SNA Output Level – Update

10 Apr

In my last posts, I reported that there was a severe roll-off in RF level that I hadn’t seen with the AD9850 in the past. I attributed it to the cheap Chinese Ebay DDS module and went about designing a workaround. I spent all week procuring, mounting and programming a MCP4018 I2C DigiPot to control the DAC current (and thus the output level) under firmware control. My plan was to build a lookup table that automatically programs the DigiPot ‘on the fly’ to correct level variations.

Well I got almost all the way through that process and was looking at how well the DigiPot controls the RF level when I discovered there was no significant uncorrected variation anymore! Well, it turns out that my initial testing was done with a random scope probe that I pulled out of a pile of old probes I had nearby. This probe had a significant high-frequency rolloff that led me to the wrong conclusion. When I measured again with a good 100 Mhz probe and confirmed with a spectrum analyzer, the rolloff was only a couple dB, easily within the correction range of the SNAP! software’s ‘normalize’ feature. I now have a couple of brand new probes on order and all the old ones are going in the trash.

I think I’ll leave the DigiPot in the hardware design as a build option and maybe use it as a software-controlled RF output level adjustment. Meanwhile, I’ll finish building the whole rig into the Altoids tin and call it a day on the hardware design. Then I can move on to getting everything documented and released to the web site.

Joe

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One Response to “SNA Output Level – Update”

  1. Ian J Downie April 10, 2015 at 6:29 pm #

    Like you, I may have to build a scope probe calibrator with a “known” output level and shape, the in built test point on my scope is too slow for HF and above. it’s only 2v p/p and 1MHz square wave.

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