TDA7529 datasheet(12/60 Pages) STMICROELECTRONICS

12 / 60 page

Function description

TDA7529

12/60

Function description

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:

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 VCC-1.2V with a quasi-exponential characteristic referred to the voltage at TCFM.

Figure 3.

Positive current diagram

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.

V_TCFM

Iout

15mA

f(e) current

AC00040

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