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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#41
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How can the same FM station appear at two different spots onthe dial?
Michael Terrell wrote:
Adjacent channel interference like that is caused by the IF bandwidth and the skirt. The IF transformers aren't brick wall, the amplitude drops away slowly outside the desired bandwidth. ** FM receivers have multiple stages of IF band limiting making the falloff very sharp outside the needed 200kHz. That allows a local station to be strong enough to cause problems. ** Nope - the FM detector ( ratio or quadrature) is also tuned to the centre of the IF strip and will not demodulate an out of band signal. ..... Phil |
#42
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How can the same FM station appear at two different spots on the dial?
William Sommerwerck wrote:
"Phil Allison" wrote in message ... William Sommerwerck wrote: Given that the FM band is 20MHz, and twice 10.7 MHz is greater than 20MHz, if the LO is above the incoming signal, images would come from stations above 107.9MHz (outside the band). If the LO were below the incoming signal, you could have in-band images starting at 98.9MHz. ** Nonsense. Long as a particular band has less width than double the IF frequency, no in-band images will occur. Did you actually read what I wrote? The second sentence says that. ** It says NOTHING the sort - ****** boy. In the third sentence, I said "If the LO were //below// the incoming signal..." Do the math: 88.1 minus 10.7 plus 21.4 equals... what? 98.8? ** Garbage. With the LO at 77.4, the image is at 66.7 The FM broadcast band does not suffer from in-band images long as the IF is 10.7MHz of higher. .... Phil |
#43
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How can the same FM station appear at two different spots on the dial?
O Ian Jackson wrote:
You don't change the mechanism for the interference, but because the two TV channel allocations are where they are, if either was used in your area you would certainly increase the possibility of interference from them. However, as I've said, I think it's unusual for the LO to be on the low side (probably for exactly this reason). ** A man who prefers his ignorant opinions to facts is a complete fool: The 6AQ8 along with the 12AT7 were the most common tubes used for LOs in FM tuners from the early 1950s onwards. http://www.r-type.org/exhib/aav0008.htm They were invariably used as low side oscillators. The fact that some what might now be considered 'highly desirable collectibles' had low-side LOs doesn't mean it became a standard. ** Never said it was "standard" - just quite common. Proves you are wrong - sonny boy. Were not several TV channels tucked right under the FM band back then ? Not the present FM band. However, in the USA FM started life between 42 to 50MHz* but this was essentially experimental. After the war, it was allocated the present band (87.8-107.9 MHz). ** Huh? What is the relevance of that crap ? ATC (AM, of course) breaking through on 97.3MHz (at least on my kitchen radio)! -- ** So that is your only case? **** off fool. ..... Phil Ian |
#44
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How can the same FM station appear at two different spots on the dial?
"Phil Allison" wrote in message
... William Sommerwerck wrote: "Phil Allison" wrote in message ... William Sommerwerck wrote: Given that the FM band is 20MHz, and twice 10.7 MHz is greater than 20MHz, if the LO is above the incoming signal, images would come from stations above 107.9MHz (outside the band). If the LO were below the incoming signal, you could have in-band images starting at 98.9MHz. ** Nonsense. Long as a particular band has less width than double the IF frequency, no in-band images will occur. Did you actually read what I wrote? The second sentence says that. ** It says NOTHING the sort - ****** boy. It says exactly that. It's amazing that someone as intelligent as you can't understand plain language. In the third sentence, I said "If the LO were //below// the incoming signal..." Do the math: 88.1 minus 10.7 plus 21.4 equals... what? 98.8? ** Garbage. With the LO at 77.4, the image is at 66.7. Uh... that's right. |
#45
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How can the same FM station appear at two different spots on the dial?
William Sommerwerck wrote:
Given that the FM band is 20MHz, and twice 10.7 MHz is greater than 20MHz, if the LO is above the incoming signal, images would come from stations above 107.9MHz (outside the band). If the LO were below the incoming signal, you could have in-band images starting at 98.9MHz. ** Nonsense. Long as a particular band has less width than double the IF frequency, no in-band images will occur. Did you actually read what I wrote? The second sentence says that. ** It says NOTHING the sort - ****** boy. It says exactly that. ** FFS learn to read you autistic idiot. MY post say there are NO in-band images at all - neither from high nor low side injection of the LO. In the third sentence, I said "If the LO were //below// the incoming signal..." Do the math: 88.1 minus 10.7 plus 21.4 equals... what? 98.8? ** Garbage. With the LO at 77.4, the image is at 66.7. Uh... that's right. ** Wot a thick head. ..... Phil |
#46
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How can the same FM station appear at two different spots onthedial?
