STA002 datasheet(18/43 Pages) STMICROELECTRONICS

18 / 43 page

order system controlled by two parameters, al-

pha-car

and

beta-car,

contained

the

CARFLTPAR register.

The natural frequency and the damping factor are

given in the following formulas:

√

m Ko KD

where

β is programmed by the carrier register

beta_car:

β = 2beta_car-4 ⋅Fsym (Fsym = 1.84MHz)

m is the reference value of the AGC2 loop (see

AGC2_REF register), KD is the phase detector

gain and Ko is the constant of the carrier NCO:

FCK

The damping factor is

ξ =

2 √



mKo KD

where

α is programmed by the carrier register al-

pha_car:

α = 2alpha_car+6

beta_car can only take value from 0 to 15; if

beta_car is 0 the loop becames a first order one.

alpha_car can take any value from 0 to 9. If both

alpha_car and beta_car are 0 then the loop is

open.

KD depends on the signal to noise ratio and is

given in the figure in next column.

(see par. 3.8 for C/N definition)

KD = 1.26 for a noise free input signal.

The natural frequency and the damping factor

can be rewritten as:

= 16.515 √

FCK √



⋅ KD

⋅ 2 beta_car

ξ = 0.0289

√

FCK 2

alpha_car

√

⋅ KD

beta_car

Table 2 gives the natural frequency and the

damping factor for the nominal amplitude m = 22,

KD = 1.26 and M_CLK = 39.02687179MHz.

In presence of noise the value of KD may be re-

duced of up to 60%; it is recommended to start

with a damping factor, without noise, greater than

the usual value of 0.7.

3.4.3 Phase and frequency detector parameter

The carrier phase error is calculated by the fol-

lowing formula :

ε = I sgn(Q) - Q sgn(I).

This value is computed (at symbol rate) if the ac-

tual I and Q components are greater than a pro-

grammed threshold otherwise the previous value

is mantained. In this way the detector outputs a

DC value proportional to the frequency offset be-

tween the incoming signal and the local oscillator.

The threshold value may be programmed by the

PFDTHR parameter inside the QPSK_CON-

TROL2 register:

TABLE 2. Carrier loop parameters (m = 22; KD = 1.26; M_CLK = 39.02687179MHz)

beta_car

3456789

fn(KHz)

0.38

0.54

0.77

1.09

1.54

2.17

3.07

4.35

6.15

8.69

alpha_car

Damping factor

0.67

1.34

2.69

5.37

10.7

0.47

0.95

1.90

3.80

7.60

0.34

0.67

1.34

2.69

5.37

0.24

0.47

0.95

1.90

3.80

0.17

0.34

0.67

1.34

2.69

0.12

0.24

0.47

0.95

1.90

0.08

0.17

0.34

0.67

1.34

0.06

0.12

0.24

0.47

0.95

0.04

0.08

0.17

0.34

0.67

0.03

0.06

0.12

0.24

0.47

C/N(dB)

0.6

0.8

(Kd)

1.2

D97AU725

Phase Detector Gain

STA002

18/43

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