When we short-circuit the oscilloscope probe and ground wire (crocodile clip), "crocodile clip bites the probe", and try to put the probe close to the circuit board, we can detect space noise from the oscilloscope.We tend to think that this is because the probe's crocodile clips the earth wire and forms an "antenna" that receives radiation from space.
In particular, if there is a DC/DC circuit on the board, we will be able to bite the probe closer to the DC/DC chip, we will be able to get a particularly clear waveform, no doubt, this is the DC/DC space noise, on the circuit board we tested, this ripple can even reach 60mVpp(when the "antenna" is very close to the DC/DC).
Therefore, if we use the ground wire (alligator clip) on the oscilloscope, in the power ripple measurement, especially the presence of DC/DC circuit board measurement, inevitably, we will be affected by the space noise caused mainly by the DC/DC circuit.Whether we measure the contact on the circuit board, or even the GND point, we will clearly see the ripple.On our test board, the ripple size is generally 15~20mVpp.It is important to note that the ripple shape is basically the same as the DC/DC noise.
Ways of resolution
A simple common technique is to measure the process, don't use such a long alligator clip and forget about it.On the ground metal ring less than 2mm apart from the probe, wrap the solder wire several times. The solder wire length should be as short as possible. The other end should be welded to the circuit board and GND close to the measuring point.
Using this simple method, we reduce the spatial noise from DC/DC to 5mVpp.It should be noted that "as short as possible" solder length is important, even if the length is 5mm more, in our test, can significantly increase the impact of spatial noise of DC/DC waveform by several times.