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84 01-0870-401B 01/03
Demodulator Application Notes
E
b
/N
0
Estimation Algorithm
Within the FEC decoder IC, the Viterbi decoder output is re-encoded and
compared to a delayed version of the original encoder input. Incoming bit errors
that are corrected will result in a difference between original input and the
encoded output. These differences are accumulated for a fixed amount of time.
This accumulation register can be read by the TDR6 microprocessor and gives an
approximation of the signal to noise level of the input signal. Once these reading
have been calibrated using an input with a known E
b
/N
0
, they are used by the
TDR6 to accurately estimate the E
b
/N
0
of the incoming signal.
Note that the estimated E
b
/N
0
reading is accurate only for relatively low E
b
/N
0
readings (less than 10). This is because there are not enough bit errors at higher
E
b
/N
0
to generate an accurate estimation.
Estimating E
b
/N
0
Using a Spectrum Analyzer
Sometimes it is desirable to be able to estimate the E
b
/N
0
of an input signal using
a spectrum analyzer. The following equation will provide a reasonable
approximation.
E
b
/ N
0
= 10 log
10
[(10
D/10
- 1)(SR/IR)]
where:
■ D = Distance in dB from top of spectrum to noise floor
■ SR = Symbol Rate (symbols/sec)
■ IR = Information Rate (bits/sec)
To determine the symbol rate issue the command
DMD.x SR.
The symbol rate is not available from the front panel.
The information rate is the TDR6 data rate parameter. To determine the data rate:
■ Using the front panel, select Details>Input>Demod>RF_x>Data Rate where
x = A, B, C, or D
■ Using a remote unit, issue the command DMD.x DR
Demodulator Low E
b
/N
0
Alarm
The TDR6 issues a “low E
b
/N
0
” fault if the E
b
/N
0
is ever 1 dB lower than the
DVB specification. You should operate with at least a 2 dB margin to allow for
rain-fade and other conditions.
Troubleshooting This section provides information that may be helpful in troubleshooting
including:
■ Using a spectrum analyzer to check the configuration
■ Troubleshooting commands
Spectrum Analyzer
Following are ways you can use a spectrum analyzer to check the configuration of
the QPSK demodulator:
■ Confirm that an input signal is present
■ Confirm that spectrum is flat
■ Confirm that width conforms to (1.35
*
symbol rate)