the phase values unless the phase is being truncated? If the phase is truncated that *will* produce errors which produces spurs.

You are over simplifying the DDS. A DDS without spurs is not a divide by N counter.

I can't tell if you don't understand, or *what* you don't understand or if you are trying to understand what I am posting. You don't seem to point to anything I say to show where it is faulty. You just disagree and post something that is not really relevant. Are you trying to get what I am saying? Should we continue or is this just an argument at this point?

The distinction between periodic and aperiodic errors is the distinction between a spur and the noise floor.

You're moving the goal posts, or maybe you really don't understand my point here, so I agree we should just let it drop.

Cheers

Phil Hobbs

I'm not sure why you brought up the "aperiodic" errors. I have explained my view on this fairly clearly and you don't dispute anything I explain. You seem to want to bring in things that aren't well defined and don't stay on topic when I try to clarify them.

But I am happy to let this drop.

That's how Yahoo operates. Buy a company, destroy it and write it off. They did it to Geocities, Broadcast.com, and others.