Message-ID: <001e01c1ee94$2553bd60$22666ec3@cestag>
From: "cesare tagliabue" <cestag@dada.it>
To: rsgb_lf_group@blacksheep.org
Subject: LF: R: Mains Harmonics ??
Date: Sun, 28 Apr 2002 11:07:05 +0200
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

            Hi Alan
        Yes, I'm aware that most of this type of noise is generated by TV or
PC supplies, nevertheless that comes also from the rectifier  of standard
supplies as well as from sodium or quicksilver high pressure lamps, I have
these last on the street facing my home. The drift observed in these lines
of noise, is compatible with the carefull regulation made here in
continental Europe by the center of Laufemburg in Germany, in fact, reducing
the drift to the fundamental frequency it appears very small, only some
milliHz. Thanks anyhow for your tips, best 73's  Cesare

Cesare Tagliabue   I 5 TGC
WW-Loc  JN53PS
e-mail: cestag@dada.it
url: http://www.dadacasa.com/i5tgc

-----Messaggio originale-----
Da: Alan Melia <Alan.Melia@btinternet.com>
A: LF-Group <rsgb_lf_group@blacksheep.org>
Data: domenica 28 aprile 2002 2.43
Oggetto: LF: Mains Harmonics ??


>Hi All, here is one to shoot at !! If as Cesare says the interference he
>gets are straight harmonics of the 50Hz mains and they are the 2750th
>harmonic, should they not be 1/ (2750)^2 down (-72dB) on the fundamental ??
>which ought to be quite small in the cables so even smaller radiated.
>
> I see drifty lines with 50Hz and 100Hz spacing, but a quick check at lower
>frequencies with teh Rx or SA shows that these are mains modulated signals
>with a funamental at 30 to 40kHz and emanate mainly from switch-mode power
>supplies, PCs and TVs being the worst offenders. If they are such, the
drift
>is due to the switcher oscillator not the mains so you cannot lock a comb
to
>it. If it genuinely is straight mains frequency harmonics then it should be
>possible to lock a switched capacitor comb filter to this.
>
>Most SMPSU QRM grids have a spacing of twice the mains frequency as there
is
>a bridge rectifer involved in the primary side of the PSU, though often the
>intermediate 50Hz is seen weakly. Local TVs seem to radiate the 40kHz via
>the braid of the aerial coax, so a standard high-pass TVI filter with a
>braid-breaker should work on reverse (Mike G3XDV has previously reported
>this solution). If I could actually get some out of the RSGB I might be
able
>to confirm this.
>
>I was able to make quite a good reduction in the locally generated lines by
>ensuring that cables were tidied up and that no aerial coax ( for any
>frequency ) ran near any mains cable. Wall-wart supplies to feed the RX
have
>the 12v cable wrapped into a coil and taped round a ferrite rod. PC suppies
>were opened and fitted with screened IEC filtered sockets (the 3amp version
>because it has the biggest inductors) This later was the most dramatic as I
>often have 4 PCs running in the house. All feeds to soundcards are isolated
>via audio transformers. All my noise now is from neighbour's TVs.  I can
>tell which channel some are watching by the cadence of the FM inducted on
>the PSU by the audio load, particularly at news time (why do they all
switch
>channel when those interesting Party Political broadcasts come on ?? could
I
>use blackmail ??....."put this filter in or I'll tell Mr Blair you switched
>him off!" ).
>
>Another gotcha is that loops dont always help, they will couple magetically
>to TV and monitor scan coils at quite large distances (10m+) , the givaway
>...TVs scan at mains and PC monitors at 70Hz or about 90Hz, both are locked
>and are "crystal" stable lines. Also the direction is at right-angles to
the
>normal loop maximum direction. Fortunately most of the hamonics of the line
>scans are outside the band.
>
>Cheers de ALan G3NYK
>alan.melia@btinternet.com
>
>
>
>