Hartley VFO with MPF102

A more elaborate Hartley design with a buffer and regulated power supply to the oscillator FET Q1. This design was also used in 1995 as the main VFO in a transceiver design, using NE602A as a mixer.
VFO Hartley
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.


MAP

Top