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ML12149 Datenblatt(PDF) 3 Page - LANSDALE Semiconductor Inc. |
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ML12149 Datenblatt(HTML) 3 Page - LANSDALE Semiconductor Inc. |
3 / 12 page www.lansdale.com Page 3 of 12 Figure 1. Simplified Schematic Q3 Q4 Q1 Q2 Q5 Q6 VCC Q QB VCC GND VREF CNTL 136Ω 200Ω 1000Ω Q2 TANK VREF LANSDALE Semiconductor, Inc. ML12149 OPERATIONAL CHARACTERISTICS A simplified schematic of the ML12149 is found in Figure 1. The oscillator incorporates positive feedback by coupling the base of transistor Q2 to the collector of transistor Q1. In order to mini- mize interaction between the VCO outputs and the oscillator tank transistor pair, a buffer is incorporated into the circuit. This differ- ential buffer is realized by the Q3 and Q4 transistor pair. The dif- ferential buffer drives the gate which contains the primary open collector outputs, Q and QB. The output is actually a current which has been set by an internal bias driver to a nominal current of 4mA. Additional circuitry is incorporated into the tail of the current source which allows the current source to be increased to approximately 10 mA. This is accommodated by the addition of a resistor which is brought out to the CNTL pin. When this pin is tied to ground, the additional current is sourced through the cur- rent source thus increasing the output amplitude of the Q/QB out- put pair. If less than 10mA of current is needed, a resistor can be added to ground which reduces the amount of current. The Q/QB outputs drive an additional differential buffer which generate the Q2 output signal. To minimize current, the circuit is realized as an emitter–follower buffer with an on chip pull down resistor. This output is intended to drive the prescaler input of the PLL synthesizer block. APPLICATION INFORMATION Figure 2 illustrates the external components necessary for the proper operation of the VCO buffer. The tank circuit configura- tion in this figure allows the VCO to be tuned across the full operating voltage of the power supply. This is very important in 3.0 V applications where it is desirable to utilize as much of the operating supply range as possible so as to minimize the VCO sensitivity (MHz/V). In most situations, it is desirable to keep the sensitivity low so the circuit will be less susceptible to external noise influences. An additional benefit to this configuration is that additional regulation/ filtering can be incorporated into the With the AC–coupled tank configuration, the Vtune voltage can be greater than the VCC voltage supplied to the device. There are four main areas that the user directly influences the performance of the VCO. These include Tank Design, Output Termination Selection, Power Supply Decoupling, and Circuit Board Layout/Grounding. The design of the tank circuit is critical to the proper operation of the VCO. This tank circuit directly impacts the main VCO operating characteristics: 1) Frequency of Operation 2) Tuning Sensitivity 3) Voltage Supply Pushing 4) Phase Noise Performance The tank circuit, in its simplest form, is realized as an LC cir- cuit which determines the VCO operating frequency. This is described in Equation 1. In the practical case, the capacitor is replaced with a varactor diode whose capacitance changes with the voltage applied, thus changing the resonant frequency at which the VCO tank operates. The capacitive component in Equation 1 also needs to include the input capacitance of the device and other circuit and parasitic ele- ments. 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 oper- ating point. Equation 1 f= o 1 2 LC √ Vcc line without compromising the tuning range of the VCO. Issue B |
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