The Theremin offers a formidable design challenge since the physical principle of operation involves the precise measurement of minute differences in capacitance. In electronic terminology, the Theremin must be able to respond to a total change of less than 4 picofarads with a resolution of no more than 30 femtofarads. These are very small values of capacitance, and as a result, the measurement process inherent to the instrument is easily compromised by minute instabilities in electronic components, chiefly affected by temperature variations. In practical terms, this means that an instrument's sensitivity and range of response may vary during performance. To overcome this problem, careful attention to the circuit design and physical construction of the instrument are essential.
Harmonic content is another consideration when choosing an instrument.
This primarily refers to the tonal coloration of the sound. Especially in the lower octaves, the output tends to be richer in odd-order harmonics, and as the pitch increases, the output becomes more sinusoidal. Tonal quality is an important consideration in selecting an instrument, but the criteria is as subjective as choosing colors for a room. My own preference is for a pure sinewave output, although I have constructed Theremins with features for harmonic selection as well. Some designs, including the ones described by Leon Theremin in his 1929 United States patent, have one or more tone control knobs.