Ceramic resonator VFO

 

The ceramic resonator VFO, or variable frequency oscillator circuit shown in Figure, illustrates a 7MH oscillator with a variable crystal oscillator (VXO). The VXO oscillator is extremely stable, but allows only a small variation in frequency, as compared with a conventional VFO. In contrast, a VFO with an LC resonant circuit can be tuned over a range of several hundred kHz, but its frequency stability will depend upon its construction, and is never as good as a crystal oscillator. The use of a ceramic resonator as a frequency determining component fulfills both requirements. The VXO oscillator is very stable yet it can vary the frequency, so the oscillator can be tuned. The range of a VXO oscillator circuit is not as wide as an LC oscillator but it offers a tuning range of 35 kHz with good frequency stability. The somewhat unusual resonant LC circuit at the collector of Q1 has two functions.

It improves the shape of the output signal and also compensates for the amplitude drop starting at approx. 7020 kHz. Transistor Q1 is a readily obtainable 2N3904 and the ceramic resonator is a Murata SFE 7.02 M2C type or equivalent. Inductor L1 consists of two coils on a T50-2 powered iron toroid. The primary coil is 8-turns, while the secondary coil is 2-turns.