TDA5204 Просмотр технического описания (PDF) - Infineon Technologies

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TDA5204

ASK Single Conversion Receiver 390MHz

Infineon Technologies 

TDA 5204 E1

preliminary

Applications

4.1 LNA and Automatic Gain Control (AGC)

The AGC extends the dynamic range of the receiver.

The automatic gain control in the TDA5204 is a narrow-band control loop which

compares the receive signal strength signal (RSSI, 0.8V to 2.8V) from the lim-

iter with a fixed threshold voltage applied to pin 23 (THRES).

In the following figure the internal circuitry of the LNA automatic gain control is

shown.

R4

R5

Pins:

24

+3V

Uthreshold

23

RSSI (0.8 - 2.8V)

OTA

VCC

Iload

RSSI > Uthreshold: Iload=4.2µA

RSSI < Uthreshold: Iload= -1.5µA

LNA

Gain control

voltage

4

UC

Uc:< 2.6V : Gain high

Uc:> 2.6V : Gain low

C

Ucmax= VCC - 0.7V

Ucmin = 1.67V

LNA_autom.wmf

Figure 4-1

LNA Automatic Gain Control Circuitry

The fixed voltage on pin 23 is generated on the external voltage divider. The

comparator is a transimpedance amplifier (OTA), which creates a positive cur-

rent (+4.2uA) in the case the RSSI level is larger than the threshold voltage.

Otherwise the current is -1.5uA. This leads to an asymmetric fast-attack and

slow-release behaviour and thus to fast reaction to the low gain mode and slow

reaction to the high gain mode.

This current is converted into a control voltage over an external capacitor C

attached to pin 4 (TAGC) which defines the gain of the LNA. The limits of the

control voltages for the LNA on pin4 are 1.67V for high gain mode and Vcc-0.7V

for low gain mode.

Wireless Components

4-2

Specification, January 2001

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