STV0299B datasheet(14/36 Pages) STMICROELECTRONICS

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STV0299B

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FUNCTIONAL DESCRIPTION (continued)

4.4 Carrier Recovery and Derotator Loop

The tracking range of the derotator is

± f

M_CLK/2

(

± f

sampling/2). The initial frequency search may

therefore be performed on several MHz ranges

without reprogramming the tuner.

Three phase detectors are selectable using

software:

• Phase detector algorithm 0: This algorithm

should only be used for BPSK reception.

• Phase detector algorithm 1: This algorithm is

used with QPSK reception, over a small range

of capture phases and with a channel noise

value over 4.5 dB.

• Phase

detector

algorithm

For

QPSK

reception, it is used after locking, to minimize

the

bit

error

rate

low

channel

noise

conditions. Algorithm 2 is recommended for

most applications.

The

loop

controlled

through

α and β

parameters.

The carrier loop control registers (the Alpha

Carrier Register, the Beta Carrier Register and the

Carrier Frequency Register) are in Addresses 13,

14, 22 and 23.

4.4.1

Loop Parameters

Like the timing loop, the carrier loop is a

second-order system where two parameters,

and

β, may be programmed with alpha_car and

beta_car respectively.

The natural frequency (fn) is:

The damping factor is:

where

α =(2+a)

⋅2b⋅214, with b ≥ 1, and

β = (4+2c+d)

⋅2e, with e ≥ 1. m2 is the reference

level in the AGC2 register.

4.4.2

Carrier Lock Detector

The carrier lock detector provides an indicator

with a high value when the carrier is locked,

dependent on the channel noise. When the carrier

is not locked, the indicator value is low.

The

indicator

value

compared

programmable 8-bit threshold (Register 15h). The

result of this comparison (1 if greater than the

threshold, else 0 if not) is written as the Carrier

Found flag (CF), and may be read in the status

routed on the output LOCK (see Register 08h).

The Lock Detector Threshold Register and Lock

Detector Value Register are in Addresses 15 and

1C.

4.4.3

Derotator Frequency

The derotator frequency can be either measured

(read operation) or forced (write operation).

Derot_freq is a 16-bit signed value.

The Derot_freq Registers are Registers 22 and

23.

4.4.4

Carrier Frequency Offset Detector

The carrier recovery loop features a carrier

frequency

offset

detector

and

two

phase

detectors. When the carrier frequency offset

detector is enabled, the central loop frequency is

modified proportionally to the carrier offset. The

gain and time constants of the detector are set by

CFD[6:4] and CFD[3:2] respectively. When the

carrier loop is about to “phase lock” with the

carrier, the frequency detector stops automatically

and the phase lock is ensured by the selected

phase

detector.

This

switchover

point

determined by the threshold CFD [1:0].

For stability reasons, the gain CFD [6:4] should

not exceed the coefficient e[3:0] of Register

BCLC.

The carrier frequency offset detector is in Address

12.

4.5 Noise Indicator

The noise indicator may be used to facilitate the

antenna pointing or to give an idea of the RF

signal quality and of the front-end installation

(dish, LNB, cable, tuner or ADC).

simple

C/N

estimator

can

easily

implemented by comparing the current indications

with a primarily-recorded look-up table.

The time constant ranges from 4 k to 256 k

symbols. The 16 MSB of the result may be read by

the microprocessor (Registers 24 and 25).

710

–

M_CLK

⋅

()

M_CLK

----------------

⋅⋅

22 10

–

α m2

---------

M_CLK

----------------

⋅

freq

()

kHz

Derot_freq

------------------------------

M_CLK

()

kHz

⋅

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