Waveform Frequency Sweep resolution, phase

[greig]

In the waveform generator, with the sweep option, it appears I can type in any start and stop sine wave frequency up to 10 MHz.  I am interested in using this as an FM chirp for ultrasound type applications.  When I specify a given sweep time, say 1 ms, and start and stop of 500kHz to 1.5 MHz, what is the frequency resolution, (or at what time scale) does the frequency change?  Secondly, is this done in a phase continuous manner?

In a related question, for the network analyzer, the output appears to step between frequencies.  Is it the case I can control the sweep time by the settle time tab and  and min periods tabs under Wavegen.  Again it appears I can list and arbitrary start/stop frequency and number of frequencies. Is each synthesized frequency controlled at some low buffer lookup table, or is it directly generated in the FPGA acting much like a higher resolution DDS.  I am trying to ascertain the spectral purity of the signals, and the extent I should add a bandpass filter to sweep over a 1-2 MHz bandwidth centered in the low single MHz range.

[attila]

Hi @greig

The Wavegen sweep creates a linear continuous frequency sweep. For other type of sweeps use the FM (frequency modulation), select Modulation after the Enable checkbox.
On Analog Discovery the carrier waveform has 4k sample buffer (16k in 4th device configuration) of 14bit resolution. The modulations use 2k sample 9 bit resolution buffers.
The carrier buffer index increment is changed by FM continuously. With 100MHz DAC rate, a sweep from 500kHz to 1.5MHz will change the increment from 1/200 to 1/66.66
Having 9bits for modulation this can change the frequency in 512 levels, like: 1kHz, 2kHz, 3kHz... 512kHz
The delays are used for low frequencies.

From the Network Analyzer help: "The analysis is performed from start to stop frequency in the specified number of steps. For each step, the WaveGen channel is set to a constant frequency and the Oscilloscope performs an acquisition. "

See the following related posts: