Controlling image tones on ZCU208

[reki]

I am trying to send signals on my ZCU208 board, and am new to the field of signal processing. My board is the ZCU208, and I am using the base overlay. I have changed it slightly to get a sense of how the development process works, but I only changed small things such as adding all the DAC and ADCs into the overlay, which shouldn’t effect my question.

When I send a signal out of a DAC using `dac_block.MixerSettings[‘Freq’] = x’, it is accompanied by image tones ~25 dBm down from the main signal . I would like to minimize these tones. One way I have read I can do this is to change the sample rate, which might move the tone up in frequency enough I can filter it out effectively. I would like to do this using dac_tile.DynamicPLLConfig().

Is there some function I can find to predict what a given sample frequency will do to the frequency of image tones for a given mixer frequency? I have read a few resources on signal processing, but this is still unclear to me.

Other settings I want to learn about with respect to images of CW tones include the datapath mode and its effect on these tones. Can I change modes from python in the overlay, or do I need to add hardware infrastructure? Does the interpolation rate effect image tones? How do I determine a good interpolation rate for a given frequency I want to transmit?

Again, I am new to this, so I welcome any and all resources you can point me to. I have read documentation pages such as PG269, DS889, DS926, UG1433, and a few others, but part of my problem is that I don’t fully know what to look for in these documents.

[joshgoldsmith]

Hi @reki

I would suggest downloading the free RFSoC book for information on this. In particular, the chapter regarding Frequency Planning.

The phenomena you are seeing are called spurs. They can be a bit of a pain to deal with at first but, once you understand the theory, you will be able to deal with them easily.

There is an accompanying notebook, which contains a Python-based frequency planner tool. You can use this to calculate where expected spurs will be on the spectrum and you can use the controls to find a favourable configuration that will keep the spurs outside of the signal of interest.

To completely get rid of spurs you will need to use filters. For DACs, this filtering is performed with analogue filters, typically something like the attached image. The idea with frequency planning is to move spurs far enough away from the main signal such that a simple low-, high-, or band-pass filter will attenuate the spurs.