I would like to get a linear FM output from the AD9910. But it should be capable to give a positive or negative chirp on demand.
I would have to correct my statement above.
As DSB has advised me, it is actually POSSIBLE to do a negative sweep in AD9910. What should be done is to make use of the image frequency.
For example, if your system clock is 1000MHz and you want to sweep between 100MHz and 200MHz and start at 200MHz, you could enter 800MHz on the lower limit and 900MHz on the upper limit. This satisfies the lower value that should be on the lower register. However, the output will be 200MHz for the 800MHz entry and 100MHz for the 900MHz entry.
When using the DRG of AD9910, it is NOT possible to start with a negative sweep.
That is because, DRCTL pin controls the rising or falling of the sweep. If DRCTL=0 at initial, the DRG output is initialized at the lower limit. Please refer to the diagram for the clearer picture.
One way I can think of is for you to hop to the upper limit first then proceed to the falling sweep.
Hope this helps.
Thank you Sittie. You are a life saver!
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