Troubleshooting Custom IQ Measurements

This topic contains information to help you troubleshoot measurements when using Custom IQ Demodulation. You should also review Making a Custom IQ Measurement for additional measurement information.

PROBLEM	POSSIBLE CAUSE	SOLUTION
Demodulation fails to lock on signal	Signal not present.	Check connections
	Carrier too far from center frequency.	Adjust center frequency.
	Input is over-loaded or under ranged.	Adjust input range. Typically, the best 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. is achieved when the range is as small as possible while avoiding overloads.
	Frequency span is too narrow.	Increase frequency span. The span should be wide enough to contain the entire transmitted signal, which depends on Symbol Rate and filtering.
	Signal has inverted frequency spectrum	Check the Mirror Frequency Spectrum setting (MeasSetup > Frequency tab).
	Wrong/mismatch in reference and measured IQ points	Verify the modulation format is correctly defined by the Active Constellation.
PULSE NOT FOUND error message	Search length too short	Increase search length, or adjust triggering to ensure that the leading and trailing edges of a pulse are fully within the search length.
	Signal is not pulsed	If the signal is not pulsed, try to use triggering to position the preamble just after the beginning of the search length. Disable pulse search (MeasSetup > Custom IQ Demod Properties > Format tab).
	I/Q gain imbalance using I+jQ receiver.	1) Check that impedance is identical for both input channels. 2) Adjust the gains for each input channel (Click Input > Calibration > Amplifier/Attenuator check box).
EVM larger than expected	Noise or interference from adjacent signals	Reduce the span to eliminate excess noise and interfering signals.
The trace data constellation symbol location shape, such as dots, is not properly displayed.	The display driver is not functioning correctly.	Contact your PC manufacturer to see if a newer version of the display driver is available. If not, try decreasing the hardware acceleration. See the Windows documentation for instructions.