USB/Serial isolation for RF devices - Page 1
Forum section: electronics / rf-microwave

author: CopperCone
author_rank: Super Contributor
timestamp: August 20, 2017, 02:37:18 am
content: So, recently connecting my SA to the computer (through a GPIB to Serial) adapter, and being interested in USB SDR devices (ADALM, which looks like it can use some external shielding), has anyone actually noticed ground loop problems in RF devices? Or interference from the computer (if a laptop is used)?I was thinking about buying (or making.... home made would probably be made better) USB/Serial isolators. It would probably be better to run the isolator off an external linear power supply, as not to couple PC emissions through any HF isolators it might have (avoiding using USB power, only signal), but anyway... (would running the USB signal at a 'low' impedance help here ?)I defiantly noticed errors in measurement in some high impedance low frequency equipment, but how about radio/microwave? I only noticed this error (DC offset) due to ground loops, running off an isolated laptop was fine (this was a resistivity sensor). I assume the time constant on the filter for the resistivity sensor I speak of is probably <1 second.

author: 4CX35000
author_rank: Regular Contributor
timestamp: August 20, 2017, 08:07:15 am
content: You can buy RS485 and RS232 to USB optical-isolated interfaces which are powered via the USB port. Links below should give you a idea of what is involved.I suggest you power the isolator from a USB hub with a reasonable sized power supply as the computer motherboard can and often limits the power which can be supplied from a USB port.https://www.blackbox.co.uk/gb-gb/fi/1619/11762/USB-Opto-Isolators/https://www.commfront.com/products/industrial-opto-isolated-usb-to-rs232-rs485-rs422-converter?variant=9075644099

author: CopperCone
author_rank: Super Contributor
timestamp: August 20, 2017, 06:00:17 pm
content: I have seen those, but have you seen actual effects of the ground loop on RF equipment?Reason being is that I need to plan my budget correctly (for now).Also, building one with quality parts is probably less then 40$. That way you can shield the power supply too, integrating it into the chassis (otherwise you would need to protect the USB hub and the cable coming from it). Fair bit of work though.

author: 4CX35000
author_rank: Regular Contributor
timestamp: August 20, 2017, 06:29:18 pm
content: If your thinking of developing your own design then that is probably the way to go as you can specify your own design to suite your needs as it would be impossible to know if any device would work on your particular SA and computer combination.

author: daqq
author_rank: Super Contributor
timestamp: August 20, 2017, 06:38:29 pm
content: Check this isolator for your USB isolation needs:http://power.murata.com/data/power/ncl/kdc_nmusb202mc.pdf

author: CopperCone
author_rank: Super Contributor
timestamp: August 22, 2017, 06:37:28 am
content: mm those fiber optic isolators by corning look real nice. I am a bit scared of disecting that cable to add isolated power but it seems difficult to beat their IR transiever.https://www.amazon.com/Optical-Cables-Corning-Cable-Meter/dp/B00JOJRF6KI wanted the power source (with heavy filtering and shielding) to be as close to the isolator as possible, to minimize loops. This means that the fiber end of the cable needs to be enclosed in the shield box, opened, insulation skinned, excess wire removed from the enclosure to reduce antenna, remove the tabs on the computer end (as the computer is less shielded the the test equipment in most cases)....I hope that cable is easy to take apart.looks like doing surgury on a fucking android, since theres fiber lines on a ultra flex cable right next to fiber conductors. power e wires are only 26 gauge