In search of RF quiet USB charging solution; 2-meters mobile

[Crom]

I'm appealing to the forum for some help.

USB charging is the truck is mandatory for me. However, I've struggled to find an RF quiet USB charging solution. I've tried (6) different off-the-shelf 12v to 5v USB products, all of which produce RF white noise (hiss) which totally destroys my 2-meter mobile radio's sensitivity. Said another way, these DC-DC converters raise the noise floor 20-30dB! I suspect, that is happening to many Amateur radio operators, and they don't even know it.

These are some of the product I've tried:

• Griffin dual port USB charger (Lots of noise, produced by 400Khz switching power supply)
• Generic DC12V to 5V 4 USB Car Charger Step-Down Power Supply Module For Phone Table PC
• SMAKN Dc/dc Converter 12v Step Down to 5v/3a Power Supply Module
• Nextrox® Car LED Display Power Supply 12V to 5V 3A DC/DC Buck Converter Module

The one DC-DC converter that seems to be okay based on my tests using human ears (No oscilloscope / Spectrum Analyzer) was this product:

• DROK DC Volt Converter 9-35V 12V/24V Step Down to 5V 8A/40W (synchronous rectification non-isolated buck module (BUCK))

Here is a video of the problem: https://www.youtube.com/watch?v=Wvz4uu5opLg

And here is one HAMS blog post on the subject.


So have any of you HAMS out there found an RF quiet USB charging solution?

 

[Frdmskr]

What kind of rig? From personal experience Icom has no front end. Kenwood is best but not perfect. Yaesu is in the middle. This is based on living in the Northern VA area where you can walk on RF.

Check these out. http://www.parelectronics.com/amateur.php

I use these on my rigs (current Yaesu FT-8900, former Icom ID 800 and Kenwood TMD700). They handle 98% of what got thrown at it. After that turn up the squelch a bit.


Sent from my iPhone using Tapatalk

 

[4x4junkie]

^^
I don't think those would help. RFI emitted by a switching converter is very wideband, and would be present across all frequencies.


My quick solution would most likely be a 7805 3-terminal linear regulator wired to a USB jack.

Not the most efficient, but would spare you all the noise.

Otherwise you could maybe try putting .01µF disc capacitors across the input & outputs of the ones you have, and/or ferrite chokes that you wind the wires through (I've dealt with this issue on some 12V LED lights I have with a good bit of success).

 

[DaveInDenver]

Using a 7805 is going to produce a lot of heat with any useful current draw. Charging a phone might consume ~1 amp, so you're going to burn off 7 watts to generate 5. Since no one will tolerate 42% efficiency anymore you really have to consider switchers, which are often at least 80% efficient. Ultimately there's no reason a SMPS has to be noisy, those are just done poorly (read: cheap). It's probably not the converter itself either but the shielding if it's that wide band. Lining the inside of the enclosure with copper tape might help, for example.

 

[Crom]

What kind of rig? From personal experience Icom has no front end. Kenwood is best but not perfect. Yaesu is in the middle. This is based on living in the Northern VA area where you can walk on RF.

Thanks. Filtering on the radio is really not an option. I need to stop the noise generated by the switching power supply, these things are so RF noisy they can interfere with FM broadcast radio. Is the noise being radiated through the air or is it being put on the DC output or DC input side? These are important questions, and there is lots to understand, and part of my problem is I don't fully understand RFI and I don't have the requiste tools to troubleshoot. This weekend I hope to read and digest this publication: A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing. The topic seems intimidating, but I'm not looking at mastering every aspect, I just want to be able to apply an effective solution at minimal cost and time to me.

^^
I don't think those would help. RFI emitted by a switching converter is very wideband, and would be present across all frequencies.

My quick solution would most likely be a 7805 3-terminal linear regulator wired to a USB jack.

Not the most efficient, but would spare you all the noise.

Otherwise you could maybe try putting .01µF disc capacitors across the input & outputs of the ones you have, and/or ferrite chokes that you wind the wires through (I've dealt with this issue on some 12V LED lights I have with a good bit of success).

Yes. I hope to add some sort of filtering like KA7OEI did here. I need to get a grasp on filtering circuits though, I have no experience with it. Do you have a suggestion on where I can learn how to build input and output filtering circuits? The linear regulator inefficiencies make that option a no-go, but I do appreciate the comment it has been suggested to me by some other hams as an effective solution.

