I've used my scope to look at noise on a power amp's rail, but that's DC, so it's easy to set the scope to AC and just look at the mV noise signal even though the DC component can be hundreds of volts (for a tube amp). What do I do if I want to look at noise coming in from the wall though? Use a capacitor so the 60Hz component is strongly attenuated? I tried it a couple of days back, and of course if I set the scope for 10s of ms and the highest voltage range, I can see the basic AC signal going way off the scale of my scope. I tried increasing the amplitude resolution while fiddling with the horizontal and vertical position controls to try and keep the trace on the screen, but that didn't work very well.
Just wondering if there is any reliable way to look at that.
Thanks,
Saurav
Just wondering if there is any reliable way to look at that.
Thanks,
Saurav
Like this?
Taken from the bottom of this page:
http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm
Thanks. My experiments with a surplus Topaz isolation transformer didn't work, the transformer's mechanical hum was louder than the 60Hz hum from my speakers. So now I'm debating trying for a DIY power cleaning/balancing solution, something I've wanted to stay away from. And before I do that I'd like to have a way of seeing the results of any experiments/products, instead of relying on my ears.
An externally hosted image should be here but it was not working when we last tested it.
Taken from the bottom of this page:
http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm
Thanks. My experiments with a surplus Topaz isolation transformer didn't work, the transformer's mechanical hum was louder than the 60Hz hum from my speakers. So now I'm debating trying for a DIY power cleaning/balancing solution, something I've wanted to stay away from. And before I do that I'd like to have a way of seeing the results of any experiments/products, instead of relying on my ears.
Yes you can do that. I guess you probably already know this but the ground of 99.9% of the scope's out there is connected to protective earth (mains ground). So DO NOT connect ground on the probe to any mains leads. If you pick the wrong one you are going to short the mains through your probe and you will have created a nice little firework and one wasted probe if the fuses in your house aren't fast enough. This might sound trivial but it happens ALL THE TIME at our student's lab with fresh people and since I am responsible for keeping all the instruments in shape, I'm not exactly too excited about it. 
A very good thing in your measurement arsenal is what is called a "sniffer". It is basically a few turns of wire (to form an air-coil) connected to a coaxial cable. With this you can pick up even very weak signals by "sniffing" through your design. It's particulary great in RF design since you can for example measure the frequency of an oscillator without loading it with the capacitance of a probe. Do a google on "sniffer" and see how you can build one yourself.
The sniffer is very good with a spectrum analyzer to basically sniff through a power supply for audio and have a look at the induced noise. An oscilloscope isn't exactly a precision instrument and the sensitivity is usually not enough. But you can give it a try.
/Magnus
A very good thing in your measurement arsenal is what is called a "sniffer". It is basically a few turns of wire (to form an air-coil) connected to a coaxial cable. With this you can pick up even very weak signals by "sniffing" through your design. It's particulary great in RF design since you can for example measure the frequency of an oscillator without loading it with the capacitance of a probe. Do a google on "sniffer" and see how you can build one yourself.
The sniffer is very good with a spectrum analyzer to basically sniff through a power supply for audio and have a look at the induced noise. An oscilloscope isn't exactly a precision instrument and the sensitivity is usually not enough. But you can give it a try.
/Magnus
An active twin-T is much better. See, for example, http://www.analogzone.com/avt08062.pdf for some design guidelines. You'll have to attenuate the line voltage to accomodate an opamp, but you could conceivably implement the circuit with some discrete HV parts. Personally, I'd just attenuate, extract the residue, then amplify it. Opamps are your friend.
Wow, I didn't know that, and I did have my probe connected across hot and neutral when I tried my experiment. I didn't see any sparks, and I think my probe still works, but I haven't tried measuring anything after that. So I should just connect the main probe lead and leave the ground floating, and rely on the connection back to safety earth to provide the reference?
Oh I see what you're saying - I got lucky and put the probe's tip on the hot line, if I'd reversed them, I'd have seen what you described. As my wife says, "Which of your lives did you use up this time?"
