VARACTORLESS_HF_MODULATOR

Published:2009/6/23 4:36:00 Author:Jessie | From:SeekIC

Traditionally, high-frequency oscillators are frequency-modulated by using a varactor. However, varactors usually require a large voltage change to achieve a reasonable capacitance change-a prob-lem in many battery-powered systems.Such a problem can be overcome by employing base-charging capacitance modulation. Resistor RI establishes Q1's current, and R2 allows control of the collector bias current byVmod The trans-mission line (T1) in the negative resistance-type oscillator determines the frequency of oscillation. T1 is a high-quality, low-loss, ceramic coaxial shorted quarter-wave transmission line. Under proper terminal impedances, a negative resistance is seen at Q1's base. T1 reacts with this negative resis-tance to produce sustained oscillations.Frequency modulation is accomplished by changing Q1's collector bias current and thus chang-ing Q1's base-charging capacitance. This effect is seen at Q1's base and causes a frequency shift in the resonators quarter-wave node.



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