TDA7705TR datasheet(10/42 Pages) STMICROELECTRONICS

10 / 42 page

Function description

TDA7705

10/42

Doc ID 15938 Rev 9

Function description

2.1

FM - mixers

The image-rejection mixer has two FM inputs, selectable through software. These inputs

feed stages with different gains, noise figures, and IIP3. They are optimized for best

performance in case of a passive tuned prestage and for a passive fixed bandpass without

tuning for low-cost application respectively.

The second input offers also the possibility of an easy addition of a weather-band

preselection filter.

The input frequency is downconverted to 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.

2.2

FM - AGC

The programmable RFAGC senses the mixer input whereas the IFAGC senses the IFADC

input to avoid overload.

The PIN diode driver is able to drive external PIN diodes with a current value as high as

15mA.

The time constant of the FM-AGC is defined by an external capacitor.

2.3

AM - LNA

The AM-LNA 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 1 k

.

2.4

AM - AGC

The programmable AM-RF-AGC senses the mixer inputs and controls the internal PIN diode

and LNA gain.

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

the signal.

The time constant of the AM-AGC is defined with an external capacitor 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 SW, DRM.

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

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