I Have noticed a problem regarding the use of LDO regulators in very low noise rf designs. It seems that a LDO regulator from say ROHM or NJRC under normal conditions, (decoupling caps as manufacturers recomendation)give a very low ripple (or possibly a damped oscillation) when placed under load, say 100 - 300 mA. The ripple is around 150mV for a load of 200mA. If i increase the caps (not good since board area is tight) the ripple reduces a little. I am using X7R ceramics so the ESR is very low.
If i replace the reg with a standard linear type, the ripple goes completely.
I know this must be the way in which the regulator has been designed, but does any one know why this happens?
The ripple is 50Khz for the ROHM ldo and 100Khz for the NJRC type.
RGS
Bagster