[RFI] DC Power supplies

Roger (K8RI) k8ri at rogerhalstead.com
Fri May 6 12:06:01 EDT 2016

A couple years ago, I purchased a 30A switching supply with adjustable 
voltage on an introductory sale price.  I wish I'd purchased two as they 
are now over double the price.  Voltage and current are displayed in two 
digital readouts. I run a 756 Pro and a couple 144/440 duobander off it. 
That power supply is pretty much RF quiet even on the spectrum analyzer, 
except when starting  and that is both visible and audible for 1 to 2 
seconds.  I assume that hash is outside the filtering range.
As it's in the shop station and I'm in the house, I'll need to look up 
the brand name when I go out.


Roger (K8RI)

On 5/4/2016 Wednesday 5:59 PM, Doug Powell wrote:
> There is no such thing as a "noise free"‎ switcher, only unaffected equipment.  The quietest switching power supplies tend to be medical grade.   You don't want RFI affecting life support equipment.
> All switchers pull pulses of current from the rectified AC Line at frequencies of a few kHz up to 1 Mhz. While this fundamental switching frequency and mostly odd harmonics can be part of the problem, the real issue is the reverse recovery time of the diodes and the storage time of the switching transistors. These times can be from a few microseconds down to 100 nanoseconds or less. The reciprocal of these recovery times is basis of the frequencies and the harmonics  they produce.  ‎Switch-mode power supply designers are always pressured for higher and higher operating frequencies and faster recovery times in order to shrink physical size and improve efficiency.   Just think about to latest generation of USB 3.0 power packs and how small they are for the amount of power they produce.
>   The high frequency pulses produced by bipolar transistor designs can easily have a fourier content in the 150 kHz to 30 MHz range. The MOSFET type switchers can typically produce additional frequencies from 10 MHz on up to more than 300 MHz. The stuff in the low end tends to be narrow band emissions and the stuff on the high end tends to be broadband noise, for the most part.  Standard emissions testing these days set limits in a range from a few 10s of kHz to 3 GHz and sometimes higher.
> So the bottom line is to find a power pack with a noise profile that works in your application.  If it is of interest to the group, I may be able to pull up some old plots from a spectrum analyzer, just to show what I mean by "noise profile".
> All the best, ~ Doug
>    Original Message
> From: WD8ARZ
> Sent: Wednesday, May 4, 2016 3:15 PM
> To: rfi at contesting.com
> Subject: Re: [RFI] DC Power supplies
> AC switching type power supplies tend to be very rfi noisy. Transformer
> type power supplies dont use the the dc to ac to dc switching that
> generate that RFI .... and should be quieter power supplies. They are
> heavier, with line isolation in most cases, and worth the extra expense
> and weight. Another bad trait for those switching power supplies is that
> there is generally not a true physical break the utility power feed
> switch. Instead they just disable the output and leave the switching
> circuits active and thus are RFI sources as long as they are plugged in.
> 73 from Bill - WD8ARZ
> South Bend, Indiana
> On 5/4/2016 2:51 PM, charlie at thegallos.com wrote:
>> Hey Gang,
>> We all know about buying old analog wall warts for power supplies to cut
>> down on RFI, but sometimes, that is NOT going to happen
>> snip snip
>> I think it might be a real good idea if we could come up with a list of
>> various "Line lump" (so they can be wired in) type power supplies at
>> various power ratings that are KNOWN to be good, and sources for them
>> I'd be more than happy to collect the list and put it up on my web site
>> 73 de KG2V - Charlie - www.thegallos.com
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