Power Line Noise Infecting Instruments?

Gentlemen,

I just fired up a spectrum analyser for the first time in many years and am horrified at the noise I'm picking up from the mains. Even with every wall wart in the house turned off it's still there and there's nothing I can do about it short of going around the neighbours and turning all their stuff off (which I imagine would not go down well). Is anyone else finding such devices virtually impossible to use without some serious additional mains filtering?

Just curious.

(this SA was made some time before switching PSUs became omnipresent, so it's possible more modern equipment has since evolved with better in- built mains filtering, of course)

Reply to
Cursitor Doom
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Make. Model? Test conditions? Specific info on background spurs?

RL

Reply to
legg

Hmm is the noise radiated, conducted or self-inflicted? (coming from the internal power supply.)

George H.

Reply to
George Herold

Before I got too bothered, I would try putting a cap across the 240 volt line and each 120 volt to ground. There should not be too much energy in the noise from switching wall warts.

Dan

Reply to
dcaster

Yup. Lately, I've been playing around with a software defined radio receiver. The amount of carriers is just amazing, from 3 MHz up to 100+ MHz. I found one of them was my computer mouse, it would burble whenever I moved the mouse. But, there are THOUSANDs more of these carriers out there. Everything has a CPU in it today, and they all have oscillators. And, as you mention, power supplies, even many wall-warts now have switching supplies in them.

Likely, any name brand analyzer, even from many years back, would have a good mains filter in it. But, the stuff goes from conducted interference to radiated whenever the agressor device is plugged into the wall socket. The house wiring becomes an antenna.

The only way to really solve it is a screened room.

Every computer has probably dozens of crystal oscillators, plus half a dozen switching supplies. Some LCD screens are strong radiators. Thermostats, refrigerators, microwave ovens, washing machines and many other home appliances now have CPUs in them. And, of course, cordless phones, tire pressure monitors, baby monitors, and who knows what else are intentional radiators, too. It's a real jungle out there.

Jon

Reply to
Jon Elson

HP 5586B. But before you go off into theorising about what may be wrong with it (I can tell you're salivating at the prospect already) I don't believe at this time that there is any fault; just a very dirty power line. I'll try to post some screen shots in due course. If it becomes apparent there *is* a fault, however, I'll repost from scratch to s.e.r and you can go nuts. ;-)

Reply to
Cursitor Doom

At this stage I'm of the opinion that it's line noise, because this particular SA uses a linear power supply (judging by the weight of it):) If it was a switcher I'd have scoped the power supply rails already. The noise is still there with the RFin socket unconnected so it's either line or an internal fault (and I don't want to go down the fault route at this stage).

Reply to
Cursitor Doom

Yes, it comes from a generation where filtering was minimal, so it might just be a cap gone down in the elementary power filter unit these things typically use. But I'm just posting a generic question about whether line noise is much worse today than it was say 25 years ago.

Reply to
Cursitor Doom

Certainly is. I gather plasma TVs are particularly bad in this respect, but fortunately they're a dying breed. :)

Reply to
Cursitor Doom

No such model. Perhaps you meant HP 8566B ?

You sure take the fun out of armchair troubleshooting. At least you have the correct procedure. Blame something or someone first. Then figure out what's wrong.

Slobber. I can't wait. Also include photos of how you're coupling the SA to the power line and what you're using for a probe or antenna.

Incidentally, my various spectrum analyzers are rather quiet and don't pickup anything from the power lines unless I use a home made LISN (Line Impedance Stabilization Network) to couple to the power line:

You might want to check if someone is using Home Plug (ethernet over power line) equipment, or some form of BPL (broadband power line) transmitter. Also, if the building has a grid tied solar power system without sufficient EMI filtering, you're going to see some noise on the line.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

My 8566B doesn't see power line junk at all. It's super well shielded and isolated.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Yeah, that's the one. Dunno how the other number got quoted. Must be advancing senility.

Mains lead to power socket of course!

'bout 2 feet of wire at the moment. Till I get this noise sorted out I'm testing on the FM radio band only.

Here ya go:

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Just a couple of snaps. You can see the s/n level is terrible. You can just make out 5 or 6 FM radio stations in the mush. They're BBC Radio transmitters line of sight with me only 10 miles away!

And now, I'll check out the links you posted to your gear......

Reply to
Cursitor Doom

You can find MIL-HDBK-419A Grounding , Bonding, and Shielding on the internet. It is a good source of information on GBS.

Dan

Reply to
dcaster

Sounds like a reference I should really have here. Have to get one.

Tomorrow, because I need to eliminate the possibility, I shall be scoping the power rails of the RF/IF sweep generator/amplifier sections of that SA. If I find anything amiss - *if* - I'll start a fresh thread on the subject over on s.e.r. Night-night all!

Reply to
Cursitor Doom

I've noticed that my 8566B is especially good at picking up noise from wall warts. Look around for active USB hubs with stout power supplies (5V 2A or so) that are running under light or no load.

