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#1
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Jerry Stuckle wrote in news:lras98$ebf$1@dont-
email.me: As for determining whether it is oscillating or not - I wouldn't trust anything short of a good spectrum analyzer. The signal could be anywhere (and changing frequency). A spectrum analyzer will still show it; a receiver won't necessarily. No clues at all? I remember when I first set up that little MAR-6 based preamp, it had some nasty side effects on the sound till I added a springy ciopper alloy strip to each side on the underside of the board so it bit gently into the aluminium case when I slid it in to it, finally curing the local RF oscillations it had before I solved this. I was wondering if similar audible sounds might be heard as a guide to other, larger scales of local feedback and oscillation. |
#2
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On 7/30/2014 5:00 PM, Lostgallifreyan wrote:
Jerry Stuckle wrote in news:lras98$ebf$1@dont- email.me: As for determining whether it is oscillating or not - I wouldn't trust anything short of a good spectrum analyzer. The signal could be anywhere (and changing frequency). A spectrum analyzer will still show it; a receiver won't necessarily. No clues at all? I remember when I first set up that little MAR-6 based preamp, it had some nasty side effects on the sound till I added a springy ciopper alloy strip to each side on the underside of the board so it bit gently into the aluminium case when I slid it in to it, finally curing the local RF oscillations it had before I solved this. I was wondering if similar audible sounds might be heard as a guide to other, larger scales of local feedback and oscillation. Not necessarily. Even if there were physical vibrations, they may be out of the audible range. But even vibrations are not common. You were lucky you could hear it. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
#3
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Jerry Stuckle wrote in news:lrbmkt$ohq$1@dont-
email.me: Not necessarily. Even if there were physical vibrations, they may be out of the audible range. But even vibrations are not common. You were lucky you could hear it. Well, it was like a flanger having a psychedilc fit. ![]() Fortunately easy to cure. I imagine that many other sources of wild RF oscillation might affect the sound like that. |
#4
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On 7/30/2014 5:08 PM, Lostgallifreyan wrote:
Jerry Stuckle wrote in news:lrbmkt$ohq$1@dont- email.me: Not necessarily. Even if there were physical vibrations, they may be out of the audible range. But even vibrations are not common. You were lucky you could hear it. Well, it was like a flanger having a psychedilc fit. ![]() Fortunately easy to cure. I imagine that many other sources of wild RF oscillation might affect the sound like that. Not really. To affect the sound, you need something which will respond to the RF in a physical vibration manner (i.e. magnetic), and the signal must be in the audio spectrum. Neither is very likely. -- ================== Remove the "x" from my email address Jerry Stuckle ================== |
#5
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Jerry Stuckle wrote in news:lrcavl$dok$2@dont-
email.me: On 7/30/2014 5:08 PM, Lostgallifreyan wrote: Jerry Stuckle wrote in news:lrbmkt$ohq$1@dont- email.me: Not necessarily. Even if there were physical vibrations, they may be out of the audible range. But even vibrations are not common. You were lucky you could hear it. Well, it was like a flanger having a psychedilc fit. ![]() Fortunately easy to cure. I imagine that many other sources of wild RF oscillation might affect the sound like that. Not really. To affect the sound, you need something which will respond to the RF in a physical vibration manner (i.e. magnetic), and the signal must be in the audio spectrum. Neither is very likely. To produce audio from RF directly all that is needed is for the difference (a beat frequency) to fall within the audio pass band of the equipment. A wildly varying frequency of oscillation could make a sweep of audio pitch. Timbre or variability of effect might give plenty of clues as to cause. That's what led me to find and fix the original oscillations in the dodgy ground plane connection in the preamp casing. If it can happen this way on one (tiny) scale, I see no general reason why it could not happen on the larger scale with feedback between antennas a hundred feet apart. Whether I can use it diagnostically is questionable, but I think it can exist. Given the number of frequencies picked up by the antenna, I find it hard to beleive that NO stray feedback oscillation would differ from any one of them by a value that puts it in the audio band. I'm fairly sure I'd hear something, especially if choosing a clean audio signal over which it woudl easily show up. |
#6
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On 7/31/2014 3:36 AM, Lostgallifreyan wrote:
Jerry Stuckle wrote in news:lrcavl$dok$2@dont- email.me: On 7/30/2014 5:08 PM, Lostgallifreyan wrote: Jerry Stuckle wrote in news:lrbmkt$ohq$1@dont- email.me: Not necessarily. Even if there were physical vibrations, they may be out of the audible range. But even vibrations are not common. You were lucky you could hear it. Well, it was like a flanger having a psychedilc fit. ![]() Fortunately easy to cure. I imagine that many other sources of wild RF oscillation might affect the sound like that. Not really. To affect the sound, you need something which will respond to the RF in a physical vibration manner (i.e. magnetic), and the signal must be in the audio spectrum. Neither is very likely. To produce audio from RF directly all that is needed is for the difference (a beat frequency) to fall within the audio pass band of the equipment. A wildly varying frequency of oscillation could make a sweep of audio pitch. Timbre or variability of effect might give plenty of clues as to cause. That's what led me to find and fix the original oscillations in the dodgy ground plane connection in the preamp casing. If it can happen this way on one (tiny) scale, I see no general reason why it could not happen