tindle@ms.uky.edu (Ken Tindle) (08/24/90)
I'm working on an audio project and have run into a nasty problem. I'm using some cassette mechanisms with a circuit of my design in a language learning system, and have magnetic coupling of noise to the ground plane. The motor is by necessity physically close to the circuit, and it is definitely a case of magnetic coupling only (not direct conduction). I can move the motor away to stop the noise, and it is being driven by a supply voltage which is completely separate from the audio circuit (there is a ground reserved only for the motors). Problem is that there's no way to permanently move the motor far enough away. The motors have an integral speed control circuit, and it could be part of the problem, too. So how do manufacturers of portable cassette players keep this trash out of the audio? They don't *seem* to do anything special as regards the motors themselves, so is there a black magic way to keep the power bus clean? I suspect there's no easy answer to this, in keeping with the ways of magic. --------------------------\ /----------------------------------------------- INTERNET:tindle@ms.uky.edu | "Could you please continue the petty bickering? BITNET: tindle@ukma.bitnet | I find it most intriguing." --- Data, Ken Tindle - Lexington, KY | Star Trek, The Next Generation, "Haven" --------------------------/ \-----------------------------------------------
fmgst@unix.cis.pitt.edu (Filip Gieszczykiewicz) (08/24/90)
<I'm using some cassette mechanisms with a circuit of my design in a <language learning system, and have magnetic coupling of noise to the <ground plane. Greetings. Hey, I'm not the only one.... :-) < <The motor is by necessity physically close to the circuit, and it is <definitely a case of magnetic coupling only (not direct conduction). <I can move the motor away to stop the noise, and it is being driven <by a supply voltage which is completely separate from the audio circuit <(there is a ground reserved only for the motors). Problem is that there's <no way to permanently move the motor far enough away. < <The motors have an integral speed control circuit, and it could be part <of the problem, too. That depends on what type of control they use. Is it one of those centrifugal (sp?) ones or a transistor one? If it's the mechanical one, the best you can do is put a capacitor across the power leads. You could also convert it to an electronic control (if you really care :-) If it's an electronic control, read below. < <So how do manufacturers of portable cassette players keep this trash out Well, for one, they do keep the motor as far away from the pre-amp as possible. One could shield the pre-amp but that may not work... (well, for me...) <of the audio? They don't *seem* to do anything special as regards the <motors themselves, so is there a black magic way to keep the power bus <clean? Oh but the do _do_ something to the motors. Not only are the motors put in a metal container (with rubber vibration "cups" (top and bottom)) but they also wrap the motor with a metal "band". It provides additional shielding. (I'm talking about more expensive motors, not the $14.95 cassette recorders :-) < <I suspect there's no easy answer to this, in keeping with the ways of magic. No magic - just a lot of wishfull thinking ;-) < <INTERNET:tindle@ms.uky.edu | "Could you please continue the petty bickering? <BITNET: tindle@ukma.bitnet | I find it most intriguing." --- Data, <Ken Tindle - Lexington, KY | Star Trek, The Next Generation, "Haven" ^ Take care. (ST:TNG sucks beans! ------+) -- _______________________________________________________________________________ "The Force will be with you, always." It _is_ with me and has been for 10 years Filip Gieszczykiewicz "A man without a dream is like a fish without water." FMGST@PITTVMS or fmgst@unix.cis.pitt.edu "My ideas. ALL MINE!!"
tindle@ms.uky.edu (Ken Tindle) (08/24/90)
In article <31302@unix.cis.pitt.edu> fmgst@unix.cis.pitt.edu (Filip Gieszczykiewicz) writes: > Greetings. Greetings to you too. > Well, for one, they do keep the motor as far away > from the pre-amp as possible. One could shield the > pre-amp but that may not work... (well, for me...) Well, when I layed out the board (which I'm now stuck with) I *did* keep the head and mic pre-amp section away from the motor. Now I find that the pre-amps don't care about the motor, but a line-level op-amp buffer circuit and the headset power amp *do.* Not very nice. I shielded the pre-amps to stop FM radio reception. > Oh but the do _do_ something to the motors. Not only are > the motors put in a metal container (with rubber vibration > "cups" (top and bottom)) but they also wrap the motor with > a metal "band". It provides additional shielding. Well, that's a good description of these motors, except the "band" is around the transistor speed regulator board inside the motor- So why is this wonderful shielding not working? Maybe magic can only be made by Sharp Corp.? Do I need to turn Japanese? > Take care. (ST:TNG sucks beans! ------+) "Well, in this case, Captain, I disagree! One hundred percent!" - Beverly Crusher, ST:TNG, "Symbiosis". --------------------------\ /----------------------------------------------- INTERNET:tindle@ms.uky.edu | "Could you please continue the petty bickering? BITNET: tindle@ukma.bitnet | I find it most intriguing." --- Data, Ken Tindle - Lexington, KY | Star Trek, The Next Generation, "Haven" --------------------------/ \-----------------------------------------------
whit@milton.u.washington.edu (John Whitmore) (08/25/90)
In article <15857@s.ms.uky.edu> tindle@ms.uky.edu (Ken Tindle) writes: >I'm working on an audio project and have run into a nasty problem. > >I'm using some cassette mechanisms with a circuit of my design in a >language learning system, and have magnetic coupling of noise to the >ground plane. > First, a sleeve around the motor will reduce the magnetic emissions considerably (most hi-fi cassette motors have a couple of deep-drawn soft steel covers that accomplish this). Secondly, the position of wires and the orientation of circuit boards can be changed. It is not unusual for an audio component to go back to the manufacturer's bench a few times before the wiring is dressed so that the stray hum pickup meets the noise specification. Lastly, DON'T use a ground plane for low frequency signals (and if you're not in the hundreds of megahertz, it's low frequency.) Instead, try to use single-point grounding as much as possible. The energy in the magnetic field is MAXIMALLY coupled into a low impedance path (like a thin ground plane). Keep any shielded covers connected to ground at EXACTLY ONE point. It's only at high frequencies that wires fail to conduct and a ground plane becomes the only way to distribute your ground. John Whitmore
jeffw@midas.WR.TEK.COM (Jeff Winslow) (08/29/90)
>The energy in the magnetic field is MAXIMALLY coupled into a low >impedance path (like a thin ground plane). Remember, magnetic fields don't really induce voltages in isolated bits of paths. High impedance, low impedance, it doesn't matter. Magnetic fields induce voltages in *loops*. How big are those loops? How are they oriented with respect to each other? If you've got magnetic interference problems, ask those questions. Jeff Winslow
myers@hpfcdj.HP.COM (Bob Myers) (08/31/90)
>Remember, magnetic fields don't really induce voltages in isolated bits >of paths. High impedance, low impedance, it doesn't matter. Magnetic fields >induce voltages in *loops*. Minor not: magnetic fields induce *current* in loops, which then by Ohm's law may result in potential differences ("voltage") *across* any two points on the loop, by the resistance of the loop. I know *you* know this, Jeff, but I get too many students who talk about "the voltage through this resistor" and other such abominations to let it go uncorrected. Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers@fc.hp.com | sentient life-form on this planet.