Varactorless high frequency modulator circuit design electronic project

Posted on Feb 7, 2014

This varactorless high frequency modulator electronic project, must be powered from a simple DC 3 volt power source ( you can use a 3 volt battery). 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 problem

Varactorless high frequency modulator circuit design electronic project
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in many battery-powered systems. Such a problem can be overcome by employing base-charging capacitance modulation. Resistor R1 establishes Ql`s current, and R2 allows control of the collector bias current by Vmod. The transmission 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 Ql`s base. T1 reacts with this negative resistance to produce sustained oscillations, Frequency modulation is accomplished by changing Q1`s collector bias current and thus changing Ql`s base-charging capacitance. This effect is "seen" at Ql`s base and causes a frequency shift in the resonators quarter-wave node.

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