VCO using relaxation oscillator in PLLs

[Junus2012]

Hello,

I would like to ask you about why all of the PLL designers use the ring oscillator (mostly differential) as the VCO unit,
while in principle the relaxation oscillators can be used as well (only those circuits those have voltage or current control ).

for example consider the relaxation oscillator shown below, where the current Is can be the output from the voltage to current converter, besides Vin is a DC voltage can be used for tuning. as you see here we dont need to have chain of delay cells and looks more simple.

Thank you

 

[sutapanaki]

For one thing, this will probably result in more jitter because of the slow dv/dt in few places in the circuit.

 

[dick_freebird]

A relaxation oscillator will have one fast edge
and one slow, generally. You'd want the fast
edge to be the active RefClk edge to tamp
down jitter. Then the relaxation oscillator is
probably lower jitter than a sine wave ref
clk, though maybe still not as nice as a crisp
"square" pulse.