I may have posed this question before, but it bares repeating. Today I put a scope on the USB output of one of those universal travel power converters. I was appalled. It was noisy as all get out. I put my horizontal cursors on to measure the envelope of the spike infested waveform, and measured something like 4.3 - 5.8 Volts.
What I'm thinking about doing is putting that through a boost converter, and then through a linear 5V regulator to try and clean it up. Any better ideas out there?
It seems more to be a ripple voltage/current you are talking about.
Linear regulator are not good at filtering ripple and are not device made for that.
A good and cheap way to improve ripple is to use what is called a capacitance multiplier circuit.
Those circuit are very good at reducing ripple without to have to add massive capacitor and have a cheap cost.
To increase effectiveness, you can add several transistor or use darlington transistor as the value of the capacitor will be (virtually) increased by the transistor gain.
A Nice youtube video explains it here.
EDIT
There is also off-the-shelve components that implements this principle, such as the AP1158.
On some applications I've had luck following the recommendations of figure 2.5 in FT 000292. That being said, on sensitive circuits with high bit ADCs, that doesn't seem enough to kick the noise from some especially noisy sources. (For example, I've noticed an iMac provided very dirty power compared to a Lenovo.) In that case a boost/linear may be the only real option.
