Spectrum analyzers typically display signals in the frequency domain, but can also be configured to display in the time domain, a feature called Zero Span.
Satsagen has been equipped with Zero Span since version 0.8.0.0.
Starting with version 0.9.2.1, the Zero Span function has been expanded and improved. In summary, the display of frequency versus time has been added in addition to power versus time, and the processing mode is no longer only by FFT, but also by IF demodulation. E.g., this process mode allows display signals up to 2 uS per division using an RTL-SDR as a receiving device.
Zero Span is activated by clicking on the Time Domain (Zero Span) button located in the Spectrum Analyzer panel.
The TimeBase knob allows you to set the time base within the range allowed by the receiving device in use and the signal processing mode settings. By extending the bandwidth controls with a click on the BW label, you can also directly set the sampling rate expressed in MSPS:
At the same time as activating Zero Span, the Time Domain tab appears below the display, where other controls related to this function are available:
The Type list sets the display to power vs. time or frequency vs. time. The Processing mode list sets the processing mode to IF demodulation, ZERO IF demodulation, or FFT. The IF decimation list sets three decimation levels, 1, 10, or 100, which allow the time base to be extended over longer times if necessary in IF or ZERO-IF demodulation modes. The Trace auto-clear button only serves a purpose if a Trigger has been selected. If there is no signal at the Trigger and the Trace auto-clear button is activated, the display is cleared after a few seconds, while if the button is deactivated, the display “freezes” at the instant of the last Trigger.
The following image is of a setup with a double device. In reception, there is an RTL-SDR dongle, and in transmission, a HackRF. A small RF cable connects the two devices. The generator modulates a 100MHz carrier in AM at 20 kHz. Satsagen is configured for the classic spectrum display:
By activating Zero Span with the same setup, the display switches to power vs. time, by setting Power vs. time from the Type list and IF demodulation from the Processing mode list:
The X-axis switches from a frequency to a time representation, while the Y-axis remains in amplitude expressed in dBm.
The image will not be still, but will scroll in one direction, as there is no synchronism between the modulation and the sampling performed. To synchronize and freeze the image, you need to activate a trigger from the Triggers tab.
The Video trigger level setting, whether positive or negative slope, should fall within a value within the modulation amplitude, in this case from approximately -48 dBm to approximately -41 dBm.
If you can’t find an optimal trigger value to get a stable image, you can try to activate a sort of hysteresis by clicking repeatedly on the Level text above the knob until it becomes Level R and a Range knob becomes available for setting the trigger intervention range:
We now switch the Generator modulation to FMW, with the same modulation frequency as before and a deviation of 30 kHz.
To view the frequency modulation, you need to go back to the Time Domain tab and select Frequency vs. time from the Type list. Furthermore, from the Triggers tab, you need to set a level between approximately -30 kHz and +30 kHz:
The X-axis remains in a time representation, while the Y-axis switches from a display of amplitude expressed in dBm to one expressed in frequency.
For correct viewing, you also need to adjust the bandwidth using the appropriate knob in the Spectrum Analyzer panel.
In the case of a frequency-modulated signal display, the minimum bandwidth value should be the maximum modulation frequency plus the deviation, multiplied by two. So in our case, the minimum bandwidth to ensure that the displayed shape does not suffer distortion is about 100 kHz.
