The mixer method would also allow for much more flexibility like measuring at different frequencies and also much closer-in than was practically possible with both the notch and the reciprocal mixing method. It now became possible to directly measure the single sideband noise of the AD DDS prototype board at It occurred to me that the noise plateau of the AD resembled the noise plots of the LDO regulators! The unfiltered band-gap reference noise however is modulating the DDS signal purely with its amplitude, because it sets the DAC bias current directly! A low noise, wideband opamp is used as the error amplifier with unity closed-loop gain. This demonstrates that a general-purpose spectrum analyzer is not good enough to show the noise sidebands of clean signal sources like the AD DDS.

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The Quadrature Mixing Method. Furthermore, the Agilent SSE also uses the mixing method.

PM-noise cannot be filtered away ad910, but AM-noise can be scrubbed of by carefully clipping the carrier with a squarer. After all, the evaluation board is the very same board that has been used to produce those excellent residual and absolute phase noise plots in the first place with!

EVAL-AD Evaluation Board | Analog Devices

Therefore, the reference clock is efaluation possible although unlikely suspect of introducing the excess noise measured with the AD prototype board. The analyzers noise figure is too high and its dynamic range is too limited to be able to do this. The answer to the question whether it is an insignificant design flaw or a pretty serious one that the AD produces orders of magnitude more AM-noise than its documented PM-noise, depends on the application where the chip is used for.

The phase noise of the TCXO is unspecified.

In order to leave these extra factors out of the measurement a different method was needed to drill down further into the problem. The tests with that board also proved that the pre-production chip is not causing the problem. Add to a parts list.


Local Oscillator

Crystal Notch Filter Blard. In my opinion, with a DDS chip that has bit amplitude control built-in, AM-noise is an important device parameter and ADI should have documented it completely like is done with PM-noise. ADI had been somewhat reluctant so far to accept my measurements, which is understandable given the reputation of the signal source analyzer from Agilent they use. This demonstrates that a general-purpose spectrum analyzer is not good enough ad99110 show the noise sidebands of clean signal sources like the AD DDS.

With the excess noise now confirmed by two entirely different measurement methods it is needed to find the root of the problem. So the change could even be made evluation a backward compatible way without invalidating existing designs that don’t care so much about the AM-noise. On the other hand if you really need a clean sinus wave output with or even without bit resolution AM, and excess AM-noise cannot be tolerated, the reality is that then there is a problem.

At that point I ecaluation to homebrew a simple PM-noise test-set in order to validate the results with yet a third and completely different measurement ad99110 They produce relatively much noise up to a few hundred KHz out and then drop off quickly towards the thermal noise floor. So ADI’s DDS design lab was contacted with a detailed report on the AD noise plateau and the effort made so far to find the source of the problem, including full details on the 2 measurement methods.

Analog Devices Direct Digital Synthesiser (DDS) Evaluation Board for AD9910

Just scrub of the AM-noise with a squarer and use the square wave instead of the sinus wave output signal. The Manufacturers reserve the right to change this Information at any time without notice. It seemed rather unlikely, but maybe also the PCB layout could play a role in the noise especially with the low noise levels we are talking about. Sending feedback, please wait The Micrel MIC is marketed as “low-noise” LDO although the graph tells that this must be some kind of relative definition of “low-noise”.


Therefore I was not too hopeful that the 4-layer evaluation board would behave much better than my 2-layer design. This turned out to be the case.

There are two strong effects. Once the board had arrived in the mail, I could not wait to power it up with my laboratory bench supply and in the next couple of days I repeated all the measurements made earlier on the double sided AD prototype board. This is a subject of a further study that I hope to report on soon! The reciprocal mixing method measures single sideband noise including phase noise, when the LO is embedded in the receiver.

The current state of the AD chip without the DACBP pin blows away for instance the plans to easily improve the 2-tone test generators I use for IMD3 measurements with a better much lower phase noise, much lower spur solution based on the AD Another elegant and simple method to measure sideband noise, including phase noise, is the Crystal Notch Filter Method.

It now became possible to directly measure the single sideband noise of the AD DDS prototype board at This is the first indication that we are not dealing with phase noise at all!