Thanks for the advice on the sniffer, I'll look that up. I'm not into this quite deep enough to invest in a spectrum analyser yet
Except that I don't trust that my opamp circuit wouldn't be adding its own noise residue. I guess if I ran it off batteries and didn't use one of the fancy 100MHz opamps that it would be fairly quiet. And I could check that on the scope too, actually.
Thanks for the link.
Well, I don't know how you Brits do it but at least here the neutral conductor and earth is tied together, usually at a central tap for a big building or group of houses. Sometimes you have to run all the way to the power station but they are tied together.
But I agree that for a proper measurement you have to measure between the hot and neutral conductors. The best way to do this is with a true differential module (available as plug-ins for many older Tektronix scope's). But you can easily build one yourself with an opamp (or preferably three opamps in the "instrument" amp configuration). If you choose a low-noise opamp the noise is going to be negligible compared to your scope. With a spectrum analyzer it is a different story.
But you would be amazed just how good a simple sniffer can be. Make a preamp for it if your scope isn't sensitive enough.
/M
I did measure powerline noise with differential probe connected to a PC sound card (Terratec EWX 24/96) input. Then I just used FFT to show the spectrum of the mains. You can see the result here.
if you don't need large bandwidth (the differential probe I used has a bandwidth of 40 MHz), you can use a differential amplifier to attenuate the mains voltage to safe and convenient level. Suitable attenuation might be about 1/100.
The series resistance must limit current from powerline to safe level, preferably much less than milliamp. Pay attention to resistor voltage ratings. It is advisable to use two or three smaller resistor in series instead of one larger due to safety reasons.
I used this kind of configuration when I did power measurements (simultaneous aquisition of the line voltage and current waveform) of computer power supply and differential probe was not conveniently accessible.
Regards,
Janne
if you don't need large bandwidth (the differential probe I used has a bandwidth of 40 MHz), you can use a differential amplifier to attenuate the mains voltage to safe and convenient level. Suitable attenuation might be about 1/100.
The series resistance must limit current from powerline to safe level, preferably much less than milliamp. Pay attention to resistor voltage ratings. It is advisable to use two or three smaller resistor in series instead of one larger due to safety reasons. I used this kind of configuration when I did power measurements (simultaneous aquisition of the line voltage and current waveform) of computer power supply and differential probe was not conveniently accessible.
Regards,
Janne
For clarification...
Correct me if I am wrong but I think a lot of mis-understanding is created by the differences in wiring between countries. In the US, at the 120vac outlet there is a neutral, hot, and earth. The earth and neutral are tied together at the fuse/breaker panel in the basement. The act of shorting neutral to ground will do no harm as they SHOULD be at the same potential. I WOULD NOT recommend trying this for recreation as there may be a wiring error somewhere in your house! The 120v at the outlet is comprised of 1/2 of the floating 240vac at the service entrance. This 240vac is 2 120vac legs with earth ground as center reference..like a center tapped transformer.
Correct me if I am wrong but..in Europe etc. where 240ac is the norm ..aren't both ac lines floating and are hot
Two completely different systems as I understand
Please correct me as I could be wrong on the 220 systems.
Correct me if I am wrong but I think a lot of mis-understanding is created by the differences in wiring between countries. In the US, at the 120vac outlet there is a neutral, hot, and earth. The earth and neutral are tied together at the fuse/breaker panel in the basement. The act of shorting neutral to ground will do no harm as they SHOULD be at the same potential. I WOULD NOT recommend trying this for recreation as there may be a wiring error somewhere in your house! The 120v at the outlet is comprised of 1/2 of the floating 240vac at the service entrance. This 240vac is 2 120vac legs with earth ground as center reference..like a center tapped transformer.
Correct me if I am wrong but..in Europe etc. where 240ac is the norm ..aren't both ac lines floating and are hot
Two completely different systems as I understand
Please correct me as I could be wrong on the 220 systems.
