STABLE_HF_VFO

Published:2009/7/24 2:34:00 Author:Jessie | From:SeekIC

This circuit is a Hartley oscillator, as identified by the fact that the feedback to the JFET transistor (Q1) is supplied by a tap on the tuning inductance (L1). The position of the tap is usually between 10 and 35 percent of the total coil length. A buffer amplifier is included, and it provides two basic functions: It boosts the low output power from the oscillator to a higher level and it isolates the oscillator from changing load impedances. The main tuning capacitor (C1) is a 100-pF air-dielectric variable unit of heavy-duty double-bearing construction, The trimmer (C2) is used to set the exact frequency, especially when using a dial that must be calibrated. Note that several of the fixed capacitors are indicated as being NPO ceramic-disk, polystyrene, or silvered mica (in order of preference). Also notice that a voltage regulator (U1) serves the oscillator. This generates very little heat; nonetheless, it is still a good idea to mount the regulator away from the actual oscillator circuit. The small trimmer capacitors (C2 and C4) are air-dielectric types, rather than mica or ceramic. The purpose of C4 is to provide dc blocking to the transistor-gate circuit. It has such a small value because we want to lightly load the LC-tuned circuit. That trimmer is adjusted from a position of minimum capacitance (i.e., with the rotor plates completely unmeshed from the stator plates), and is then advanced to a higher capacitance as the oscillator is turned on and off. The correct position is the lowest value that allows the oscillator to start immediately every time that power is applied.



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