Thread: power supplies with minimal radio interferance?

I think either Mickz or OldSpark can give a definite answer, but it seems the idea of having the power supply in its own shielded case, with the pass through caps, is that it removes noise from even being near the motherboard as well as other peripherals.

So maybe one option is to put the power supply into the computer case, but still have it inside its own chamber inside there, and still use the pass through caps.

I have the CPU and PSU mounted inside an aluminium case.

I carried out extensities tests with most noise filtering methods.

Changing from a linear PSU (no noise) to AC and then DC PSU’s in an attempt to pin down all sources of FM and GPS interference.

The results vary with CPU clock frequency, power and type of HDD. It varies with the layout and position of all leads going to and from the unit and with alternator voltage cycling around the inverter boost/cut change over point.

When PSU and HDD noise had been reduced, in all cases the PC was still a problem when in close proximity to a GPS receiver. With the exception of a very low power Notebook/Laptop.

Bench testing revealed that it was just too hard to get nil interference with a powerful PC anywhere near the GPS.

The fact that the PSU is in the same shielded case as the MB made little difference to PSU noise.

The PSU has to be in its own shielded enclosure with feed through caps. NO amount of “after the fact” ferrite filtering will work at these frequencies.

I now have the shielded PSU and the MB mounted together in an aluminium case in the boot. The lead length from PSU to MB is around 2 inches. Even under a 6m x 6m carport with 3 sides enclosed in metal I get almost every satellite position at high strength with the USB GPS RX mounted under the centre dash panel. There is no interference to FM at all.

Chassis earthing layout, while certainly effective for car ignition and other lower frequency impulse noise is less effective when it comes to suppressing this short wavelength interference.

It depends on so many things. There are a lot of benefits in having the PC in the boot, the only disadvantage I was worried about was the video quality over a long cable, however in my case the quality is unchanged.

Once again it’s worth pointing out that cutting a hole in the lid of an enclosure has almost no effect on noise in the FM/GPS range. A completely shielded enclosure (by itself) is ineffective at removing high frequency interference from travelling along every lead that exits or enters the case, which is what VHF - UHF noise does very well.

well dam....I may just leave it in the trunk then

Making a case to fit the board, psu, drives, etc...and then run the cables thru the pass-through caps...I just don't have the room right now...have to re-measure the dash cavity.

They don't make cables with this sort of filtering built in do they?
Are these commonly found on equipment such as DirecTV receivers, etc?

will the SMT versions work just as well as the others listed? Is there a reason not to use the SMT?

Unfortunately this type of noise can’t be effectively removed once it’s on the cable.

SMT units are for a different purpose and by virtue of their size are severely current limited.

Feed throughs caps filter by bypassing noise to ground as the leads pass from inside the shielded case through the case wall and into the outside world.

Some PSU's may not need them if you have everything in a case in the boot. The M4 definitely needs them if you want a really quite system.

This bypass scheme is really only for the PSU – The PC in a case in the boot is usually fine. However having the PC in the dash with most of the car wiring running behind or close to it will be a problem for front mounted GPS receivers.

A few pictures of the M4-ATX supply with RF case mod along with MB, 60GB SSD and Battery ISO.

The case is smaller than my COMPAQ laptop but it’s about 3 times as thick though. . It’s made of very heave gauge aluminium. The fans all run at reduced voltage so are very quite. Everything runs cool.

The unit will be hidden behind a false carpeted panel with a speaker grill allowing air into the fan.

Found an interesting note on EMC while reading some data on GPS interference.

Quote: “The GPS band is far beyond the 1GHz limit that applies to almost all EMC regulations. So even if a device is compliant with respect to EMC regulations it might severely disturb a GPS receiver.

Some notes on CPU clocks typically generating interference into GPS region. By the way, the waveforms I looked at in a 12v ATX PSU that boosts down to 7 volts or so are something to be very afraid of.

And once again a quote: ”One of the most difficult tasks in electrical engineering is to design a system that is essentially free of measurable interference.”

There is some technical data that reveals why a GPS receiver is so sensitive to this interference, especially when you realise the level of signal that is at the input of the receiver.

What? You mean a simple low power jammer could render positioning systems useless?
How will people find their stolen cars?

LOL - I recently responded to someone that did not understand why "alarms" etc were still being built with the GPS tracking systems that exist...
But many do not understand my preference for terrestrial land lines...
Especially when "satellites are free"!

I share your feelings for copper wire. Hell I still build valve communication gear and yes, I won’t be putting the paper map away just yet.

nice case...did you make that yourself or have it fabbed? The Travla case I have is steel, about the same thickness as a high-end tower, doesn't bend easily and was tough as nails to cut through. The power supply is mounted inside the case as well...

Is there anything I could do to further reduce interference since it's all inside the case other than using the pass-thru caps? Maybe put the PSU inside it's own case inside the travla case??

Setting the motherboard speeds (fsb, pci-e, sata, memory and cpu speed) to something outside the fm range wouldn't help would it? (for noise generated by the MB)