TDA5235
Functional Description
2.4.6.4 Digital Automatic Gain Control Unit (AGC)
Automatic Gain Control (AGC) is necessary mainly because of the limited dynamic range
of the on-chip bandpass filter (BPF). The dynamic range reduces to less than 60dB in
case of minimum BPF bandwidth.
AGC is used to cover the following cases:
1. ASK demodulation at large input signals
2. RSSI reading at large input signals
3. Improve IIP3 performance in either FSK or ASK mode
The 1st IF buffer (PPFBUF, see Figure 3) can be fine tuned "manually" by means of 4
bits thus optimizing the overall gain to the application (attenuation of 0dB to -12dB by
means of IFATT0 to IFATT15 in DDC mode; SDC mode has lower IFATT range). This
buffer allows the production spread of external components to be trimmed.
The gain of the 2nd IF path is set to three different values by means of an AGC algorithm.
Depending on whether the receiver is used in single down conversion or in double down
conversion mode the gain control in the 2nd IF path is either after the 2nd poly-phase
network or in front of the 2nd mixer.
The AGC action is illustrated in the RSSI curve below:
Analog (blue) &
digital (black )
RSSI output
Mixer2
saturation
BPF bypassed
AGC OFF
Max. B W
BPF
saturation
Min. B W
AGC ON
margin
Analog AGC
attack point
hysteresis
Analog AGC
decay point
Max. B W
Max. FE gain
(IFATT 0)
Front-end
Min. B W
noise x gain
Min. FE gain
(IFATT 15)
Limiter
noise floor
Input power
Figure 12
Analog RSSI output curve with AGC action ON (blue) vs. OFF (black)
Data Sheet
30
V1.0, 2010-02-19
INFINEON [ Infineon ]