Using a 7805 is going to produce a lot of heat with any useful current draw. Charging a phone might consume ~1 amp, so you're going to burn off 7 watts to generate 5. Since no one will tolerate 42% efficiency anymore you really have to consider switchers, which are often at least 80% efficient. Ultimately there's no reason a SMPS has to be noisy, those are just done poorly (read: cheap). It's probably not the converter itself either but the shielding if it's that wide band. Lining the inside of the enclosure with copper tape might help, for example.

Dave, I appreciate your comment here. I've tried wrapping some of these BUCK modules in aluminum foil as an experiment, it did not seem to help. I think the noise is being generated on the DC output side, but I don't have any real tools to verify. My tools consist of multimeter and handheld Inductive amplifier.

In KA7OEI's post linked above, he wrote his solution was to:

[QUOTE="KA7OEI]What I finally did was to construct my very own, custom, car USB power adapter using a pair of 3 amp switching converters, placed inside a die-cast aluminum box with extensive input and output L/C filtering: This has proven to be absolutely clean in terms of RFI [/QUOTE]

I'm not opposed to assembling some filters, but I need to learn how first. I also don't have a lot of time... So it would be nice though if there was a product in the market for hams to charge USB products that is RF quiet. Based on my months of research I have yet to find one.

Side question, in my testing, the one BUCK module that seemed to be RF quiet was a module that was described as "synchronous rectification non-isolated buck module (BUCK)" it was a more expensive module vs. the others I tried. I'm currently using it in the truck as a hardline +5v to a SPOT Trace product. That module is powered 24/7 and does not interefere with 2M radio as far as I can tell. Now for my question, Why would this module be RF quiet vs. the others? Is it the synchronous rectification property? Or perhaps better quality components? Or is it just impossible to tell without seeing the circuit? And remember I don't have any real RF analysis tools!

 

[DaveInDenver]

Synchronous rectification might help with RFI, although it's mostly to improve converter efficiency. Not all switchers are the same, there are literally hundreds of topologies and configurations; buck, boost, buck-boost, flyback, forward and then there are continuous and discontinuous modes of them. Add in the range of switching frequencies you can select, different filtering options, quality of components, layout, PCB quality etc. You usually have to sacrifice some efficiency when you design for good EMI/EMC.

It's really a monster task to try and predict with high certainty which particular power supply will be quiet generally. You can generalize to some extent, select a fundamental that is not a harmonic of your desired band. So if you can find the spec make sure it's not switching a multiple of 150MHz. There are low power monolythic chips that do a very good job at being quiet. They're used in telecommunications, radios, servers, places that highly value low EMI. But one thing you can use is often cost. Quiet, efficient, cheap - pick two. Most USB converters select efficient and cheap...

http://www.onsemi.com/pub_link/Collateral/AND8428-D.PDF
http://www.testequity.com/documents/pdf/switching-mode-power-supply-analysis.pdf
http://www.elmac.co.uk/pdfs/CE_SMPS_updated05.pdf

 

[craig333]

What kind of range are you talking about? I have the http://www.westmarine.com/buy/blue-sea-systems--12v-dual-usb-charger-socket-panel-mount--14152532 in my camper and I don't notice any rfi in my truck mounted radio.

 

[mm58]

Would it be possible to "wrap " the charger with aluminum foil or wire screen, making a crude Faraday cage- suppressing the rfi being emitted? Just a thought. ..

 

[4x4junkie]

Yes. I hope to add some sort of filtering like KA7OEI did here. I need to get a grasp on filtering circuits though, I have no experience with it. Do you have a suggestion on where I can learn how to build input and output filtering circuits? The linear regulator inefficiencies make that option a no-go, but I do appreciate the comment it has been suggested to me by some other hams as an effective solution.

Much of my knowledge initially came from printed sources such as the ARRL Handbook (a little before the internet became popular). The Handbook isn't as detailed as some other documentation that is available, however it should still serve as a great place to start your learning, as much of it is written in layman's terms. There's also another good-sized ARRL book available specifically for dealing with just RFI/EMI issues, which I have not read myself, but I would expect to provide a lot of info without it getting too overly technical at first. As you pick up more & more on it, the other (more detailed) stuff such as in the article you linked to should all begin to make more sense (and be easier to digest). At least that has been the case for me anyway.

