| The Phase Analysis tool is used for analyzing the characteristics of phase modulated signals, and to a limited extent, M-ary PAM signals. The Phase Analysis tools menu really consists of three tools tightly integrated into one menu. These are Asynchronous mode phase plane, Synchronous mode phase plane, and phase modulation symbol rate tool. When starting signal analysis, one is trying to determine the characteristics of an unknown signal. The normal starting point for this is FFT. To make this easy, the FFT is available directly from the Phase Analysis tools menu. The FFT is used for determining an estimate of the signal center frequency, and signal bandwidth. When called from the Phase Analysis menu, the "Hold/Measurem." menu of the FFT has the option to "Set Filter". This takes the information from the measurement cursors, and uses this information to configure the center frequency and bandwidth. This setting is then used for the phase analysis tools. This sets up a pre-filter, allowing the tools to be used on signals where there is out of band interference, or when there are more than one simultaneous channels. The display can started and stopped by selecting "Start" / "Hold". The menu changes value depending on if the system is active. The menu displays the function that execute when selected. So when "Hold" is displayed, then the system is running, and selecting the menu will pause processing
Back to the Phase Analysis menu, the "Center: xxx" allows the center frequency for the demodulator to be adjusted. An error in the center frequency normally results in a phase plane that is rotated by an amount proportional to the frequency error. The "Hold Time" adjusts the number of points displayed on the screen. Increasing the hold time increases the amount of time a dot will remain in the image before being overwritten by a new value. In ASYNC mode, the signal is not demodulated at all. Instead, the phase of the signal is visually 'compared' with a reference signal. Providing the correct reference signal is selected, this will result in a phase display that provides an indication of the type of PSK or PAM signal. The points will trace the path taken as the signal phases change. At the nominal signal mapping points, there is normally an accumulation of data points, providing a visual clue the total signal mapping. Multi-channel DPSK signals often have a very narrow channel signal bandwidth. This results in the phase of the signal never remaining constant, and so the accumulation of signal points in the phase plane is not visible. To quantify such signals use SYNC Mode. The frequency of the reference signal is set using the "Ref I/Q: xxx" option. Note that if the reference frequency is incorrect, the display rotates at a rate that is the difference between the reference frequency, and the true carrier frequency of the signal. When adjusting the reference signal, the changes are 'live'. This means that changes you make have immediate effect, and resulting change in the phase display is immediately visible on the display. For ASYNChronous mode, the symbol rate measurement is not required. The sampling rate is independent of symbol rate. Three sampling rates are selectable. For low baud rate signals, better results are obtained with a lower sampling rate. The "Select Mode" menu item is disabled in the "Demodulator" menu for ASYNC mode. In summary, the basic steps for analyzing a suspected (D)PSK signal is as follows:
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