Re: LF: Noise R75 - rs232 from smpsu -

["James Moritz" <james.moritz@btopenworld.com>]

Dear Laurence, LF Group,

----- Original Message ----- 
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Tuesday, August 03, 2010 11:35 AM
Subject: Re: LF: Noise R75 - rs232 from smpsu -


> Hi Laurence I have found some problems with the "two wire" versions where
> the powered instrument has no proper ground connection....VLF receivers 
> etc.
> But often this can be cured by grounding one side of the low-voltage 
> output.
My experience would agree with Alan - several times with laptops, and also a 
"compact" desktop with external PSU, making a direct connection between the 
mains ground and the chassis of the computer has made a substantial noise 
reduction. It seems to be fairly common that there is no ground  continuity 
between input and output of PSU. With no ground connection to the output 
side of the PSU, any common-mode noise due to the PSU has to find its way to 
earth via the RF connections usually, and can get into the signal path in 
the process. Winding as many turns as possible of the DC output lead round a 
large split ferrite core can also be helpful in reducing the noise current. 
I find >10 turns are needed to make a useful difference.
Cheers, Jim Moritz
73 de M0BMU