on the larger scale with feedback between antennas a hundred feet apart. Whether I can use it diagnostically is questionable, but I think it can exist. Given the number of frequencies picked up by the antenna, I find it hard to beleive that NO stray feedback oscillation would differ from any one of them by a value that puts it in the audio band. I'm fairly sure I'd hear something, especially if choosing a clean audio signal over which it woudl easily show up. Yes, I know how it works - I've held both amateur and commercial licenses for well over 40 years and worked on almost everything from $40 CB sets to multimillion dollar mainframe computers. I've even done some design in my free time. I suggest you not try to teach those who know more than you your "facts". The facts a to have the beat frequency, you need two signals within 15kHz or so of each other. Those signals must be mixed, which means at least one must be non-linear. This will give you a beat frequency in the audio spectrum. Once you have this, you need something to generate a strong enough magnetic field in the audio frequency range to act as a driver, and something close enough and made of a magnetic material to vibrate. I've seen a fair number of spurious emissions over the years (mostly from VHF/UHF radios). Every one of them has been RF, and none of them have created audio oscillations. A few months ago we even had a case right here where a VHF radio in a county bus had a spur on the input to the local 2 meter ham repeater. Once again, no indication in the bus this was occurring. The county found out about it only after the hams contacted them. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
#7
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Jerry Stuckle wrote in news:lrdc2v$b15$1@dont-
email.me: The facts a to have the beat frequency, you need two signals within 15kHz or so of each other. Those signals must be mixed, which means at least one must be non-linear. This will give you a beat frequency in the audio spectrum. Ok, that nonlinear bit makes sense. If the signals do not do that then I'd have nothing to detect the way I heard it. When I said audio I didn't mean direct emission from something driven by strong RF. I was thinking it might be heard from a weak signal added to the output audio signal, as happened with that preamp's casing before I improved its connection to the PCB's ground plane. The sound was a kind of whistling squeal, almost white-noise at times. I imagined that feedbacking antennas might also produce this, and that's the core of my question, whether or not this is true, or useful even if it is. By the way, I'm not trying to teach you anything. I know you know these things or I'd not be here asking stuff. I was just stating what I know to try to get to the bottom of this, and so you had some basis for pitching a reply based on what you have reason to think I might understand. |
#8
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On Thu, 31 Jul 2014 08:16:29 -0400, Jerry Stuckle
wrote: Yes, I know how it works - I've held both amateur and commercial licenses for well over 40 years and worked on almost everything from $40 CB sets to multimillion dollar mainframe computers. Well let's see... I got my novice license when I was 15, so that means I've had a ham license (various call signs) for 51 years. My FCC First Class and now GROL for about 46 years. I've even done some design in my free time. My time is not free. I did RF design and ran a 2way shop for about 11 years. I guess that having more years of experience entitles me to tell you how to run your life. I suggest you not try to teach those who know more than you your "facts". Yep. Quantity is a good substitute for quality. Unfortunately, I have to agree with most of your comments. The facts a to have the beat frequency, you need two signals within 15kHz or so of each other. Those signals must be mixed, which means at least one must be non-linear. Signals are not linear or non-linear, but are usually called "distorted" if not a clean sine wave. The circuitry through which the signals pass can be linear or non-linear. Linear circuits do not mix, unless overloaded. If the mixing circuit is non-linear, you can use harmonics to create more mix combinations that result in 15KHz frequency difference. n*f1 +/- m*f2 = beat_freq where n and m are integers. This will give you a beat frequency in the audio spectrum. Yep. However, my hearing is kinda marginal and needs something lower than about 10-12KHz. Once you have this, you need something to generate a strong enough magnetic field in the audio frequency range to act as a driver, and something close enough and made of a magnetic material to vibrate. True. There has to be an electric to audio transducer somewhere. However, there are situations where the mechanism is obscure. For example, when I moved into my house in Ben Lomond in 1973, the 200MHz radar on nearby Mt Umunhum was running megawatts pointed straight at me. Every time the rotating dish went by, my hi-fi would produce buzz out of the speakers at the pulse repetition frequency. That was easy enough to understand. However, the coils inside my kitchen electric oven also went twang as the beam went by, which was more difficult to explain. There had to be a rusty connection or bolt, but I couldn't find one. Maybe the oven or coils were resonant at 200 Mhz. I never did figure out how it worked. I've seen a fair number of spurious emissions over the years (mostly from VHF/UHF radios). Every one of them has been RF, and none of them have created audio oscillations. When I transmit on my VHF HT near my comptah speakers, they buzz loudly. Does that count? A few months ago we even had a case right here where a VHF radio in a county bus had a spur on the input to the local 2 meter ham repeater. Once again, no indication in the bus this was occurring. The county found out about it only after the hams contacted them. The county should have had a new digital or narrow band radio. Was this a new radio? I usually look at the output on a spectrum analyzer before letting something go out the door. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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