ST7540 datasheet(27/44 Pages) STMICROELECTRONICS

27 / 44 page

ST7540

Functional description

27/44

In the analog domain, the signal is filtered in order to reduce the output signal spectrum and

to reduce the harmonic distortion. The transition between a symbol and the following is done

at the end of the on-going half FSK sinewave cycle.

Figure 16. Transmitting path block diagram

●

Automatic Level Control (ALC)

The Automatic Level Control Block (ALC) is a variable gain amplifier (with 32 non linear

discrete steps) controlled by two analog feed backs acting at the same time. The ALC

gain range is 0dB to 30 dB and the gain change is clocked at 5KHz. Each step

increases or reduces the voltage of 1dB (Typ).

Two are the control loops acting to define the ALC gain:

–

A Voltage Control loop

–

A Current Control Loop

The Voltage control loop acts to keep the Peak-to-Peak Voltage constant on Vsense.

The gain adjustment is related to the result of a peak detection between the Voltage

waveform on Vsense and two internal Voltage references. It is possible to protect the

Voltage Control Loop against noise by freezing the output level (see Section 7.5:

Output voltage level freeze).

–

If Vsense < VsenseTH - VsenseHYST

The next gain level is increased by 1 step

–If VsenseTH - VsenseHYST < Vsense < VsenseTH + VsenseHYST No Gain Change

–

If Vsense > VsenseTH + VsenseHYST

The next gain level is decreased by 1 step

TRANSMISSION

FILTER

FSK

MODULATOR

D-TYPE

FLIP

FLOP

DAC

TIMER

THERMAL

SENSOR

VOLTAGE

LOOP

CURRENT

LOOP

CLR/T GENERATOR

Vsense

PA_IN-

PA_OUT

TX_OUT

CLR/T

TxD

Bit 14

Bits 0-5

Bits 0-2

Bits 7-8

Band Pass

ALC

D03IN1420

PA_IN+

BU/THERM

Bits 17 & 21

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