USB Hub Noise Filter: Reduce 500mV Amplitude & 10MHz Frequency

In summary: I see you are using a USB hub to power the sensor. Are you sure that is not adding noise?The noise frequency is up to 10MHz with 0.5V amplitude. It is a very noisy USB hub. I am not sure how to select a propper inductor. I know that inductors are not perfect and there is a stray capacitance present in them. I want to try a relatively large inductor, but I am worried that parallel capacitor will become a dominant component.
  • #36
chakr said:
The noise generated inside the hub affects circuits inside.
Well, on the picture you have attached previously that terrible thing on the screen of your 'scope is picked up through a classic one-turn loop? ...

... to be honest, your circuit with that LDO looks 'good enough' for most application. Of course, it is not able to make up for excess mistreat, that would require more circuits added.
If you are interested in your hub then maybe you should dissect that instead? (I'm too interested. Slightly.)
But my practical side keeps saying that you should just dispose of it.

Ps.: just one important remark. That's no HUB. That's a CHARGER. Things are not expected to work when attached...
 
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  • #37
I
Rive said:
Well, on the picture you have attached previously that terrible thing on the screen of your 'scope is picked up through a classic one-turn loop? ...

... to be honest, your circuit with that LDO looks 'good enough' for most application. Of course, it is not able to make up for excess mistreat, that would require more circuits added.
If you are interested in your hub then maybe you should dissect that instead? (I'm too interested. Slightly.)
But my practical side keeps saying that you should just dispose of it.

Ps.: just one important remark. That's no HUB. That's a CHARGER. Things are not expected to work when attached...

I agree, my mistake. The problem is that people plug in the device into anything that has a USB port in it and not everything works :).
 
  • #38
chakr said:
II agree, my mistake. The problem is that people plug in the device into anything that has a USB port in it and not everything works :).

Which is why immunity is part of any EMC spec...

The world is a noisy place so products that are expected to work in that world need to not cease operating at the first hint of some radiated or conducted EMI.

Input filters are your friend.
 
  • #40
Rive said:
Ps.: just one important remark. That's no HUB. That's a CHARGER. Things are not expected to work when attached...

looks to me like it has eight or ten USB slots on it

so i don't know what it is for sure

did he ever give us details on it ?

Isn't USB standard 5 volts DC at about an amp ?
 
  • #41
jim hardy said:
I don't think you can get a good measurement anyplace near that USB hub.
You did the experiment i was hoping you would

View attachment 230023

that thing is radiating so intensely it induces voltage in your 'scope probe. A few feet of separation might help.

I'd say try powering your gizmo from a battery if you can find one that's not over the 6 volt absolute maximum rating and preferably not over the 5.5 recommended maximum operating.
Maybe series up three AA's ?
If that helps get a 4.5 or so volt "Wall Wart" that's linear (line frequency transformer and rectifier inside) . You can tell them from switchers they're about twice as heavy. Thrift shops are full of them .

First you might try placing a copper clad iron skillet over the USB hub for an EMI shield. That ought to be nice and lossy at 100 khz

Keep on having fun ! It feels so good when you conquer a pesky problem like this.

old jim
Yes, in the picture it is the hub. It is actually a 5V charger. The unit has an option to run from AA batteries. Sometimes it needs to run from AC. This charger has both conducted and EMI noise. Such a poor design. I opened it. It looked inside very unattractive.
 
  • #42
Hi everyone,
I am placing an order for some inductor chokes and ceramic capacitors. If I am successful, or should I say when I succeed, I will post my solution. I am also placing an order for a demo board with my LDO to verify if I make a layout error.
Thank you, everyone. It was a great help.
 
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  • #43
gnurf said:
https://psearch.en.murata.com/capacitor/product/GRM188R60J106ME47#.html
That 10μF capacitor, with 5V applied, is less than 3μF. See the third graph "DC bias characteristics" in the datasheet.

Also the capacitor has a self resonant frequency of 2MHz, so it will be less effective above that frequency. See the first graph "Frequency characteristics |Z|" in the datasheet.

The root problem seems to be that switching power supply, the scope trace indicates it is bottom-of-the-barrel quality. Replace it.

Cheers,
Tom
 
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  • #44
jim hardy said:
Isn't USB standard 5 volts DC at about an amp ?
That's the general idea, yes.

However, since most device which people would connect to a charger has battery (what can make up for many problems from a wicked input voltage), something 5V-ish is frequently considered market-ready as long as it can provide charging current for cheap (and at least a week long).
 
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  • #45
Here's a 5 volt wall-wart with an old fashioned transformer/rectifier/linear regulator

upload_2018-8-31_3-26-51.png


https://www.jameco.com/z/DDU050100H...Single-Output-5-Volt-1-Amp-5-Watt_168605.html

it can't make EMI like a switcher does

but it has a round barrel connector not USB.
 

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  • #47
Tom.G said:
That 10μF capacitor, with 5V applied, is less than 3μF. See the third graph "DC bias characteristics" in the datasheet.

Also the capacitor has a self resonant frequency of 2MHz, so it will be less effective above that frequency. See the first graph "Frequency characteristics |Z|" in the datasheet.

The root problem seems to be that switching power supply, the scope trace indicates it is bottom-of-the-barrel quality. Replace it.

Cheers,
Tom
I am going to find a better capacitor. RF type.
 
  • #48
I would try adding something like a 100nF in parallel. Just solder it to the existing capacitor.
 
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  • #49
CWatters said:
I would try adding something like a 100nF in parallel. Just solder it to the existing capacitor.
Yes, as in a disc ceramic. Put one at each of the connectors in the hub too, they will probably do more good there than in your external device.
 
  • #50
Hi, I tried different ways to fight the noise. In the end, the only thing that actually helped is Earth ground. I connected my device ground return to an Earth ground through Y-type capacitors. It reduced the noise to acceptable levels. The only thing is that this solution is not practical in my case.
 

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