STA400A datasheet(16/117 Pages) STMICROELECTRONICS

16 / 117 page

STA400A

16/117

Figure 9. RRC Filter Passband Ripple

Carrier Phase/Frequency Error Detector

The carrier Phase/Frequency Error Detector (PFD) measures the error between the sampled symbols and the

quadrant bisector. The error is calculated by the following formula : PHerr = I x Sgn(Q) - Q x Sgn(I), where Sgn(.)

is the sign function.

This value is computed at symbol rate if the actual I and Q components are greater than a programmed thresh-

old otherwise the previous value is maintained. In this way the detector outputs a DC value proportional to the

frequency offset between the incoming signal and the local oscillator. In the steady state, when the carrier loop

is locked (and, therefore, the phase error is small), the circuit behaves like a pure phase detector while during

the acquisition phase it behaves like a PFD.

The threshold value may be programmed by the PFDTHR register. The signal level after the AGC2 loop must

be taken into account when setting the threshold. The default preset for this parameter is about the 20% of the

AGC2 reference value (see AGC2REF register).

The carrier Phase and Frequency Detector Gain (Kd) characteristic as a function of the Carrier to Noise ratio is

given in fig.10. Kd = 1.24 is the value for a noise free input signal and may be reduce up to 50% of its maximum

value in a low C/N condition.

Figure 10. Carrier Phase/Frequency Detector Gain

100

200

300

400

500

600

700

800

900

1000

0.5

Frequency (kHz)

0.5

0.6

0.7

0.8

0.9

1.1

1.2

1.3

C/N (dB)

PFD Gain

STA400A Datasheet(PDF) 16 Page - STMicroelectronics