Audio interference ID request: all multiples of 172Hz (attached) - Page 1
Forum section: electronics / rf-microwave

author: Whales
author_rank: Super Contributor
timestamp: February 24, 2022, 04:51:36 am
content: Attached are some audio recordings containing some interference. Do you have any idea what it could be?Constant: 172Hz and all of its multiples. Occasional: bursts of what sound like data, lasting a couple of seconds. Might both be related to each other or not.Thoughts:* 172Khz is a common monitor refresh rate, but that's a factor of 1000 off, it doesn't divide nicely AFAIK* 172Hz ~= 170Hz -> accidentally demodulated RTTY??BackgroundI am working with a group that's using phototransistors as insect shadow microphones. ie phototransistor followed by opamps and then an audio ADC.In some locations and times of day our recordings are beautiful. At other times and locations they're full of interference, from internal sources (problems in our design), external light sources (incandescent lightbulbs flicker at 100Hz) and external EMI sources (grr).Identifying and fixing these problems is a pain because we're not allowed to bring the insect species we study into Australia. I'm trying to diagnose this EMI issue remotely with a team on the other side of the planet, but sadly the problem is not consistent. For a few hours you have beautiful recordings, followed by these noise issues, then magically it disappears (eg) at exactly 3:00AM localtime.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 24, 2022, 09:21:49 am
content: getting 172 Hz and many of the multiples points to some pulsed interference. My first though was GSM or DECT radio signals, but these use 216 and 100 Hz pulse frequency. It could still be a similar TDMA radio transmission.Another possibly source could be SMPS working in a pulsed, low power mode. This could be coupled to something like street lights and actually cause light pulses at that frequency.LIDAR systems could also use a comparable frequency.Similar some local RADAR system could use such a pulse frequency.A first test would be to see if the interference is light or more radio type.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 24, 2022, 11:57:53 am
content: Ethernet Cable??

author: mjs
author_rank: Regular Contributor
timestamp: February 24, 2022, 01:59:54 pm
content: GSM has 577 μs slots corresponding to 173.3 Hz. Strange coincidence ?A few hints:- Grounded metal foil shielf around electronics, just enough opening to get the light to sensor- ferrite+cap filters on all signal and power wires entering the shield- JFET or CMOS amplifiers (bipolars rectify RF to cause things like this)

author: Whales
author_rank: Super Contributor
timestamp: February 25, 2022, 10:12:11 am
content: Quote from: Kleinstein on February 24, 2022, 09:21:49 amHmm. I initially ruled out GSM, because it doesn't sound like how I remember that sounding.Background pulses sound similar to this 5G sample.Nothing like 2G CDMA IS-95These GSM samples have much bigger gaps between dominant frequencies than 172HzI can't find a good AM demodulated source of DECT to listen to, butthis modified recordingsuggests it should be short pulses and the standard is apparently "24 time slots every 10ms frame", which means 2400Hz.QuoteI've been thinking of lighting; but I didn't consideroutdoorlighting. Hmm, I wonder what might be scheduled to turn off at 3AM? I'm going to have to see if the exact 3AM interference shutoff repeats in future tests.QuoteRadar crossed my mind, scaring the hell out of me ("Dear military installation (to whom it may regard), I am sending you this letter today..."). I need to check some maps for nearby military, marine and aviation installations, that's an idea.I know for certain that the indoor lights are fine, they don't cause interference, but if it's an outside light shining in through a window... ty for the suggestion.Quote from: Wallace Gasiewicz on February 24, 2022, 11:57:53 amSo the 172Hz could be the presence-detect, then the short periods of hash could be data?I'm doubtful. The short hash periods sound like very slow data, ie a few hertz, not anything even approaching 1Mbit.I just tried probing some (off/no-link) ethernet jacks. My cheap switch sends pulses seldom (less than once a second?) whilst my laptop sends out what sounds like morse code (attached for your security amusement). It has a fundamental around 190Hz -- perhaps every ethernet chipset uses its own freq for "carrier detect"? I am going to have to find where the "carrier detect" standards are detailed.Quote from: mjs on February 24, 2022, 01:59:54 pmOoh OK.My experience with GSM is that it sounds a lot more bursty, however. Not super continuous (not missing a single beat) for hours.QuoteWe've got new design with a ground plane in the PCB and sensors mounted directly on the board (not hanging off long cables), so far tests suggest it's a lot less interference prone. (By tests I meansimply trying to operate it in my housewhich ironically has turned out to be useful for EMI rejection tests).Ty for the tip on bipolar opamps. I've put an "EMI immune" FET opamp on an alternate audio route in the new design, going to test whether or not that's had an effect.Sadly it's going to take quite a while before we can ship updated designs overseas; and we're not even sure if that will be enough.