Troubleshooting DOCSIS 4.0 Downstream Measurements

This topic contains information to help you troubleshoot measurements when using DOCSIS 4.0 Downstream Demodulation. You should also review Making a DOCSIS 4.0 Downstream 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. The measurement hardware center frequency must be within a few subcarriers of the signal center frequency to lock properly. (The subcarrier spacing is equal to the OFDM System Sample Frequency divided by the FFT Length.)
	Input is over-loaded or under ranged.	Adjust input range. Typically, the best EVM 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 all subcarriers being analyzed.
	Incorrect OFDM System Sample Frequency.	Set correct OFDM System Sample Frequency as specified in the signal standard being demodulated.
	OFDM signal has inverted frequency spectrum	Check the Mirror Frequency Spectrum setting (MeasSetup > DOCSIS 4.0 Downstream Demod Properties > Advanced tab).
	Wrong modulation format	Set the correct modulation format in the Resource Modulation array, or set the Resource Modulation array values to zero so that the analyzer auto-detects the modulation format.
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 > DOCSIS 4.0 Downstream 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	Signal is transmitting incorrect pilot bits.	If the signal is transmitting incorrect pilot bits, the CPE level will be high, both in the Syms/Errs summary and on the Common Pilot Error trace. Turning off Pilot Track Timing, and possibly Pilot Track Phase, may help (MeasSetup > DOCSIS 4.0 Downstream Demod Properties > Advanced tab).
	Signal has longer than usual transition region between symbols	Change Symbol Time Adjustment to be more negative (MeasSetup > DOCSIS 4.0 Downstream Demod Properties > Advanced tab).
	Noise or interference from adjacent signals.	Reduce the span to eliminate excess noise and interfering signals.
	OFDM signal has inverted frequency spectrum	Try inverting the frequency spectrum (MeasSetup > DOCSIS 4.0 Downstream Demod Properties > Advanced tab). It is possible for the VSA to appear to synchronize to a signal with inverted frequency content, because of symmetry properties of the preamble of some signal types (for example 802.11a can sometimes show this).
EVM increasing from beginning of burst to end (visible on Error Vector Time trace)	Signal amplitude varies during the burst.	Turn on Pilot Track Amplitude (MeasSetup > DOCSIS 4.0 Downstream Demod Properties > Advanced tab).
EVM good, but Sync Correlation coefficient not good	Center frequency not close enough to signal carrier frequency	Adjust center frequency.
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.