Film-Tech Forum ARCHIVE: Using an oscilloscope to measure AC line noise?

my profile | my password | search | faq & rules | forum home

»	Film-Tech Forum ARCHIVE » Operations » Film Handlers' Forum » Using an oscilloscope to measure AC line noise?

Author

Topic: Using an oscilloscope to measure AC line noise?

Nic Margherio
Film Handler

Posts: 91
From: St. Louis MO, USA
Registered: Dec 1999

posted 04-07-2002 01:06 PM

I am seeking advice on using a 20Mhz oscilloscope to get an idea of AC line conditions before and after an AC conditioner. Really all I'd like to see is if the thing is doing me any good. Any ideas? Can I measure ground and neutral noise? Thanks for any input.

| IP: Logged

Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 04-07-2002 01:26 PM

I would wonder how you would see transient events (that is, momentary and non-repetitive) lasting only microseconds unless your scope was of the capture type.

| IP: Logged

Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 04-07-2002 05:53 PM

Steve is right...normally a line conditioner may be as simple as something that has capacitors arranged in a Pi configuration (one across the line and one on each leg to ground). Better ones also have MOVs to clamp any voltage exceeding the MOVs rating (surge supression). The best ones work by providing a high impedance to the spike yet don't affect normal operation.

I suppose that if you had something like a UPS with pure sine-wave output (or a motor-generator set up) you could see an improvement if your normal input line is crap.

We use Tripplite's IBAR-12 surge-suppressor on all of our racks (rack mount with 12 protected outlets, including two in front for test equipment). I don't think we have had ANY digital processors fail that have been plugged into them (card failure) that were not manufacturing defects (failed within the first 90 days of installation, ticking sounds in the Cat. 675A...etc). Now we have had a theatre with CP-500s reset 6 units out of 8 to factory defaults after the power in the the theatre repeatedly reset itself over a short period of time (seconds). Mark G will probably chime in here. This was not a power surge though.

Steve

------------------
"Old projectionists never die, they just changeover!"

| IP: Logged

John Walsh
Film God

Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999

posted 04-07-2002 06:27 PM

There are some relatively inexpensive data acquisition products (a circuirt board with software that is used with a PC.) There are ones that will "capture" line voltages and store what it sees for you to look at later. "Relatively inexpensive" still means $600-$800, though there may be some stripped down products.

You could only use the scope if the noise was constant, and "A-B" the input to the AC conditioner with the output. But, most AC conditioners are meant to stop spikes and other transitory AC noise problems lasting a few hundred milli-seconds.

I guess the only good test of that AC conditioner is to see if the problems you bought it for all go away.

| IP: Logged

Josh Jones
Redhat

Posts: 1207
From: Plano, TX
Registered: Apr 2000

posted 04-07-2002 08:36 PM

I almost wonder if a spectrum analyzer would be a better choice, it would be easier to see the spikes, something used in RF service. I doubt that a RTA would do the job.

just a thought

Josh

------------------
"Film is made of silver, video is made of rust"
'nuf said

| IP: Logged

John Anastasio
Master Film Handler

Posts: 325
From: Trenton, NJ, USA
Registered: Sep 2000

posted 04-07-2002 09:30 PM

Josh, a spectrum analyzer is only good on an RF signal that is fairly continuous. I use one frequently to set the proper levels between aural and visual carriers on TV modulators. You'd only really be able to diagnose transients or broadband noise if that noise was continuous enough to be displayed and if it had definite frequency components, otherwise you'd just see some spikes that wouldn't tell you much. A scope coupled to the power line would tell you more, but you'd have to be constantly looking at it to detect a transient problem. What you need is a power line monitor. We had a nasty problem once with a 50Kw FM transmitter that kept going goofy every day at around the same time and we couldn't figure out what was going on. When we put a recording power line monitor at the site, we were able to trace the problem to a crappy motor on a freight elevator in the building that was pulling all three phases down when it started up, dumping the transmitter. There are digital devices now that will constantly record the lines and then show you a readout later.Changes in voltage, line frequency or current and ambient spikes show up very clearly.

| IP: Logged

John Anastasio
Master Film Handler

Posts: 325
From: Trenton, NJ, USA
Registered: Sep 2000

posted 04-07-2002 09:43 PM

In answer to the original question, which I seem to have lost sight of, yes...you'd be able to see the difference between the input and the output on a dual trace scope. Ground or neutral noise? Well, you won't see anything on a scope if you connect the probe to the ground connector...you'll just end up with a dead short. That noise is relative to the waveform, so it will show up in the waveform itself. Use small coils of wire wrapped around the input and output lines and hook 'em up to the scope inputs. You'll see the "before" and "after" pictures on the screen.

| IP: Logged

Sam Hunter
Jedi Master Film Handler

Posts: 779
From: West Monroe, LA, USA
Registered: Jan 2002

posted 04-08-2002 11:51 AM

From my Electronics experience with Communications systems and dispatch centers I have came to the conclusion to assume all AC Power is dirty and to go ahead and add Filter/Backup to CPU based installations.

| IP: Logged

All times are Central (GMT -6:00)

Printer-friendly view of this topic

The Film-Tech Forums are designed for various members related to the cinema industry to express their opinions, viewpoints and testimonials on various products, services and events based upon speculation, personal knowledge and factual information through use, therefore all views represented here allow no liability upon the publishers of this web site and the owners of said views assume no liability for any ill will resulting from these postings. The posts made here are for educational as well as entertainment purposes and as such anyone viewing this portion of the website must accept these views as statements of the author of that opinion and agrees to release the authors from any and all liability.