Phil Allison wrote: Michael Terrell wrote: Adjacent channel interference like that is caused by the IF bandwidth and the skirt. The IF transformers aren't brick wall, the amplitude drops away slowly outside the desired bandwidth. ** FM receivers have multiple stages of IF band limiting making the falloff very sharp outside the needed 200kHz. That allows a local station to be strong enough to cause problems. ** Nope - the FM detector ( ratio or quadrature) is also tuned to the centre of the IF strip and will not demodulate an out of band signal. More Phil****, as always. It's a damned good thing you didn't design and build deep space telemetry equipment. Even business radios used expensive crystal filters to reduce adjacent channel interference, instead of 50 cent IF transformers. Digitally tuned FM receivers can still receive an adjacent channel, but more than a channel away it becomes quite distorted. Go back to hacking old stereos. -- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge. |
#47
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How can the same FM station appear at two different spots onthe dial?
Michael Terrell wrote:
Phil Allison wrote: Michael Terrell wrote: Adjacent channel interference like that is caused by the IF bandwidth and the skirt. The IF transformers aren't brick wall, the amplitude drops away slowly outside the desired bandwidth. ** FM receivers have multiple stages of IF band limiting making the falloff very sharp outside the needed 200kHz. That allows a local station to be strong enough to cause problems. ** Nope - the FM detector ( ratio or quadrature) is also tuned to the centre of the IF strip and will not demodulate an out of band signal. More Phil****, as always. It's a damned good thing you didn't design and build deep space telemetry equipment. ** Desperate liars resort to abuse when they have no case. Even business radios used expensive crystal filters to reduce adjacent channel interference, ** Not one bit relevant to *wide band* FM broadcast receivers. instead of 50 cent IF transformers. ** Which, when used in multiples, produce sharp roll ofsf at the skirts of the pass band. Digitally tuned FM receivers can still receive an adjacent channel, ** More irrelevance, it matters not how the LO is tuned. but more than a channel away it becomes quite distorted. ** Tuned FM detectors are like that. Go back to hacking old stereos. ** Go back to washing dunny floors, you pathetic ass. ..... Phil |
#48
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How can the same FM station appear at two different spots onthedial?
Phil Allison wrote: Michael Terrell wrote: Phil Allison wrote: Michael Terrell wrote: Adjacent channel interference like that is caused by the IF bandwidth and the skirt. The IF transformers aren't brick wall, the amplitude drops away slowly outside the desired bandwidth. ** FM receivers have multiple stages of IF band limiting making the falloff very sharp outside the needed 200kHz. That allows a local station to be strong enough to cause problems. ** Nope - the FM detector ( ratio or quadrature) is also tuned to the centre of the IF strip and will not demodulate an out of band signal. More Phil****, as always. It's a damned good thing you didn't design and build deep space telemetry equipment. ** Desperate liars resort to abuse when they have no case. DING! DING! DING! Whenever you have no clue you start your 'Angry Dumbass Dance', and call people a liar. All you do is prove what a fool you are. Even business radios used expensive crystal filters to reduce adjacent channel interference, ** Not one bit relevant to *wide band* FM broadcast receivers. Only if you have the I.Q. of a rusty doorknob. instead of 50 cent IF transformers. ** Which, when used in multiples, produce sharp roll ofsf at the skirts of the pass band. Define sharp. If it is too sharp, it causes distortion in the recovered signal. Each transformer has an insertion loss in the IF stages. FM radios use just enough tuned circuits to get barely acceptable performance. Even the cheap Murata ceramic filters have a sloppy skirt. The only advantage is that they don't need aligned during manufacturing. Cram your bull**** and look at it on a network analyzer. Oh, that's right. You have no real test equipment, just junk from a '70s TV shop. You want to talk wideband? One of the Telemetry products we manufactured had an IF range from 1 KHz to 20 MHz bandwidth at the -3 dB points. They had to be aligned on a network analyzer, or with a calibrated sweep generator to achieve the proper skirt. Digitally tuned FM receivers can still receive an adjacent channel, ** More irrelevance, it matters not how the LO is tuned. Sigh. More of your stupidity. Digital has no AFC, so it can't be pulled off of center. but more than a channel away it becomes quite distorted. ** Tuned FM detectors are like that. Go back to hacking old stereos. ** Go back to washing dunny floors, you pathetic ass. Whatever the hell that crap means, but I guess that you've heard it all your life from people around you. Read a damned book on receiver design. Analog IF bandwidth is specified at the -3 dB points. That wouldn't be possible without a skirt. Brick wall requires a FIR filter or another digital filter that processes a digitized input. The design I worked on digitized to 50 to 890 MHz range for a 70 MHz IF. That was followed by a pair of FIR filters for the IF and another pair for the output bandwidth. -- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge. |
#49
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How can the same FM station appear at two different spots onthe dial?
