1. Proper grounding of all electronic equipment is of paramount importance. In a modern station, many items of equipment are connected: transceiver, power amplifier, PC, control boxes, switch boxes, power sources. Each of these devices must be grounded with a separate connection to the common ground point, thus forming a star ground connection.
We recommend to follow guidelines in this article.
The proper grounding of PCs and laptops is often overlooked. A separate ground connection should be run from the PC's case (including laptops) to the station's common ground point. The best place for grounding a PC/laptop is a screw with a good connection to the case, for example the retaining screw on a D-sub connector (e.g, VGA output) on a laptop or the screws holding the power supply in a desktop system.
It is absolutely important to prevent ground currents from flowing through the ground connection through a signal cable. If you use microHAM "keyers" a good test is to remove the DB15/DB37 connector and USB cable from the keyer. Measure the resistance from the shell of the DB15/DB37 to the shell of the USB cable; there must be NO MORE than 5 ohms between them.
Note: many PC manufacturers fail to provide an adequate connection between the shell of the PC-side USB connector and the PC case. If this is the case, a connection can be established by placing a folded piece of aluminum foil between the shell of the USB connector and the PC case.
2. Power all your equipment from a single wall outlet. The protective earth often exhibits excessive noise between different power outlets, sometimes often due to unrelated equipment powered from the same branch circuit. It is always better to stay safe and avoid this source of noise/interference. It is also a good idea to check the mains distribution, tighten all screws, check proper earthing etc.
3. Sometimes, the USB cable can be a source of RF interference - the cable might have inadequate shielding or the USB transceiver in PC might be improperly designed. Data flowing inside the cable can be reflected as a common mode current on the shield of the cable and be radiated as a significant "digital noise." If this is the source of your problems, eliminate it using ferrite chokes on both ends of the cable; two or three turns through a #31 mix FT240 toroid are always better than snap-on cores of unknown mix.
4. Often, the cause of RFI problems is a common mode current flowing through the antenna feedline into the shack. It is a common misconception that all what a feedline needs is to achieve a near-perfect SWR. However, this does not guarantee low common mode currents, which after entering the shack are radiated from the feedline, induced to any nearby metal object, or conducted into any of the interconnected equipment. Feedline common mode issues are indicated by problems on one or two bands, problems that move around when the feedline is moved, cases where different equipment is effected each band and where the problem changes based on power level. The solution is to use common mode chokes to prevent the current from entering the shack. This topic has been given thorough treatments in recent works by W1HIS and K9YC.