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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have a tektronics 2465 4 channel scope. It works pretty well. I noticed when making some AC measurments that when I have the scale set low such as 0.2 v/div without the probes being connected to anything I get 60hz 1volt noise on the screen. Is there anyway to get rid of this or filter it out, or is this a limitation of the scope?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Short-circuit the input. If the signal goes away then it's just pickup, no problem. If the signal does not go away then it's a 'scope problem, and you should consider replacing the electrolytic capacitors.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
The noise does go away when I ground the lead. Something puzzles me, still. I have a line that is carrying about 100ma across a power triode and a 240ohm resistor. I am measuring the AC voltage across the resistors using two identical probes, inverting one and adding it to the other. The channel inputs are set to AC. I also have a digital volt meter across the same resistors. The AC component is taken using a 470uf capacitor to block the DC component.
Across the resistor I measure the AC voltage and the scope and meter correlate. When I see 2vac on the scope I am getting about 2vac on the meter. I can turn this AC component on and off. As I add a DC bias the scope signal appears to get a clipped looking signal, and the AC component gets smaller and smaller, eventually getting lost in this "noise". The digital meter still appears to read correct readings according the the specs on the tube. I won't see the signal on the scope, but the digital meter still picks up the signal.
When I don't have the AC signal on, and I see about 2vac of "noise" the digital meter correctly reads close to 0 volts AC.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Are you sure everything is grounded properly? Are you using different outlets for the scope and the project you're testing? You might find that something you thought was grounded properly in fact isn't. I'd check to see what kind of voltage difference is on the scope ground and your equipment ground, your 2V could be there.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
As you add a dc component you are exceeding the linear input range of your 'scope. Switch off the (invert and add) and look at each waveform separately on the 'scope as you increase the dc component the trace will go off the screen and evntually overload the 'scope input.
Put 100 nF etc. in series with each input channel then you can do Invert+add, since now you are only allowing the ac component through.
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60hz noise on oscilloscope
