CS3810 Amphion Semiconductor Ltd., CS3810 Datasheet - Page 7

• Manufacturer Part Number: CS3810
• Description: 32 Qam Demodulator
• Manufacturer: Amphion Semiconductor Ltd.

The input to the demodulator may be fed directly from an A/
D converter. It samples the received spectrum at double the
symbol rate.
An AGC loop is implemented with an external gain control
element in order to achieve the desired receiver dynamic
range and maintain proper input level to the ADC. The AGC
module determines the average power of the input signal and
compares it to a programmed threshold. A pulse width
modulated signal is output by the AGC module. The PWM
stream is externally integrated and can be used as a AGC
control voltage.
The received data stream then enters the symbol timing
recovery loop, which consists of resampling circuitry and
filtering, crosstalk removal, IQ imbalance adjustment and a
Digital Phase Locked Loop (DPLL). A combined resampling /
matched filter is used to enable transfer of the input data
stream from the input data domain to the symbol rate domain.
The recovered symbol rate data then enters the carrier
recovery loop of the demodulator. This consists of a de-
rotation module, removal of DC crosstalk, adjustment of any
I/Q imbalance, equalization and the DPLL. As the
constellation of the recovered symbol rate data can still be
rotating at this stage it is necessary to de-rotate this prior to
equalization.
After de-rotation the symbol rate data is applied to an
adaptive equaliser to remove transmission related distortions.
The recovered symbols are fed to the 64 state TCM decoder.
The user can select the puncture rate, lock thresholds or even
to bypass the TCM decoder. The TCM decoder provides BER
estimates which may be read via the microprocessor interface.
CARRIER RECOVERY
TIMING RECOVERY
AGC
TCM
DEMODULATOR OVERVIEW
Prior to RS decoding the block boundaries are recovered. The
sync detection circuitry searches for a unique word similar to
the scheme employed by Intelsat IESS-308. Once locked the
sync detection module also repacks the TCM output stream
into 8 bit Reed Solomon symbols.
The delineated blocks are de-interleaved and Reed Solomon
decoded. The corrected errors are reported and reflected on
the RSERR_U and RSERR_L output ports during a statistics
gathering period.
A self synchronizing descrambler utilizing a 2
operates on the data stream thus de randomizing it to recover
the original modulated input message. Data from the
descrambler is written to a FIFO on the symbol rate clock and
output on RXDATA on the rising edge of the RXUSRCLK.
A number of lock indicators are provided as status registers
bits to enable the overall synchronisation status of the
demodulator to be monitored. These are:
•
•
•
•
Symbol timing recovery loop lock
Carrier recovery loop lock
TCM decoder synchronized
Unique word sync detection lock
REED SOLOMON DECODING
LOCK INDICATION
DESCRAMBLING
20
-1 pattern
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TM