Is the noise being radiated through the air or is it being put on the DC output or DC input side?

It's most likely being radiated by the wires exiting the device (and the noise subsequently is being received through the air by your antenna). For sure this would have to be suppressed at the device that is radiating the noise (the USB charger). Wrapping the device itself in foil is unlikely to do anything though since it's the wires radiating the RFI. Ferrite chokes are what's needed here (possibly along with the .01µF caps I mentioned, which these would have to go internal to the unit across each of the devices input & output connections, and to it's chassis ground).

Using a 7805 is going to produce a lot of heat with any useful current draw. Charging a phone might consume ~1 amp, so you're going to burn off 7 watts to generate 5. Since no one will tolerate 42% efficiency anymore you really have to consider switchers, which are often at least 80% efficient. Ultimately there's no reason a SMPS has to be noisy, those are just done poorly (read: cheap). It's probably not the converter itself either but the shielding if it's that wide band. Lining the inside of the enclosure with copper tape might help, for example.

Yeah... I did say "Not the most efficient..."

For one, maybe two devices, I don't see where this would be a problem (and is why I mentioned it). But if numerous devices are all being charged at once, then yeah, I s'pose you'd be wasting a lot of energy as heat.

 

[DaveInDenver]

Yeah... I did say "Not the most efficient..."

For one, maybe two devices, I don't see where this would be a problem (and is why I mentioned it). But if numerous devices are all being charged at once, then yeah, I s'pose you'd be wasting a lot of energy as heat.

At 250mA with a 7805 dropping from 12V you're generating 1.75W, which means without a heat sink the device will rise 113.75 degrees C above ambient (the thermal resistance junction-to-air is 65 C/W). If it's room temp of 25 you'll be roughly at 138C, which will probably shut down the 7805. With a heat sink the temp rise is 5 C/W + the heat sink thermal resistance. So bolt it to something, a chunk of aluminum or a real heat sink.

 

[mm58]

Another shielding idea:

Extract just the outer braided shield from a length of antenna coax and use it as a jacket over the USB charger cord

 

[Crom]

What kind of range are you talking about? I have the http://www.westmarine.com/buy/blue-sea-systems--12v-dual-usb-charger-socket-panel-mount--14152532 in my camper and I don't notice any rfi in my truck mounted radio.

Range between USB and radio is about 3'.

From what I've read (see link below) that unit has the same problem described in this thread. Although the specific complaint below was that the blue sea was wiping out FM broadcast stations.

http://forum.ih8mud.com/threads/blue-sea-dual-usb-causing-rfi-any-solutions.799250/

In your case, you've been spared for some reason, maybe its harmonics doesn't affect 2M, or its happening to you and you don't even know it! Best way to test is to remove power from blue sea usb charger, tune up a weak station on 2M radio, plug your blue sea back in and if that station is wiped out, you're affected. Another symptom is hiss during strong station reception. Repeat the same test above while tuned up to a strong repeater for example, with USB charger powered you get a hiss sound, remove power to USB and strong station comes in crystal clear, and the difference would be very noticeable!

 

[craig333]

In my unscientific test I took my ft-60 into the camper. 2m and 70cm I could detect some slight rfi within two feet. Not much and nothing at all further than that. I have a little inverter with a usb port that I plugged into the truck, sits about six inches from the 8800 and didn't notice any rfi there either. I also grabbed my portable superadio and took it into camper next to the usb. It did have some interference on am but again on fm, it was just slight, certainly not enough to wipe out even weak stations. I wish I had better equipment but theres my subjective experience.

 

[lugueto]

That's weird, I have two of those Griffin dual-usb chargers, they are both within 12in of my radio and I get zero noise when using them. The only noise I get on my vehicles is from the Rigid Industries Duallys, significant noise at that, and they're at least six feet away from the radio.

I have a general question though, when using ferrite chokes, do you have to use them in the wires of the equipment that emits interference or in the radio's wiring or antenna coax???

 

[4x4junkie]

^^
In general, they go on the device that is emitting the RFI (the USB charger in this case).

Use of chokes and/or filters on a device that is receiving interference is generally reserved for when the fundamental (desired) output of a transmitter causes the device to malfunction (not the case here).