TDA7540N datasheet(34/76 Pages) STMICROELECTRONICS

34 / 76 page

Functional description

TDA7540N

34/76

VQual = 0.8b (VNoise-0.8 V)+ a (VREF1-VMpout).

The noise-signal is the PEAK-signal without additional influences (see noise blanker

description). The factor 'a' can be programmed from 0.6 to 1.05(QDC) and the factor b can

be programmed from 6dB to 15dB ( QNG). The output is a low impedance output able to

drive external circuitry as well as simply fed to an AD-converter for RDS applications.

4.3.5

AFS control and stereo decoder mute

The TDA7540N is supplied with several functionality to support AF-checks using the stereo

decoder. The additional pin (AFS) is implemented in order to speed up the stereo decoder

AF-functions compared to IIC controlling.

The block diagramm of AFS function is shown in Figure 17.

In order to separate the different functions of the AFS pin, two different logic thresholds are

implemented. Below the higher threshold voltage (2.4V) only the multipath-detector is

switched into small time constant (internal logical signal MPfast).

Below the lower threshold voltage (0.8V) the full AFS function is activated. The MPXIN pin is

switched into high impedance mode (internal signal AFSMute), which avoids any clicks

during the jump condition. If the stereo decoder is not muted, it is possible at the same time

to evaluate the noise- and multipath-content of the alternate frequency using the Quality

detector output.

Furthermore the AFS pin does also freeze the condition of pilot locking and magnitude

(internal signal PDhold). The Pdhold signal is defined by Vth1 or Vth2, dependent on the

PDH signal.

4.4

PLL and IF counter section

4.4.1

PLL frequency synthesizer block

This part contains a frequency synthesizer and a loop filter for the radio tuning system. Only

one VCO is required to build a complete PLL system for FM world tuning and AM

upconversion (Figure 9). For auto search stop operation an IF counter system is available.

The PLL counter works in a two stages configuration. The first stage is a swallow counter

with a two modulus (32/33) precounter. The second stage is an 11-bit programmable

counter.

The circuit receives the scaling factors for the programmable counters and the values of the

reference frequencies via an I2C-Bus interface.The reference frequency is generated by an

adjustable internal (XTAL) oscillator followed by the reference divider. The main reference

and step-frequencies are free selectable (RC, PC).

Output signals of the phase detector are switching the programmable current sources. The

loop filter integrates their currents to a DC voltage.

The values of the current sources are programmable by 6 bits also received via the I2C Bus

(A, B, CURRH, LPF).

To minimize the noise induced by the digital part of the system, a special guard

configuration is implemented.

The loop gain can be set for different conditions by setting the current values of the

chargepump generator.

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