TEA6100 Просмотр технического описания (PDF) - Philips Electronics

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TEA6100

FM/IF system and microcomputer-based tuning interface

Philips Electronics 

Philips Semiconductors

FM/IF system and microcomputer-based

tuning interface

Product specification

TEA6100

In the AM mode, the level information at pin 3 cannot be directly used owing to AM modulation on the output signal of

the level amplifier. This signal requires filtering, which is achieved by the following method:

• the multiplexer is switched to a position which causes the signal to be applied to the attenuator

• after attenuation the signal is fed to an amplifier (the resultant gain of attenuator and amplifier is unity), after

amplification the signal is filtered by an internal resistor and external capacitor

• after filtering the signal is applied to ADC2 and is externally available

In AM mode pin 5 contains the level information.

The voltages on pin 3 and 5 are converted into two 3-bit digital words by the ADC, which can then be read out by the

I2C bus. The meaning of the 3- bit words is shown in Table 1.

Table 1 3-bit words

WORD

1

2

FM

multipath

level

POSITION

AM

level without modulation

level with modulation

The FM modulated signal is converted into an audio signal by the symmetrical quadrature detector. The main advantage

of such a detector is that it requires few external components.

An FM signal requires good AM suppression, and as a result, the IF amplifiers must act as limiters. To achieve good

suppression on small input signals the IF amplifiers must have a high gain and thus a high sensitivity. High sensitivity is

an undesirable property when used in car radio applications, this problem is solved by having an externally adjustable

mute stage to control the overall sensitivity of the device.

The IF mute stage is controlled by the level amplifier (soft muting) and is only active in FM mode. If the input falls below

a predetermined level, the mute stage becomes active. To avoid the 'ON/OFF' effect of the audio signal due to

fluctuations of the input signal, the mute stage is activated rapidly but de-activated slowly. The mute stage is de-activated

slowly, via a current source and an external capacitor at pin 2, to avoid aggressive behaviour of the audio signal. It is

possible to adjust the '−3 dB limiting point' of the audio output via the level voltage due to the level signal being externally

adjustable. If hard muting is required then pin 2 must be switched to ground.

The 8-bit counter allows accurate stop information to be obtained, because exact tuning is achieved when the measured

frequency is equal to the centre frequency of the IF filter.

To measure the input frequency, the number of pulses which occur in a defined time must be counted. This defined time

is refered to as 'window'. A wide window indicates a long measuring time and therefore a high accuracy. The counter

resolution is defined as Hertz per count. Due to the TEA6100 having to measure the IF frequencies of AM and FM, the

counter resolution must be adjustable (different channel spacing). The counter resolution depends on the setting of

dividers 1 (N1), divider 2 (N2) and the reference frequency (Fref). The divider ratios of N1 and N2 are controlled by

software (see section PROGRAMMING INFORMATION). In Table 3 the window and counter resolution has been

calculated for a reference frequency of 40 kHz. The accuracy is controlled by bit 7 of the input word. Although the

resolution is the same for bit 7 = logic 0 and bit 7 = logic 1, the width of the window doubles when bit 7 = logic 1.

• bit 7 = 0, accuracy = ± counter resolution

• bit 7 = 1, accuracy = ± 1⁄2 counter resolution

August 1987

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