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MC100EL1648DTR2 bảng dữ liệu(PDF) 8 Page - ON Semiconductor |
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8 / 16 page MC100EL1648 http://onsemi.com 8 FIXED FREQUENCY MODE The MC100EL1648 external tank circuit components are used to determine the desired frequency of operation as shown in Figure 8, tank option #2. The tank circuit components have direct impact on the tuning sensitivity, IEE, and phase noise performance. Fixed frequency of the tank circuit is usually realized by an inductor and capacitor (LC network) that contains a high Quality factor (Q). The plotted curve indicates various fixed frequencies obtained with a single inductor and variable capacitor. The Q of the components in the tank circuit has a direct impact on the resulting phase noise of the oscillator. In general, when the Q is high the oscillator will result in lower phase noise. Figure 8. Fixed Frequency LC Tank CAPACITANCE (pF) 470 370 270 170 70 −30 0.3 300 500 1000 2000 10000 Measured Frequency (MHz) Calculated Frequency (MHz) 570 0 0.1 mF C L 8 (10) 1 (12) 4 (3) VCC 3 (1) 2 (14) Test Point FOUT Tank #2 5 (5) 6 (7) 7 (8) VEE 0.1 mF 0.1 mF 0.01 mF 100 mF 0.1 mF 0.1 mF Note 1 Capacitor for tank may be variable type. (See Tank Circuit #3.) Note 2 Use high impedance probe (> 1 MW ). L = Micro Metal torroid #T20−22, 8 turns #30 Enameled Copper wire (@ 40 nH) C = 3.0−35 pF Variable Capacitance (@ 10 pF) 8 pin (14 pin) lead package QL ≥ 100 Only high quality surface−mount RF chip capacitors should be used in the tank circuit at high frequencies. These capacitors should have very low dielectric loss (high−Q). At a minimum, the capacitors selected should be operating at 100 MHz below their series resonance point. As the desired frequency of operation increases, the values of the tank capacitor will decrease since the series resonance point is a function of the capacitance value. Typically, the inductor is realized as a surface−mount chip or a wound coil. In addition, the lead inductance and board inductance and capacitance also have an impact on the final operating point. The following equation will help to choose the appropriate values for your tank circuit design. f0 + 1 2p LT *CT Where LT = Total Inductance CT = Total Capacitance Figure 9 and Figure 10 represent the ideal curve of inductance/capacitance versus frequency with one known tank component. This helps the designer of the tank circuit to choose desired value of inductor/capacitor component for the wanted frequency. The lead inductance and board inductance and capacitance will also have an impact on the tank component values (inductor and capacitor). Figure 9. Capacitor Value Known (5 pF) Inductance vs. Frequency with 5 pF Cap 5 10 15 20 25 30 35 40 45 50 0 700 1000 1300 160 400 FREQUENCY (MHz) Figure 10. Inductor Value Known (4 nH) Capacitance vs. Frequency with 4 nH Inductance 5 10 15 20 25 30 35 40 45 50 0 700 1000 1300 160 400 FREQUENCY (Hz) |
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