Oscillation in LD1117 voltage regulator appears to be the root cause

OUTPUT CAPACITOR ON REGULATOR IS CRITICAL TO STABILITY

This type of linear voltage regulator requires an external capacitor to achieve stability in its feedback loop, otherwise it can oscillate. The data sheet recommended a 4.7 uF ceramic capacitor of XR5 or XR7 dielectric type, which is what I used. 

Almost every power supply component is best with the lowest equivalent series resistance (ESR). However, I see reports that too low an ESR can lead to oscillation with this regulator. 

DATASHEET FOR THE REGULATOR IS NOT VERY HELPFUL

The data sheet recommended 4.7uF ceramic with XR5 or XR7, however those dielectrics are know for very low ESR. There is no discussion of having a high enough ESR, other than the charts buried near the end of the sheet. 

The data sheet shows how the regulator behaves with load variations but the chart provided has a rise time of 5 microseconds, much much slower than the rise time of the 80-100 ns pulses. Thus I don't know how this behaves with higher speed circuits driven by the regulator. 

Two charts show the stability region for the output capacitor, one for 5V output and the other for 1.2V output. My regulator is 3.3V, but I guess I can interpolate between the two charts since the manufacturer doesn't provide a specific datasheet for my part. It shows ESR at 100KHz frequency but my operating frequency is up over 10MHz. 

The bottom bound at 100 Khz seems to be an ESR of roughly.015 ohms so the next question is what ESR does my output capacitor exhibit at my actual operating frequency.

MY CHOSEN CAPACITOR HAS AN ESR THAT IS VERY LOW

The ESR is way, way below that level from approximately14KHz all the way up to roughly 70MHz. Thus any signal changes in that range risk oscillation of the regulator, exactly what I am seeing. Yikes. The smoking gun. 

SOME POSSIBLE FIXES FOR THE PROBLEM

I could choose a different voltage regulator part that doesn't have this issue, as long as it will drop in on the footprint of the existing PCB. I found a great part, Microchip Technology MIC39100-3.3 but its footprint is incompatible. It is SOT-223 just like the other one, but the middle pin and tab are ground on this one and output on the old part. It would require a new PCB version to work properly. 

The data sheet, however, tells me that a low ESR ceramic capacitor, like the one recommended for the old regulator, would also cause instability in this regulator. Instead, the recommend a tantalum of perhaps 10 uF as long as the ESR is 2 ohms or less. The datasheet was much, much more helpful than the prior regulator datasheet. 

Higher than 2 ohms, instability. Fractional ohms like my ceramic XR7 also unstable. However, the correct part, in the goldilocks zone of ESR, should work properly. 

I could source a 4.7uF or larger capacitor in the same 0805 footprint with a much higher ESR. In fact the same 10uF tantalum as for the MIC39100-3.3 should work properly on the existing LD1117 regulator chip.  

Avoiding changes to the PCB is desirable. First, it saves the delay waiting for a new board. Second, it avoids the labor of moving all the components over from one board to another. Third, it avoids all the connectivity testing I have to do verifying the SMD soldering. 

My first task is to do some testing to verify that it was the regulator oscillating, by adding a series resistor to the decoupling capacitor temporarily, or by removing the regulator chip and using a solid external power supply. Based on that, I can then try a higher ESR capacitor with the LD1117 and validate its good behavior.