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Demodulator Application Notes
01-0870-401B 01/03 85
■ Measure C/N to estimate E
b
/N
0
where
❒ C is carrier power
❒ N is noise floor
■ Calculate the estimated E
b
/N
0
Refer to the equation for estimating E
b
/N
0
using a spectrum analyzer in a
previous section of this appendix.
■ Transmit pure carrier to precisely locate the center frequency
If the demodulator is supplying the DC to the LNB, then the spectrum analyzer must monitor the
L-band input using a T-junction. Insure that the spectrum analyzer input is protected using a
DC-block. Since the spectrum analyzer has an input impedance of 50 ohms, this method will also
double-terminate the input signal (i.e. lower it by 3 dB).
The demodulator will acquire only if the input carrier frequency is equal to [(RF – LO) ±2 MHz].
Once acquired, it is recommended that you optimize the RF parameter using the carrier offset status
parameter. For the optimization procedure, refer to the chapter on configuring the TDR6.
Diagnostic Commands
This section describes commands you can use for troubleshooting the
demodulator.
■ G4 – Use this command to display the demodulator IC registers.
Syntax – DMD.x G4 where x designates the demodulator that is the object of
the command.
■ Register Values – The register values are all in hexadecimal. The two
registers that are most useful for debug are Register 6 and Register 31.
■ Register 6 (Power Level) – This register should be almost equal to A0. This
indicates that the AGC loop is locked and the signal level is optimum at the
analog-to-digital converter inputs.
■ Register 31 (Status) – Only the three least significant bits are active.
❒ Status[0] = 1 if the symbol clock is locked
❒ Status[1] = 1 if the carrier phase is locked
❒ Status[2] = 1 if the carrier frequency is locked
Note that the state of these three bits are reflected in the status LED’s.
■ G3 – Use this command to display the three decoder error count registers.
Syntax – DMD.x G3 where x designates the demodulator that is the object of
the command.
■ Register Values – The register values are all in hexadecimal.
■ Reed-Solomon Corrected – This register displays the total number of byte
errors corrected by the Reed-Solomon decoder. It is reset to zero upon
reaching max count.
Any substantial channel noise will cause this register to be greater than zero.
■ Reed-Solomon Uncorrected – This register displays the total number of byte
errors that were detected, but not corrected, by the Reed-Solomon decoder. It
is reset to zero upon reaching max count. For normal operation this register
must be zero.
Any non-corrected byte errors will result in some form of degradation in the
decoder.
NOTE