optimize varactor tuned oscillators

Posted on Feb 5, 2014

Millimeter-wave frequency bands are attractive for their wide available bandwidths. There are a number of ways to generate these signals but, for each type of oscillator, it is desirable to be able to tune the source electronically, as well as in a defined and controlled manner. By using a suitable reactive device (such as a varactor diode) for tuning these millimeter-wave oscillators,

optimize varactor tuned oscillators
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the relationship between an applied voltage and the resulting frequency can be precisely defined. This can aid the design engineer in achieving the required spectral performance at these higher frequencies. Oscillators developed for use at millimeter-wave frequencies are typically designed around waveguide housings. Electronic tuning of a waveguide-type oscillator can be accomplished in a number of ways, some of which are shown in Fig. 1. Additional information can be found in the technical literature (ref. 1) and in a recent article (ref. 2). The tuning sensitivity of a millimeter-wave varactor-tuned oscillator (VTO) can be estimated by means of relatively simple models, and this article hopes to provide some sights into the tuning relationship. Usually, the approach is to keep the fixed capacity with the varactor diode as large as possible, using the varactor diode to control the resonant frequency to the greatest extent. A varactor diode is essentially an active device with positive-negative (PN) junction which has reverse bias applied. This results in a movement of charge carried away from the junction, so that this region is referred to as the depletion layer. The depletion layer has charge on either side of the junction and acts like a parallel-plate capacitor (Fig. 1). The capacitance relationship for a parallel-plate capacitor is C = µA/W, where: The typical fabrication process for a varactor diode results in a...

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