A low noise, wideband opamp is used as the error amplifier with unity closed-loop gain. In retrospect this is all rather logical. When I designed my 2-layer board I have thoroughly examined the evaluation board gerbers available for download at www. The DAC bias is defaulted to 20mA with value That is dB relative to the 1. In that configuration the DDS signal is squared up with the 74AC04 squarer on the frontend board before it drives the FSA H-Mode mixer switches, which at least in theory should have scrubbed of the AM-noise to a large degree. Please select an existing parts list.
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Thank you for your feedback. Therefore, the reference clock is a possible although unlikely suspect of introducing the excess noise measured with the AD prototype board. Notch Filter Method results Another elegant evaluuation simple method to measure sideband noise, including phase noise, is the Crystal Notch Filter Method. The Manufacturers disclaim all warranties including implied warranties of merchantability or fitness for a particular purpose and are not liable for any damages arising from your use of or your inability to use the Information downloaded from this website.
With the excess noise now confirmed by two entirely different measurement methods it is needed to find the root of the problem.
EVAL-AD Evaluation Board | Analog Devices
Usually, more expensive 4-layer boards are deployed with such an application, which for sure is more convenient when doing the layout and gives much more freedom in assigning complete layers as power planes and ground planes.
This is new information. Given the impressive spur results, or better said the lack evalustion spurs with the AD, I was getting convinced that the AD board was a winner!
The expectations in evaaluation “phase noise department” were pretty high because of the estimated performance of the Wenzel based 1GHz clock source and the residual and absolute phase noise graphs provided by ADI. 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.
In order to leave these extra factors out of the measurement a different method was needed to drill down further into the problem. There is a remarkable resemblance with this graph and the observed AD noise plateau! But still the evidence is quite convincing that a big capacitor is really needed on a “DACBP” pin to take out boad noise plateau caused by the internal band-gap reference of the DAC. Save this item to a new parts list. AD chip revision Suspicion existed regarding the actual DDS chip revision being used on my prototype board, because it is a pre-production chip obtained through evluation early sample program before the AD was officially released.
One eavluation the methods, which is especially convenient when measuring LO phase in a receiver, is the Reciprocal Mixing Method. PCB Layout 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.
The five main areas to look for the source of the noise are: It is unclear at this moment why the squarer on the frontend board does not seem to sufficiently suppress the AD’s excess AM-noise. 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.
None of the experiments made so far had shown a significant change in the noise plateau observed with the AD The five main areas to look for the source of the noise are:. Therefore I was not too hopeful that the 4-layer evaluation board would behave much better than my 2-layer design. In general PM-noise is much more important than AM-noise.
boadd Add to a parts list. If the internal band-gap device is not thoroughly filtered, then the noise will end up directly in the output signal of the DAC. Just scrub of the AM-noise with a squarer and use the square wave instead of the sinus wave output signal. The result, the red series, does not even come close to the required level, the blue series, let alone the maximum achievable plot, the green series.
Whether it will be actually done is something that only ADI can tell! Also the cutoff point is at roughly the same offset.
Analog Devices Direct Digital Synthesiser (DDS) Evaluation Board for AD9910
However there is a vast improvement possible when the bypass capacitor is added. Save to parts list Evalutaion to parts list. Second, the cutoff frequency offset changes by as much as an octave from KHz to KHz, which is very significant!
It also shows that the reciprocal mixing method is working quite well, because the notch filter method confirms its results.
In fact the single sideband noise of the AD is about 20dB stronger than we could theoretically hope for given the residual measurements at This frequency is chosen because of the availability of inexpensive standard crystals needed for the crystal filter used in this method.