The common Gunn diode oscillator "(circuit diagrams)" illustrates how negative resistance oscillators work. NPN transistor voltage converter based on the blocking oscillator. One represents the input frequency, the other the PLL's output voltage-controlled oscillator (VCO) frequency. Piezoelectric resonators are sold as separate components for use in crystal oscillator circuits. Low cost ballasts contain only a simple oscillator and series resonant LC circuit. RLC circuits have many applications as oscillator circuits. One topic was amplitude control in a Wien bridge oscillator. Coherent techniques generate frequencies derived from a single, stable master oscillator. This resulted in detector manufacturers responding by changing their local oscillator frequency. Johnson described three ways of producing an integrated one-transistor oscillator. Consider a harmonic oscillator (e.g. a pendulum). Consider the one-dimensional quantum harmonic oscillator. First consider the simpler bosonic case of the phonons of the quantum harmonic oscillator. It was employed in a circuit called a relaxation oscillator. The pendulum was the first harmonic oscillator used by man.