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. Therefore an improvised way of measuring phase noise was needed. This result eliminated the noisy power supply theory at least for now. This test scrubbed yet another possibly noisy candidate from the list! 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.
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My homebrew regulators are using a band-gap voltage reference that is de-noised with an RC filter cutting off at 0.
One of the big questions remaining now is why ADI cannot reproduce the noise av9910 that I can measure so easily with two different but simple and reliable methods in my home-lab! This frequency is chosen because of the availability of inexpensive standard crystals needed for the crystal filter used in this method. With the excess noise blard confirmed by two entirely different measurement methods it is needed to find the root of 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. Save to parts list Save to parts list.
The following graph is grabbed from the datasheet of the AD 1. The following experiment is finally the start of a breakthrough in nailing the source of the noise. In order to eliminate the reference clock as the source of the problem the following simple tests have been done:. The analyzers noise figure is too high and its dynamic range is too limited to be able to do this. Apart from these points, the problem can also be caused by the measurement methods themselves.
Each time the DDS sideband noise was measured for any changes. Evaluqtion product detailed below complies with the specifications published by RS Components.
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.
Given the impressive spur results, or better said the lack of spurs with the AD, I was getting convinced that the AD board was evaouation winner! Also the cutoff point is at roughly the same offset. None of the experiments made so far had shown a significant change in the noise plateau observed with the AD At this point there is quite a bit of evidence that the excessive sideband noise is originating from my AD DDS prototype board, now that the receiver is not part of the measurement system anymore.
There are two strong effects.
Analog Devices Direct Digital Synthesiser (DDS) Evaluation Board for AD9910
My AD prototype board is a simple double sided board. The Micrel MIC is marketed as “low-noise” LDO although ad9190 graph tells that this must be some kind of relative definition of “low-noise”.
Furthermore, the Agilent SSE also uses the mixing method. Although chances are slim when two different methods produce quite similar results, it cannot be ruled out completely. The evaluation board now at my disposal had a production chip soldered to it. However there is a vast improvement possible when the bypass capacitor is added.
With the 3Hz resolution bandwidth of the analyzer, its noise floor is reached at 50Hz away from the carrier. Now the pieces of the puzzle are finally coming together.
Without the bypass capacitor, the AD is noticeably worse than the AD, but it shows the same characteristic plateau effect with a cutoff frequency at an offset of around KHz from the carrier. 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.
The reciprocal mixing method measures single sideband noise including phase noise, when the LO is embedded in the receiver. One of the methods, which is especially convenient when measuring LO phase in a receiver, is the Reciprocal Mixing Method. wd9910
EVAL-AD Evaluation Board | Analog Devices
The Quadrature Mixing Method. The phase noise of the TCXO is unspecified. Add to a parts list. Can AD be fixed? The results together with the baseline phase noise and the residual noise of the AD at In retrospect this is all rather logical.