This electronic project circuit is a very simple varactorless high frequency modulator . 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 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.
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) .