sine wave or TLL/CMOS output is preferred for crystal oscillator?

[yakex]

Hi,
I have two choices: sine wave or TLL/CMOS, for the output of oscillator, which one is better?
The output of oscillator is applied to the reference input of PLL, which play the role of clock generator. The PLL can accept both sine and TLL/CMOS, so I don't know which one is better.
Thank you in advance.

 

[yoramgr]

If your PLL is driving digital circuits, then I think that TTL/CMOS output will be better, because the slope is sharper, therefore less sensitive to noise, therefore will carry less jitter into the PLL.
If your PLL drives radio circuits, TTL/CMOS might be too noisy for the radio circuits.

 

[yakex]

yoramgr: thanks

 

[tlihu]

Better check the PFD (Phase-Frequency-Detector), which accepts reference clock as its input, in the PLL to see if it uses sine wave or TTL/CMOS square wave.

 

[biff44]

Some PLL's specify a digital waveshape for LOW frequency clocks. This is because the sine wave clock transitions through the PLL's input threshold voltage too slowly, and it has trouble locking to it without phase noise. This frequency is typically for clocks below 10 MHz.

Higher frequency inputs to the PLL clock input could be either type, as even high speed clocks tend to have rounded over corners and look a little like a sine wave to begin with.