TDA5240 Datasheet, PDF (31/284 Pages) Infineon Technologies AG – Enhanced Sensitivity Multi-Channel Quad-Configuration Receiver

TDA5240
Functional Description
Digital RSSI, AGC and Delog:
In order to match the analog RSSI signal to the digital RSSI output a correction is
necessary. It adds an offset (RSSIOFFS) and modifies the slope (RSSISLOPE) such
that standardized AGC levels and an appropriate DELOG table can be applied.
Upon entering the AGC unit the digital RSSI signal is passed through a Peak Memory
Filter (PMF). This filter has programmable up and down integration time constants
(PMFUP, PMFDN) to set attack respectively decay time. The integration time for decay
time must be significantly longer than the attack time in order to avoid the AGC interfering
with the ASK modulation.
The integrator is followed by two digital Schmitt triggers with programmable thresholds
(AGCTLO; AGCTUP) - one Schmitt trigger for each of the two attack thresholds (two
digital AGC switching points). The hysteresis of the Schmitt triggers is programmable
(AGCHYS) and sets the decay threshold. The Schmitt triggers control both the analog
gain as well as the corresponding (programmable) digital gain correction (DGC).
The difference ("error") signal in the PMF is actually a normalized version of the
modulation. This signal is then used as input for the DELOG table.
AGC threshold programming
The SFR description for the AGC thresholds are in dBs. The first value to set is the AGC
threshold offset in AGCTHOFFS.
This value is the offset relative to 0 input (no noise, no signal), which for the default
setting of gain, and assuming typical insertion loss of matching network and ceramic
filter, can be extrapolated to be approximately -143dBm.
In this case the default setting of the AGCTHOFFS of 63.9dB corresponds to an input
power of approximately -79dBm (= -143dBm + 63.9dB).
The low (digital) AGC threshold is then -79 + 12.8dB (default AGCTLO) = -66dBm and
the upper (digital) AGC threshold is -79 + 25.6 (default AGCTUP) = -53dBm.
Therefore a margin of about 6dB is indicated before a degradation of the linearity of the
2^nd IF can be observed when using the 50kHz BPF or even about 16dB when using the
300kHz BPF.
The input power level at which the AGC switches back to maximum gain is -66dBm -
21.3dB (default AGCHYS) = -87dBm. This provides enough margin against the minimum
sensitivity.
Data Sheet
31
V4.0, 2010-02-19