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9 / 16 page MC100EL1648 http://onsemi.com 9 VOLTAGE CONTROLLED MODE The tank circuit configuration presented in Figure 11, Voltage Controlled Varactor Mode, allows the VCO to be tuned across the full operating voltage of the power supply. Deriving from Figure 6, the tank capacitor, C, is replaced with a varactor diode whose capacitance changes with the voltage applied, thus changing the resonant frequency at which the VCO tank operates as shown in Figure 3, tank option #1. The capacitive component in Equation 1 also needs to include the input capacitance of the device and other circuit and parasitic elements. Figure 11. Voltage Controlled Varactor Mode 50 70 90 110 130 150 170 190 02 46 8 10 Vin, INPUT VOLTAGE (V) Figure 12. Plot 1. Dual Varactor MMBV609, VIN vs. Frequency C L 4 (3) VCC 3 (1) 2 (14) VIN FOUT Tank #1 8 (10) 1 (12) * 0.1 mF0.1 mF 5 (5) 6 (7) 7 (8) VEE 0.1 mF 0.1 mF 0.01 mF 100 mF ** 1 KW *Use high impedance probe (>1.0 MegW must be used). **The 1200 W resistor and the scope termination imped- ance constitute a 25:1 attenuator probe. Coax shall be CT−070−50 or equivalent. L = Micro Metal torroid #T20−22, 8 turns #30 Enameled Copper wire (@ 40 nH) C = MMBV609 8 pin (14 pin) lead package When operating the oscillator in the voltage controlled mode with Tank Circuit #1 (Figure 3), it should be noted that the cathode of the varactor diode (D), pin 8 (for 8 lead package) or pin 10 (for 14 lead package) should be biased at least 1.4 V above VEE. Typical transfer characteristics employing the capacitance of the varactor diode (plus the input capacitance of the device, about 6.0 pF typical) in the voltage controlled mode is shown in Plot 1, Dual Varactor MMBV609 Vin vs. Frequency. Figure 6, Figure 7, and Figure 8 show the accuracy of the measured frequency with the different variable capacitance values. The 1.0 k W resistor in Figure 11 is used to protect the varactor diode during testing. It is not necessary as long as the dc input voltage does not cause the diode to become forward biased. The tuning range of the oscillator in the voltage controlled mode may be calculated as follows: f max f min + CD(max) ) CS CD(min) ) CS Where f min + 1 2p ( L(CD(max) ) CS ) Where CS = Shunt Capacitance (input plus external capacitance) CD = Varactor Capacitance as a function of bias voltage Good RF and low−frequency bypassing is necessary on the device power supply pins. Capacitors on the AGC pin and the input varactor trace should be used to bypass the AGC point and the VCO input (varactor diode), guaranteeing only dc levels at these points. For output frequency operation between 1.0 MHz and 50 MHz, a 0.1 mF capacitor is sufficient. At higher frequencies, smaller values of capacitance should be used; at lower frequencies, larger values of capacitance. At high frequencies, the value of bypass capacitors depends directly on the physical layout of the system. All bypassing should be as close to the package pins as possible to minimize unwanted lead inductance. Several different capacitors may be needed to bypass various frequencies. |
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