When powering my RFID reader board using a SMPS without GND-PE connection, I can observe quite an amount of common mode noise:
~340Vpp from mains.
~4Vpp @ resonant frequency of the antenna. (This vanishes when disconnecting the antenna)
This common mode noise (4Vpp) is I believe the cause for read-range degradation. With a GND-PE connection, the common mode noise is eliminated as well, but I'm interested in addressing the issue on the board rather than at the Power supply if possible (especially since low-power PSUs rarely include a GND-PE connection).
The power supply used is of a rather cheap variable voltage type product link and has settings from 3V to 12V rated @ 1A. I currently use it, set at 12V. Power draw is between 40-65mA. It is of the "double insulated" type, with two connectors for the power sockets L+N, and no PE connection.
The Scope is set to AC coupling, the little crocodile-connector from the scope-probe is disconnected/removed and the measurements were taken using the tip of the probe at various points on the board, all exhibiting a similar common mode noise pattern as seen above. Regardless of GND/+5V/+12V rail.
I probed differential noise as well, which is in the oder of a few mV synchronous to the Antenna driver and is exhibited stronger closer to the Antenna driver power input pins.
P.s. this Question is in continuation to: RFID reader sensitivity/read range - How does a "hard" ground increase sensitivity/read range?