TDA5225 Datasheet, PDF (29/176 Pages) Infineon Technologies AG – Enhanced Sensitivity Multi-Channel Quad-Configuration Receiver

TDA5225
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 1^st 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 2^nd 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 2^nd IF path is either after the 2^nd poly-phase
network or in front of the 2^nd mixer.
The AGC action is illustrated in the RSSI curve below:
Analog (blue) &
digital (black )
RSSI output
Mixer2
saturation
BPF
saturation
Analog AGC
attack point
Max. B W
Min. B W
BPF bypassed
AGC OFF
margin
h y s ter e s is
Analog AGC
decay point
Front- end
noise x gain
Max. B W
Min. B W
Limiter
noise floor
Max . FE gain
(IFA TT 0)
Min. FE gain
(IFA TT 15)
AGC ON
Input power
Figure 12 Analog RSSI output curve with AGC action ON (blue) vs. OFF (black)
Data Sheet
29
V1.0, 2010-02-19