Cisco 7246 - uBR Router Hardware Installation Manual page 175

Chapter 6 Troubleshooting
Scan the output for the value corresponding to the "SNR_estimate" variable. If this value is at least
Step 2
35 dB, then you have an optimized signal. If the value is less than 34 dB, you must adjust the upconverter
at the cable headend.
Verify that the line card DS output level is correct (+32 dBmV or +42 dBmV, depending on model).
a.
b. Make sure that the uBR7200 Series line card IF input to the upconverter is within an acceptable
range (check the upconverter manufacturer's specifications).
Check the upconverter RF output level. A typical setting is +55 to +58 dBmV.
c.
Make sure that the upconverter RF output center frequency has been set correctly.
d.
When using a GI C6U upconverter, the displayed frequency will be 1.75 MHz below the channel's actual
Note
center frequency.
Make sure that the proper amount of attenuation has been installed between the upconverter RF
e.
output and headend combiner input so that the digitally modulated carrier's average power level is
6 dB to 10 dB below the level of the analog TV channels.
Make sure that the digitally modulated carrier's RF average power level is in the –15 dBmV to
f.
+15 dBmV range at the input to the cable router.
The exact value displayed will vary from cable interface to cable interface; however, the values collected
Note
from measurement to measurement on the same router will be consistent.
If the previous steps do not identify the problem, check the downstream digitally modulated carrier using
Step 3
a QAM analyzer. Several third-party test equipment manufacturers have these instruments available.
Measure modulation error ratio (MER), which should be 35 dB or greater in the headend.
a.
Check pre- and post-FEC bit error rate (BER). There should be no bit errors in the headend. If bit
b.
errors are observed, one likely cause is sweep transmitter interference.
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c. Evaluate the digitally modulated carrier's constellation. Look for evidence of:
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OL-5421-03
Check the sweep transmitter to make sure that adequate guard bands have been programmed
around the digitally modulated carrier.
Bit errors also may be caused by clipping of the digitally modulated carrier in the upconverter
or other active device, including amplifiers and optical transmitters.
Gain compression
Phase noise
I-Q (inphase and quadrature) imbalance
Coherent interference
Excessive noise
Clipping
Cisco uBR7200 Series Universal Broadband Router Hardware Installation Guide
About Verifying the Downstream Signal
6-11