Em interferences - what oscilloscope do I need to see them?

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Hi all, you'll like this. I have build a MCU-driven circuitry for driving those ancient split-flap units from a train station. My problem is that each display unit is driven by set of eight 48V signals that power a built-in relay which switches 110V AC built-in motor (1W-2W-3W, depending on unit size). When the display unit reaches the appropriate position, it turns the relay off, stopping the AC mottor.

 

When the relay goes off and the motor stops, it must induce all kind of EM noises that apparently propagates to my 5V circuitry (wasn't a problem back in 70's), namely to one 74HC164's clock input. I know of various techniques of supressing these transient effects and have indeed employed some (ferrite cores on both ends of cables, 100n decoupling capacitors for each IC), but still the problems persist, because I don't know what frequencies I'm supressing.

 

Instead of blindly trying snubbers, various capacitances, inductors everywhere, shielding, multiple PCB layers etc., i would like to SEE the transient effects on an oscilloscope and I want to SEE the corrective effect of any action. I'd like to spend like $500. 

 

The Question: Are oscilloscopes in this price category able to record such transients? I have NO IDEA what frequencies may be involved here. Do I need 1Gsamples/s? Do I need an equipment for $100.000?

 

I'm already fed up with EMI, it is destroying my second project, but this time I'm not giving up :-) I don't really need to complete it anytime soon, it's 80% learning.

 

 

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Ferrite beads and decoupling caps are for curing other problems. First up, do you have catch diodes on the relay coils?
Does the problem only occur when you're switching the 110vac to the motors? Ie: is it the relays or the load causing the problem. There's also circuit layout to consider. Where are the currents flowing?
A schematic and a picture of your setup might help.

The scope will show the transients. Your problem is broadband, not with a specific frequency, so don't confuse emissions with susceptance.

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Well the relays are inductive loads, so they need reverse polarity protection diodes across them to begin with. Otherwise you could easily induce voltages way over 1000V into your micro.

I don't think it's the frequencies you need to worry about, so any scope 20Mhz or above should easily be able to see what's going on.

You can buy a pretty decent 50Mhz or 100Mhz scope for just £250 here in the UK such as USB Based PicoScope or Rigol desktop scopes.

 

*EDIT*

Having said that, I can;t be held responsible if you spend $$$ on a scope and still can't see what's going on! ;-)

SpiderKenny
@spiderelectron
www.spider-e.com

 

Last Edited: Mon. Feb 15, 2016 - 09:51 PM
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The tool usually used to track EMI is a spectrum analyzer. A regular oscilloscope is not usually sensitive enough or is it usually able to trigger on such events. Usually.

 

Further, it is often difficult to correlate what you see in the time domain (scope) to what you see in frequency domain (analyzer).

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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ka7ehk wrote:
The tool usually used to track EMI is a spectrum analyzer.
Add a tracking generator to it (usually an option) to ease EMC diagnosis.

A Vector Network Analyzer (VNA) is an all-in-one for such.

For this thread's case the edge rates may be low enough such that don't need much bandwidth.

There are some relatively inexpensive spectrum analyzers and VNAs with reduced bandwidth.

Otherwise might only need a week or two to troubleshoot; so, rent the instrument.


High Frequency Measurements Web Page
Douglas C. Smith

Technical Tidbit - March-April 2014
Troubleshooting Radiated and Conducted Immunity Problems in the Development Lab

http://emcesd.com/tt2014/tt040114.htm

"Dare to be naïve." - Buckminster Fuller

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matkoliptak wrote:
... namely to one 74HC164's clock input. I know of various techniques of supressing these transient effects and have indeed employed some (ferrite cores on both ends of cables, 100n decoupling capacitors for each IC), but still the problems persist, because I don't know what frequencies I'm supressing.
Do need to be especially careful with clock inputs; not too much dV/dt AND not too little (fixes: low pass filter (LPF), Schmitt trigger).

EFT is akin to ESD and lightning (the rise and fall times are roughly similar).

Try a low pass filter that's a T (R-C-R) (2K-47pF-200).

Repetitive EFT can eventually short the IC's ESD protection; a T LPF is a quick patch in lieu of a rail-tied ESD/lightning TVS.

matkoliptak wrote:
... i would like to SEE the transient effects on an oscilloscope and I want to SEE the corrective effect of any action. I'd like to spend like $500.
There are some low price spectrum analyzers.

The following are from the US; browse down for products from EU and China :

Featured Image

RF Explorer – Low-Cost, Handheld, RF Spectrum Analyzers

http://rfexplorer.com/

 

Tektronix announces affordable portable spectrum analyzer

by

November 04, 2014

http://www.embedded.com/electronics-products/electronic-product-reviews/debug-and-optimization/4436877/Tektronix-announces-affordable-portable-spectrum-analyzer

...

The new RSA306 features a broad 9 kHz to 6.2 GHz frequency range, 40 MHz real time bandwidth, and weighs in at just 1.2 pounds.

...

(3490USD)

...

http://www.mouser.com/new/tektronix/tektronix-rsa306-spectrum-analyzer/ 


Spectrum Analyzer,Aaronia, Antennas, Screening, EMC,EMV

Handheld & USB Spectrum Analyzer, RF Analyzers, EMF Testers    

RF Spectrum Analyzer

http://www.aaronia.com/products/spectrum-analyzers/

SIGLENT Technologies America, Inc

Spectrum Analyzer

http://www.siglentamerica.com/pd.aspx?tid=227&T=1

"Dare to be naïve." - Buckminster Fuller

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Embedded

Siglent's SDS 2000X oscilloscopes

by Jack Ganssle

July 25, 2018

http://www.embedded.com/electronics-blogs/break-points/4442429/Siglent-s-SDS-2000X-oscilloscopes

...

... but the scope and SA (spectrum analyzer) are very different animals, the scope being basically an amplifier and A/D, while the latter is more like a superheterodyne radio, which achieves enormous gain and selectivity.

...

P.S.

At the moment they’re having a promotion where you get a higher-speed scope for the price of a slower version.

SIGLENT Technologies America, Inc

Home

News&Events

Siglent Activity

Get Free Bandwidth Upgrade/Options now!

2016-07-06

http://www.siglentamerica.com/qyxwxx.aspx?id=4767&sid=216

...

3, This promotion is in effect between between 8 July and 8 October, 2016.

...

 

"Dare to be naïve." - Buckminster Fuller

Last Edited: Wed. Jul 27, 2016 - 04:40 AM
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matkoliptak wrote:
... EM noises that apparently propagates to my 5V circuitry (wasn't a problem back in 70's), namely to one 74HC164's ... 

 

Well, back in the '70s, you wouldn't have had 74HC - would you?

 

It would've been real old TTL - which doesn't have the sensitive, high-impedance inputs of CMOS ...

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Cool project. I loved watching those change. The original logic that operated the thing might have been all switches and relays.

 

Put diodes across your relay coils. Separate your 48 and 5 volt supplies carefully. Use LOTS of bypass caps. Maybe even use opto couplers between the relay drivers and the cmos so they're not actually in the same system.

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