I might have a blown line filter cap in the 8566, for that matter. It's a common-enough affliction in older test gear, but usually they go "bang" and smell up the room when they fail...

-- john, KE5FX

Reply to
John Miles, KE5FX

As I get older, my customers tend to do the same, at the same rate. Some of them seem to have trouble accurately describing their hardware. Fortunately, many of them have smartphones with cameras. I ask them to take a photo and email it to me. None seem to know how to resize the photo, which results in huge, out of focus, shaky, and misaligned photos. With the help of my Ouija Board, image enhancement software, and pure guesswork, I can usually decode the model numbers. If you can't read the model number, try posting a photo of the serial number tag or nameplate.

Clever. Guess the wrong connection on the power plug and you get

240VAC on the case of the spectrum analyzer. LISN boxes were contrived to keep you alive a little longer and to provide a constant termination impedance across the various LF and HF bands so that measurement levels on different power lines and using different test equipment all result in similar numbers. As an added bonus, they're also reasonably well shielded, but I'll get to that shortly.

I'll assume 2 feet of unshielded wire which is a good simulation of a crude antenna. It will help pickup all kinds of interesting OTA (over the air) signals which you don't want to see. Coax cable is helpful for removing these problem signals from your life.

Those are FM stations being picked up by your 2ft wire antenna, which are NOT coming from the power line.

With 100Khz of IF bandwidth, you'll get plenty of base line noise to help hide any signals that you might want to see. Despite that, I see at least 94dB of dynamic range (20dB attenuator + 74dB on the screen), which lacking a suitable reference could be considered a kind of SNR (signal to noise ratio). That's doing quite well methinks.

Well, with 20dB of input attenuation and a 2ft antenna, I'm surprised that you can see anything. Must be some strong FM signals. Switch to

10dB attenuation and you'll see more stations. However, don't try 0dB or your risk blowing out your front end. I haven't seen a photo of your derangement yet. If it's what a I suspect, it won't take much of a wiring error to blow out the front end. Burning out the input attenuator is cheaper than blowing out the HP custom broadband amps.

Don't bother. Go find a book or web pile on how to use a spectrum analyzer. Learn how it's done and what to expect before you destroy it by randomly poking probes into the AC power receptacle and using wire antennas instead coax cables and probes. etc...

RF is magic. Repeat it until you believe it.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

I purposely bought a 20 year old digital camera for this very reason. It's a 3Mp Olympus which gives fabulously sharp results and by default creates very small, manageable images that can slip through the meanest email box. Those pictures were taken with it. Only cost me 15 quid!! My other tips for sharp images: use a tripod and take the pic using the camera's delay-timing function so you don't have to press a button which destabilises the camera at the critical moment.

A. It's the *correct* power lead. B. Think I'd notice 50Hz hum! This is *much* higher.

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I didn't say they were. The *noise* I suspect to be coming from the power line *via* the power lead.

a.) I am *not* probing the mains!!!

b.) A short wire antenna is perfectly fine for examining the immediate radio environment -certainly between 85 & 110Mhz!

c.) I've had several spectrum analyzers over the years and currently have at least 3 within reach. I'll check out your links, but I doubt there is much I don't know about SA operation!

Reply to
Cursitor Doom

Thank you, John. Yes, I'm striving to eliminate the obvious things before I start tearing it down assuming some fault with it. It may easily just be a more modern add-on mains filter it requires.

Reply to
Cursitor Doom

In Europe, there are large number of powerful (100-500 kW) short wave transmitters especially in the 49 m (6 MHz) band, which will induce quite a significant voltages into a random wire, like the 60 cm test lead or into the house wiring.

When there are a lot of strong signals and even a small amount of nonlinearity, there are going to be a lot of harmonic and intermodulation distortion, generating a continuous noise floor, when viewed with a wide band receiver/SA.

To check if the interference products are generated in the receiver, insert a 10 dB attenuator in front of the receiver. The wanted signal should drop by 10 dB, but the noise floor by up to 30 dB. This is a clear indication of front end overload. Check the whole spectrum from

100 kHz to 100 MHz with 5 kHz bandwidth for any strong signals even close to the maximum input voltage. Pay special attention to the 6 MHz area during the night.

If the ratio between wanted signals and the noise floor remains constant, it is likely that those interference signals are generated outside the SA, e.g. in oxidized connections in the mains wiring.

Just one other observation. The ring mains system as used in the UK will create a loop antenna, which might be at resonance at some low-HF frequency, making a large RF current to flow. Any resistive line or connection losses will generate strong RF voltages.

Reply to
upsidedown

Is this differential or common mode noise ?

If common mode noise, at low (50/150 Hz) frequencies the quality of the PE connector to real ground is determined by the conductor resistance. However at 100 MHz (wavelength 3 m) the conductor is several wavelengths long and hence useless for VHF grounding. If both L and N conductors are riding on top of some high RF voltage, the PE to ground conductor doesn't help a lot :-) Perform the measurements in another room closer to the mains distribution panel and grounding electrode and check if there are any significant reduction of noise.

Reply to
upsidedown

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