|Jul3-12, 06:26 PM||#18|
Fixing noise spikes caused by AC switching
Can you draw a circuit of your test setup that includes grounds in the wall?
If I understand your posts correctly the scope shots are a differential voltage across a 100 ohm resistor. To get 0.4Vpp on the resistor over the air something has to be injecting 4mA into your circuit. I find this extremely unlikely.
I think it is much more likely a reference is moving somewhere due to a ground loop. But if the scope shots are differential then there would have to be some kind of non-zero common mode gain to see those shots which is also kind of odd but possible at a couple of MHz.
In short. I am pretty confused, as I suspect others are, and I think a high level schematic of the test setup would go a long way toward clarifying things.
|Jul3-12, 06:36 PM||#19|
This could also be a probing error.
For example, see figure 1-9, figure 6-5, and figure 6-10 in the publication below. These are various ways the probe ground can pick up noise in circuit. As the probe is AC coupled in the shots its hard to know which is applicable. Also notice in figure 6-10, the noise is transmitted over the air into the loop which includes the ground probe.
|Jul3-12, 07:49 PM||#20|
that fig 6.5 is worth a thousand words.
I have been searching for words to express this thought -
Electricians sometimes get creative when wiring a room. If the line and neutral to an outlet get run physically apart from one another, that creates a loop. So long as they're run together in the same "romex" or same conduit the area of the loop is quite small, but if they're separated they make a loop antenna that 'broadcasts' in accordance with whatever current is running around it. At 60 hz not much energy goes into the air, but a small antenna works better at higher frequencies.
I don't know quite how to check for this without getting inside the walls...
but here's some thouhts -
a lamp dimmer is quite a noisy thing, electrically. A plain incandescent lamp from thrift store, powered through a lamp dimmer, will generate electrical noise. A cheap transistor radio set to AM band in between stations will give you an idea how much of that noise makes it into the air.
I'd try that lamp dimmer-radio test at several outlets around the troublesome room and again in one of the well behaved rooms.
If a striking difference jumps out at you, maybe that's a thought path to look into. If not you will have wasted an hour or two.
I used to wander around the power plant looking for fields with a loop , ten turns on 1/10 square meter area (14 inch diameter), connected to a battery powered scope. If your dimmer/radio finds something worth pursuing, that simple experiment would let you put a number on it.
you have my sympathy on this one, bro !
|Jul12-12, 07:16 PM||#21|
The wiring in this room should be "decent". Everything I'm looking at is on the same wall as the electrical panel and the walls are massive concrete so everything is in conduits on the surface. The outlets where I'm plugging in the fluorescent light (or whatever I switch to generate the spike) are along a wall on one of these "modular outlet bars"---basically a steel channel where you can put outlets along it anywhere you want and fill in the rest with covers. I should be able to open this and see, but the standard practice here is to put two or three circuits into these channels and wire the outlets 1, 2, 3, 1, 2, 3, etc. So I doubt there is any funny business as far as neutrals going in physically different directions or, as it was the case at my house, a guy using the ground wire as a second neutral so he could put a switch for part of a circuit without running another "romex". But I should be able to check.
I know there are other places where the is funny business---we have a "home made" control panel, and about 1/4 of the status lights has a different neutral than the rest of the panel, and this neutral goes "through" another instrument (when I sent the instrument out for calibration that quarter of the panel didn't work and that's when I discovered it). Perhaps a worthwhile test, but I'm sure if I turn off everything in the room (including this cockamamy panel) I will still see this problem.
|Jul12-12, 07:18 PM||#22|
I think the next thing for me to try is to carefully solder a twisted pair shielded cable to a BNC, short the two wires when it's connected to the scope, and see what I get.
I should probably look for another scope just to be safe....
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