TDA7786 datasheet(12/54 Pages) STMICROELECTRONICS

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Function description

TDA7786, TDA7786M

12/54

DocID026122 Rev 5

Function description

2.1

FM - mixers

The FM Image Rejection mixer has two single ended inputs, selectable through software.

They are designed to achieve best performance both in case of a passive tuned

preselection and of a passive fixed band-pass preselection without tuning for lower cost

applications.

The input frequency is down-converted to very low IF with high image rejection.

The tuned application is supported by an 8-bit tuning DAC. The alignment of the DAC is

performed automatically on-chip.

2.2

FM - AGC

The programmable RFAGC senses the mixer input to avoid overload.

When the RFAGC threshold is reached, the PIN diode output is activated in order to

attenuate the incoming RF signal

The PIN diode driver is able to drive external PIN diodes with up to 15 mA current.

The time constant of the FM AGC is defined by the combination of an external capacitor and

internal currents. There are two programmable attack and decay time constants.

2.3

AM - LNA

The integrated AM LNA feature is integrated with low-noise and high IIP2 and IIP3. The gain

of the LNA is controlled by the AGC. The maximum gain is set with an external resistor,

typically 26 dB with 470 Ω.

2.4

AM - AGC

The programmable AM RFAGC senses the mixer inputs and controls the internal PIN

diodes and LNA gains.

Firstly the LNA gain is reduced by about 10 dB, and then the PIN diodes are activated to

further attenuate the signal.

The time constant of the 2nd order AM AGC LPF is defined by both external components

and programmable internal currents.

2.5

AM - Mixers

The image rejection mixer has two AM inputs selectable via software. It easily supports low-

cost applications for extended frequency bands like short-waves.

The input frequency is converted to low IF with high image rejection.

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