A neat way to display filtered noise? - Page 1
Forum section: electronics / projects

author: oxcart
author_rank: Contributor
timestamp: July 13, 2015, 06:41:41 pm
content: I just scored a couple of Schaffner FN-2090-1 emi filters, one for proper use(mains here can get noisy sometimes), the other to play a bit.And I keep thinking: is there a quick way to "see" (a led, an arduino counter..) how much garbage is the filter suppressing?My uneducated guess is that two wideband power measurements before-and-after would do the job, but I keep wondering if there's a much more simple solution..?(PS: I was the ex user dilbert, but the free mail service I used ages ago got messed up)(PS2: My company, Polar, sells our own workstations and consultancy, I'll clarify it in the signature after an espresso)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 13, 2015, 09:09:06 pm
content: Put a scope on it, with the opposite bandwidth (i.e., a high pass, so you aren't also reading the 120V+ line voltage). Be careful that you aren't measuring common mode noise ... or that you are, as the case may be.Inside equipment, it's in the 1-100V range (or less, it can always be less). It has to be mV on the line to meet FCC Part 15 or IEC 61000-4 series regulations. Line noise in an industrial environment (Part 18 I think, or respective IEC stuff) can be much worse, plus the many likely unregulated sources (VFDs, switching gear, furnaces, etc.), to the extent that you can sense whole volts by holding up the probe (and I don't mean just at 50/60Hz).Tim

author: oxcart
author_rank: Contributor
timestamp: July 13, 2015, 09:19:13 pm
content: Thanks for confirming my scope idea! But i was asking if there's a simple, qualitative way to show it, eg. a glowing led put somewhere somehow...?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 14, 2015, 03:29:50 am
content: An RF probe circuit would be more suggestive (e.g., small RF rectifier diode, and some inductors to carry the DC as needed), maybe with a meter. It's not a very selective method though.Tim

author: oxcart
author_rank: Contributor
timestamp: July 14, 2015, 04:22:39 pm
content: Quote from: T3sl4co1l on July 14, 2015, 03:29:50 amThank you! I wasn't looking for selectiveness, just..it's working, it's eating stuff. Can you show me how to wire the things you mentioned..?

author: AndyC_772
author_rank: Super Contributor
timestamp: July 14, 2015, 07:49:33 pm
content: If the LED in some circuit comes on, does that mean the filter is working at 10kHz? Or 100kHz? Or 1MHz? What attenuation is it providing? Is it clamping transients? You can't possibly hope to evaluate it in any meaningful way using an LED.It's an EMC product, so if you need to know anything, it's the effectiveness of the filter across a range of frequencies. A 'simple' way to test it is to hook it to a network analyser, run a frequency sweep across the range of interest, and plot the result. Dead easy, if you have the kit. If you don't, maybe you can do something similar with a function generator and a scope.If you don't have any of the proper equipment, then you're poking around in the dark and can't hope to learn anything meaningful.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 14, 2015, 09:09:21 pm
content: If you're expecting signals in the 100mV range (give or take), a pair of schottky diodes (e.g. BAT54S?) and a coupling cap (essentially the rightmost portion of the above circuit) will do. I'd suggest using a sensitive meter rather than LEDs. But you can marry that to a DC amplifier to light an LED if you like.http://www.zen22142.zen.co.uk/Circuits/rf/sfsm.htmThis is about as simple as it gets. For EMC purposes, you'll probably want much more inductance (100uH?).Tim

author: oxcart
author_rank: Contributor
timestamp: August 10, 2015, 11:34:40 am
content: Thank you guys