TDA7513T datasheet(27/84 Pages) STMICROELECTRONICS

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TDA7513T

Functional description

27/84

Functional description

3.1

FM section

3.1.1

Mixer1, AGC and 1st IF

Mixer1 is a wide dynamic range stage with low noise and large input signal performance.

The mixer1 tank center frequency can be adjusted by software (IF1T). The AGC operates on

different sensitivities and bandwidths (FMAGC) in order to improve the input sensitivity and

dynamic range (keyed AGC). The output signals of AGC are controlled voltage and current

for pre-amplifier and pre-stage PIN diode attenuator. Two 10.7MHz amplifiers (IFG1 - fixed

gain - and IFG2 - programmable) correct the IF ceramic insertion loss.

3.1.2

Mixer2, limiter and demodulator

In this 2nd mixer stage the first 10.7MHz IF is converted into the second 450kHz IF. A multi-

stage limiter generates signals for the complete integrated demodulator without external

tank. MPX output DC offset compensation is possible via software.

3.1.3

Quality detection and ISS field strength

Parallel to the mixer2 input a 10.7MHz limiter generates a signal for the digital IF counter

and a fieldstrength output signal. This internal unfiltered fieldstrength is used for adjacent

channel and multipath detection. The behaviour of this output signal can be corrected for DC

offset (SL). The internally generated unfiltered fieldstrength is filtered at pin #SMETERTC

and used for softmute function, FM AGC keying and generation of ISS filter switching signal

for weak input level (sm). (See

Figure 1.)

3.1.4

Adjacent channel detector

The input of the adjacent channel detector is AC coupled to the internal unfiltered

fieldstrength. A programmable and configurable highpass or bandpass filter (ACF) and

amplifier (ACG) followed by a rectifier measure the adjacent channel content. This voltage is

compared with an adjustable threshold (ACWTH, ACNTH) comparator (comparator1). The

output signal of this comparator generates a DC level at PIN15 with a programmable time

constant. Time constant control (TISS) for the adjacent channel is made by linearly charging

and discharging an external capacitor following. The charge current is fixed and the

discharge current is controlled by I2C bus. This level produces digital signals (ac, ac+) after

comparing by the following comparator4. The adjacent channel information after filtering

and rectification is available as analog output on pin #TUNQUALITY (the gain can be

selected via I2C bus) in combination with multipath content information. It is possible to

enable adjacent channel content information output only via I2C bus control.

3.1.5

Multipath detector

The input of the multipath detector is AC coupled to the internal unfiltered fieldstrength. A

programmable band-pass filter (MPF) and amplifier (MPG) followed by a rectifier measures

the multipath content. This voltage is compared with an adjustable threshold (MPTH)

comparator (comparator2). The output signal of this comparator2 is used to disable the

adjacent channel detector control of the ISS filter in case of strong multipath, which would

otherwise result in bandwidth reduction because of the multipath-induced high-frequency

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