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On the subject of reducing RFI when using USB SDRs..

I have currently have four systems using USB SDRs and a pair of them in my radio shed/workshop. The SDRs in the workshop were suffering with RFI that appeared to be from noise getting into the SDRs from the USB cables. All the antennas, coaxial feeders and RF devices are bonded to the "Radio Earth" system which is earthed to a copper ground spike. After some checking and measuring with various meters, spectum analyser and an oscilloscope I have found that the noise was reduced by using good quality, well shielded USB cables from PC to Powered USB Hub and from the hub to the SDRs with ferrite chokes fitted on them at the PC end AND at the SDRs.

There was still extraneous noise getting into my RF gear, including my SDRs, ICOM VHF, AOR wideband scanners, hacker VHF/LW/MW and Westminster LM/MW radios. I found that a significant amount of the RFI/noise was originating from the domestic AC mains supply with varying amounts of ripple, spikes and RFI present on ALL conductors.

Curing the noise. This was initially reduced with mains conditioning filters. but there was still some extraneous noise that was getting into the SDRs and other radios raising the noise floor more than calculated when running pre-amps. The amps are good quality inline 14dB and 18dB wideband amps fed by Bias-Tee power injectors with smoothed,regulated DC power supplies. DC power to both the amps and the USB HUB are now fitted with RF Chokes.

The radio shed/workshop is fed 240VAC mains power via a dedicated spur from the domestic split-CU (protected by 30ms MCB/RCBO) in the house. The feeder cable to the workshop is earthed at the supply end to the domestic mains earth in the CU. The feeder cable shielding is earthed at the feeder end but NOT at the workshop CU. The workshop mains is a "T-T earth system" where the 240VAC mains earth for the workshop is ISOLATED from the TNC Domestic supply earth and bonded to its own dedicated copper ground spike(s). (This is NOT connected to the Radio Earth) If you don`t understand exactly how TNC/TNC-S and T-T protective earthing works DO NOT just have a go. Earthing power systems requires much measurement and calculation to ensure electrical safety. If in doubt, call a sparky out!

The Laptop PC that runs the SDR software is itself grounded to the "Radio Earth" and its 19VDC power supply lead is also fitted with ferrite chokes. Note: the laptop power supply unit is earthed normally through its AC power lead to the AC MAINS T-T EARTH of the workshop.

This seems like a lot of work but it has significantly reduced the noise floor level on all my radio equipment not just the SDRs, this has improved signal to noise ratios and made a significant difference in operable range especially when running amps/pre-amps.

The USB HUB and SDRs are also housed in a steel enclosure that is bonded to the workshop Radio Earth to stop RFI produced by other equipment in the workshop getting to the SDRs.