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SX1223I073TRT Datasheet(PDF) 9 Page - Semtech Corporation |
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SX1223I073TRT Datasheet(HTML) 9 Page - Semtech Corporation |
9 / 25 page © Semtech 2007 www.semtech.com 9 SX1223 the user according to the selected frequency band, whereas the three VCO_IB bits can be either forced by the user or set automatically by the circuit which will select the combination having the best phase noise. This automatic setting can be enabled by setting the three VCO_IB bits to ‘0’. Table 2 lists the bias setting used for the different VCO_freq settings in automatic mode. When any of the VCO_IB bit is set to 1, it will overrule the automatic setting. RF frequency VCO_IB2 VCO_IB1 VCO_IB0 VCO_freq1 VCO_freq0 425/850 MHz 1 1 1 0 0 434/868 MHz 1 0 1 0 1 457/915 MHz 0 1 1 1 0 475/950 MHz 0 0 0 1 1 Table 2: VCO bit settings. The bias bits optimize the phase noise, and the frequency bits control a capacitor bank in the VCO. The tuning range, the RF frequency versus varactor voltage, is dependent on the VCO frequency setting, and is shown in Figure 3. When the tuning voltage is in the range from 1 to 1.6V, the VCO gain is at its maximum, approximately 65- 70 MHz/V. It is then recommended that the varactor voltage is kept as much as possible in this range. Figure 3 : RF frequency vs. varactor voltage and VCO frequency bit The input capacitance at the varactor pin must be taken into considerations when designing the PLL loop filter. This can be critical when designing a loop filter with high bandwidth, which gives relatively small component values. The input capacitance is approximately 6 pF. For test purposes, the VCO can be bypassed by applying a differential local oscillator (LO) signal to the device on pin CPOUT and VARIN. A resistor of 18 k to ground and a series capacitor of 47 pF are needed on both pins for proper biasing. The register bit VCO_by must be set to ‘1’. VCO gain Vdd=3.3V, LDO_en=1, VCO_IB=0 800 820 840 860 880 900 920 940 960 980 1000 1020 1040 0 0.5 1 1.5 2 2.5 Vvaractor [V] Freq [MHz] VCO_freq=11 VCO_freq=10 VCO_freq=01 VCO_freq=00 |
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