On 03/07/13 19:50, Jim K9YC Brown wrote:
On 3/7/2013 3:07 PM, KM5VI wrote:
I thought that conventional power supplies (transformer-bridge-filter)
were
relatively clean on reflected harmonics?
Nope! Power line harmonics are the result of current flowing in
relatively short pulses at the positive and negative peaks of the 60 Hz
cycle to charge the input filter capacitor, which is used in both linear
and switch-mode power supplies.
It's quite simple -- capacitor input filter => strong harmonic current
on the mains power line. These are harmonics of the mains power
frequency, either 50 or 60 Hz, depending on where you live.
What switch mode power supplies add is RF noise -- they start with
rectified DC to a capacitor input filter, then use that DC to generate
square waves at a much higher frequency (typically 10-20 kHz), then
rectify and filter that to feed the equipment. The RF noise consists of
harmonics of the square wave, modulated by random noise added to
"dither" (frequency modulate) the square wave. Dithering is a sort of
"cheat" to get around EMC regulations (FCC, and EU) -- instead of a
strong carrier at each harmonic, the noise is spread over a wider
bandwidth. The result is "humps" of noise that slowly drift up and down
the band with a weak carrier in the middle of the hump.
So you are saying our tube HPA's hV power supplies have a near unity
power factor?
--
Ron KA4INM - Youvan's corollary:
Every action results in unwanted side effects.
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