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19 page 
CC1100E SWRS082 Page 19 of 92 signals are joined together (C131, C121, L121 and L131 for the 470 MHz reference design [3], and L121, L131, C121, L122, C131, C122 and L132 for the 950 MHz reference design [4]) form a balun that converts the differential RF signal on the CC1100E to a single-ended RF signal. C124 is needed for DC blocking. Together with an appropriate LC network, the balun components also transform the impedance to match a 50 load. C125 provides DC blocking and is only needed if there is a DC path in the antenna. For the 950 MHz reference design, this component may also be used for additional filtering, see section 7.5 below. Suggested values for 470 MHz, and 950 MHz are listed in Table 19. The balun and LC filter component values and their placement are important to keep the performance optimized. It is highly recommended to follow the CC1100E EM schematics for the reference designs are available for download from the TI website. 7.3 Crystal A crystal in the frequency range 26-27 MHz must be connected between the XOSC_Q1 and XOSC_Q2 pins. The oscillator is designed for parallel mode operation of the crystal. In addition, loading capacitors (C81 and C101) for the crystal are required. The loading capacitor values depend on the total load capacitance, CL, specified for the crystal. The total load capacitance seen between the crystal terminals should equal CL for the crystal to oscillate at the specified frequency. parasitic L C C C C 101 81 1 1 1 The parasitic capacitance is constituted by pin input capacitance and PCB stray capacitance. Total parasitic capacitance is typically 2.5 pF. The crystal oscillator is amplitude regulated. This means that a high current is used to start up the oscillations. When the amplitude builds up, the current is reduced to what is necessary to maintain approximately 0.4 Vpp signal swing. This ensures a fast start-up, and keeps the drive level to a minimum. The ESR of the crystal should be within the specification in order to ensure a reliable start-up (see Section The initial tolerance, temperature drift, aging and load pulling should be carefully specified in order to meet the required frequency accuracy in a certain application. 7.4 Reference Signal The chip can alternatively be operated with a reference signal from 26 to 27 MHz instead of a crystal. This input clock can either be a full- swing digital signal (0 V to VDD) or a sine wave of maximum 1 V peak-peak amplitude. The reference signal must be connected to the XOSC_Q1 input. The sine wave must be connected to XOSC_Q1 using a serial capacitor. This capacitor can be omitted when using a full-swing digital signal. The XOSC_Q2 line must be left un-connected. C81 and C101 can be omitted when using a reference signal. 7.5 Additional Filtering In the 950 MHz reference design, C126 and L125 together with C125 build an optional filter to reduce emission at 770 MHz. This filter is necessary for applications with an external antenna connector that target compliance with ARIB STD-T96. If this filtering is not necessary, C125 will work as a DC block (only necessary if there is a DC path in the antenna). C126 and L125 should in that case be left unmounted. Additional external components (e.g. an RF SAW filter) may be used in order to improve the performance in specific applications. 7.6 Power Supply Decoupling The power supply must be properly decoupled close to the supply pins. Note that decoupling capacitors are not shown in the application circuit. The placement and the size of the decoupling capacitors are very important to achieve the optimum performance. The should be followed closely. |