On 10/28/2014 08:40 AM, Michael Black wrote:
On Tue, 28 Oct 2014, dave wrote: On 10/27/2014 01:48 PM, whit3rd wrote: On Monday, October 27, 2014 12:01:45 PM UTC-7, David Platt wrote: In article , micky wrote: How can the same FM station appear at two different spots on the dial? (3) Intermodulation. If you have two strong stations nearby, their signals can mix (either in the receiver front end, or elsewhere) and create "spurious" signals located on either side of their true locations on the dial. These spur signals will often be noisy and distorted. This sounds very likely; if it is due to front-end nonlinearity, it's possible to test/treat it by inserting an attenuator between the FM antenna and the receiver (assuming the receiver has a plug-in antenna). Lower the signal level, and the spurious response should go away. Alternately, one can attenuate (filter) either the interfering FM station or the (presumably AM) difference-frequency station: this can be done with a lossy antenna+load placed near your radio, so can apply without access to antenna terminals. I used to work on Radio Row in Houston. One day the FCC came to visit KILT FM 100.1 because they were causing squeals on the aeronautical band. It wasn't any of the station's pro gear making the interference; it was an old console FM receiver in the station lobby. 100.1 + 21.4 = 121.5. Radio row was on the direct approach to Hobby Airport or this old mis-aligned radio would have never been busted. I thought that was some of the basis of the ban on electronic devices on airplanes. Certainly there is folklore that when AM/FM transistor portables became cheap and available, suddenly people were using them on airplanes, and that did or could have caused interference, precisely because the local oscillator radiated and in the aircraft band. It's murky whether that was the specific cause of the rule or not, and probably made murkier since it's been forty years since I read about this. Michael Cheap consumer radios are using pretty exotic methods to reduce the mass and the current draw these days. Any oscillators are deep into the chipset. |
#50
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How can the same FM station appear at two different spots onthe dial?
On 10/28/2014 11:11 PM, micky wrote:
On Tue, 28 Oct 2014 16:39:16 -0400, Michael Black wrote: On Tue, 28 Oct 2014, Paul Drahn wrote: On 10/27/2014 9:32 AM, micky wrote: How can the same FM station appear at two different spots on the dial? To the person who complained recently that I was off topic, I'm sorry. This is not about any repair it would be feasible to make. It's only about electronics. Where I live there are two FM radio stations, 88.1 which is only a few miles away, and 88.5 which is 40 or 50 miles away. Right now, only my expensive KLM radio plus any car radio gets the second one well, but I've had some cheap radios that do almost as well. I lose track of which radios those are, so I'll start tuning at 88.1 and tune up very gradually. After a period of silence, when I get above 88.5 to what I'd estimate is 88.6 or .7 or .8 I get 88.1 again. How is that happening? I know about harmonics, but that doesn't apply, does it? P.S. This means 88.5 doesn't come in at all. I've tried stretching out the power cord, which on the cheap radios is usually the antenna. Sometimes that helps but on most of the radios, 88.5 won't come in at all. P.P.S. 88.1 is WYPR Baltimore. 88.5 is WAMU in DC. Sometimes they play the same thing, like during the top of the hour news, Diane Rehm, etc. although WAMU is on a 5 or 10 second delay most of the time. Because the topic and the voices can be the same it means I can't tell for a while if I've gotten 88.5 or just another 'instance' of 88.1. If you have the AFC on, the station will pop up at different dial locations depending on which direction you are tuning. At least my old portable does. I think I've noticed this too. But the AFC wasn't on, because that would have made it almost impossible to get a weak station like 88.5. Well, I'm calling it weak because most radios won't get it, but Wikip says that it's 50,000 watts ERF (sp?) but 88.1 is only 15,500 watts. (also ERF? It didnt' say.) So maybe I'm calling it weak because it farther away, in DC, not Baltimore where I live, but actually, there are places north of here, farther from DC, the Westminster, Md. area, where 88.5 comes in well and 88.1 barely comes in. A friend moved to Finksberg and she had to change to 88.5. But maybe the FCC makes them arrange their antennas so that in the city of Baltimore and its populous suburbs, 88.5 doesn't overpower 88.1. But the frequencies are different, and there's no Baltimore 88.5, so why would 88.5's antennas have to avoid the populous part of Baltimore, or any part? I was thinking along that line, except thinking of pointing out that for whatever reasons, not great selectivity or a noisy synthesizer, a station can be heard on more than one frequency. But, I can't recall that happening when there's an adjacent station, then the first station being received further up. If that second station wasn't there, AFC is a good suggestion, and something we might not think of much anymore, with so many fm receivers digitally tuned. But I'd think it would "lock" to the statino further up, that presumably is stronger at that point than the first station. Michael The fcc.info link I sent lets you search the FCC database in a friendly gui fashion |
#51
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How can the same FM station appear at two different spots onthe dial?
