TDA9873HS datasheet(7/40 Pages) PHILIPS

7 / 40 page

2000 Apr 04

Philips Semiconductors

Product speciﬁcation

Multistandard dual carrier stereo sound decoder

TDA9873H

FUNCTIONAL DESCRIPTION

FM demodulators

The FM demodulators are Narrow-Band Phase-Locked

Loops (NBPLLs) with external loop ﬁlters, to provide the

required selectivity. To achieve good selectivity, linear

Phase Detectors (PDs) and constant input levels are

required. The intercarrier signal from the input terminal is

fed via high-pass filters and gain controlled amplifiers to

the phase detectors. A carrier cancellation circuit placed

before the amplifier for the second PLL is used to reduce

the first sound carrier. The PD output signals control the

integrated relaxation oscillators via the loop filters.

The frequency range is approximately 4 to 7 MHz. As a

result of locking, the oscillator frequency tracks with the

modulation of the input signal and the oscillator control

voltages are superimposed by the AF voltages. Using this

method, the FM-PLLs operate as FM demodulators.

The AF voltages are present at the loop filters and fed via

buffers with 0 dB gain to the audio amplifiers.

The supported standards and their characteristics are

given in Table 1.

Digital acquisition help

A narrow-band PLL requires a measure to lock to the

wanted input signal. Each relaxation oscillator of the three

integrated PLLs (first and second sound carriers and pilot

carrier) has a wide frequency range. To guarantee correct

locking of the PLL with respect to the catching range, the

digital acquisition help provides individual control until the

VCO frequency is within the standard and PLL dependent

lock-in window, related to the standard dependent carriers.

It ensures that the oscillator frequency of the FM-PLL is

within

±225 kHz of the sound carrier to be demodulated.

The pilot carrier frequency window is

±150 Hz.

The working principal of the digital acquisition help is as

follows. The VCOs are connected, one at a time, to a

down-counter. The counter start value is standard

dependent and predefined for each of the three PLLs.

After a given counting time the stop value of the

down-counter is probed.

If the stop value is lower (higher) than the expected value

range, the VCO frequency is higher (lower) than the lock-in

window. A negative (positive) control current is injected

into the loop filter for a short time, thereby decreasing

(increasing) the VCO frequency by a proportional value.

If the stop value meets the expected value range, the VCO

frequency is within the defined lock-in window and no

control current is injected into the loop filter.

In an endless circle the VCO of the next PLL will be

connected to the down-counter and the described

procedure starts again.

The whole tracing as well as the counting time itself is

derived from the external frequency reference. The cycle

time is 256

µs.

Auto mute

If a sound carrier is missed, acquisition pulses are

generated when the NBPLL frequency leaves the window

edges. To avoid noise at the audio output, an I2C-bus

switchable mute-enable stage is built in. If auto mute is

enabled via the I2C-bus, the circuit mutes immediately

after the first acquisition pulse. If a sound carrier occurs

(no further acquisition pulses), the mute stage

automatically returns to active mode after 40 ms.

If the first sound carrier is not present, the second audio

channel will also be muted.

Audio preampliﬁer

The AF preamplifiers are operational amplifiers with

internal feedback, high gain and high common mode

rejection. The AF voltages from the PLL demodulators

(small output signals) are amplified by approximately

34 dB. Using a DC operating point control circuit, the

AF amplifiers are decoupled from the PLL DC voltage.

The amplified AF signals are available at the output

terminals and fed via external decoupling capacitors to the

stereo decoder input terminals.

Stereo decoder

The input circuit incorporates a soft-mute stage which is

controlled by the FM-PLL acquisition circuit. The auto

mute function can be disabled via the I2C-bus.

The AF output voltage is 500 mV (RMS) for 54%

modulation, clipping therefore may occur at high

over-modulation. If more headroom is required the input

signal can be attenuated by 6 dB via the I2C-bus.

A stereo adjustment (see Fig.6) is incorporated to correct

the FM demodulator output voltage spread (see Table 19).

If no I2C-bus adjustment is required (potentiometer

adjustment or no adjustment) the default value should be

0 dB for B/G, M and D/K (2) standard. For the standards

D/K (1) and D/K (3) the second sound carrier frequency is

below the first sound carrier which results in a lower

AF output level for the second sound carrier. In this state,

a gain of +0.1 dB for D/K (1) and +0.2 dB for D/K (3) is

preferred.

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