A Dual-Band Switchable IF VCO For GSM/PCS HandsetsSource: Skyworks Solutions, Inc.
Many of today's handset cellular telephones are multifunctional, multiband units. They are complex RF systems with frequency plans requiring multiple RF sources. To accomplish this, the number of VCOs can be increased, however this is expensive and requires more PCB area. This approach strongly contradicts current market trends. A straightforward solution proposed in this application note is using switchable (multiband) VCOs.
Skyworks application note APN1007, Switchable Dual-Band 170/470 MHz VCO for Handset Cellular Applications, discusses a switchable Colpitts VCO design switching between the 170 MHz and 420 MHz range. Described here is a solution for higher frequency range switching, 450/640 MHz, using a DC Cascode Colpitts configuration for the VCO. This design is optimized for the lowest phase noise meeting GSM/PCS handset requirements.
The Colpitts VCO Fundamentals
Fundamental Colpitts VCO operation is illustrated in Figures 1a and 1b. Figure 1a shows the Colpitts VCO circuit as it is usually implemented. In Figure 1b, the same circuit is shown as a common emitter amplifier with parallel feedback. The transistor junction and package capacitors CEB, CCB and CCE are separated from the rest of the transistor parasitic components to demonstrate their direct effect on the VCO tank circuit.
In a real low noise VCO circuit, the capacitor, CVAR, may have a more complicated structure including series and parallel connected discrete capacitors used to set required oscillation frequency and tuning sensitivity. The parallel connection of resonator inductor, LRES, and varactor capacitive branch, CVAR, constitutes a parallel resonator (or simply resonator). A fundamental property of the parallel resonator in a Colpitts VCO implementation is that it always shows inductive impedance at the oscillation frequency. This means that its parallel resonant frequency is always above the oscillation frequency.
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Application Note: A Dual-Band Switchable IF VCO For GSM/PCS Handsets