On 17 Oct 2003 11:28:37 GMT, (W1RFI) wrote:

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Switch-mode power supplies can also be noisy, but they generally have noise
that ranges from reasonably coherent (carrier-like) to broad, buzzy clumps of
noise, spaced every N kHz, where N is the free-running rate of the switcher
frequency. N is typically 10-50 kHz or so. Some switchers make noise that is
fairly close to gaussian-distributed noise, with a pretty uniform
distibribution of power vs frequency (power spectral density). One key
diagnostic for a switcher is that the N kHz tends to drift around a bit. A
series of noisy carriers or clumps of noise that are spaced every 15 kHz, for
example, may drift up the band as the unit is turned on and warms up over an
hour or so. If so, it is almost certainly a switcher.

Watch out for Silicon Controlled Rectifiers (SCRs) used for power
switching. They switch on the during a portion of the positive
swinging cycle generating a *sometimes* huge pulse and are usually
used in pairs. They are always 60 cycle, but as there is one on each
polarity there will be two equally spaced pulses appearing as if they
were a single 120 cycle repetition. OTOH a 3-phase system might look
a bit confusing. However the pulse train "should" be symmetrical
based on 120 degree spacing of a 60 cycle base which might give the
appearance of 180 cps. These things are bad enough that the pulses
from one system can cause false triggering in another SCR system
nearby.

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Precip static and in particular, snow static does a very good job of
imitating the sound of some bpl, *but* it looks entirely different
when seen as a time domain display. The static will be a display of
relatively evenly spaced pulses that will *usually* increase in
frequency over time. A single unterminated half wave 75 meter dipole
can blank out a whole city block on some frequencies. The voltages
generated can make the ignition coil in a car look puny.

Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)

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Thanks for your interest.

73,
Ed Hare, W1RFI