Need help investigating RF over-the-air signal - Page 1
Forum section: electronics / rf-microwave

author: homem_do_saco
author_rank: Contributor
timestamp: June 29, 2020, 02:31:55 pm
content: Hi all.So yesterday i felt like attaching an antenna to my oscilloscope. Lots of FM stations, fun stuff.The time waveform showed a regular burst and I have been unable to identify the source. Do you guys know what it can be?img 9:BW limit off, max sampling rate.img 10 onwards: BW limit on (so signal should be bellow 20MHz), delayed timebase showing increasing detail.

author: radiolistener
author_rank: Super Contributor
timestamp: June 29, 2020, 05:37:14 pm
content: It may be self excitation oscillations of your oscillosope input frontend.If you can you make RAW recording of this signal with your scope, share it on google drive. It will be easier to analyse it with spectrum analysis.

author: 0culus
author_rank: Super Contributor
timestamp: June 29, 2020, 06:40:38 pm
content: Do you have any phone chargers or other SMPS nearby? LED lamps near your bench? Could be hash from a poorly designed+poorly shielded SMPS.

author: homem_do_saco
author_rank: Contributor
timestamp: June 29, 2020, 07:00:13 pm
content: Don`t think it is SMPS noise because I unplugged the antenna from the BNC and the trace went flat. I`m compressing a CSV with the waveform,will post momentarily.Here it is:https://drive.google.com/file/d/1Tsa0cvdFc8ZIyz8g5ZJbwV5GpUMOPvV8/view?usp=sharing

author: 0culus
author_rank: Super Contributor
timestamp: June 29, 2020, 07:28:21 pm
content: Quote from: homem_do_saco on June 29, 2020, 07:00:13 pmNot necessarily...I used to have a lamp on my bench that used incandescent bulbs. I got an LED replacement for it and soon noticed that I was getting really high levels of noise on the bench. The bulb was radiatingstrongly. I got rid of it.

author: homem_do_saco
author_rank: Contributor
timestamp: June 29, 2020, 07:53:45 pm
content: So I unplugged all SMPSs on the bench, noise is still present. I took another measurement, now on normal mode, instead of HighRes, and yet another regular noise burst appeared. The original is still there, though.

author: 0culus
author_rank: Super Contributor
timestamp: June 29, 2020, 08:39:34 pm
content: Does your scope have FFT, or do you have a spectrum analyzer? A frequency domain view of what's going on would be more instructive.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 29, 2020, 11:27:37 pm
content: Overseen some wireless sensor device nearby? Is there any change with location within your building?

author: radiolistener
author_rank: Super Contributor
timestamp: June 29, 2020, 11:56:20 pm
content: here is spectrum

author: radiolistener
author_rank: Super Contributor
timestamp: June 30, 2020, 12:44:21 am
content: I'm not sure what is this, but it has very wide bandwidth and probably sitting at about 2.871 MHz.I have thoughts that this could be alias from frequency higher than 500 MHz. If so, it may be alias from 997.129 MHz / 1002.871 MHz / 1997.129 MHz / 2002.871 MHz or even higher.If you have RTLSDRv3 dongle, you can find the source more easy. Because this recording is too short to see more detail picture...

author: homem_do_saco
author_rank: Contributor
timestamp: June 30, 2020, 01:09:25 am
content: Quote from: Neomys Sapiens on June 29, 2020, 11:27:37 pmGonna walk the scope around the house and get back to you.Quote from: radiolistener on June 30, 2020, 12:44:21 amReally makes you wonder what is happening when the scope says it`s 100MHz bandwidth and 350+ MHz signals are -20 dB only. Talk about smooth rollover...But seriously, do you think aliasing is probable? Seems too far for me....I did a bunch of single shots and was measuring the time difference between the first 2 peaks, and got a consistent 40us. I guess that if it was some kind of powerline comms the signal should be there no matter if the antenna is connected or not, right?Also @radiolistener, I suppose that spectrum was taken on the whole signal I uploaded? Perhaps it would be cleaner if done only on the burst, although with a lower resolution?

