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Message-ID: <3.0.5.32.19991119182201.00a15280@sunf0>
Date: Fri, 19 Nov 1999 18:22:01 +0000
To: rsgb_lf_group@blacksheep.org
From: "Simon Lloyd-Hughes" <simon.lloyd-hughes@rd.bbc.co.uk>
Subject: Re: LF: Receivers and power-line noise
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The Elektor magazine always carries details at the back on emc measures to
be taken on psu's.


At 11:38 18/11/99 -0000, you wrote:
>Hi all,
>
>This may well have been mentioned before on this reflector, so apologies to
>those concerned if it's going back over old ground.
>
>For anyone plagued by power line "hash" when receiving on LF, particularly
>when using wire antennas, it's worth checking the cleanliness of the DC
>power supply which is being used to power the receiving setup.
>
>I have found out the hard way that the rectifier diodes in some power
>supplies can generate truly enormous amounts of harmonic hash at these
>frequencies. I have not checked too closely but I assume this is simply
>being re-radiated through the transformer and into the mains wiring. There
>is EC EMC legislation relevant to this aspect of power supply design, but
>I'm not sure how effective this is in "special cases" such as our very weak
>signal conditions.
>
>I have tried several power units ranging from simple regulated "plug-top"
>types to more complex heavy current 13.8V PSUs specifically designed for
>powering radio gear. I currently use a small double-insulated (i.e. no mains
>earth connection) plug-top PSU to power my receiver. This was originally so
>bad that it completely wiped out reception on my AKD HF3 below about 2MHz
>(note that this PSU was actually intended for this receiver!). In this case
>the problem was cured by the simple expedient of fitting a 0.1uF ceramic
>capacitor across the transformer secondary feed to the bridge rectifier, but
>this is not necessarily a cure for all situations. Lots of lossy ferrite
>etc. on the mains side may be called for!
>
>My Daiwa 13.8V 12A unit is much better in this respect but does still
>introduce a small level of extra noise. This is not really significant
>enough to warrant modification though. A slightly cheaper grade of 13.8V 5A
>PSU was tested and this proved too noisy to be usable without extensive
>modification.
>
>Finally, I have a multi-output variable voltage lab-type PSU for bench work
>which has also proved too noisy to power the LF gear as it stands.
>
>I suppose the surest way to eliminate this kind of noise would be to run the
>receiving gear from batteries, which would also get round the problem of
>noise directly coupled from the mains. I would be interested to know how
>many other people have encountered similar problems with LF reception.
>
>73
>
>Kevin, G1HDQ
>
>
>
>