Have you got confusion about DDS linear sweep mode? Is it real linear? May it be step by step like gradin?
Thank you for elaborating it.
Yes you are right, sweep signals can be generated with analog circuitry like VCO or digitally like the DDS.
Since the input voltage to control the VCO is linear and continuous, you would also expect a sinusoidal output which is linear and continuous. Thus, indeed there exists an instantaneous frequency between lower and upper frequency because each voltage value corresponds to a certain frequency out of VCO. So we can say it is real linear sweep.
In digital (like in our DDS):
In order to do frequency sweeping, the frequency accumulator requires digital values as set on the registers.
To sweep from F0 to F1, values for Delta FTW, F0, F1 and ramp rate must be set. The falling/rising DFTW and ramp rate dictates your sweep. So, there is no continuous sweep of frequency since it just jumps from DFTW to next DFTW at ramp rate. So, we can say it is not real linear sweep (as your definition says).
Hope this clears your confusion.
Yes, the sweep mode in DDS is linear. By linear, we mean that the increment is constant based on the Falling/Rising Delta FTW (example, in AD9956 DDS) value set on the register.
Is it ok if you elaborate "step by step like gradin"? I'm not quite sure I got your "gradin" term well.
Thank you very much.
Thanks for your reply. For "Step by step like gradin", I mean the frequency increases from F0 to F1 step by step with a minimun Delta FTW like 1Hz. In real linear sweep mode, frequency should be continuous and any accurate frequency between F0 and F1 exists physically. The analog VCO can generate this kind of linear sweep as long as motivated by a linear control voltage such as triangle wave. At the sight of Delta FTW, I got confused about its' linear, is it real?
I'm not sure if I got your question correctly but I hope this would be of help. In the DDS case, F0 and F1 may approach or hit the exact value wherein the accuracy is limited by the resolution of the DDS. A DDS can be viewed as a frequency divider that has a divisor which are integers from 1 to 2^n . The divisor is an integer, not a real number unlike the continuous counter part and it is a consequence of the fact that DDS is digital. The "VCO" in the DDS is actually a DAC. The data being fed to the DAC is the waveform plots of the sinusoid which is derived from the phase information, the FTW. In order to do a frequency sweep, what the DDS does is to increment/decrement the FTW at a rate submultiple of your CLK frequency. So there is no control voltage in DDS, there is only an increment or decrement of FTW between F0 and F1. This also implies that the frequency sweep is not continuous, rather it is discrete-time.
Thanks a lot!
I got it. That is to say a 32-bit DFTW DDS such as AD9554 can generate better-linearity sweep than AD9556 with a 24-bit DFTW, I am right?
Yes you are right!
That is because you now have 8-bits more resolution in AD9954 than in AD9956. This means much smaller frequency increment.
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