Jųan, As you code is executing you may be reading eeprom or sitting in a tight loop looking at an input port or writing data to a lcd display for update. These all change the electrical path of the circuit. Infact you have more than 1 source of oscillator in your circuit if you are using an LCD or VFD for display. If you mount the display inside an enclosure the RF emissions from you circuitry are comming out of the opening for the display. Some times if they are strong enough you may be required to put RF screening in front of the display. Better pieces of test equipment that use displays use this method to cut down on the emissions.

Dave Purola,
N8NTA