How to compare relaxation oscillator and ring oscillator?

Hi all, I want to design VCO in MHz. ring oscillator is one choice. But I don't understand relaxation oscillator.
1. Does it have better phase noise compared to Ring osc.?
2. What's the drawback of relaxation oscillator? I can only find bad temperature frequency stability.
3. What's its advantage?
3. What's the control signal for relaxation oscillator? Someone said it only support current control but not voltage control.

Is there some book or paper which introduce relaxation oscillator in detail?

Thanks!

I believe a RO will have better phase noise -if- it operates
on sharp edges. But a current starved RO is not that. The
slowing of edges is exactly the principle there.

Relaxation oscillator works with sharp compares on one
slow and one fast "edge". The slow edge particularly will
transform voltage noise to phase noise, badly. The fast
edge on the other hand is a challenge to keep tight delay
across PVT (relaxation oscillators are popular in PWM
controllers, but I tend to use triangle wave myself - which
is another option for you at few MHZ in CMOS, two cheesy
comparators, one SRFF, and a fat inverter driving a RC
network bang, bang. See NE555, and shave off all the
microns left of the decimal point, heh. Oh, and keep the
2-tap VDD-VSS divider scheme. With a low-TC thin film
resistor and a MIM cap you can just about eliminate
supply variation, temperature becomes a comparator
bias tempco problem and process, you could even
consider e-trim if you wanted.

For variable frequency, change fixed resistor to a pair
(up, down) of switched current source/sink devices
and make -their- tempco whatever suits you (CTAT
with MIM, maybe PTAT with junction caps, etc.).

The issue with most relaxation oscillator designs is that they have fast sensor amplifiers and slow slews. Wide bandwidth together with hysteresis translates to the generation of unnecessarily high noise voltages. You could in principle slow the detector amplifier to match the slew, but this makes for tricky design, particularly if you want to adjust the relaxation oscillator over a wide range. A ring oscillator generally consists of a sequence of similar single-stage amplifiers. The slew rate and the effective bandwidth of simple single-stage amplifiers are intimitely related, particularly for CMOS stages; consequently, the thermal noise of such oscillators is well controlled. High frequency CMOS inverters with symmetrical slews will also give good suppression of flicker noise, as the time between opposing transitions is small, allowing any flicker noise to self-compensate; the difficulty here is maintaining symmetrical slews and gains, which makes balanced differential arrangements attractive - but note that you still need single stage sections to take full benefit.
You will notice that fast edges are not really necessary if the amplifier speed is suitably matched to the slew. So much so that a three-stage ring oscillator and a seventeen stage oscillator with identical total current and total active device area will give the same noise performance, in spite of the slower edges needed to generate the same oscillation frequency*.
Note however that many popular arrangements for maintaining differential operation add more noise than is strictly necessary...
*Additional capacitances are needed to slow the slews of this three-stage oscillator. Long rings are therefore more economic for the generation of lower frequencies.