USB and RF noise - Page 1
Forum section: electronics / projects

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 26, 2021, 05:05:50 am
content: I have USB (2.0) cable running through a conduit with an RG-58 (I think). The RG-58 is hooked up to an aviation radio. The USB is hooked up on one side to a Cubieboard 2 (kind of like a Raspberry Pi) and on the other side it's hooked to a powered USB hub. The USB hub in turn is connected to a TTGO T-beam (ESP32 based board), a CP2102 USB->serial converter, and a Bluetooth dongle. It appears that when the USB hub is powered and the USB cable is connected, it is constantly breaking squelch on certain frequencies (126.1MHz in particular) when certain software is running on the Cubieboard which is listening to the ESP32 and CP2102.In other words, the noise only starts after the software is running and listening to the data, AND the USB hub is powered AND connected to the Cubieboard. If you pull the USB cable noise stops. Shutoff the power to the hub and the noise stops. Don't run the software, and there is no noise. I find this perplexing.This may be wrong, but I'd think the T-beam, CP2102 and Bluetooth would still be operating even if the USB hub isn't powered because the Cubieboard would supply power to them, I doubt they need more than 100mA. So I don't see why unpowering the hub would make a difference. Similarly, if these devices were making noise, I wouldn't think disconnecting the USB cable (at the Cubieboard end) would make a difference. It seems obvious that the coupling is between the USB cable and the RG-58... but would a USB device (like the T-Beam or CP2102) only transmit information on the USB cable if a receiver is listening? Is a handshake or acknowledge required for a USB device to send data? Would this handshake/acknowledgement happen as a result of software or hardware?Lastly, other than throwing ferrite beads onto the cable, does anyone have any suggested solution?

author: magic
author_rank: Super Contributor
timestamp: September 26, 2021, 05:13:50 am
content: USB is 100% host-driven. Without the host asking for data, no device transmits anything. A DC pullup is applied to the lines to indicate connection/presence.Device activity may also be host-driven. Whatever that T-Beam thing is, it may be doing stuff in response to host queries which it wouldn't be doing otherwise on its own.Any chance of a ground loop involving the coax and USB?

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 27, 2021, 07:37:32 pm
content: By "ground loop" I presume you mean a USB cable is grounded on both ends? I presume this is guaranteed, but perhaps the shield doesn't have to be. A friend suggested replacing my 12V->5V DC/DC converter with a transformer isolated converter and that I could test it by using a floating lab supply to replace the converter for testing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2021, 09:19:48 pm
content: And to be clear, any change in configuration can interrupt the interaction -- dropping hub power might not kill communications directly/immediately (the hub chip may still be powered by the host cable for example), but the peripherals might not be receiving full power, so they don't respond to messages correctly, or at all, and the host driver may show a dropped connection (device disconnected). Basically equivalent to Windows producing the ba-dump sound when this state changes (operating/disabled).Which, YMMV, I've seen embedded Linux platforms where interrupting USB causes the driver to fail silently and it has to be manually cycled to restore function, it doesn't auto-retry. No idea if there's a setting for that or what. Not that that should be a primary issue, but leakage works both ways and maybe the transmitter (if applicable) can knock it out, too.Does the power go through the same conduit, and are there any ground connections -- is this in a vehicle perhaps, so things may end up grounded by virtue of mounting hardware?Could you make a drawing showing everything in the conduit, and what power/ground connections are made at each end?Also, have you tested the radio on transmit, does that knock out the USB?It's entirely possible that the USB connectors or cables have poor shielding, and noise is leaking out, at pretty much any connector. It's not obvious if this is leaking into the antenna cable (or whatever it is?) directly, or being radiated and then picked up by the antenna. RG-58 is notoriously leaky, enough that it might be a problem when paired with a noisy cable like that. But there are plenty of other ways for noise to get into the radio system, it's just one of many possibilities.Also, do you have a rough idea of how much signal is being received? If it's only like +6dB from baseline, yeah, ferrite beads may prove useful. If it's just screaming, there's not much you can do with filtering alone, it'll be leaky shields or something and the only meaningful change is improving them (or adding more).To that end, you can add cable grounding clips. This works by first stripping back some of the outer jacket, exposing the ground braid. (So, this is NOT a sealed environment anymore; that will take more work to achieve!) Clip the braid into a finger contact, much like a fuseholder; there are sources for these (cable grounds) specifically, though I don't have any links offhand unfortunately. The clip must be placed somewhere that grounding will be effective; for example, at the ends of the conduit, assuming it's metallic. The low ground impedance also makes ferrite beads more effective: FB adds series impedance to the cable, on the order of 30 to 300 ohms (depending on size, or more with multiple passes/turns through the core), and noise, from the source towards the grounding point, acts as an impedance divider. The ground point may have an impedance of 1-10 ohms at this frequency. So a reduction of 20-40dB is reasonable in this way, more with multiple clips (and FBs between) or careful construction.Tim

