Single-Ended Output Families

Sinewave and Clipped Sinewave

Sinewave outputs are the “natural” output of a crystal oscillator circuit and they usually represent the maximum level of spectral purity that one can expect from an oscillator. By definition, a pure sine wave has only a single or fundamental frequency with, in the ideal case, no harmonics present. There is no “standard” output level associated with sinewave outputs as there is in the case of the other output types, with the waveform of a sine input defined (for a given frequency) by the amplitude alone, usually expressed as output power in dBm. Sinewave outputs are meant to drive 50Ω impedance loads and the PC trace should also be designed to a 50Ω impedance. Most logic output sources are derived form a sine or clipped sinewave source which degrades the phase noise performance. A sinewave output is the ideal for demanding low phase noise applications.

Clipped sinewaves are formed by limiting a sinewave output, “squaring off” the wave at its maximum and minimum extent. Limiting a sinewave in this way introduces additional harmonics, which reduces the spectral purity of the waveform, but can offer a means to take advantage of the fast rising and falling edges in systems that cannot tolerate the full swing of a large amplitude sinewave. Clipped sinewave devices draw less power than full digital logic outputs and, for this reason, are popular in TCXO designs where the additional power dissipation of a CMOS stage would influence the thermal gradients in the IC. Clipped sinewave TCXO’s are meant to drive a 10pF 10K load.