In order to determine how much noise you have on Vcc, do a popper measurement and read of the Vpp. Sounds simple, right? And maybe you already did that.
The best paper I know of that describes how to do this measurement is this one:
In short: Use a coax cable soldered directly to your board. Run the 50R coax into your oscilloscope set to 50R input impedance. Select AC-coupling. A bandwidth that is adequate (minimum 500 MHz). And infinite persistence.
If you made your measurement using a high impedance probe with a long "pig-tail" for ground - you may have extra noise not related to your Vcc noise picked up. When in doubt, always do the null-experiement: touch the probe tip to the ground point, so both tip and ground of the probe touches the same point on the board. If you don't get a flat line, something is being picked up by inductive coupling into the loop formed by probe and ground lead.
Provided your measurement is done reasonably well, your noise is about 200mVpp (read from the oscilloscope, why your computer plot shows about 100-something you have to read about in the manual of your equipment).
How to reduce noise? Well if you are talking about reducing error in the measurement, you should follow the above ideas. If you are talking about reducing the noise on the Vcc itself there are a number of techniques. Search PDN design to learn a lot more about power planes and bypass capacitors.
Your next question may be: Is 200mVpp too much? Well since I don't know what parts you are using, let me go by example. Suppose the datasheet of this device calls for 3.3V +/-5% for the Vcc supply. You have +/-165mV as the limit. Let's assume you have a 2% accuracy of your DC regulator. And let's assume you have a 0-1% distribution drop in the connections between the regulator and the device (cables, connectors, traces, filters etc.). That leaves 2% to the AC-noise/ripple or +/-66mV (132mVpp).
In this example you have too much noise - if your measurements are done right.
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