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TEA5757HL

Self Tuned Radio (STR)

Philips Electronics 

Philips Semiconductors

Self Tuned Radio (STR)

Product specification

TEA5757HL; TEA5759HL

FUNCTIONAL DESCRIPTION

The TEA5757HL; TEA5759HL is an integrated AM/FM

stereo radio circuit including digital tuning and control

functions.

The radio

The AM circuit incorporates a double balanced mixer, a

one-pin low-voltage oscillator (up to 30 MHz) and is

designed for distributed selectivity.

The AM input is designed to be connected to the top of a

tuned circuit. AGC controls the IF amplification and for

large signals it lowers the input impedance of the AM

front-end.

The first AM selectivity can be an IF-Tank (IFT) as well as

an IFT combined with a ceramic filter; the second one is an

IFT.

The FM circuit incorporates a tuned RF stage, a double

balanced mixer, a one-pin oscillator and is designed for

distributed IF ceramic filters. The FM quadrature detector

uses a ceramic resonator.

The TEA5757HL; TEA5759HL can also be used with an

external FM front-end circuit. The external front-end is

activated by the FM-ON/OFF signal. The AFC circuit in the

TEA5757HL; TEA5759HL provides a tuning voltage to

drive the VCO of the external FM front-end. The frequency

of the external VCO is counted in the Self Tuned Radio

(STR) tuning system.

The PLL stereo decoder incorporates a signal dependent

stereo-blend circuit and a soft-mute circuit.

Tuning

The tuning concept of Self Tuned Radio (STR) is based on

FUZZY LOGIC: it mimics hand tuning (hand tuning is a

combination of coarse and fine tuning to the qualitatively

best frequency position). As a consequence the tuning

system is very fast.

The tuning algorithm, which is controlled by the sequential

circuit (see Fig.1), is completely integrated; so there are

only a few external components needed.

The bus and the microcontroller can be kept very simple.

The bus only consists of three wires (BUS-CLOCK, DATA

and WRITE-ENABLE). The microcontroller must basically

give two instructions:

• Preset operation

• Search operation.

PRESET OPERATION

In preset mode, the microcontroller has to load information

such as frequency band, frequency and mono/stereo. This

information has to be sent via the bus to the STR.

The internal algorithm controls the tuning sequence as

follows:

1. The information is loaded into a shift register, a

last-station memory and the counter.

2. The Automatic Frequency Control (AFC) is switched

off.

3. The counter starts counting the frequency and the

tuning voltage is varied until the desired frequency

roughly equals the real frequency.

4. The AFC is then switched on and the counter is

switched off.

5. The real frequency is more precisely tuned to the

desired frequency.

After the AFC has tuned the real frequency to the desired

frequency an in-lock signal can be generated. In order to

get a reliable in-lock signal, there are two parameters

measured: the field strength and the S-curve. The field

strength indicates the strength of the station and by

looking at the S-curve the system can distinguish false

in-locks from real in-locks (false in-locks occur on the

wrong slope of the S-curve).

In the event of fading or pulling the in-lock signal becomes

logic 0 and the synthesizer will be switched on again and

the algorithm will be repeated.

SEARCH OPERATION

During a search operation, the only action the

microcontroller has to take is: sending the desired band

plus the direction and the search sensitivity level to the

STR. The search operation is performed by the charge

pump until an in-lock signal is generated (combination of

measuring the field strength and the S-curve). The AFC

then fine tunes to the station. The frequency belonging to

the found station will be counted by the counter and written

into the last-station memory and the shift register of the

counter. At this time the frequency is available in the shift

register and can be read by the microcontroller.

The microcontroller decides whether the frequency is

within the desired frequency band. If so, this frequency can

be stored under a preset and if not, a new search action

should be started.

2000 Feb 02

7

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