[Fixed]: 43 MHz burst noise every 63us in the Lab, (How to find the source)?

[HighVoltage]

I have a very strange situation in my lab.
First I thought it came from a high power 3 phase 400V frequency changer, until I looked at the details of the signal and turned the power to the frequency changer off. Actually I turned all power off to lab and confirmed the strange burst signal with a battery powered scope.

Here is the situation:

The scope is connected to a DC output circuit via BNC cable
The Scope probe is set 1:1
The circuit is not powered on and works only as an Antenna for this burst signal.
Every 63 us there is a burst of about 300 mV pk/pk on the scope
Zooming in to the signal, shows the burst frequency of about 43 MHz

My question is, how to find the source.
May be someone here with HF experience knows what kind of burst signal this is.

I am in a business park in Germany with neighbors to the left and the right of me.

Thanks for any suggestions.

[tom66]

It's nearly 15.875kHz or the horizontal refresh rate of a CRT. It's a bit of a stretch but do any of your neighbours use old CCTV systems with CRTs? Or just CRTs in general? (PAL format... not monitors as they use different retrace rates.) The retrace return could potentially cause something like that... though that's particularly bad.

[Ian.M]

I hope you have already tried putting *ALL* electronic devices on your person into a metal tin with a tight fitting lid or wrapping them in at least two layers of foil and putting them as far away as you reasonably can from the device under test.

I'd start by confirming the signal is still present with a different scope or a spectrum analyser.   Then see if you can pick it up with a small tuned loop antennae which will eliminate the possibility that the excitation is actually at a different frequency, but a semiconductor junction or metal oxide layer in your unpowered device is providing enough nonlinearity to act as a mixer.  After that you are into what the radio amateurs call "foxhunting".   Although you are at the lower end of the VHF band, HF foxhunting techniques are more likely to scale well to 43MHz.  The transmitter may be distant so check for the signal outside the building and up and down the street a bit before you spend a lot of time looking for a local source.

[hammy]

My question is, how to find the source.
May be someone here with HF experience knows what kind of burst signal this is.
Thanks for any suggestions.

In Germany you can call the "BNetzA" for such problems. (Search for "Funkstörungen + Bundesnetzagentur" with your favorite search engine.) You pay nothing for this service (in general).

Good luck!
hammy

[mij59]

No rf expert, the signal is quite strong, so the source is likely to be very close.
Maybe its the lcd backlight of the scope.

[KJDS]

43MHz is about 6m wavelength, so lambda/4 is 1.5m.

You should be able to get that far apart with two scope probes with a couple of turns of wire attached. Move these around comparing the phase and you'll be able to tell the direction. Do some triangulation from a few places to work out where it's coming from.

I do know someone who spent several months trying to remove some 15ish kHz spurs from the RF synth they were designing, which were coming from the CRT of the spec ana he was using to measure it.

[HighVoltage]

It's nearly 15.875kHz or the horizontal refresh rate of a CRT. It's a bit of a stretch but do any of your neighbours use old CCTV systems with CRTs? Or just CRTs in general? (PAL format... not monitors as they use different retrace rates.) The retrace return could potentially cause something like that... though that's particularly bad.

I did ask all the neighbors for old CCTV and other old electronics.
They all claimed that there is nothing.
I  might have to take my scope to them to see if it shows up.

[HighVoltage]

My question is, how to find the source.
May be someone here with HF experience knows what kind of burst signal this is.
Thanks for any suggestions.

In Germany you can call the "BNetzA" for such problems. (Search for "Funkstörungen + Bundesnetzagentur" with your favorite search engine.) You pay nothing for this service (in general).

Good luck!
hammy

Thanks, I just called the "Bundesnetzagentur" and they will probably come already tomorrow.
What a surprise, a fast acting government agency.
I am looking forward to their finding.

[HighVoltage]

No rf expert, the signal is quite strong, so the source is likely to be very close.
Maybe its the lcd backlight of the scope.

I tried 3 different scopes, all show the same signal.

If I wrap the scope and the DUT is Aluminum foil, the signal shows stronger on the scope.
The if I ground the Aluminum foil to the scope BNC ground, the signal is gone completely.

Thanks for all your suggestions.
May be the people from the government can figure it out tomorrow.

[electr_peter]

Handheld spectrum analyser + directional antenna + GPS receiver. Anritsu even has special SW for this.

[donmr]

Clipping the ground lead to the tip of a common probe makes a nice loop antenna.  You can move it around and rotate it to get an idea of the location and/or direction of the signal source.

The 43 MHz part my be ringing in your circuit and/or probe.

We had a signal like that one a bench just last week, it was a cheap wall wart power transformer.

[envisionelec]

Perhaps some telemetry equipment is nearby?

[SL4P]

Many years ago, we had nearby airport approach radar causing a repetitive pulse like this, but the repetition rate was much slower.  Of course technology has moved on since then...

[Gixy]

Many years ago, we had nearby airport approach radar causing a repetitive pulse like this, but the repetition rate was much slower.  Of course technology has moved on since then...

Yes! I saw that 25 years ago. The radar pulses generated false read on magnetic disks as the reading heads were moving by themselves when the radar pulsed. We have been obliged to put all the computer centre into a Faraday cage. Good old times... Nowadays radars are digital so field levels are lower and frequencies higher, but who knows?

