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Re: [RFI] 12vdc native laptop

To: kd4e@verizon.net, rfi@contesting.com
Subject: Re: [RFI] 12vdc native laptop
From: "FoMoCo EMC" <fomoco_emc@hotmail.com>
Date: Fri, 26 May 2006 14:35:35 -0400
List-post: <mailto:rfi@contesting.com>
Most computer noise in a notebook computer that bothers you in the HF band 
will be from Switch Mode Power Supplies (SMPS).  A typical notebook has a 
few power supplies that can give you grief.  First there is the the AC/DC 
power cube which is external to the notebook and has over a meter of cable 
going to each of the AC and DC connectors which makes for a more efficient 

Then there is the DC to DC supply in the laptop that takes the DC input 
(around 18V) and generates the +/- 12V, 5V, 3.3V, 1.8V or whatever the 
processor is running on.  It is essentially several separate SMPS.

Next is the supply that takes 5V and generates the HV AC for the LCD 
backlight.  The LCD itself may also have a SMPS that generates the various 
bias voltages used in the panel itself.

By its nature SMPS noise is broadband so it will wipe out more than a few 
kilohertz and repeat itself at harmonics of the switch frequency.  When 
hunting down these issues use a divide and conquer approach.  For every 
frequency that causes interference you will need to remove cables, disable 
features, etc to determine the root cause of the issue.

For example,  if you hear a 20 kHz wide signal in the radio and it repeats 
itself every 50 kHz it shows the characteristics of a SMPS.  Remove the AC 
power cube and run the laptop on batteries.  If it went away then you pretty 
much have it isolated.  If not keep going until you find it.

I don't recommend tearing your laptop apart to fix it but that depends on 
your comfort level.

Remember to disconnect your receive antenna from the receiver to verify that 
you are hearing the interference through the antenna and not through the 
radio cabinet or power line.

Hope this helps but I don't expect it to be easy.

>It appears that the LCD displays may be of
>concern in both laptops apart from the power
>supply problems.
> > I would expect more noise from a hardwired NIC than from an 802.11x 
> > LAN which operates in the 2.5 GHz range.
>The IBM was noisy without any NIC installed.

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