Omnipresent glitch - Page 1
Forum section: electronics / beginners

author: JuiceKing
author_rank: Regular Contributor
timestamp: July 08, 2012, 10:25:34 pm
content: Hi,I noticed a periodic glitch that is present in the output of my PSUs and even signal generators, whether powered on or not. As long as they are plugged in, the glitch is there. It was there yesterday and it's still here now.It's pretty big: 200-300mv peak-to-peak, and is a burst of 50ns waves about 500+ns (see attached). It also seems very regular - I counted a frequency of approx. 1.2Hz.I'm seeing in various scopes. I tried unplugging my computer in case it was putting the glitch into the mains circuit, but no, that didn't make a difference. Again, this is coming out of PSUs and signal generators even when they are turned off. If I disconnect them from the mains, then the glitch disappears.Any idea where this might be coming from or how to get rid of it?Thanks,- Ken

author: Rufus
author_rank: Super Contributor
timestamp: July 08, 2012, 11:54:24 pm
content: Quote from: KenSchwarz on July 08, 2012, 10:25:34 pmMost likely on your mains or possibly in the air. Do you see anything with a big loop of wire on the probe? Do you live near any radar sources?On the mains what could be repeating at about 1.2Hz? Look for clues. Can you measure the rate accurately is it stable? If it ever goes away can you correlate it with anything else? Probably something arcing. A relaxation oscillator from a high voltage source charging and breaking down? A gas igniter not shutting down? An insect killer?If the arc is big enough it might not be that local. Can you hear it on am radio?

author: JuiceKing
author_rank: Regular Contributor
timestamp: July 09, 2012, 02:30:16 am
content: Quote from: Rufus on July 08, 2012, 11:54:24 pmIt IS in the air. An 18 inch loop of wire on the probe is all it takes--coming through loud and clear. It is certainly regular. I have no idea where it could be coming from. We do have an Air Force base (Hanscom) about five miles from here.Even with this very rudimentary antenna, I have a much easier time triggering and have explored this signal. It's quite complicated, with multiple levels of modulation:1. Level 1 - 1.18 Hz glitches2. Level 2 - Single glitch - It's a modulated 1.8 msec-long burst of 122 Khz sub-bursts3. Level 3 - There are two patterns in the 122 Khz sub-bursts - digital encoding, perhaps?4. Level 4 - 36 Mhz carrierSearching Google for "36 Mhz carrier" indicates that this might be satellite communication. Anybody familiar with this?- Ken

author: amyk
author_rank: Super Contributor
timestamp: July 09, 2012, 07:16:14 am
content: It's probably not satellite, the carrier frequency is far too low -- they use bands in the microwave GHz ranges.A check of the FCC frequency allocation shows 36MHz to be used for land mobile communications.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 09, 2012, 07:26:07 am
content: Your Rigol has a "pretend" Spectrum Analyser function with its FFT capability.It should show you 36MHz OK.There might be more to identify it in the Frequency domain.

author: bradleytron
author_rank: Regular Contributor
timestamp: July 09, 2012, 07:37:05 am
content: It might be worthwhile to try your equipment at another location, e.g., work place or perhaps a friend's house. Also it may be worthwhile to contact the military and report it especially if it occurs over a large area.

author: JuiceKing
author_rank: Regular Contributor
timestamp: July 09, 2012, 11:29:16 am
content: Quote from: amyk on July 09, 2012, 07:16:14 amI think I found a relevant document here:http://www.ntia.doc.gov/files/ntia/Spectrum_Use_Summary_Master-06212010.pdf.Next to Hanscom Air Force Base is Minuteman National Park. Could be that, too. The descriptions are very general.35-36Mhz:Non-Federal: Fixed Land MobileMilitary: The military agencies operate communication systems in this band on a non-interference basis for tactical and training operations.36-37Mhz:Non-Federal:Military: The military agencies operate radio communication systems in this band for networks providing command and control for combat, combat support, and combat service support as part of tactical and training operations. They also operate tactical air-to-ground and air-to-air communication systems in this band for military close air support missions.Federal: The federal agencies operate land mobile radio communication systems in this band to support national park management, law enforcement, public safety nets, contingencies, and natural resources management.

