Eliminating AC Noise

[CycloWizard]

I have recently constructed a new instrument using some 5V optical flags. The optical flags are identical - two from the same package, and are positioned about two inches apart on the instrument. One is getting significant AC noise (right at 60 Hz, so I believe it's AC anyway). I'm using the instrument on a large, vibration-free optical bench. Because of how I built the instrument, it's not really feasible to add shielding to the electrical connections on the instrument. Any ideas on why one flag is picking up the noise but the other is not? I'm guessing it has to do with the circuits that each flag is connected to, but I'm not really that familiar with the electrical aspects of instrumentation. Also, any ideas on how to cut down on said noise? Any help is greatly appreciated.

 

[PowerEngineer]

Optical flags?

My first guess would be that the noise is coming from some sort of grounding problem, rather than being induced by outside electromagnetic radiation.

 

[f95toli]

Yes, it is probably a ground loop.
Shielding is not THAT important at low frequencies as long as you avoid big pickup-loops (which is easy, just use twisted pairs). Moreover, a Faraday cage won't help much against inductive pickup at 60 Hz unless you are using e.g. mu-metal shields.

Draw your circuit on a piece of paper and try to figure out if there are any ground loops.
Or, use batteries. At work we use ordinary 12V lead-acid car ackumulators for all low noise electronics (ore-amps etc) which works very well. In your case you just need a cheap 7805 to get the voltage down from 12V to 5V (if you use a low dropout regulator you can also use a 6V ack.

 

[CycloWizard]

Thanks for the advice guys. I know that I had grounded everything to my data acquisition card, but maybe one of the connections is bad. I'll look into it. :beer:

 

[wwswimming]

Originally posted by: f95toli
Yes, it is probably a ground loop.
Shielding is not THAT important at low frequencies as long as you avoid big pickup-loops (which is easy, just use twisted pairs). Moreover, a Faraday cage won't help much against inductive pickup at 60 Hz unless you are using e.g. mu-metal shields.

mu-metal works well but you don't have to use mu-metal.

i forget the term, austenitic, martensitic, cold-rolled.

how about - magnet-izeable. the kind of steel that a magnet is attracted to. like a computer case. that will substantially attenuate radiated EMI.

for conducted EMI, line filters set to capture the frequency in question. for 60 Hz that might involve some large filter components.

 

[Born2bwire]

Originally posted by: wwswimming

Originally posted by: f95toli
Yes, it is probably a ground loop.
Shielding is not THAT important at low frequencies as long as you avoid big pickup-loops (which is easy, just use twisted pairs). Moreover, a Faraday cage won't help much against inductive pickup at 60 Hz unless you are using e.g. mu-metal shields.

mu-metal works well but you don't have to use mu-metal.

i forget the term, austenitic, martensitic, cold-rolled.

how about - magnet-izeable. the kind of steel that a magnet is attracted to. like a computer case. that will substantially attenuate radiated EMI.

for conducted EMI, line filters set to capture the frequency in question. for 60 Hz that might involve some large filter components.

60 Hz lies in the quasi-static region. A standard Faraday cage is not going to be much good because the magnetic field is decoupled from the electric field in addition to the fact that the device lies in the source's near field. The only real way to deflect a magnetic field is to use an opposing magnetic field (which they usually do in magnetically shielded speakers), or a thick surrounding of a mu-metal to concentrate the field away from the device. But in this case, as it already has been mentioned, I have usually found that if you have a 60 Hz noise that it is a result of a ground loop or noise.

 

[CycloWizard]

I took a look at it today and it looks like the insulation came partially off of the signal wire from one of the flags and was touching a piece of random metal. I fixed that and added some capacitance to the signal circuit, so hopefully that will take care of things. I'll try it out tomorrow to be sure. As always, you guys knew it well ahead of time. Thanks for the help. :beer: