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#1
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On 7/30/2014 1:16 AM, Lostgallifreyan wrote:
Jerry Stuckle wrote in : The problem with what you're trying to do is you can easily get RF feedback between the output antenna and the input antenna, causing oscillations. And those oscillations may not be in the FM band - they may be on an adjacent band (like aircraft, which is just above the FM band). That's my main concern, I want it to be well behaved, or it won't happen. It has simple 1-pole LC filters to limit it to the FM broadcast band, but I will leave it intact and not cobble it into another purpose because it's very good at its original intent. If I do experiment further, it looks like the best thing to watch out for is any unexpected oscillation frequencies despite its filters (LPF on input, HPF on output), and deliberately orienting a radiating dipole to minimise feedback. Is it easy (or possible) to catch clues of such bad behaviour while listeing using somethign like a Tecsun PL-390 or other radio using those new DSP IC's? I ask that because they're cheap, easy to get, and fairly consistent, and include a usefully specified signal strength meter. If so, what should I consider to be a warnign sign? But the amplifier you're trying to use is meant to feed a receiver directly, not another antenna. So output is going to be very low (on the order of microwatts) - much lower than any amplifier which feeds an antenna. Agreed. ![]() I only tried because the last thing I want to do is pollute the spectrum. Start small... 1-pole LC filters won't be narrow enough to limit potential oscillations to the FM band. You'll need much more than that. 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. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
#2
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Jerry Stuckle wrote in news:lras98$ebf$1@dont-
email.me: As for determining whether it is oscillating or not - I wouldn't trust anything short of a good spectrum analyzer. The signal could be anywhere (and changing frequency). A spectrum analyzer will still show it; a receiver won't necessarily. No clues at all? I remember when I first set up that little MAR-6 based preamp, it had some nasty side effects on the sound till I added a springy ciopper alloy strip to each side on the underside of the board so it bit gently into the aluminium case when I slid it in to it, finally curing the local RF oscillations it had before I solved this. I was wondering if similar audible sounds might be heard as a guide to other, larger scales of local feedback and oscillation. |
#3
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On 7/30/2014 5:00 PM, Lostgallifreyan wrote:
Jerry Stuckle wrote in news:lras98$ebf$1@dont- email.me: As for determining whether it is oscillating or not - I wouldn't trust anything short of a good spectrum analyzer. The signal could be anywhere (and changing frequency). A spectrum analyzer will still show it; a receiver won't necessarily. No clues at all? I remember when I first set up that little MAR-6 based preamp, it had some nasty side effects on the sound till I added a springy ciopper alloy strip to each side on the underside of the board so it bit gently into the aluminium case when I slid it in to it, finally curing the local RF oscillations it had before I solved this. I was wondering if similar audible sounds might be heard as a guide to other, larger scales of local feedback and oscillation. Not necessarily. Even if there were physical vibrations, they may be out of the audible range. But even vibrations are not common. You were lucky you could hear it. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
#4
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Jerry Stuckle wrote in news:lrbmkt$ohq$1@dont-
email.me: Not necessarily. Even if there were physical vibrations, they may be out of the audible range. But even vibrations are not common. You were lucky you could hear it. Well, it was like a flanger having a psychedilc fit. ![]() Fortunately easy to cure. I imagine that many other sources of wild RF oscillation might affect the sound like that. |
#5
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On 7/30/2014 5:08 PM, Lostgallifreyan wrote:
Jerry Stuckle wrote in news:lrbmkt$ohq$1@dont- email.me: Not necessarily. Even if there were physical vibrations, they may be out of the audible range. But even vibrations are not common. You were lucky you could hear it. Well, it was like a flanger having a psychedilc fit. ![]() Fortunately easy to cure. I imagine that many other sources of wild RF oscillation might affect the sound like that. Not really. To affect the sound, you need something which will respond to the RF in a physical vibration manner (i.e. magnetic), and the signal must be in the audio spectrum. Neither is very likely. -- ================== Remove the "x" from my email address Jerry Stuckle ================== |
#6
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Jerry Stuckle wrote in news:lrcavl$dok$2@dont-
email.me: On 7/30/2014 5:08 PM, Lostgallifreyan wrote: Jerry Stuckle wrote in news:lrbmkt$ohq$1@dont- email.me: Not necessarily. Even if there were physical vibrations, they may be out of the audible range. But even vibrations are not common. You were lucky you could hear it. Well, it was like a flanger having a psychedilc fit. ![]() Fortunately easy to cure. I imagine that many other sources of wild RF oscillation might affect the sound like that. Not really. To affect the sound, you need something which will respond to the RF in a physical vibration manner (i.e. magnetic), and the signal must be in the audio spectrum. Neither is very likely. To produce audio from RF directly all that is needed is for the difference (a beat frequency) to fall within the audio pass band of the equipment. A wildly varying frequency of oscillation could make a sweep of audio pitch. Timbre or variability of effect might give plenty of clues as to cause. That's what led me to find and fix the original oscillations in the dodgy ground plane connection in the preamp casing. If it can happen this way on one (tiny) scale, I see no general reason why it could not happen on the larger scale with feedback between antennas a hundred feet apart. Whether I can use it diagnostically is questionable, but I think it can exist. Given the number of frequencies picked up by the antenna, I find it hard to beleive that NO stray feedback oscillation would differ from any one of them by a value that puts it in the audio band. I'm fairly sure I'd hear something, especially if choosing a clean audio signal over which it woudl easily show up. |
#7
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On 7/31/2014 3:36 AM, Lostgallifreyan wrote:
Jerry Stuckle wrote in news:lrcavl$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 ================== |
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