STABLE_INDUCTIVE_TUNED_OSCILLATOR

Published:2009/7/21 2:56:00 Author:Jessie | From:SeekIC

In frequency-shift keying (FSK) applications, when modulating high data rates, coupling data to the varactor can pose a problem. A duplex filter is necessary to prevent RF loading or leaking of the oscillator tank while transferring the fast data edges to the varactor. These two constraints contradict one another-especially when the edge's rise time approaches that of the carrier frequency. If an RF choke or a large-value resistor is used to inject data to the varactor, then the edges of the data will be severely distorted by RC or inductive-capacitive (LC) time constants. The loss of these rise times can result in duty-cycle distortion as seen by the threshold level detector in the receiver. To improve upon the traditional VCO techniques, varying the inductor provides increased tuning sensitivity and better resonator Q. Inductive tuning uses the coupling or leakage inductance characteristics of a transformer. Thus, tuning capacitor C4 can be changed to a fixed, high-quality capacitor. Use of a negative -temperature -coefficient capacitor here can balance out the active device's capacitance variation with temperature. The tuning inductor (L1) is now a transformer. At about 48 MHz, it is a nine-turn, center-tapped coil of AWG28 wound on a 725-10 Micrometals core. Other transformers also can be implemented, depending on operating frequencies and performance requirements. The Q for this inductor is about 100, which is more than sufficient to isolate the active device to prevent its pulling the oscillator tank over the extended temperature range of -40 to 85℃. The inductance variation or stability with temperature is dependent on the core material of L1.



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