[max666]

In Germany you can call the "BNetzA" for such problems. (Search for "Funkstörungen + Bundesnetzagentur" with your favorite search engine.) You pay nothing for this service (in general).

Good luck!
hammy

Funny, I remember people being afraid of them, when they saw those guys driving around in their vans, because they thought they would hunt down people who aren't paying "Rundfunkgebühr" (radio & TV licence fee). And would be able to tell if you're receiving a radio broadcasting.

Who would have thought they could actually be useful to the general public 

[tszaboo]

Do you live close to Italy?
http://web.tiscali.it/vcoletti/ref/43chan.html
They say 100Km, but 300mV is a LOT.

[HighVoltage]

Funny, I remember people being afraid of them, when they saw those guys driving around in their vans, because they thought they would hunt down people who aren't paying "Rundfunkgebühr" (radio & TV licence fee). And would be able to tell if you're receiving a radio broadcasting.

Who would have thought they could actually be useful to the general public 

So true, I remember those days and I had a very strange feeling, when I called them.
Actually they called back today and confirmed an appointment for next week and will bring their van of equipment to find the source.
I am looking forward to those "Beamte" and may be they turn out to be useful.

[HighVoltage]

Clipping the ground lead to the tip of a common probe makes a nice loop antenna.  You can move it around and rotate it to get an idea of the location and/or direction of the signal source.

The 43 MHz part my be ringing in your circuit and/or probe.

We had a signal like that one a bench just last week, it was a cheap wall wart power transformer.

No, the 43 MHz is not part of my circuit at all.
I can insert just a 50 cm wire in to the scope input and get almost the same signal

Can you really get 300 mV Pk/Pk from a broken wall wart?
I am no HF expert at all, but this level I am measuring seems pretty high.

[HighVoltage]

Many years ago, we had nearby airport approach radar causing a repetitive pulse like this, but the repetition rate was much slower.  Of course technology has moved on since then...

The nearest big airport is about 10 km away
The nearest high power over land lines are about 1 km away with cell phone distribution on top of the power line pillars.
Otherwise I am surrounded by forest and farmland

[hammy]

So true, I remember those days and I had a very strange feeling, when I called them.

Mostly fairy tales.

Actually they called back today and confirmed an appointment for next week and will bring their van of equipment to find the source.
I am looking forward to those "Beamte" and may be they turn out to be useful.

I called the BNetzA twice for radiointerference from the neighbourhood. I wasn't able to work CW from 80m up to 20m any more. I was able to narrow down the source, but I was not able to get a responsible contact person.  So these guys made it on the official way ... it took some time, several appointments ... but then they "cleaned up" the whole street. My amateur radio station is now in a very quite area, and my neighbours are far more sensitiv on this topic now!

Give them a chance. They are very nice and helpful people. We had a couple of talks over some cups of coffee, quite impressive the stories they can tell! 

[ivan747]

My question is, how to find the source.
May be someone here with HF experience knows what kind of burst signal this is.
Thanks for any suggestions.

In Germany you can call the "BNetzA" for such problems. (Search for "Funkstörungen + Bundesnetzagentur" with your favorite search engine.) You pay nothing for this service (in general).

Good luck!
hammy

Thanks, I just called the "Bundesnetzagentur" and they will probably come already tomorrow.
What a surprise, a fast acting government agency.
I am looking forward to their finding.

Gotta love Germany!

[HighVoltage]

The official government people from the "Bundes-Netz-Agentur" finally came and analyzed the signal.
They had a  Rhode & Schwarz handheld spectrum analyzer with a really old antenna and walked around the building and neighborhood. It took them about an hour to locate the source in a different building, about 100 m away from my suite.

The source was identified as a bad server power supply in a big business server rack.
The company was advised to replace or repair the PSU.

Well, I hope they will fix it and the problem is solved.
The agency will come again in about two weeks time to check if the bad signal is taken care of.

It was an interesting event and these people had lots of really interesting and funny stories to tell of their encounters.


 

[tom66]

That's great to hear, I'm amazed that they went that far. +1 to them for actually finding the source!

[babysitter]

Those guys once visited the digipeater i was co-sysopping years ago to check if we keep to our licensed parameters.

They had a lot of stories to tell. Intersting was their car, too!

Not one of those cliche VW busses with a fishbone on top, it was a car for covert operations.
It was a big white Mercedes Combi with tinted glass panels which got new license plates regularly.
On top there was a antenna unit that was camouflaged as a luggage rack with grandmas suitcase under a tarp.On the inside, the control unit was sitting in the glove box, which was also able to remote-control some of these slightly hidden fixed intercept stations. Due to the equiment weight in the back, it required a reinforced rear axle. Direction finding doesn't take long, they talked about single digit milisecond transmissions sufficient with that system.

If called, those guys are very effective. But very expensive if the interference is your own fault.
A shop owner next my workplace reported that they came due to mobile network interference coming from his computer shop and he was afraid that he made a badly radiating computer, but finally the culprit was his wireless phone which he wasn't responsible for, from a big german manufacturer.

[hammy]

It is not unusual to find the source for interferences up to 300m away.

The owner of this power supply will get an letter about the issue they found. If it exceeds the limit, the owner is forced to fix the problem in a given period, otherwise the have to shut down the faulty device. It's the law.
Don't worry. They follow the procedure and they do everything by the book. It is impossible to outrun the BNetzA! 

Cheers
hammy