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...it depends. The crystal alone cannot oscillate - it needs an amplifier and a closed loop. In this loop the crystal constitutes the frequency selective circuit. And it depends on the amplifier and its amplitude stabilizing circuitry if the amplifier output is in the linear or in the saturated region.
Most oscillator circuits use nonlinear limiting rather than a control loop to define the amplitude, thus the output is can't be clean sine wave. If you refer to usual crystal oscillator devices, they have square wave output except for some special high frequency oscillators.
It depends on which application you are using, as far as my experience with VXCO's, if an oscillator is generating a clock signal to a FPGA, it will/should be a square wave.
You will get a square wave, but when you detect the signal of the terminals of Crystal, it should be a sine wave.
Added after 29 minutes:
FvM said:
Most oscillator circuits use nonlinear limiting rather than a control loop to define the amplitude, thus the output is can't be clean sine wave. If you refer to usual crystal oscillator devices, they have square wave output except for some special high frequency oscillators.
I think we'd better have a agreement on the concetp, before we discuss the detail.
The original question is for "Cyrstal oscillator", not VCO/VCXO/TCXO or some other oscillater circuit. So, for OSC(cystal oscillato) the output is definitely sinewave, while for others it depends on detail structure.
It depends on where is the output point. The crystal itself behaves like a tuned circuit of very high Q thus passing only its fundamental frequency and the harmonics associated with it at a much attenuated amplitude.
If you were to look at the input side of the crystal you may well see some sort of amplitude limited square wave. On the output side (without loading the circuit with your measuring probe too much) you will see a sine wave of the fundamental with the harmonic components added. These extra components may be down 20dB or more so it may not be that visible on a time domain instrument like an oscilloscope, but will be very obvious on a spectrum analyzer.
If the crystal output passes through another amplifying stage it may be distorted intentionally by overdriving to a prescaler or other. Many oscillators use more than just one active device. If you study high stability circuits such as the RLC half-bridge and such you will find up to 4 or more transistors in the basic oscillator design and another one or two to produce a TTL output.
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