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. AM-noise versus PM-noise One of the big questions remaining now is why ADI cannot reproduce the noise plateau that I can measure so easily with two different but simple and reliable methods in my home-lab! At 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 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 The Manufacturers reserve the right to change this Information at any time without notice. Now the pieces of the puzzle are finally coming together.

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One of the methods, which is especially convenient when measuring LO phase in a receiver, is the Reciprocal Mixing Method.

The noise sidebands of the AD DDS prototype board are measured using this method when tuned to AM-noise versus PM-noise One of the big questions evaluatino now is why ADI cannot reproduce the noise plateau that I can measure so easily with two different but simple and reliable methods in my home-lab! Add to a parts list. The Quadrature Mixing Method.

ADI could not explain my noard and were not much convinced of the existence of the noise plateau as their sophisticated test equipment did not show it. The product detailed below complies with the specifications published by RS Components. The reciprocal mixing method measures single sideband noise including phase noise, when the LO is embedded in the receiver. Excess Noise Sources With the excess noise now confirmed by two entirely different measurement methods it is needed to find the root of the problem.


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

This turned out to be the case. The tests with that board also proved that the pre-production chip is not causing the problem.

The other observation that can be made is that the notch method shows a sudden reduction in the noise level starting about KHz away from the carrier and outwards. It occurred to me that the qd9910 plateau of the AD resembled the noise plots of the LDO regulators! There are two strong effects.

The phase noise of the TCXO is unspecified.

This is new information. 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.

Therefore I put especially the notch method through quite a lot of scrutiny, but nothing wrong could be found. This firmly indicates that ac9910 reference clock is not the cause of the excess sideband noise of the DDS. Please select an existing parts list.

Local Oscillator

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. The side of the chip that is related to most of the analog functionality has actually no less than 7 unused pins available in the current chip revision.


In retrospect this is all rather logical. Second, the cutoff frequency offset changes by as much as an octave from KHz to KHz, which is very significant!

Therefore I was not too hopeful that the 4-layer evaluation board would behave much better than my 2-layer design. This frequency is chosen because of the availability of inexpensive standard crystals needed for the crystal filter used in this method. Programming this value to 0 reduces the bias current to about 8.

EVAL-AD Evaluation Board | Analog Devices

However there is a vast improvement possible when the bypass capacitor is added. Whether it will be actually done is something that only ADI can tell! The expectations in the “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.

RoHS Certificate of Compliance.

Save to parts list Save to parts list. So already from the start, AM-noise was not considered to be an issue at evalhation, which turned out to be a wrong assumption. Furthermore, the Agilent SSE also uses the mixing method.

The unfiltered band-gap reference noise however is modulating the DDS signal purely with its amplitude, because evaluattion sets the DAC bias current directly! If the internal band-gap device is not thoroughly filtered, then the noise will end up directly in the output signal of the DAC.