Monday, May 26, 2014

How USB car power adapters can ruin 2 meter mobile reception!

For an update on this article, see the posting about a "Quiet" vehicle USB power supply - LINK.

For decades now, the ubiquitous power adapter found in vehicles has been the "Cigarette Lighter Plug" - a 12 volt receptacle found in pretty much every vehicle made for more than more than half a century.

Now, enter the USB connector.

Not necessarily for data connectivity, these provide 5 volts at up to 3 amps and are used to power/charge about anything from a cell phone, smart phone, tablet, game device, E-reader - even modern Handie-Talkie - to name but a small number of possibilities!  If your vehicle is fairly new, it will already be equipped with a couple of these, but if not, you'll probably have to get something like the one shown in Figure 1, below.
Figure 1:  A typical USB power adapter, available practically everywhere -
and if you have one in your car, it's probably clobbering
YOUR 2 meter reception!
Click on the image for a slightly larger version.

These things seem to be everywhere, and they are pretty much all alike:  They are small and fit pretty much entirely inside the cigarette lighter plug receptacle itself.

While the two USB outlets have different current ratings printed on them, they are actually connected in parallel - but on the two center pins (the "data" pins of the USB connector) there are different resistances that indicate to the device connected to it how much current they can pull:  Not all devices actually pay attention to these "programming resistances" but some (particularly some Apple [tm] devices) do - although plugging into the "wrong" outlet will (supposedly) result in just lower charging rate.

What's wrong with these USB power devices?

Functionally, these devices - at least the ones at which I have looked - seem to "work" just fine:  They output a "reasonably" clean 5 volt source of charging voltage (perhaps 100 millivolts of ripple at 3 amps - nothing that these devices can't deal with, and cleaner than many "wall" chargers) so what's wrong with them?

Nothing - unless you want to listen to the radio.

I got one of these to charge my cell phone and power the GPS receiver in my car to replace the "Y" cigarette lighter adapter that I'd been using in my car - a rather awkward affair that was always in the way of the gearshift and something that kept un-plugging itself due to its swinging around under its own weight, and into it was plugged the original Garmin power adapter and an OEM Motorola car charger - both of which were "reasonably" clean - causing a little bit of QRM only in the weakest-signal areas.

Immediately upon plugging in this new device, pictured in Figure 1, I noticed that something was amiss:

2 meter reception practically disappeared!

70cm reception wasn't as badly affected - but I did notice an impact there as well - and I even noticed that broadcast FM reception was adversely affected when I went up some of the local canyons, all of this clearly evident when I unplugged the adapter and suddenly, everything was "normal" again.

Attempts at modification:

The first thing I did was attempt to modify this adapter to reduce its emission of "grunge" which took the form of a white noise "hiss" across the spectrum - at least at VHF and above - and I did the following:
  • The addition of a choke (10 uH) on the DC input of the switching regulator.
  • The addition of an SMD monolithic capacitor on the input of the switching regulator.
This and the above removed all traces of noise on the DC input as measured using an oscilloscope,  but a quick check back in the car revealed that this made no difference in the amount of QRM being generated.

I should have known better:  Removing the USB cables then revealed that the "grunge" was being radiated almost entirely on the DC output rather than the DC input, so I went back to the the workbench and did more modification:
Figure 2:
Modifications made to the bottom of the board to make it "quieter" from
and RF standpoint.  Some of the SMD caps can be seen, as can the
RFI filter with integrated beads.
Click on the image for a slightly larger version.

  • I added SMD monolithic capacitors to better-bypass the DC output
  • I also noticed that the "grounding" of the DC input, that of the main switching circuit and of the DC output were not solidly connected to each other - that is, they were connected through some rather narrow traces that could offer some potentially high impedances.  I bolstered these using pieces of copper strap and wire to reduce these possibly high-impedance paths.
  • Routed the DC output through an integrated RFI filter consisting of two ferrite beads and a ceramic capacitor - the red/orange thing that can be seen in Figure 2, above.
Figure 3:
Modifications made to the top of the board.  In the upper-
right corner can be seen the added RF choke while
some of the added grounding can be seen along
the bottom edge.  The cardboard was added to prevent
the added grounding and components from
shorting out when the unit was reassembled.
Click on the image for a slightly larger version.

Doing the modifications did help a bit - but the unit still caused a considerable amount of degradation to 2 meter reception.

How bad was it?

Making the modifications depicted in Figure 2 and Figure 3 reduced the RFI by about 15-20dB which meant that I could now hear strong, local repeaters around town OK, but more distant repeaters with the weaker signal were still significantly degraded.

After modifications, how bad was the degradation on 2 meters?

To answer that question I connected a communications test set to the mobile rig via an "Iso-Tee" and ran a SINAD test with the GPS receiver and a cell phone connected.

The result?

18 dB of degradation on 2 meters.

Still pretty bad - And this was after modification.

Prior to modification, the degradation was on the order of 30-35dB on 2 meters!  I didn't make any measurements on 70cm or quantify the degradation on the FM broadcast band.

If I disconnected the USB cable to the GPS receiver, the degradation dropped to about 12 dB, which made sense since that particular cable ran up onto the dashboard, while the phone sat on the console, down between the passenger and driver seats.

While I was at it, I tried adding some snap-on ferrites to the USB cables to on the output of the adapter, but I could find little actual improvement - but that wasn't too surprising as these devices (the ferrites) are generally better at keeping RFI from getting into devices than getting out of them.

By re-routing USB cables, I could cause the RFI level to vary, but I couldn't get it much better than the 12 dB value mentioned above, but I could make it much worse - well into the 20's.

What to do?

Figure 4: 
 Another typical 5 volt car-type power adapter.  I have several that look
like this on the outside, but on the inside, who knows what you'll find?  One of
them was a well-constructed, well-filtered switching regulator while another
switcher was utter garbage - while yet another one consisted of nothing other
than a 78L05 regulator and a couple of electrolytic capacitors:  While it was
"quiet" from an RF standpoint, but capable of no more than 100 mA
of current, at most - barely enough to charge a phone, slowly, and
not nearly enough to power up the GPS receiver.
At the moment, I don't have any suggestion as to what brand of car USB power converter is "clean" in terms of RFI, but it would suggest that one should avoid those like the one picture in Figure 1.

In looking around, I've also tested a number of other units - some of them being much larger, such as the one pictured in Figure 4, below.  I have several of these - all of them look pretty much alike on the outside but inside, none of them look alike.  One of them came with an old cell phone as is quite well constructed with what looks like proper filtering on the input and output - but with a rating of only 500 mA, it wouldn't be able to power anything but the GPS receiver, alone - if that - let alone several devices at once.

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 unit has proven to be absolutely clean in terms of RFI and you can read about that device here - LINK.

In the meantime, if you have noticed that your 2 meter reception seems to have gone to hell, try unplugging your USB power adapter for a few seconds and see if that has anything to do with it...

How quiet are the USB power converters built into recent-vintage vehicles?

I have no idea, but the limited reports that I have indicate that they do not seem to be anywhere near as bad as the things depicted above!


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