Living near a cellphone tower causing noise on Oscope measurements? - Page 1
Forum section: electronics / beginners

author: Plasmateur
author_rank: Regular Contributor
timestamp: November 12, 2020, 06:54:09 pm
content: I am getting about 50mV peak to peak noise at harmonics of 50MHz no matter where I plug in my oscilloscope anywhere in my house.I wanted to make a noise measurement of a linear power supply (should this be done at 50Ohm termination btw?).Noise is supposed to be well below the values I'm measuring.There is a cellphone tower about a mile or so away. Is the noise just all throughout the mains? If so, how do I deal with this?Thank you.

author: TimNJ
author_rank: Super Contributor
timestamp: November 12, 2020, 10:14:32 pm
content: Do you have a probe attached to the scope when you see this noise? If so, is the noise still present when the probe is connected to the circuit to be tested?If you are trying to measure a device with already low noise, keep the probe's loop area as small as possible. One way to do this is to run a pair of tightly twisted pair wire to the circuit you want to test. Connect the twisted pair to a female BNC. Then use a scope-tip to BNC adapter like this one:https://www.digikey.com/en/products/detail/cal-test-electronics/CT3655/5029183?s=N4IgTCBcDaIMIBUDMA2ArGkBdAvkAThis is for using a standard 1X probe with 1-megaohm input impedance.If your scope has switchable input input impedance, it's tempting to use the 50-ohm input to reduce the noise picked up by nearby magnetic fields. But, if the power supply you are trying to measure is above ~5V, you risk damaging the scope's 50-ohm input. Also, consider if it makes sense to load the circuit under test with 50 ohms. You could try to using AC coupling with 50-ohm input, but then, depending on the scope, you might severely attenuate frequency content below 100KHz or so. The cut-off could be anywhere between 10KHz-1MHz, typically.So, there's a bunch of gotchas there too...I'd just stick to 1X probe with 1-megaohm input impedance, and be careful to make a good setup.

author: Plasmateur
author_rank: Regular Contributor
timestamp: November 12, 2020, 11:54:53 pm
content: Quote from: TimNJ on November 12, 2020, 10:14:32 pmI attached what I am seeing in 3 different scenarios.Oscope @ 1MOhm Bnc to banana adapter to power supplyOscope @ 1MOhm Bnc to banana as an antennaOscope @ 1MOhm Bnc to Bnc to signal generatorI don't currently have a 50Ohm plus BNC-Tee to attempt that measurement. Does anyone else see something "in the air" like this?EDIT: I attempted this with 20MHz filter on Scope as well, but only a slight improvement.

author: james_s
author_rank: Super Contributor
timestamp: November 13, 2020, 12:44:28 am
content: This is a problem when living near radio and TV broadcast towers as well. I've seen people make a test chamber out of a cookie tin or similar metal container with small openings to feed through probes and power for the DUT.

author: fordem
author_rank: Regular Contributor
timestamp: November 13, 2020, 02:42:02 am
content: 50MHz interference from a cell tower, I'd say that's not very likely.A couple of decades back I had a situation where I had a signal "swamping" my TV reception, the signal was strong enough that I could pick it up just holding the scope probe up in the air, don't remember the frequency, never found the source, but it was clearly a CW transmitter being used outside of the amateur bands.I called a friend who was a licensed ham, gave him the frequency and he called me back from the shack to confirm that the signal he was receiving was the same one I was - the only additional info he could provide was that the operator had an "eastern european fist" - the way he handled the key was typical of some eastern european countries - we surmised the transmitter was at an embassy or consulate close by.

author: radiolistener
author_rank: Super Contributor
timestamp: November 13, 2020, 06:43:25 am
content: it looks like this carrier come from your PSU. Did you tried to power off your PSU? Try to remove PSU from mains, does it helps to get rid of this interference?

author: tautech
author_rank: Super Contributor
timestamp: November 13, 2020, 06:54:08 am
content: Sanity check; what does a low amplitude 40 MHz sinewave from the SDG2042X look like ?The same on all channels ?

author: Plasmateur
author_rank: Regular Contributor
timestamp: November 13, 2020, 06:58:02 pm
content: Quote from: tautech on November 13, 2020, 06:54:08 amI tried two different cases where the power supply was plugged into two different channels.EDIT: I also had the Power supply unplugged and used a 90MHz sinewave.

author: Plasmateur
author_rank: Regular Contributor
timestamp: November 13, 2020, 06:59:21 pm
content: Quote from: radiolistener on November 13, 2020, 06:43:25 amYes, slightly, now I have a cleaner sinewave.

author: TimFox
author_rank: Super Contributor
timestamp: November 13, 2020, 10:14:09 pm
content: Before I retired, my company had two locations: the first was near (< 2 miles) from an AM broadcast antenna (roughly 1 MHz), and the second was maybe 5 miles from an FM broadcast antenna (roughly 100 MHz). We could see the result of this on our analog boards when not enclosed in shield boxes. The AM interference was easy to identify, since the "syllabic" modulation envelope was obvious on the CRO. Looking carefully at the FM interference, one could see the modulation as a time jitter on the carrier waveform. For a while, we had a commercial RF shield room for NMR work that totally removed this interference.Now that I do audio hobby work at my house, about 1 mile from a different AM antenna, I have the same problem. I built a small shield enclosure using two aluminum baking pans that had rectangular flanges, hinging them on one side. I cut a large rectangular hole in one of the pans, closing it up with an aluminum plate with appropriate connectors (BNCs and binding posts), so that I could use another set of connectors if necessary. When closed, the other three edges of the pans are clamped together with small spring clips.

author: tautech
author_rank: Super Contributor
timestamp: November 14, 2020, 12:03:46 pm
content: Quote from: Plasmateur on November 13, 2020, 06:58:02 pmOK let's work within the known ratings of your equipment.Again 40 MHz sinewave from the SDG2042X into as many channels as the cabling and Tees you have allow. Check all channels are the same even if the capture you post cannot show all 4.All channels same settings, SDG2042X channels phase locked and use the mirror channel feature if needs be.V/div settings low enough to see a good sinewave (doesn't matter if they cross some...they shouldn't but will push into each others amplitude. 2 or 3 periods is enough and screenshots are better than pics.Only these 2 devices ON then other devices can be powered to check if they have an influence on the signal.This is just a sanity check that should give firm data if the 2 instruments are OK and then you can trust them to go searching for where the interference is coming from.I once had a customer that was certain his new scope was broken yet when we worked through the basic sanity checks it was found a SMPS wallwart some meters sway was the culprit.Eliminate the simple then go after the prize.

author: wizard69
author_rank: Super Contributor
timestamp: November 15, 2020, 07:59:41 am
content: Do you have any industrial operations near by? I'm not sure how accurately you can measure this noise but there is a good possibility that it might be a harmonic of an industrial process machine running at 13.56 MHz or 27.12 MHz. At one time we had multiple tools running 13.56 at 500 watts and sometimes they would have failures in the RF system.