author: Rufus
author_rank: Super Contributor
timestamp: July 09, 2012, 01:42:51 pm
content: Quote from: KenSchwarz on July 09, 2012, 02:30:16 amStrange, could still be on the mains and the loop is just some capacitance to work against. If it is coming from a direction and actually picked up by the loop it should show nuls when you rotate the loop.If you have any clip on ferrites does clipping one on the scope mains lead make any difference?Do you or close neighbours have any power line networking adapters?

author: JuiceKing
author_rank: Regular Contributor
timestamp: July 10, 2012, 12:09:12 pm
content: Quote from: Rufus on July 09, 2012, 01:42:51 pmOK, more sleuthing...It doesn't show nulls when I rotate the loop. I "improved" reception of the loop by taping it to a 12" ruler as a frame and was able to get close to 1V peak-to-peak signals!It doesn't go away when I turn off every electronic appliance I can find in the the room or nearby, including computers, wireless phones, wireless mice, WIFI router, etc.It DOES go away when I move the oscilloscope and loop to the other side of the house. OK, there's a real clue!I found a spot for the antenna in the middle of the house where the glitch is strongly or weakly present depending on which plug I use to power the scope. So, it does seem to be in the power or at least related to the orientation of the power cord.We don't use power line networking adapters; I know what they are because I did use them at one time. I have no idea if our neighbors do--they might. Still, if they were the source of the trouble, you'd figure it would be seen throughout the house.I looked into the various protocols used for power line networking. It looks in many ways like it might be X10, a primitive one that goes back to 1970. As far as I know, there are no X10 devices in the house, but it's possible that our burglar alarm has it. (It's ancient...I check on-line doc and there's no mention of it, but who knows?)Like X10, I'm seeing pulses are about 1ms long (check) at 120kHz (check). But, I'm seeing that two-pattern modulated 36Mhz sub-carrier inside the 120kHz signal that I don't see mentioned in the X10 discussion. Newer protocols, such as HomePlug AV and UPA DHS can't be it because their maximum carrier frequencies are 30Mhz and 32Mhz respectively.

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 10, 2012, 12:52:07 pm
content: Quote from: JuiceKing on July 10, 2012, 12:09:12 pmThat's probably because it's connected to just one of the phase lines.

author: Rufus
author_rank: Super Contributor
timestamp: July 10, 2012, 12:57:44 pm
content: Quote from: JuiceKing on July 10, 2012, 12:09:12 pmStill interesting. If it was some form of communication signal I don't like the way the envelope of the 122kHz 'pulses' tails off. I am thinking it looks more like a noisy switch mode power supply for something that needs a pulse of power every 1.2 seconds. The 122kHz would be the switching edges and the 36MHz just a circuit resonance they are stimulating. Maybe the 1.2 seconds is a low power burst mode.If you suspect the alarm and can't turn off the power maybe you can increase the power requirement by checking alarm status or something and see if your signal change.

author: JuiceKing
author_rank: Regular Contributor
timestamp: July 10, 2012, 01:31:21 pm
content: Quote from: Rufus on July 10, 2012, 12:57:44 pmThanks, this helps! I'm guessing now that this is coming from a switching power supply that is keeping a battery charged up. When I get home, I'll check the toothbrush, shaver, and two UPS units (one for the computer and one for network storage). I did unplug one of the UPS from the wall earlier, but not the other. More later...

author: grenert
author_rank: Frequent Contributor
timestamp: July 10, 2012, 03:37:57 pm
content: FWIW, I used to have a BK Precision DDS function generator that would periodically send out a burst from its output when switched off. Only unplugging would get it to stop. Unfortunately, I don't recall the characteristics of the burst.

author: JuiceKing
author_rank: Regular Contributor
timestamp: July 11, 2012, 04:55:31 pm
content: Quote from: JuiceKing on July 10, 2012, 01:31:21 pmWell, it wasn't the shaver or the toothbrush, and it wasn't the UPS either. I've tried unplugging just about everything on this side of the house to no avail. I give up for now. Thanks for the help, guys!