In Canada and the USA, we don't use differential breakers. Just a standard breaker on the "hot" lead. We also have ground fault interrupters that sense current faults in the common and ground for wet locations (bathroom, hot tubs ...). We do break both phases on 208 V appliances like stoves and dryers.
By tying the ground to common in North America, you may create a great ground loop. Don't do it. To measure noise, just use your probe without the ground. You already have a ground ref via the grounded plug.
It may be interesting to look at the noise current. Use a wide band current sense coil and the twin t notch filter. You don't have to attenuate anything then.
BTW, any normal power supply that rectifies the waveform will generate quite a lot of noise on the AC. We should be more concerned about our own power supplies in our equipment than what's outside. A good power supply design will reject outside noise, and inside noise that the supply itself creates. Most equipment I see have poorly designed power supplies.
-Chris
By tying the ground to common in North America, you may create a great ground loop. Don't do it. To measure noise, just use your probe without the ground. You already have a ground ref via the grounded plug.
It may be interesting to look at the noise current. Use a wide band current sense coil and the twin t notch filter. You don't have to attenuate anything then.
BTW, any normal power supply that rectifies the waveform will generate quite a lot of noise on the AC. We should be more concerned about our own power supplies in our equipment than what's outside. A good power supply design will reject outside noise, and inside noise that the supply itself creates. Most equipment I see have poorly designed power supplies.
-Chris
Ground and neutral is connected and the next transformer, and often at the houses power inlet box. Connecting it at the power plant or so will not cause much, as there are some transformers inbetween. You should not connect ground and neutral behind the differential breaker, but before is ok.
Just a note of caution with this Earth-Neutral thing:
Do not connect earth and neutral together directly at any point, eg using a 'modified' plug, thinking that if they are bonded once, more must be better. The reason is fairly subtle: the earth wire exists as a separate wire inside your house for fault currents only - an escape path. If it was connected to the neutral it woud effectively become exposed to normal load current. If your neutral develops a fault, everything would continue to operate as normal except that all the items normally bonded to earth would float up to some indeterminate potential, possibly dangerously high, because the 'earth' is now a service conductor. Think about that for a bit...
Do not connect earth and neutral together directly at any point, eg using a 'modified' plug, thinking that if they are bonded once, more must be better. The reason is fairly subtle: the earth wire exists as a separate wire inside your house for fault currents only - an escape path. If it was connected to the neutral it woud effectively become exposed to normal load current. If your neutral develops a fault, everything would continue to operate as normal except that all the items normally bonded to earth would float up to some indeterminate potential, possibly dangerously high, because the 'earth' is now a service conductor. Think about that for a bit...
Ther situation is actually quite complex, as a number of different earthing systems are in use in the UK.
In some cases, the neutral will be fairly solidly tied to earth, in others (particularly rural locations), the earth is little more than a protective current-sensing connection which is used to trip the RCD in case of faults. There's usually a local physical earth, but depending on its implementation and maintenance it may be next to useless as a fault current sink.
BTW, in the latter case, there's often some advantage in providing an independent physical earth for signal screening purposes, but be careful not to mix them up!
The mains 'earth' is often the primary carrier of noise in such intallations - it's common for an earth fault in a neighbour's house to trip one's own RCD, so imagine how much rubbish is regularly imposed on the safety earth. It's instructive to scope the differential between mains earth and physical earth - but BE SURE that you know what you're doing, and are aware that there's the possibility of a large PD between the two.
One thing that I realise is that any discussion about mains in such an international forum can be complex and confusing due to the fact that there are so many systems out there. Apples and oranges as they say. When I said it is possible to short neutral to ground with no consequence in the US, I certainly did not mean for anyone to do it. I was just making the point that they are at the same potential. They SHOULD be bonded together right in your house at the panel. If they are not, you certainly have a wiring/grounding/safety issue.
Anyone unsure should get one of those plug in thingys
Anyone unsure should get one of those plug in thingys
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