[RFI] Another source containing PWM

Roger (K8RI) k8ri at rogerhalstead.com
Fri Apr 21 00:34:29 EDT 2017

Just something to keep in mind

Working on the latest build, where absolutely nothing seems to fit, I 
noticed on all the new variable speed fans (CPU and Cabinet) the letters 
PWM.  That computer has 7 fans, 4 of which are PWM. (forgot to count the 
one on the 850W PS which is variable speed so it's 8 fans, 5 PWM) These 
fans each use less than 5W at full speed and can be run directly off 12 
VDC at full speed, so if necessary, all wires carrying  PWM can be 
eliminated except for the chips and traces on the motherboard.

"So Far" and I put that in quotes, I've never noticed noise from them, 
but they might just be a problem in the plastic cased, consumer targeted 
computers. Normally problems come in through the antenna(s), but many 
pieces of ham equipment, including the newer top end rigs are connected 
directly to the computers via CAT-5 networks, serial ports, and USB ports.

When I had the noise problem last year (it disappeared) everything got 
into the receiver. It seemed like turning  anything on and off changed 
the noise.  Even my true sin wave generator, 3KVA line conditioner 
(Dedicated 30A circuit) appeared to be generating 20 to 40 over RFI.  
The power supplies for my servers and USB drives were all generating 
horrendous noise.  40 over with NO Antenna connected or interconnecting 
coax connected to the receiver.

Will the general use of PWM in thousands, if not millions of computers 
cause Tremendous RFI, raise the noise floor, cause intermittent 
problems, or go unnoticed?  Compared to variable speed furnace and air 
conditioner fans, the power is minuscule, but the routes into the 
station can be direct and numerous.


Roger (K8RI)

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