Trimless IF VCO: Part 2: New ICs Simplify Implementation

Posted on Feb 5, 2014

Presents a family of IF voltage-controlled oscillators (VCOs) to cover the frequency range from 45MHz to 650MHz. The ICs are in 6-pin SOT23 packages. Phase noise is -100dBc/Hz for the MAX2608 at 300MHz to 500MHz. An off-chip inductor sets the operating frequency. The output stage can be matched with resistors or lossless techniques. Desig

Trimless IF VCO: Part 2: New ICs Simplify Implementation
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ning a VCO for use with a fixed intermediate frequency (IF) can be daunting. Fortunately, VCO ICs from Maxim (MAX2605-MAX2609) can simplify the task. Compared to conventional discrete-device VCOs, the Maxim parts cost less and require less PC board space. In a traditional IF VCO design, the oscillator core and the output buffer stage are formed by discrete transistors, resistors, capacitors, and inductors (Figure 1). The tank is built from a networkconsisting of the frequency-setting inductor, varactors, coupling capacitors, and feedback capacitors. The output stage uses reactive elements to match the output impedance to a particular load impedance. To ensure a successful design, the component values not only must establish a desired nominal oscillation frequency, but they must also guarantee an adequate tuning range, proper biasing, oscillator startup under all conditions, and proper output-stage performance. Problems can occur even with a good first-order design because of the trade-off that exists among current consumption, startup margin, frequency tuning range, and phase noise. A major disadvantage of discrete IF VCO designs is the amount of PC board area required. Much effort must be expended in optimizing the layout to below 6mm x 10mm. Furthermore, the PC board layout has a critical effect on the VCO`s performance and design accuracy. The layout contains parasitic capacitances and inductances that affect the...

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