STV0196 datasheet(19/23 Pages) STMICROELECTRONICS

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The C/N indicator register permanently reports a

value S which depends on the C/N level at the input

of the STV0196B.

The C/N indicator offers a programmable sensitivity

which allows a reliable C/N estimation over a wide

Eb/No range (4dB to 16dB typically) ; this is par-

ticularly useful to optimize the dish positioning.

Remark : In this note, we have assumed that :

⋅ 2 (PR), PR : Puncture Rate

The sensitivity of the C/N indicator is dependant on

the SN bits of the register VSEARCH (Hex06) and

on the AGC function reference level ”m”.

A - SUGGESTED PROCEDURE TO RELIABLY

ESTIMATE THE ACTUAL C/N

As no simple mathematical low ensumes a good

matching between the C/N indicator and the actual

C/N, the method relies on a comparaison of the

value S (reported by the C/N indicator) with a

reference look-up table which has been realized

under well controlled conditions.

Basically there are 3 steps in the C/N estimation

software.

To collect C/N indication (un der adapted

conditions).

Indication scaling and correction versus the

puncture rate

Comparaison with the look-up table

A.1 - To collect C/N Indication

The purpose of this first step is to collect the C/N

indicator with the appropriate sensitivity (SN bits

and AGC reference level m).

Basically :

- The value reported by the C/N indicator is propor-

tional to the Number of bits (at the output of the

VITERBI decoder) selected by the SN bits.

- The AGC reference level is only changed to ap-

preciate the high Eb/No ratios. This second pa-

rameter has to be used with some care.

Procedure : Before to make an estimation, the

VSTATUS register (internal address Hex 08) must

be checked to make sure that :

- a carrier is actually present (bit 7)

- puncture rate is found (bit 4)

- puncture rate is known (bits 0-1-2)

Remark : Optionally, it is possible to make an

estimation without informations about the puncture

rate (useful when the dish is still very far from

optimum position), in such case the puncture rate

is forced.

The C/N indicator register has no overflow detec-

tion, so it is necessary to start the measure with the

lowest sensitivity (SN = 00) and to gradually in-

crease it (using SN bits). Due to the noise, the result

S of the measure may have a lot of dispersion,

consequently it is recommended to measure S

several times (typically 100 times) and to calculate

the average value.

Remark : The requred duration tW between two

readings of the register must be higher than :

tW (Min.)

BR = 2 (Fs) x (PR)

with

BC : Bit Count (selected by SN bits)

Fs : Symbol Rate

PR : Puncture Rate

When the current average value of the measure S

is lower than 63, the measure is done again with a

higher sensitivity.

With this care the new C/N

measure S does not overflow t he counter

(the counting time is multiplied by 4 at each step).

In practice some margin is given to this threshold :

a higher sensitivity is selected when the average

value of S is lower than 60.

Wh en the maximum SN value is reached

(SN = 11

⇔ to 65 536 bits at the output of the

VITERBI decoder), the sensitivity can be further

increased by lowering the AGC reference level

( p ar a m et er m, i nt er nal addr es s Hex11,

bit 0 to bit 5).

Remark : There is the need to change the AGC

reference level only in case of high C/N conditions,

then to change the reference level has no important

influence on the bit error rate (BER). In other words,

a completete C/N estimation can be run during the

operation of the receiver.

When the highest possible sensitivity is found

the result S (average value) is ready for further

process.

ANNEXE 1 : C/N ESTIMATION

STV0196B

19/23

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