author: radiolistener
author_rank: Super Contributor
timestamp: June 30, 2020, 01:24:18 am
content: Quote from: homem_do_saco on June 30, 2020, 01:09:25 amwith lower sample rate it will be better. Try to decrease time scale and find lower sample rate but keep 24 megapoints. For example 500 MS or 50 MS and 24 megapoints it will give more detail about low frequency. If you can provide such recording, it will explain more detail about low frequencies...By the way you have a lot of FM stations. The most strongest FM station in your location is 102.1 MHz.

author: homem_do_saco
author_rank: Contributor
timestamp: June 30, 2020, 03:38:52 am
content: Bursts at lower Sample rate:https://drive.google.com/file/d/16MY2ovF1JS8ttjayTR6hY87P6RE2tPTh/view?usp=sharing

author: David Hess
author_rank: Super Contributor
timestamp: June 30, 2020, 07:21:21 am
content: Use the line trigger source to see if the bursts are power line related.

author: radiolistener
author_rank: Super Contributor
timestamp: June 30, 2020, 10:01:04 am
content: Quote from: homem_do_saco on June 30, 2020, 03:38:52 amIt looks like some very bad Chinese SMPS noise. Can you take recording with more slow sample rate?By the way, despite the fact you're enabled 20 MHz LPF, I still see FM stations through aliases. And FM reception is pretty clean, I can hear radio presenter voice at 30.1 MHz (alias from 94.9 MHz). On other FM stations I can hear music

author: aristarchus
author_rank: Regular Contributor
timestamp: June 30, 2020, 11:19:20 am
content: Nowadays it is impossible to have a noiseless environment1.SMPS (lots of things, display/monitor PSU, shack lamp, various LEDs, mobile chargers etc)2.Ethernet over the power Line3.VDSL (yes, VDSL..)4.Inverters without mains filtering (again so many of them, eg seen our natural gas boiler and our elevator on my scope..)IMHO, the only way to make sure this is not from your house/appartment, is to run the scope in batteries and shut down the general mains switch, others may be pissed, but tell them it is for a good ..scientific reason :-))

author: homem_do_saco
author_rank: Contributor
timestamp: June 30, 2020, 01:05:52 pm
content: Quote from: David Hess on June 30, 2020, 07:21:21 amDid that, the bursts do not seem coherent.While I do understand that SMPSs produces high freq bursts, would the time difference between spikes1be so consistent (really don`t know, please educate me on SMPS dynamics)? As I understand, the spikes are load dependent, relying on the feedback pin.1- I`m talking about time difference between spikesinsidethe bursts, not the time difference between bursts. Img 11 in the original post is the most representative.This time consistency suggests, for me, some kind of package structure, transmitted at 2.8-3 MHz, as the spectrum shows. Could well be from the powerline, I just don`t think it`s "dumb" noise.

author: radiolistener
author_rank: Super Contributor
timestamp: June 30, 2020, 03:10:12 pm
content: Quote from: homem_do_saco on June 30, 2020, 01:05:52 pmBut you can see on the spectrum it has random frequency deviation and it's spectrum looks very similar to high power switching mode power supply. Most of all this is some kind of switching mode power supply for some high power machine, maybe water pump or something like that. It has bad quality and no filtering, probably it is cheap Chinese device which was incorrectly installed by uneducated and unlicensed personnel...

author: homem_do_saco
author_rank: Contributor
timestamp: June 30, 2020, 05:52:25 pm
content: I think I got it:2 850-3 025 AERONAUTICAL MOBILE (R)http://handle.itu.int/11.1004/020.1000/1.43.48.en.101

author: radiolistener
author_rank: Super Contributor
timestamp: June 30, 2020, 10:31:55 pm
content: Quote from: homem_do_saco on June 30, 2020, 05:52:25 pmNo, the noise has about 200-300 kHz high power bandwidth and also affects 2000 - 8500 kHz with smaller noise power.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 02, 2020, 01:05:22 am
content: Ethernet cords are good radiators of noise. I hear a bunch of freq on my ham radio, they go away if I disconnect the one ethernet cord in use in the house. It goes from the modem/router to a computer. I think the Freq is different than yours, but try disconnecting ethernet cords if you are using any and see if that makes any difference.Wally

author: Lord of nothing
author_rank: Super Contributor
timestamp: July 03, 2020, 04:43:48 pm
content: Only when you use stupid UDP Cable like the Ami Like to use for no good reason!I use Double Shield Ethernet Cable + Glasfiber one and have no Problem on HF (until my Dad want to use his powerline shhoottt).