MT312CSLASHCG datasheet(53/90 Pages) ZARLINK

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Automatic Gain Control

MT312

8.2 Automatic Gain Control Read Registers

8.2.1 Measured Signal Level at MT312 Input. Register 19 (R)

B7-0:

SIG LEVEL[11:4]:

This register provides a measurement of the MT312 input signal level. It

contains the 8 MSBs. The remaining 4 LSBs are contained in SIG LEV L

conditions, it should only be necessary to read the high byte to determine the

incoming signal level. If further accuracy is required, then the remaining bits of

the lower byte should be read and the 12 bits combined into one number.

When AGC is controlling the signal level, there is a direct relationship between SIG LEVEL and AGC REF:

SIG LEVEL * 8 = AGC REF

NOTE: the signal level is measured at the output of the ADC before any digital ﬁltering takes place. Hence the

reading includes all noise and other signal channels passed by the SAW or baseband ﬁlter.

8.2.2 Measured AGC Feed Back Value. Registers 108 - 110 (R)

AGC[13:0]:

These two registers provide a measurement of the AGC error feed back value by the MT312 to

the front end. Reading the bytes does NOT reset the value.This measurement can be used to

provide an indication of the signal level at the input to the tuner.

To avoid having too large a number, the following formula extracts a number less than 10000:

Tuner input signal level = AGC[13-6] * 4 + AGC[5-4] / 64.

ERR DB[9:0]: The ERR DB is the difference between the expected signal level deﬁned by AGC REF and

received signal level. This is in a non-linear logarithmic scale (hence the notation DB).

The way H/M/L registers work within the QPSK block is as follows. When you read the H register the 24-bit

value is dumped into a shadow register. You don't have to read M and L after this. However, what you must

NOT do is to read M and L (or just L of a 24 or 16-bit register) without reading H. The safest solution is to read

H/M/L in that order.

NAME

ADR

Def

hex

SIG LEVEL

SIG LEVEL[11:4] - Signal level at MT312 input

NAME

ADR

Def

hex

AGC H

108

AGC[13:6] - Front end AGC (high byte)

AGC M

109

AGC[5:0] - Front end AGC (low byte)

ERR DB[9:8]

AGC L

110

ERR DB[7:0] - Error difference (low byte)

MT312C/CG Datasheet(PDF) 53 Page - Zarlink Semiconductor Inc

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