TDA5204E1 Infineon Technologies, TDA5204E1 Datasheet - Page 21

• Manufacturer Part Number: TDA5204E1
• Description: ASK Single Conversion Receiver 390MHz
• Manufacturer: Infineon Technologies

Wireless Components
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.
LNA_autom.wmf
Figure 4-1
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.
LNA Automatic Gain Control Circuitry
RSSI > U
RSSI < U
threshold
threshold
Pins:
: I
: I
4 - 2
load
load
=4.2µA
= -1.5µA
C
24
4
+3V
U
I
C
load
R4
U
U
U
U
c
c
cmax
cmin
:< 2.6V : Gain high
:> 2.6V : Gain low
23
= V
= 1.67V
OTA
CC
U
threshold
- 0.7V
Gain control
R5
voltage
VCC
RSSI (0.8 - 2.8V)
L N A
Specification, January 2001
TDA 5204 E1
Applications
preliminary