I'm using an LM317T to regulate 5 V to 3.38 V for a microcontroller. R1 is 330 ohms and R2 is 560 ohms. There isn't any input capacitor and the output capacitor is 470 µF 25V, because I had that value in a pack of 50. The microcontroller is drawing at least 50 mA, so I'm fairly sure I'm meeting the minimum load specifications. The regulator is being powered from a computer SMPS, but I had a very similar configuration set up last time running off an SMPS without any problems.
Here's a scope trace of an I/O pin to show you what I mean:
The ripple is about 60 mVp-p, but as you can see it also happens when the pin is low. I'm worried it might affect the microcontroller itself (for example, make it unstable.)
What I'm more worried about are these large bursts of noise which occur seemingly randomly about 100 times per second:
It turns out this noise is coming from my power line network adapters; it's leaking through from the power line into the computer SMPS and to the 5 V output.
Make sure that there is a small ceramic decoupling capacitor at the supply pin(s) of the microcontroller. Something 100nF - 1uF should be OK. (The 470uF cannot deal very well with HF noise).
How is the grounding on the microcontroller side? Is there a large ground plane?
Is the circuit return earth-referenced? Is the scope connected to the same earth reference?
If you don't have a spring-clip for the ground, and don't mind abusing a probe, you can solder short pieces of solid wire from the supply rail to the probe tip and ground ring (under the probe tip) which will cut down on CM noise pickup dramatically.
In general, most ripple measurements in the switching power world are done with short probes (or direct coax) with 100nF and 10uF ceramic / tantalum caps shunting the probe and the scope set @ 20MHz bandwidth limit. (CM noise <> PARD)