Usually HF filters are designed using inductors with cores. In this way a medium/high inductance value is obtained in small size components. But using cores has a limitation.
The mirror technique is a very old technique to be used with PCBs when failing in radiated EMC tests. An easy solution to avoid changes in layout if the technique can be applied to your product.
Clock signals from 1 to 100MHz are usually responsible for radiated EMC problems in HF/VHF range. Harmonics are the culprits, but think in current, not in voltage.
Inductive loads and interrupted currents are an explosive combination. High voltages, arcing, and HF broadband noise are some typical effects. The phenomena behind these transients is complex, but it is not difficult to understand the fundamentals and how to minimize the effects.
Snubbers are RC networks that are really useful for protecting components (transistors, diodes, etc.) and reducing EMI, especially in switching applications.
RF signals entering in a system and being rectified can interfere seriously. That has been classically the case of audio noise in speakers because of mobile phones using TDMA technology. The problem is now very common in products with wireless RF funcionality in products with audio areas.
Whenever an electronic circuit is first energized, transients occur in current and voltage waveforms. These start-up transients can affect the electrical and thermal behavior of components and circuits with serious reliability, EMI, and random effects. Try to characterize how your circuits start and stop.
Common mode currents are in the origin of many typical problems in EMI/EMC and RF electronics. The best way to understand those currents is to visualize them in your scope or spectrum analyzer.
If you are evaluating a design (i.e. power supply) from the EMI/EMC point of view, avoid replacing the real load with some kind of “equivalent resistor.” Differences can be really impressive.
Parasitic oscillations are one of the four typical causes for emissions in EMI/EMC problems. Try to reduce the gain or break the feedback and the problem could be solved at low cost.