Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!seismo!rutgers!ames!ucbcad!ucbvax!CORY.BERKELEY.EDU!dillon
From: dillon@CORY.BERKELEY.EDU (Matt Dillon)
Newsgroups: comp.sys.amiga
Subject: Re: ASDG Fast ram chirps?!
Message-ID: <8708250447.AA18383@cory.Berkeley.EDU>
Date: Tue, 25-Aug-87 00:47:29 EDT
Article-I.D.: cory.8708250447.AA18383
Posted: Tue Aug 25 00:47:29 1987
Date-Received: Wed, 26-Aug-87 01:59:34 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 47

>Why chirping noises?
>The mini-Rack C has a very simple power supply just sufficient to support
>a majority of Zorro-I type boards without drawing power from the Amiga.  When
>any device accesses the address lines of the Amiga all power sources will 
>experience brief Mhz type power surges as digital gates switch on and off.
>A more expensive power supply wont reflect these surges too much as noise.
>As to why Mhz type surges produces audible Khz type sounds you'd have to
>refer to Mr. Fourier Series and Transform for the accurate answer.
>                                Wilson Cheung

The digital switching has nothing to do with it.  Not only are the frequencies
five orders of magnitude higher than we could hear, but the bypass capacitors
handle the spikes before they get anywhere near the power supply.

I don't have a mini-Rack, but I can guess it's got a switching power supply.
Switching power supplies work by gating power through an inductor and 
capacitor.  You simply wait until the current in the inductor reaches maximum,
then gate the power off.  An inductors is analogous to momentum.... the
current keeps on flowing even when the power is cut... right into the 
capacitor charging it and bringing up the voltage at the node.  By switching
the power on and off at some frequency, you can build up the voltage to
(theoretically), infinity.  Realistically, the capacitor can only hold so
much before it arcs between the plates.  In anycase, switching power supplies
are used for two reasons:  (1) no transformer, (2) they can regulate up (as in
step up the voltage), (3) they are usually extremely efficient... as in 85%
typical.  

Oops, that's three reasons.    So what does this have to do with chirping?
Simple... the frequencies used to switch are quite low, and with sufficient
load can get down near 60hz or even lower.  With all that power going between
the capacitor and inductor you hear it quite well.  Switching power supplies
are notorious for being noisy, especially when loaded down.  Typical switching
power supplies oscillate at around 20-30Khz when not loaded terribly, which is
beyond the hearing of most people.

Well, I've said this much, I might as well finish.  The drain reason the
frequency goes down when the power supply gets loaded is an artifact of the
way it works.... ALL the power a switching power supply uses comes directly
from that capacitor (or capacitors).  The more power you pull from it, the
faster the capacitor drains (causing the voltage to go down a little... a
couple of mV), and it must oscillate slower to build up more current in the
inductor... 


				Well, a simplistic explanation anyway.

				-Matt