Q: Is it possible to start a negative sweep using the digital ramp generator of the DDS?
A: Yes, it is possible. Most of the DDSs of ADI are capable of frequency ramp or frequency chirp. One way to perform this is using the on-chip digital ramp generator (DRG) present in some the DDSs. Although not all DDSs perform sweeping the same way, the following are common parameters set for sweeping function:
- Start frequency (Startfrequency)
- Rising step size
- Rising ramp rate
- End frequency (Endfrequency)
- Note: Some don’t specify the end frequency and instead use the number of increment registers.
- Falling step size
- Falling ramp rate
One requirement is the start frequency must always be less than the end frequency (Startfrequency > Endfrequency). So one would expect that the initial sweep of the generator is rising, that is from start frequency to end frequency. However, there are instances in which a falling sweep is needed at initial ramp. Since we are limited to the register value that Startfrequency > Endfrequency, this can be done by using the image frequency or alias frequency.
The DAC image of the frequency output occur as a result of the sampling function and are attenuated by the sin(x)/x function.
Image frequencies are located at N*FSYSCLK + M*FOUT, where M=+/-1 and N = 1,2,3,…
Take note that a low pass filter with cut-off frequency of 45% the system clock is to be implemented at the output of the DDS to capture frequencies at the 1st nyquist zone. Special filter must be implemented if images higher than the nyquist frequencies are needed.
To illustrate, here’s an example.
System clock is 1000MHz
Sweep starts at 300MHz and ends at 100 MHz
You could enter 700 MHz for the lower limit (Startfrequency) and 900MHz for the upper limit (Endfrequency). This satisfies entering a lower value in the start frequency register. The available output would then be 700MHz to 900MHz. However, because of the low pass filter, what’s being captured is the image frequencies at the first nyquist zone, that is the 300MHz and 100Mhz. So the output will have an actual falling sweep from 300 MHz to 100MHz.