Message-ID: <007401c18027$30ba9a60$96b51bca@xtr743187>
From: "Vernall" <vernall@xtra.co.nz>
To: rsgb_lf_group@blacksheep.org
References: <200112081848.fB8ImJV13723@smtp.wanadoo.nl>
Subject: Re: Re: LF: Mains hum
Date: Sun, 9 Dec 2001 09:29:55 +1300
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Hi all,

In the discussions there has been mention of mains hum on interconnections,
and mains hum on VLF/LF radio reception.  Both are fair game.

Mains hum on cables
---------------------
The main method of control is to use suitable audio isolating transformers.
These can be salvaged from surplus modem cards.  Some of these have quite
good frequency response (telephone audio bandwidth) but others could be
lacking in low end response and attenuate 50 Hz signal that is actually
present in a signal.  For the likes of Argo viewing, it is the CW sidetone
frequency that is of most interest, and this is usually an easy frequency
for an audio transformer to handle (even if it is designed for 600 ohm
applications, sound cards outputs are lo Z but inputs are hi Z (47 k
ohms?)).

The little transformers can have some leakage flux, so do not place them
near mains transformers as otherwise there will be unwanted flux linkages to
the "isolating" transformer.  I have also had situations where an RF choke
used to control RF feedback in a microphone pre-amplifier was the reason for
more 50 Hz signal appearing in the microphone output (due to the sum of flux
linkages inthe many turns of the RFC).

There are several magazine articles for interfacing radios with PCs, for the
likes of HF RTTY or PSK-31 operation.  As well as audio isolation being
recommended for Tx audio and Rx audio, the PTT isolation can be done with an
opto-isolator.

VLF & LF mains noise
-----------------------
Probably comes from switchmode power supplies and lamp dimmer type of
domestic apparatus, and is received as a radio signal (can be verified by
unplugging the antenna feed).  Looks like 50 or 100 Hz "tramlines" on an
Argo display.  Even if the switching frequency of a switchmode power supply
is unstable, the spacing between the lines tracks at 50 or 100 Hz spacing,
giving "curved tramlines".  As the switchmode supplies have frequencies in
the VLF and LF range, that is where most spurious emissions occur, and
reduce in density at higher frequencies.

Having an antenna separated from mains wiring is the most effective way to
reduce pickup of "mains junk" relative to desired radio signals.

73, Bob ZL2CA