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Maxim > Design Support > Technical Documents > Application Notes > Wireless and RF > APP 2032
Keywords: rf, if, vco, rf design, rfic, voltage controlled oscillator, rf ics, integrated circuits, radio frequency
APPLICATION NOTE 2032
Trimless IF VCO: Part 1: Design Considerations
Sep 02, 2002
Abstract: 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 fixed-frequency 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. Typically, 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 laser-trimmed (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 cost-effective 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, phase noise, current consumption, cost, etc.). For higher
volume, cost-sensitive 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
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