Could anyone help me a bit sort this out?
I have implemented the following schematic on a PCB for the decoupling of a supply rail of an Ethernet Switch.
More specifically it is the IO supply of the SSMII interface. This implementation is also what the manufacturer proposes. The 3.3V comes from a step-down DC-DC converter.
I have noticed that the power supply after the L (so IC side) is not very clean. When measured with the oscilloscope on pin 1 of L462, I had the following waveform
The frequency measured by the oscilloscope is not always the same. It varies a bit between 25MHz and 60MHz. Although this was true before making the following changes (the measurement was taken with These)
- Substitute one 10nF and one 100nF with two 330pF capacitors
- Add an additional 4.7uF capacitor (all ceramic) at the L462.1 pin.
On the other side, the L462.2 side, all Looks quieter.
What can I additionally do in order to have a cleaner power supply? Use an additional capacitor? Replace with another value? Normally at these frequencies the 100nF and 10nF are indeed effective, no?
Here also the layout. The supply rail after the bead (the relative big component upper left) is coloured with yellow. They are all connected through a small plane on an inner layer. It is probably not ideal that many capacitors share the same via, but at this area there are not any more GND balls. Also probably the light green capacitors should be turned horizontally and be connected to the two GND vias on the right side of the image.
So, I took some new measurements by using the technique of soldering a Coax cable directly on a capacitor and the results can be seen in the waveforms below.
First at L462.1:
and second at the L462.2 pin:
I mean, apart from the frequency (here I detected more clearly the 31.25MHz, which is the half of the 62.5MHz with which the data signals of this Interface work), it is still clear that the disturbances are there and that they are filtered away from the main board's supply with the ferrite bead.
Then, lastly, I substituted the ferrite bead with a 0-Ohm resistor:
I think it is clear that the disturbances come from the IC and that without the filtering the situation would be worse.
So the question remains, how to make the power supply quieter?