A Tunable Resonator with Constant Rate of Trimming

An inductor connected to a bridge of five capacitors forms a single-resonant circuit in which the natural frequency can be set independently (almost) by two separate controls. Such a resonator is useful in amplifiers and oscillators that need to be tuned to many separate frequencies. This arrangement is particularly advantageous when the frequency must be preset in a digital manner, as in a decade oscillator. The characteristics of this circuit are described and analyzed, and numerical examples are presented to demonstrate the accuracy of the approximation involved. In a decade oscillator tuning from 9.00 to 9.99 MHz, the trimming error is ±9 Hz. In a beat-frequency oscillator using two resonators of this kind, the frequency can be set to a repeatability of one part n 106, with a trimming error of one part in 105.