SWEPT_OSCILLATOR_FOR_HAM_BAND_ANTENNA_TUNING

Published:2009/7/13 3:39:00 Author:May | From:SeekIC

Transistor Q1 acts as a constant-current generator, charging capacitor C1 at a rate determined by the value of R1. The high input impedance of U1b ensures that the constant-current generator is not excessively loaded and forwards the voltage level of C1 to U1c. U1c is configured as a comparator, and output pin 8 goes low when pin 9 rises above 9 V. This switching action applies a trigger pulse via capacitor C2 to activate the monostable multivibrator, U2. The resulting high pulse from pin 3 of U2 provides a discharge path for capacitor C1 through turned-on transistor Q2. A continuous sweep ramp with reset is produced at pin 7 of Ulb at a rate determined by RIC1 (approximately 75 Hz). The swept oscillator section consistsof a dual-gate MOSFET transistor in a Colpitts configuration, followed by a buffer amplifier that provides the RF from the selected frequency range to an output amplifier and short antenna. As the output sawtooth waveform from U2, pin 7, is applied to the varactor, the capacitance of the tank circuit decreases from approximately 62 pF at the low end of the ramp to a very low value at the maximum sweep level of about 9.2 V. The on-off switching action of the sweep waveform modulates the generated RF to produce an obnoxious buzz, which is easy to differentiate from other low-level signals at the selected receiver frequency when adjusting the tuner or antenna.



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