APN1005

www.DataSheet.co.kr APPLICATION NOTE APN1005: A Balanced Wideband VCO for Set-Top TV Tuner Applications Introduction Modern set-top TV DBS tuner systems require more channel coverage, while maintaining competitive prices. This situation creates tough design goals: to improve performance and simplify design. Balanced VCO configuration could be a competitive circuit solution, since it provides the widest tuning range with practical circuitry and layout. However, tuning margins would be further improved by optimizing the varactor manufacturing process. Skyworks has developed such a process to satisfy the most ambitious wideband design goals. In this publication, we will address the design of the balancedtype voltage control oscillator (VCO) based on the newly developed varactor SMV1265-011 with the unique set of capacitance tuning ratios and Q-quality. VCO Model Figure 1 shows the VCO model built for open loop analysis in Libra Series IV including the SMV1265-011 varactor model. The circuit schematic in Figure 2 shows a pair of transistors in a single feedback loop, connected so that collector currents would be 180° shifted (ideally). A pair of back-to-back connected SMV1265-011 varactors is used, rather than a single one. This allows lower capacitance at the high-voltage range, without changing the tuning ratio. The reason is that, apart from package capacitance, certain mounting fringing capacitances, though small, may strongly affect higher frequency margins. The effects o...