Simple RF Signal Generator

Posted on Feb 5, 2014

This circuit came out of some work I did for the two antenna analysers which are described elsewhere on this site. Those instruments both include an RF signal generator. This is used as the signal source to drive the impedance bridge and detectors used to measure impedance. Those instruments ended up with somewhat different oscillator designs due

Simple RF Signal Generator
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to other conflicting design issues. The signal generator outlined here is much simpler, and quite easy to build. The design is based around the Franklin oscillator configuration. This is perhaps not as well known as other types of oscillator, such as the Colpitts or Hartley. This is also called a "source-coupled FET oscillator" in some books and articles. This oscillator, shown in outline form above, uses two active devices. Some designs I`ve seen use two capacitors to couple into and out of the resonant circuit, but in practice, only a single capacitor is actually required with this circuit. This simple two-FET circuit forms the heart of this Franklin oscillator. It`s really a two-stage amplifier with feedback. A common drain FET first stage is followed by a common gate FET second stage. The first stage gives high gain with a high input impedance to minimise loading on the oscillator`s tuned circuit components. The coupling capacitor is usually a small value, typically less than 10 pF. The second common gate FET stage has no voltage gain. The low input impedance of this stage permits the stage to be fed from the first FET`s source resistor. The output of this stage provides the feedback loop to permit oscillation, and since the output impedance of this stage is high, this minimises loading again on the oscillator`s tuned circuit. This is enhanced to some extent by the use of an RF choke which is a high impedance at RF but...

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