author: bson
author_rank: Unknown
timestamp: September 27, 2021, 10:29:38 pm
content: If you have significant return currents on the USB shield braid it will act as an antenna. This is why it's important for return currents to be directed to the internal GND wires, not the shield. The 126MHz frequency may be a function of the electrical length of the USB cable. Or if the USB shield is capacitively coupled there may be a transition band around there where return currents aren't fully blocked the capacitor. (In other words, it's too small.) Or, it lacks a little bit of resistance like a few Ω to discourage its use (along with a capacitor to fully block higher frequencies).

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 28, 2021, 03:51:31 am
content: Quote from: T3sl4co1l on September 27, 2021, 09:19:48 pmThanks for the long well thought out answer. I'll try to respond as best as I can...1) Yes, it's an aircraft, and yes, there's a ton of stuff in the conduit. There are, I think, two USB cables, a power and ground cable, two shielded audio cables, a second pair of power/ground, and there's probably one more wire (possibly two) in addition to the coax. I don't know if the coax is RG-58. It should be 50 ohms and it is similar in size and flexibility to RG-58, but I didn't look at it that carefully. The chassis itself is non-conductive, so any conduction is through wiring.Power/Ground comes through the conduit on 10 gauge wire. The ground goes to a terminal strip. Power goes to a SPST switch where it then goes to 4 parallel breakers. One pair of power (from one of the breakers) and ground (from the terminal strip) goes back down the conduit to power a switching converter which in turn powers the hub. The radio is powered off a separate breaker and terminal strip ground. A third pair of power/ground may be going through the conduit, however, if that is the case the circuit would be open.One of the USB cables is hooked up to an Arduino which emulates a keyboard -- this is not the proximate cause of the problem. The other goes to the powered USB hub which if disconnected will cause the RF noise to go away.2) I have not tested the radio in transmit in that sense. The radio transmits fine, but I have not verified the behavior on the USB. My general assumption is that's not an issue. It likely doesn't take much power from the USB to cause the receiver to break squelch. When it's actually receiving a real signal, the USB noise isn't particularly noticeable, and when it breaks squelch with no signal present it just sounds like background static, I don't hear anything that resembles a birdie or a coherent noise.3) The noise problem is frequency dependent. Tune the radio a couple MHz away will make the noise go away. The radio is a single conversion superheterodyne with 21.4 MHz high side injection, so I think it's reasonable to presume that the noise is in the passband (120-130 MHz) or the LO (140-150MHz), and NOT at the IF.4) The antenna is likely 15 feet away. I very much doubt that the antenna is involved in this. I think either the USB is putting noise into the feedline or into the ground or something like that.5) The conduit is metal (and I think it should be grounded), although there are also sections of it which may be plastic.Beyond that, I didn't follow your comments about grounding clips. It will be a massive pain in the butt to mess around in the conduit. I wasn't clear if you were talking about adding grounding clips to the USB shield or to the coax shield or how you would effectively ground the grounding clip given the frequencies involved. I could probably add ferrite beads to both sides of the USB cable.

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 28, 2021, 03:57:30 am
content: Quote from: bson on September 27, 2021, 10:29:38 pmI'm not clear how I would control this. The only thing I can think to do is ensure that the shield is only grounded on one end, with the other one floating. Without an end-end path, that seems like it would force all the current through the internal wire. The coaxial cable will be more problematic. I do not have ready access to the antenna end of the coaxial cable unfortunately.