Re: DDS spectral "purity" ?

George N2APB
 

Hi guys,

FWIW, I tried many different output amplifier circuits in the early days of my DDS-xx plug-in modules (DDS-30, DDS Daughtercard) in order to provide clean-spectrum at the desired max output level (+16 dBm) … ranging from 2-stage discrete bipolar transistors, to 4 different MMICs, to the LT1253 op amp … and ultimately settling on the use of the AD8008, which is still with the DDS-60 module today.

I was convinced that the downfall of (my) attempts with the MMICS back then was the UHF parasitic oscillations (that are so hard to tame in homebrewer/experimenter designs) that were mixing down to create a lot of ‘spectral gudge’ in the HF region of interest. Further, those oscillations helped to make those MMIC devices run hot, and provided unwanted “energy” that would be detected in the wideband reflectometer circuits in which the DDS card was being used as a signal source, thus providing misleading measurements of Return Loss, Z, etc.

So, with the assistance of Jim Kortge back then, we carefully crafted a low-noise op amp design that seemed to provide the best results overall for the design goals … and that has stayed to this day. (And I also source the AD9851 chips directly from Analog Devices to ensure that I’m not getting noisy/fall-out parts that the offshore suppliers might provide.)

73, George N2APB

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From: PHSNA@... [mailto:PHSNA@...]
Sent: Sunday, July 10, 2016 5:35 AM
To: PHSNA@...
Subject: [PHSNA] Re: DDS spectral "purity" ?


Hi all,
Over the last week or so I have been doing quite a lot of measurements myself. They also confirm that the MMIC ruins what is otherwise quite a good spectral output, and I am thinking about various ways to fix it.
So I would have been very interested to see Jim's video. Unfortunately though clicking on the link does not run the file.Evidently my browser does not know what to do with that extension. (I haven't come across it before either.)
Related to this; I discovered a long time ago that the chinese modules (and possibly the AD985x itself) generates a whole forest of junk at about -60dBm (ish) while it changes frequency, and for about a second or so afterwards. In any sweep mode it does this all the time of course!
Tony (G3PTD)...

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