I am trying to eliminate RFI coming from a raspberry pi 4. The Rtlsdr(enclosed in a metal case) is directly plugged into the pi without any USB extension cable. The frequency of interest is at 1420.40575Mhz, few strong spikes that can be seen in the picture attached can be removed easily with a median filter, but the broadband peak around 1.4199e9 and groups of spikes at 1.4208e9 isn't. I am guessing that the raspberry pi is conducting interference straight into the Rtlsdr, or the problem could also be the Rtlsdr itself as it is a counterfeit... I don't think the RFI is coming from the switching power supply, as I record similar patterns of interference when I tried laptop running on battery+rtlsdr. What lengths of USB extension cable do you guys suggest? I am looking at a 6.5mm diameter USB extension cable, from ugreen(no affiliation), that offers good shielding according to the specs.
1.Rtlsdr and Rtlsdr+biast+lna with rpi.
2.Rtlsdr+biast+lna with rpi and laptop.