STV0196 datasheet(11/23 Pages) STMICROELECTRONICS

11 / 23 page

VIII - AGC CONTROL

The modulusof the input is compared to a program-

mable threshold; the difference is scaled by the

AGC coefficient, then integrated; the result is con-

verted into a pulse density modulation signal to

drive the AGC output ; it may be filtered by a simple

analogue filter to control the gain command of any

amplifier before the A to D converter.

The 8 integrator MSB’s 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

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

the input signal power, whatever this signal is, and

can be used to point the antenna.

The coefficient may be reset by programmation; in

that case, the AGC reduces to a programmable

voltage synthesiser.

The AGC reference level ”m” value impacts the

value of the following functions :

- carrier to noise indicator (see paragraph VII)

- the carrier loop (see paragraph V.2)

- the timing loop (paragraph IV.2)

- carrier offset evaluator (paragraph VI)

Control Registers

Internal Addresses : Hex11

Iagc

Invert

signal

Reserved

AGC reference

level (”m”)

Internal Addresses : Hex12

AGC integrator value (signed)

(Read/write register)

Internal Addresses : Hex13

00000010

Reserved

G[2..0] :

AGC coefficient

The 8 bit signed value in the integrator is the image

of the AGC output; reading this value gives an

image of the RF signal power.

A constant error on the modulus leads to a ramp at

the output of the integrator with value :

AGC_Int = 2

AGC_Coeff-16 . error

As a consequence, for the reset conditions, a con-

stant signal of null value (error = 24) should cause

the output AGC duty cycle to go from 100% to 0%

in 2

22 symbol periods, or 8.7ms at 20MBauds.

If Iagc is set, the sign of the integrator is inverted.

IX - VITERBI DECODER AND SYNCHRONIZATION

The convolutives codes are generated by the

polynoms Gx = 171oct and Gy = 133oct.

The Viterbi decoder computes for each symbol the

metrics of the four possible paths, proportional to

the square of the Euclidian distance between the

received I and Q and the theoretical symbol value.

The puncture rate and phase are estimated on the

error rate basis.

Five rates are allowed and may be enabled/dis-

abled through register programming :

1/2, 2/3, 3/4,5/6, 7/8.

In Mode B, 7/8 is replaced by 6/7.

For each enabled rate, the current error rate is

compared to a programmable threshold; if it is

greater, anotherphase (or another rate) is tried until

the good rate is obtained.

A programmable hysteresis is added to avoid to

loose the phase during short term perturbation.

The rate may also be imposed by the external

software, and the phase is incremented only on

micro request ; the error rate may be read at any

time in order to use other algorithm than imple-

mented.

The decoder is accessed via a set of 9 registers :

Threshold Registers (VTH0 to VTH4)

Internal Address : Hex1 (VTH0) to 5 (VTH4)

Reset Value : Hex20

Threshold

Value

VTH0 0 Th6 Th5 Th4 Th3 Th2 Th1 Th0

rate 1/2

VTH1 0 Th6 Th5 Th4 Th3 Th2 Th1 Th0

rate 2/3

VTH2 0 Th6 Th5 Th4 Th3 Th2 Th1 Th0

rate 3/4

VTH3 0 Th6 Th5 Th4 Th3 Th2 Th1 Th0

rate 5/6

VTH4 0 Th6 Th5 Th4 Th3 Th2 Th1 Th0

rate 7/8

or 6/7

For each register, bits 6 to 0 represent an error rate

threshold : the average number of errors happen-

ing during 256 bit periods; the maximum program-

mable value is 127/256 (higher error rates are of

no practical use).

Puncture Rate Enable register

Internal Address : Hex09

Reset Value : Hex10 (Mode A)

E4E3E2E1E0

E4 : enablePuncturedRate 7/8(Mode A) or 6/7(Mode B)

E3 : enable Punctured Rate 5/6

E2 : enable Punctured Rate 3/4

E1 : enable Punctured Rate 2/3

E0 : enable Basic Rate 1/2

FUNCTIONAL DESCRIPTION (continued)

STV0196B

11/23

STV0196 Datasheet(PDF) 11 Page - STMicroelectronics

STV0196 Datasheet(HTML) 11 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL