Neil Mula


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  • Basic VHF Tuning Circuit

  • Created: Dec 14, 2016

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A frequency tuning circuit or frequency tuner is an approach of electronic device to receive VHF and other Radio Frequency (RF) band. There are several ways in VHF tuning in which this can be done through crystal set, varactor diode, and superheterodyne receiver with tuning selected by adjustment of the frequency of a local oscillator. In the design provided, the tuning is done through varactor diode. The BB659C varactor or variable capacitance diode features high capacitance ratio, low series inductance, low series resistance, and excellent uniformity and matching due to “in-line” matching assembly procedure.

The circuit design is comprised of varactor tuning and Colpitts oscillator circuit. The Colpitts oscillator shown is a practical common base Colpitts oscillator (Q1 base connected ground with load resistor) is used in radio communications. This Colpitts oscillator exhibits inductor and a series combination of C3 and C4 forms the parallel resonant tank circuit. The varactor tuning circuit incorporates RC circuit where R1 value is dependent to the required frequency signal and tuning voltage. The tuning voltage input is connected to the reverse biased diode or the varicap diode. This varactor diode has high impedance, which suits to any tank circuit. It is connected to the Colpitts oscillator through C2 decoupling capacitor that its capacitance can vary with reverse voltage bias and this gives a variable capacitor that is voltage-controlled.

This circuit is suitable for radio communications or FM broadcast system. It can be part of the development of electronic devices that require VHF tuning circuit.



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