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Chirp Generation on AD9914 DDS

Hi all, 

I'm trying to understand the parameters of the chirp generation on AD9914, mainly the rising and falling steps and rate. Am I correct if for example on the evaluation software the boxes for rising step and rising rate are used to generate a say 10ms chirp, and the falling step and rate are zero (or near to zero) then the generated chirp is a typical sawtooth chirp, figure a, below. If the falling steps and rates are then used, the chirp is a triangular shape? 

If triangular chirp is generated, is there anyway to implement a short constant frequency between the rising and falling frequencies? (figure c below) 

The other question I have is, are there guidelines or any analysis on what the steps and rate should be for a given bandwidth? 

Cheers



Edit
[edited by: AAmiri at 1:50 PM (GMT -5) on 4 Feb 2022]
  • The answers to your questions follow, but first, be aware of a bug under the SWEEP tab regarding the Rising/Falling Sweep Ramp Rate entry boxes. Namely, the function of the boxes is reversed. What you enter for a rising rate, the software loads into the falling rate register, and vice versa.

    To obtain sweep profile a), simply check the No Dwell High box. Then, when you initiate an up sweep, the DRG will return to the lower sweep limit upon reaching the upper sweep limit. Note the falling sweep parameters are rendered meaningless in this mode. Note you can also generate an inverted version of a) by checking the No Dwell Low box, instead.

    To obtain sweep profile b), simply check both the No Dwell High and No Dwell Low boxes. This causes the DRG to generate a continuous up/down ramp. You have independent control of the up/down slopes via the step size and step rate parameters. Note the slope is result of two parameters: step and rate. So there are a multitude of ways to implement the same slope. For most applications, for a given chirp range (lower & upper limit) and a given chirp period, it makes sense to use the smallest step size and the fastest step rate that meets the range and period requirement.

    To obtain sweep profile c) requires more intervention on the part of an external "controller". The AD9914 does not have a built-in "dwell time" feature. That said, the first step for obtaining sweep profile c) is to make sure the No Dwell feature is disabled (that is, both No Dwell boxes are unchecked) and program the desired up and down slope settings. To get the dwell times, the controller needs to know the up/down chirp periods, which are deterministic (a function of the step size and rate values). In operation, the controller would trigger an up sweep and the AD9914 would automatically dwell at the end of the up sweep. The controller has to keep track elapsed time so it can trigger a down sweep at t = up period + dwell time. Likewise for the down sweep. This, of course, would be a repetitive process.

    There are no "guidelines" for setting the sweep parameters. It is completely dependent on the requirements of the specific application.

  • Thanks for the info, Kenny. 

    For the sweep profile a), how can I make it continuous? 

    Thanks 

  • 1) Program the desired up slope.

    2) Program a very steep down slope (1 or 2 frequency steps at the highest rate).

    3) Set both No Dwell bits.

    The DRG may start ramping automatically as soon as you set both No Dwell bits (I can't recall for sure). If not, then force an edge (polarity doesn't matter) on the DRCTL pin to start the DRG.

  • Fantastic, thank you.