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STA002

STARMAN™ CHANNEL DECODER

STMicroelectronics 

STA002

QPSK_CONTROL2 Register

Internal address: 81 H

Reset Value: 90H

MSB

b7 b6 b5 b4 b3 b2

PFDTHR

CNTHR

LSB

b1 b0

SN

The threshold value depends on the signal level

at the Nyquist filter output. A good value for this

parameter is given by: PFDTHR = 0.4 AGC2REF

where AGC2REF is the reference value for the

AGC2 loop.

3.4.4 Internal ramp parameter

In presence of a frequency offset greater than the

pull-in range of the carrier loop or in presence of

low signal to noise ratio the tracking performance

of the loop itself may became rather slow. To

help the loop in tracking this frequency offset an

internal ramp can be activated by I2C bus.

This ramp can be switched on or off by setting

the SWON parameter 1 or 0 respectively. When

SWON=0 the output value of the ramp is null.

The sweep rate can be calculated by the follow-

ing formula:

dF

dt

=

2swstep

stepper +

1

Fc2k

226

where swstep can only take 0 and 1 values and

stepper can be programmed in a range from 0 to 15.

MSB

LSB

X X b5 b4 b3 b2 b1 b0

b5 : SWON; 1 = 2 ramp on; 0 = 2 ramp off

b4 : SWSTEP

b3 - b0 : STEPPER

Ramp control register (RAMPCTRL)

Internal address: 95 H

Reset Value: 01H

3.5. AGC1

3.5.1 AGC1 control

To avoid a degradation of the signal to noise ratio

a constant IF level is necessary at the channel

decoder input.

The AGC1 outputs a signal to control the Variable

Gain Amplifier in the RF Front-End in order to

mantain a fixed level at the ADC input.

The input signal power (computed after the A/D

conversion) is compared to a programmable

threshold; the difference is scaled by the

AGC1BETA coefficient then integrated.

The result is converted into a pulse width modula-

tion signal to drive the AGC output pin; it may be

filtered by a simple RC filter to control the gain

command of a variable gain amplifier before the

A to D conversion.

The 8 integrator MSB’s (AGC1_ INTG register)

may be read or written at any time by the micro;

when written, the LSB’s are reset.

The integrator value is the level of the AGC out-

put, after low pass filtering; it gives an image of

the input signal power. The sign of the loop can

be controlled by the AGC1CHS control bit in the

QPSK_CONTROL1 register in order to adapt the

loop to a positive or negative slope of the variable

gain amplifier.

3.5.2 Registers

AGC1 reference level register (AGC1_REF)

Internal address: 83 H 82H

Reset Value : 01H 06H

MSB

LSB

X X X X X X b9 b8

AGC1_REF2

MSB

LSB

b7 b6 b5 b4 b3 b2 b1 b0

AGC1_REF1

This register is divided into two bytes. The LSB

byte is named AGC1_REF1, the MSB is named

AGC1_REF2.

The reset value of this register (262) maintains

the peak signal input level equal to the half range

of the ADC.

AGC1 integrator gain register (AGC1_BETA)

Internal address: 84 H

Reset Value: 00H

MSB

LSB

X X X X X b2 b1 b0

AGC1_BETA

The AGC1 loop gain βAGC1 is given by:

bAGC1 = 2AGC1_BETA

The parameter AGC1_BETA can only take values

from 0 to 5. When AGC1_BETA is set to "111"

the loop gain is null. This condition is useful to

open the AGC1 loop.

19/43

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