Clock Adjust (Digital Demod)

Range: -0.5 symbols to +0.5 symbols

Clock Adjust determines when the VSA's digital demodulator samples the I/Q trajectory.

The digital demodulator used in this VSA does not require symbol-clock timing signals to determine the location of symbols (detection decision points). Instead, the digital demodulator uses an algorithm to determine symbol locations.

Some digital communications systems contain non-linearities that can bias the digital demodulator's estimation of the symbol clock position. Use clock adjust to compensate for this "offset" and obtain a lower EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. (Error Vector Magnitude).

The clock adjust, or offset, is entered in symbols. By default, the offset is zero (0) symbols. An offset can be specified from -0.5 to +0.5 symbols.

To see the effects of changing clock adjust, display a constellation diagram or eye diagram. Increasing (or decreasing) clock adjust:

Shifts the detection-decision points in the constellation diagram.
Slides the eye diagram right or left (remember that in the eye diagram, the symbol locations remain in a fixed location on the display; therefore, changing the location of symbols has the effect of horizontally shifting the eye diagram).

Additional Tips

Here are some additional tips that may help in using clock adjust:

Specifying a clock adjust only affects the I/Q Meas trace data. It does not affect the I/Q Ref trace data.
Use the eye diagram with an eye length of one (1) to observe the accuracy of the symbol clock timing. The EVM (Error Vector Magnitude) in the symbol table summary can be monitored while adjusting clock adjust to obtain the optimum symbol timing.
Clock Adjust is reset to 0.0 when Preset is selected.