Does the datasheet for the 1117 suggest ceramic caps for input and output? I suspect ceramics have too little esr for your regulator.
The 1117's I have specify tantalums for input and output caps.
Yes, it specifically mentions that ceramic capacitors can be used and that they both must have an ESR between 0.15R and 0.5R. I've not been able to find out the expected ESR of the capacitors I've used, the datasheets for them don't seem to mention it.
I guess if this does look like typical LDO oscillation then it can't be much else, but if I change the input and output caps to something with a higher ESR, won't the remaining 1uF decoupling cap still make the combined resistance too low? or doesn't it work like that?
Here's what the AP1117 datasheet says:
Input Capacitors
To ensure stable operation, the input supply must be low impedance up to a frequency of a few MHz. This requires a closely placed input decoupling capacitor of 4.7µF minimum. This can be either ceramic or solid tantalum. The ESR of this capacitor must be less than 0.5?.
Output Capacitor
The AP1117 also requires a closely placed output capacitor as part of the device frequency compensation. As part of its improved performance over industry standard 1117, the AP1117 is suitable for use with MLCC (Multi-Layer Ceramic Chip) capacitors. A minimum output capacitor of 4.7µF ceramic X7R or 4.7µF solid tantalum is required. Aluminum electrolytic can be used but a minimum of 47µF is required. When using Aluminum electrolytic it is still recommended to also use a 1µF MLCC in parallel. The ESR of the output capacitors must be less than 0.5?. The AP1117 is stable when using the correct value of MLCC capacitors. When using MLCC capacitors X7R dielectric is recommended. Do not use Y5V dielectrics.
Capacitor Track Length
Both input and output capacitors must be placed close to the AP1117. PCB traces not longer than 10mm are recommended between the AP1117 and the capacitors.