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 |
---|---|---|
|
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 ( ). |
|
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. |
|
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 ( ). |
|
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 check box). |
|
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 ( tab). |
|
Signal has longer than usual transition region between symbols |
Change Symbol Time Adjustment to be more negative ( 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 ( ). 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). |
|
Signal amplitude varies during the burst. |
Turn on Pilot Track Amplitude ( ). |
|
Center frequency not close enough to signal carrier frequency |
Adjust center frequency. |
|
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. |
See Also
Making a DOCSIS 4.0 Downstream Measurement