Are there certain supply domains for which having an LDO is less critical?
That depends on the power supply sensitivity of the supply domain the LDO is providing power to. Most often the VCO supply or analog supplies domain are the most sensitive and digital supplies are the least sensitive. For example a VCO supply may have a pushing specification of 3 MHz/V. This means 1mV of 10KHz ripple on the supply will cause 3KHz of frequency modulation. If we assume small signal modulation the single sided spur 10KHz offset from the carrier will be generated is:
spur dBc= 20log(deltaF/(sqrt2*Fm))
deltaF=supply ripple * pushing gain
Fm = the frequency of modulation (the ripple frequency)
Noise will have the same modulation impact. Also be careful with switching supplies the frequency of the switching supply could generate a spur.
Retrieving data ...