I measured it with a CRO in the physics lab and I was wondering what could be its source
That is a period of about 62 hours or 2.5 days! Are you sure you measured this with an oscilloscope, or does CRO mean something else to you? Maybe the motion of the planets, earth, moon, etc. I don't know? Do you mean a period of 4.5 microseconds? That would be 222kHz perhaps EMI from a computer, other electronic gear, short wave radio, etc. If you can clarify, I might be of more help. Edit: OK; I have a couple of ideas, but it can be DIFFICULT to find. I spent a long time trying to track down the source of 70MHz in my lab. Turns out there is a TV station (channel 4) transmitting about 1 mile away. Channel 4 occupies the space between 67.31MHz and 71.75MHz. 222 KHz has a wavelength of about 1350 meters. It takes a pretty big antenna to launch that as a traveling wave so I'm going guess its a near field source. A good way to look for it is to make a small loop antenna with a few turns of magnet wire (maybe a few 10s of cm in diameter) and hook it to your oscilloscope. Walk around with it putting it near any and all electrical equipment that is in or near your lab. Make sure to change the orientation of the coil when you are moving around. You are basically playing a game of hot and cold. Try to maximize the signal and in theory you should be near the source. Also, be suspicious of the lights. I know their source is 60Hz, but some electronic ballasts for fluorescents run at a pretty high frequency, at least 10's of KHz, you could be seeing a harmonic. Just try flipping the lights on and off looking for a change in amplitude of the signal on the scope. If this doesn't find it, then perhps it is far field? I don't know of any commercial braoadcasts at this frequency though. The low end of AM radio is around 500KHz. Maybe you have an active Ham radio operator in the neighborhood? I wouldn't think he would be transmitting all of the time though is this 'noise' always present? Hope this helps. Let me know if you find it. Dave
