from one antenna determines frequency (pitch), and the distance from the other controls amplitude (volume). Higher notes are played by moving the hand closer to the pitch antenna. Louder notes are played by moving the hand away from the volume antenna. Most frequently, the right hand controls the pitch and the left controls the volume, although some performers reverse this arrangement. Some low-cost theremins use a conventional, knob operated volume control and have only the pitch antenna. While commonly called antennas, they are not used for receiving or broadcasting radio waves, but act as plates of capacitors. The theremin uses the heterodyne principle to generate an audio signal. The instrument's pitch circuitry includes two radio frequency oscillators set below 500 kHz to minimize radio interference. One oscillator operates at a fixed frequency. The frequency of the other oscillator is almost identical, and is controlled by the performer's distance from the pitch control antenna. The performer's hand acts as the grounded plate (the performer's body being the connection to ground) of a variable capacitor in an L-C (inductance-capacitance) circuit, which is part of the oscillator and determines its frequency. In the simplest designs, the antenna is directly coupled to the tuned circuit of the oscillator and the 'pitch field', that is the change of note with distance, is highly nonlinear, as the capacitance change with distance is far greater near the antenna. In such systems, when the antenna is removed, the oscillator moves up in frequency. To partly linearise the pitch field, the antenna may be wired in series with an inductor to form a series tuned circuit, resonating with the parallel combination of the antenna's intrinsic capacitance and the capacitance of the player's hand in proximity to the antenna. This series tuned circuit is then connected in parallel with the parallel tuned circuit of the variable pitch oscillator. With the antenna circuit disconnected, the oscillator is tuned to a frequency slightly higher than the stand alone resonant frequency of the antenna circuit. At that frequency, the antenna and its linearisation coil present an inductive impedance; and when connected, behaves as an inductor in parallel with the oscillator. Thus, connecting the antenna and linearising coil raises the oscillation frequency. Close to the resonant frequency of the antenna circuit, the effective inductance is small, and the effect on the oscillator is greatest; farther from it, the effective inductance is larger, and fractional