With the 3Hz resolution bandwidth of the analyzer, its noise floor is reached at 50Hz away from the carrier. The noise sidebands of the AD DDS prototype board are measured using this method when tuned to 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”. Each time the DDS sideband noise was measured for any changes. Because it is impossible to test the effect of a decoupling capacitor on the AD by the lack of the very pin, I reverted to the AD to see how much effect the bypass capacitor can give with that device.
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EVAL-AD Evaluation Board | Analog Devices
The reciprocal mixing method evaluates the DDS while it is embedded in the receiver. Suspicion existed regarding the actual DDS chip revision being used on my prototype board, because it is a pre-production chip obtained through the early sample program before the AD was officially released. This seems like a relatively simple change to an otherwise great DDS chip. Apart from eva,uation points, the problem can also be caused by the measurement methods themselves.
The reason why ADI did this “optimization” is unclear. The noise sidebands of the AD DDS prototype board are measured using this method when tuned to But still the similarity between the noise plateaus of the LDO regulator and the AD cannot be ignored.
When I designed my 2-layer board I have thoroughly examined the evaluation board gerbers available for download at www. Given the lower working frequency of The reciprocal mixing method showed the noise plateau from the start. Because it is impossible to test the effect of a decoupling capacitor on the AD by the lack of the very pin, I reverted to the AD to see how much effect the bypass capacitor can give with that device. Notch Filter Method results Another elegant and simple method to measure sideband noise, including phase noise, is the Crystal Notch Filter Method.
Analog Devices Direct Digital Synthesiser (DDS) Evaluation Board for AD9910
There is absolutely no difference of the resulting noise plots within the accuracy of the measurement method 1dB. The noise level of such a voltage regulators is very low, roughly identical to the input noise voltage of the opamp, which in this case is only 4. 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.
It really only measures PM-noise.
This is the first indication that we are not dealing with phase noise at all! First, the plateau comes up consistently by 2 to 4dB’s, which is significant. It also shows that the reciprocal mixing method is working quite well, because the notch filter method confirms its results. There is a remarkable resemblance with this graph and the observed AD noise plateau!
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.
I replaced the onboard regulator voltages one by one except 3. In order to leave these extra factors out of the measurement a different method was needed evaluatino drill down further into the problem. Furthermore the really low spur levels gave confidence in the double sided PCB layout and the other design choices made with this board. The unit is only specified for good frequency stability, therefore its phase noise is probably not in the same class as the Evaluatiln oscillator, but still no difference in the end result.
Can AD be fixed? In order to confirm or eliminate this theory I have designed and build “ultra-low-noise” discrete voltage regulators, because no truly “low-noise” off-the-shelf regulator IC’s could be found!
The product detailed below complies with the specifications published by RS Components.
So already from the start, AM-noise was not considered to be an issue at all, which turned out to be a wrong assumption. This is a subject of a further study that I hope to report on soon!
This turned out to be the case. If the internal band-gap device is not thoroughly filtered, then the noise will end up directly in the output signal of the DAC. A hoard noise, wideband opamp is used as the error amplifier with unity closed-loop gain. Evaluatkon that point I decided to homebrew a simple PM-noise test-set in order to validate the results with yet a third and completely different measurement method: The five main areas to look for the source of the noise are:.
This firmly indicates that the reference clock is not the cause of the excess sideband noise of the DDS.