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Cisco uBR7225VXR Universal Broadband Router Hardware Installation Guide
OL-17309-02
Chapter6 Troubleshooting
Verifying the Downstream Signal
snr_estimate 352(TenthdB), ber_estimate 0, lock_threshold 26000
QAM in lock, FEC in lock, qam_mode QAM_64
This shows a downstream frequency of 555 MHz, 64-QAM downstream, and an SNR estimate of
35.2 dB.
Note Use the debug cable mac log verbose and show controllers cable 0 mac state commands when
troubleshooting the cable interface’s initialization sequence and locking on the downstream signal.
Step2 Scan the output for the value corresponding to the “SNR_estimate” variable. If this value is at least
35 dB, then you have an optimized signal. If the value is less than 34dB, you must adjust the cable
interface line card.
a. Verify that the line card DS output level is correct (+32 dBmV or +42 dBmV).
b. Make sure that the Cisco uBR7225VXR line card IF input to the upconverter is within an acceptable
range (check the upconverter manufacturer’s specifications).
c. Check the upconverter RF output level. A typical setting is +55 to +58 dBmV.
d. Make sure that the upconverter RF output center frequency has been set correctly.
e. Make sure that the proper amount of attenuation has been installed between the upconverter RF
output and the 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.
f. Make sure that the digitally modulated carrier’s RF average power level is in the –15 dBmV to
+15 dBmV range at the input to the cable router.
Note The exact value displayed will vary from cableinterface to cable interface; however, the values collected
from measurement to measurement on the same router will be consistent.
Step3 If the previous steps do not identify the problem, check the downstream digitally modulated carrier using
a QAM analyzer. Several third-party test equipment manufacturers have these instruments available.
a. Measure modulation error ratio (MER), which should be 35 dB or greater in the headend.
b. Check pre- and post-FEC bit error rate (BER). There should be no bit errors in the headend. If bit
errors are observed, one likely cause is sweep transmitter interference.
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Check the sweep transmitter to make sure that adequate guard bands have been programmed
around the digitally modulated carrier.
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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.
c. Evaluate the digitally modulated carrier’s constellation. Look for evidence of:
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Gain compression
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Phase noise
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Inphase and quadrature (I-Q) imbalance
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Coherent interference
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Excessive noise
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Clipping