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TEA5757HL Datasheet(PDF) 7 Page - NXP Semiconductors |
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TEA5757HL Datasheet(HTML) 7 Page - NXP Semiconductors |
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7 / 40 page  2000 Feb 02 7 Philips Semiconductors Product specification Self Tuned Radio (STR) 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. |
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