TDA7529_11 Datasheet, PDF (12/61 Pages) STMicroelectronics – RF front-end for AM/FM DSP car-radio with IF sampling

Function description
3
Function description
TDA7529
3.1
IMR mixer and active balun output
The IMR mixer has two FM inputs (referred as mode 1 / mode 2) and one AM input
selectable by software. The FM inputs differ by their gains, noise figures, IIP3 and maximum
signal handling capability. The mode 1 FM input (with the higher gain, lower IIP3 and lower
noise figure) is normally coupled with passive antenna input stages; the mode 2 FM input is
normally used for input stages featuring an external preamplifier.
There are two single ended outputs of the IMR mixer: Balunout1 has a 4 dB higher gain than
Balunout2. It is not recommended to use both outputs in parallel.
The Balun1 pin is the current mixer output over an internal resistor. The LC filter at Balun1
can be realized with a low cost SMD-coil (Q ~ 4).
3.2
FM RF-AGC
The FM AGC system is controlled by a peak detector, whose gain can be varied by the
keyed AGC. The latter function is meant to be controlled by a D/A converter in the back-end
part of the system.
The time constant of the FM RF-AGC is defined by an external capacitor connected to
TCFM and programmable internal currents. The currents can be selected independently for
AGC attack and decay. By this the ratio between the attack and the decay time can be
programmed between 0.4 and 250.
The FM RF-AGC has two output pins to drive one PIN diode attenuator and the external
preamplifier gain control.
The AGC outputs can be programmed to the following modes:
1. Positive current I=f(e): after reaching the AGC threshold voltage, the current output
delivers a current I=f(e) up to 15mA in a voltage range from 0.1V (@10µA sink current)
up to V_CC-1.2V with a quasi-exponential characteristic referred to the voltage at TCFM.
Figure 3.
Positive current diagram
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FE CURRENT
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2. Pos/neg current I = f(e): below the AGC threshold voltage the AGC output sinks a
constant current of -5 mA. When the RF input level crosses the AGC threshold voltage,
the current is reduced down to 0 mA with a quasi-logarithmic behavior. At half control
voltage the current becomes positive and reaches up to 15mA following an exponential
function.
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Doc ID 13311 Rev 4