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Spectrum Analyser Help

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LAN_403

Avalanche

Thu Jul 20 2006, 02:10PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Ok... I'm posting this from work because no-one here can figure out what is wrong with our analyser, and wondered if you guys could help.

I am trying to do some conducted emissions tests, using the analyser and an artificial mains network, both are plugged into an isolation transformer. The attachment shows some unexplainable peaks, at around 2-3Mhz but I cannot figure out where they are coming from! Nothing was plugged into the input of the analyser when this was taken, and putting a 50R resistor on the input makes no difference. Ignore the very large peak to the left, that is 0Mhz

I'm thinking it might be out of calibration, but it seemed strange that we should get results this extreme, thought i'd better ask before cracking the lid off and messing about too much.

Any ideas?

Thanks!

Tom540

Thu Jul 20 2006, 02:51PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

Maybe your just picking up some other transmission from some whwere else. If you got the sensitivity up high. I know I can pick up all kinds of weird stuff at work. When you said you put a 50 ohm resistor on the imput you mean the cap type right. In that case it seems it might be screwed up. I don't know if i'd go poking around inside a $8,000 analyzer though.

Avalanche

Thu Jul 20 2006, 04:25PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Yeah, I thought about it being some kind of transmission but applying the resistor did not dampen it at all, so I ruled that out. It was just an ordinary resistor. It might help if I attach the attachment this time as well

Not sure if it is a major problem with the analyser, a calibration issue or some kind of conducted noise from the mains, which would seem strange because it's running through an isolation xfrmr. I'm not sure if I have everything grounded properly either...

HV Enthusiast

Thu Jul 20 2006, 05:27PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Make a reference plot without the EUT connected or powered up and then you should be able to see what is coming from the environment vs. the equipment. Keep in mind, lots of equipment runs at 2.4GHz and in the vicinity.

Tom540

Thu Jul 20 2006, 06:39PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

It seems like it would haveto be comming from inside of the unit itself since analyzers aren't all that sensitive. In order to pick up FM radio stations I have to attach an antenna and bring the amplitude waay up. Maybe put it inside a faraday cage and see if you still get it. Just maybe make a small one. Then you'll know for sure

Steve Conner

Thu Jul 20 2006, 11:17PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Do you see the strange results with the spectrum analyser running by itself? or only with the LISN attachment plugged in?

if the latter, do you see the problem all the time, or only when the LISN is connected to the mains?

Avalanche

Fri Jul 21 2006, 11:09AM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Thanks for the suggestions

I tried it earlier with everything disconnected, and only the analyser plugged directly into the wall - no change.

I also tried fitting an aerial, but could not cause any increase in amplitude of the peaks, and fitting the analyser into an equipment cabinet for screening did not help anything either. It's like they are fixed (and I'm not viewing a stored trace either, that was one suggestion from the boss

). The peaks are just there all the time, no matter what.

We're thinking the analyser must be messed up, which is a shame because it's a nice piece of kit

Carbon_Rod

Sat Jul 22 2006, 07:03AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

"2-3Mhz"
Was the equipment functioning inside a relatively clean temperature controlled environment prior to failing? Equipment exposure to radiation, humidity, dust/tailings, and extreme Hot/Cold temperatures do all sorts of things. Also, high frequency shockwaves have been known to cause physical damage to insulators -- and on rare occasion even cause canned crystals to sing all sorts of garbage. If external factors can be removed the probability of identifying localized causes may increase. Like a sick switch mode supply, failed MOV, filter cap, cracks in ceramic, or rf shield leaking cross talk

Of course it could be setup wrong too. ;-]
Cheers,

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