Beginner's USB question - Page 1
Forum section: electronics / beginners

author: josefmtd
author_rank: Contributor
timestamp: October 05, 2018, 06:29:07 am
content: Hello everyone, I am still a beginner in designing boards and recently I created my first board with USB and it didn't go as smooth as I planned.I designed a board with USB and ended up messing up with the USB port pinout. I tried to work around it and soldered a wire directly to the CH340, as attached. In my board, there is a FM radio module, and every time I use the radio to transmit, it creates noise in the USB 5V pin and it can go up to 5.5V, which according to the datasheet is out of the range of the normal operating mode of the CH340.This also happens when I use my handheld transceiver nearby the USB connection. Is it safe to assume that this is only caused by my poorly made USB cable I soldered directly to the PCB?Is there any improvement that I can add to my USB port circuitry to reduce impact of RF interference from my radio to the USB connection? What I have tried is adding a 10nF capacitor I have laying around and ended up making the oscillation worse.The rest of the circuitry works fine, the only issue is the USB loses connection in presence of RF signal nearby.Thanks for your input

author: Mario87
author_rank: Frequent Contributor
timestamp: October 05, 2018, 11:31:45 am
content: What is the frequency of the noise? an RC filter may work to filter it out, or even just some shielding??

author: malagas_on_fire
author_rank: Frequent Contributor
timestamp: October 05, 2018, 12:09:04 pm
content: As stated in the previous comment you could use a shielded USB cable such as this for example:https://goo.gl/images/txxtx3They can be found on good quality USB A to micro / mini B cables that have a ferrite bead.

author: vidarr
author_rank: Frequent Contributor
timestamp: October 05, 2018, 01:43:09 pm
content: A shield and one of these ferrite beads like said above (or something like it, smaller, etc). There are ferrite components you can use in any old electronics you have to recycle.

author: jeroen79
author_rank: Frequent Contributor
timestamp: October 06, 2018, 10:13:39 am
content: The unshielded wires look quite long.Can you shorten them a bit?What went wrong with the USB connector?I see some traces going to the chip from the connector.If you layed out the traces wrong then I would remove them and solder in short wires instead.

author: malagas_on_fire
author_rank: Frequent Contributor
timestamp: October 06, 2018, 11:56:47 am
content: Quote from: malagas_on_fire on October 05, 2018, 12:09:04 pmI add another example of a shielded USB cable.https://goo.gl/images/JdwcnCThe Sony phones from 2011 ~2013 have USB cables with ferrite beads on each terminals.

author: hsn93
author_rank: Regular Contributor
timestamp: October 07, 2018, 11:34:12 am
content: https://www.ftdichip.com/Support/Documents/AppNotes/AN_146_USB_Hardware_Design_Guidelines_for_FTDI_ICs.pdfsection 2.2

author: josefmtd
author_rank: Contributor
timestamp: October 08, 2018, 01:22:48 am
content: Quote from: Mario87 on October 05, 2018, 11:31:45 amSomewhere around 148MHz, which is consistent with the radio my FM transceiver is transmitting atQuote from: jeroen79 on October 06, 2018, 10:13:39 amI tried shorting them out a bit, but it didn't help as much as I expected it would be, yes the micro USB layout was wrong, I mistook the D- and D+ line so I had to solder the USB wire directly on the chip.Quote from: vidarr on October 05, 2018, 01:43:09 pmWould adding a SMD ferrite bead between the USB power and the USB TTL IC supply pin help?Quote from: malagas_on_fire on October 06, 2018, 11:56:47 amThanks, I'll be sure to use a decent USB to alleviate this issue, the poorly made USB that I tried making is definitely badQuote from: hsn93 on October 07, 2018, 11:34:12 amThanks for this application note!