STA400A datasheet(21/117 Pages) STMICROELECTRONICS

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STA400A

C/N Estimator

This block gives the signal to noise ratio at the Nyquist filter output. It computes the statistic (mean and variance)

of a single component of the demodulated complex signal and writes the estimated C/N value in dB in the CN

This register has a fixed point format. The integer part of the number is stored in the six MSBs (bit7-2) and the

fractional part in the two LSBs (bit1-0). The C/N estimation is correct only when the QPSK demodulator is

locked.

Carrier Null Offset

This block is used to enhance the acquisition performance of the carrier loop. It may be enabled/disabled setting

bit-6 of the QPSK_CTRL register. The block consists of a FSM that is activated when the carrier loop has

reached the lock condition (low level on at the Lock Detector output). The FSM checks if the lock detector output

is low for a programmable period (see NULOFS_WIN register) and then it writes the locked carrier frequency

value (read from the CARFREQ register) into the Carrier NCO. It resets also the carrier loop filter integrator and

disables the frequency sweep (see fig.6).

In this way the demodulator works with a null carrier offset resulting in a faster re-acquisition time of the carrier

in case of a signal drop-out. The NULOFS_DELTAF register gives the possibility to subtract a programmed fre-

quency from the locked carrier before writing the new value in the carrier NCO.

The flow diagram of the CarrierNullOffset FSM is showed in fig.13

Flag Register

A flag register is provided in the QPSK demodulator. This is a read only register containing specific status bits

of the demodulator. It gives information on the lock staus and on the operation of the CarrierNullOffset and Ti-

mintgCtrl blocks.

Figure 13. CarrierNullOffset FSM Flow Diagram

demodulator locked (LOCK=0)

demodulator

unlocked

system reset

block disable

demodulator locked for

a time less than a fixed

value (NULOFS_WIN)

demodulator locked for a time equal to the

fixed value (NULOFS_WIN)

no system reset

and block enable

Reading lock

Time counter active

Frequency.Sweep Disabled

Carrier Loop Filte Integrator Cleared

Writing New IF Frequency in the Carrier NCO:

NEW_IF_FREQ= CARFREQ-NULOFS_DELTAF

Clear release in the carrier loop

filter integrator

system reset

block disable (NULOFS_EN=0)

demodulator unlocked (LOCK=1)

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