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T5760-TG Datasheet(PDF) 4 Page - ATMEL Corporation |
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T5760-TG Datasheet(HTML) 4 Page - ATMEL Corporation |
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4 / 32 page  T5760 T5761 / Rev. A2, 19-Oct-00 Preliminary Information 4 (32) The value of CL should be optimized for the individual board layout to achieve the exact value of fXTO (the best way is to use a crystal with known load resonance fre- quency to find the right value for this capacitor) and hereby of fLO. When designing the system in terms of re- ceiving bandwidth and local oscillator accuracy, the accuracy of the crystal and the XTO must be considered. If a crystal with $30 ppm adjustment tolerance at 25_C , $50ppm over Temperature –40_C to 105_C, $10 ppm of total aging and a CM ( motional capacitance ) of 7 fF is used, an additional XTO pulling of $30 ppm has to be added. The resulting total LO tolerance of $120ppm agrees with the receiving bandwidth specification of the T5760/T5761 if the T5750 has also a total LO tolerance of $120 ppm. DVCC XTAL TEST 3 TEST 2 n.c. V C S L Figure 4. XTO peripherals The nominal frequency fLO is determined by the RF input frequency fRF and the IF frequency fIF using the following formula (low side injection): fLO = fRF – fIF To determine fLO , the construction of the IF filter must be considered at this point. The nominal IF frequency is fIF = 950 kHz. To achieve a good accuracy of the filter corner frequencies, the filter is tuned by the crystal fre- quency fXTO. This means that there is a fixed relation between fIF and fLO. fIF = fLO / 915 The relation is designed to achieve the nominal IF fre- quency of fIF = 950 kHz for the 868.3 MHz version. For the 915 MHz version an IF frequency of fIF = 1.0 MHz results. The RF input either from an antenna or from a RF genera- tor must be transformed to the RF input Pin LNA_IN. The input impedance of that pin is provided in the electrical parameters. The parasitic board inductances and capaci- tances influence the input matching. The RF receiver T5760/T5761 exhibits its highest sensitivity if the LNA is power matched. This makes the matching to an SAW filter as well as to 50 W or an antenna more easy. Figure 33 shows a typical input matching network for fRF = 868.3 MHz to 50 W. Figure 34 illustrates an according input matching for 868.3 MHz to an SAW. The input matching network shown in Figure 33 is the reference net- work for the parameters given in the electrical characteristics. Analog Signal Processing IF Filter The signals coming from the RF front end are filtered by the fully integrated 4th-order IF filter. The IF center fre- quency is fIF = 950 kHz for applications where fRF = 868.3 MHz and fIF =1.0 MHz for fRF = 915 MHz. The nominal bandwidth is 600 kHz. Limiting RSSI Amplifier The subsequent RSSI amplifier enhances the output signal of the IF amplifier before it is fed into the demod- ulator. The dynamic range of this amplifier is DRRSSI = 60 dB. If the RSSI amplifier is operated within its linear range, the best S/N ratio is maintained in ASK mode. If the dynamic range is exceeded by the transmitter signal, the S/N ratio is defined by the ratio of the maxi- mum RSSI output voltage and the RSSI output voltage due to a disturber. The dynamic range of the RSSI ampli- fier is exceeded if the RF input signal is about 60 dB higher compared to the RF input signal at full sensitivity. In FSK mode the S/N ratio is not affected by the dynamic range of the RSSI amplifier, because only the hard limited signal from a high gain limiting amplifier is used by the demodulator. The output voltage of the RSSI amplifier is internally compared to a threshold voltage VTh_red. VTh_red is deter- mined by the value of the external resistor RSens. RSens is connected between Pin SENS and GND or VS. The output of the comparator is fed into the digital control logic. By this means it is possible to operate the receiver at a lower sensitivity. If RSens is connected to GND, the receiver switches to full sensitivity. It is also possible to connect the Pin SENS di- rectly to GND to get the maximum sensitivity. If RSens is connected to VS, the receiver operates at a lower sensitivity. The reduced sensitivity is defined by the value of RSens, the maximum sensitivity by the signal-to- noise ratio of the LNA input. The reduced sensitivity depends on the signal strength at the output of the RSSI amplifier. Since different RF input networks may exhibit slightly different values for the LNA gain, the sensitivity values given in the electrical characteristics refer to a specific input matching. This matching is illustrated in figure 33 |
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