AN2032| Application Note
Maxim/Dallas > App Notes > WIRELESS, RF, AND CABLE Keywords: rf, if, vco, rf design, rfic, voltage controlled oscillator, rf ics, integrated circuits, radio frequency
Feb 05, 2001
APPLICATION NOTE 2032
Trimless IF VCO: Part 1: Design Considerations
This application note explores the design fundamentals needed to implement a trimless, fixed-frequency, IF voltage-controlled oscillator (VCO) and points out the challenges in guaranteeing proper circuit operation. VCOs are essential components in the architecture of most wireless systems. In dual-conversion approaches, a fixedfrequency IF VCO is required to control the frequency translation from IF to baseband and/or baseband to IF. Additional Information: Trimless IF VCO: Part 2: New ICs Simplify Implementation Part 1 of this two-part article explores the design fundamentals needed to implement a trimless, fixed-frequency, IF voltage-controlled oscillator (VCO) and points out the challenges in guaranteeing proper circuit operation. VCOs are essential components in the architecture of most wireless systems. In dual-conversion approaches, a fixed-frequency IF VCO is required to control the frequency translation from IF to baseband and/or baseband to IF. Dual-conversion systems require two oscillators. Typica
lly, the first (RF VCO) tunes over the full range of input channel frequencies, and the second (IF VCO) operates at a single frequency established by the frequency plan. The RF VCO is available as a module, IC, or discrete-component circuit, with modules and ICs being more common. For IF VCOs, small, cost-effective modules are nearly absent from the market. Probable reasons include the need for many arbitrary IF frequencies and the need for large-valued inductors that cannot be lasertrimmed (adjusted) in production. As a result, the IF VCO is usually implemented as a discrete circuit or as part of an IC. Maxim has pioneered a new VCO IC intended for use in wireless systems whose other board-level RF/IF ICs lack that function. Part 2 of this article will introduce the IC, discuss its development, and detail the simple and costeffective applications it makes possible. A discrete-component VCO offers sufficient degrees of freedom to meet the performance objectives of most systems (tuning range, output power, ph
ase noise, current consumption, cost, etc.). For higher volume, costsensitive modern products, however, production-line adjustment of the oscillation frequency is not acceptable. The RF engineer is therefore compelled to devise a VCO that requires no adjustments during assembly, i.e., a trimless VCO. The design is not trivial. In addition to an understanding of VCO design fundamentals, it requires substantial RF engineering effort to ensure that the design is properly centered and that the oscillator tunes to the desired frequency over all allowed variations in component values, temperature, and supply voltage. The following discussion, while explaining pertinent issues in the design of a trimless IF VCO, seeks to develop an appreciation for the magnitude of the task.
VCO Topology
While several oscillator topologies are viable for construction of a practical RF VCO, the one that has proven successful in many commercial VCO modules and countless discrete VCO circuits is the Colpitts commoncollector topology (Figure 1). This topology is useful for a wide range of operating frequencies, from IF to RF.
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