On 10/29/2014 02:30 AM, Arfa Daily wrote:
snip Well, I'm calling it weak because most radios won't get it, but Wikip says that it's 50,000 watts ERF (sp?) but 88.1 is only 15,500 watts. (also ERF? It didnt' say.) ERP - Effective Radiated Power ? The 'real' transmitter output multiplied by the 'gain' of the transmitting antenna. So maybe I'm calling it weak because it farther away, in DC, not Baltimore where I live, There are many many factors that affect the propagation of a VHF signal over a lower frequency one, some of which will degrade that signal, and others of which can, under the right conditions, enhance it. VHF signal reception is a lottery, once you are outside the designed service area of the station. but actually, there are places north of here, farther from DC, the Westminster, Md. area, where 88.5 comes in well and 88.1 barely comes in. A friend moved to Finksberg and she had to change to 88.5. But maybe the FCC makes them arrange their antennas so that in the city of Baltimore and its populous suburbs, 88.5 doesn't overpower 88.1. But the frequencies are different, and there's no Baltimore 88.5, so why would 88.5's antennas have to avoid the populous part of Baltimore, or any part? Without seeing a published map of the station's service area, it's impossible to say. However, something as simple as a tall building in the direction of the transmitting site, can be enough to cast a 'radio shadow' across a large swathe of territory on the 'downstream' side Arfa Michael An obstruction in the energy field creates Fresnel Zones on the side of the obstruction opposite the antenna. These are alternating peaks higher than normal and dips lower than normal. The distance between the peaks and nulls (Fresnel Zones) is determined by the distance between the antenna and the obstruction. |
#52
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How can the same FM station appear at two different spots on the dial?
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#53
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How can the same FM station appear at two different spots on the dial?
David Platt wrote:
Commercial FM is generally allowed a +/- 75 kHz carrier deviation. Due to the way FM works, and due to the fact that the station is transmitting a stereo subcarrier (centered on 38 kHz, with its own sidebands going out as much as 15 kHz on either side), ** It's worth pointing out that there is only ONE carrier for an FM broadcast signal. At any instant in time, there is only one frequency to deal with and one signal voltage coming from the detector. With FM stereo, the detector's output includes supersonic signals up to 50 KHz or so. The supersonic stuff provides the L-R difference signal. A lot of FM radios/receivers have fairly "broad" intermediate- frequency filters... e.g. one or two crystal filters with 220 kHz or even 250 kHz bandwidth. ** Crystal filters are a tad expensive for an FM radio - so designers make do with tuned transformers and Ceramic filters in the 10.7MHz amplifier stages. Even a budget FM tuner will have at least one or two of each along with a tuned RF stage to provide good out of band and nearby signal rejection for normal use. FM DXing is NOT normal use. Such broad receptivity lets almost all of the "desired" station's signal in... and that's good for low-distortion stereo reception since you get the whole stereo subcarrier. Unfortunately, if there's a strong signal on the "alternate" channel (200 kHz away), that signal's outer sidebands will end up getting through the filter, and will probably affect the stereo subcarrier and increase distortion or "break through" into audibility. ** There are never two, strong FM signals separated by 200KHz - authorities govern frequency allocations on the band so as to prevent this. If you're trying to tune in a weak, distant signal that's on an "adjacent" channel to a strong local (100 kHz away) the problem is even worse. ** Only mad FM DXers have that issue. There are ways to work around this: - Use an FM tuner which has a narrower IF bandwidth. Better tuners often have a wide/narrow switch setting, with the narrow setting using different (or more) crystal filters with reduced bandwidth - 200, 180, 150, or even 110 kHz. ** Yep - mad FM DXers sometimes do that. - Use a directional FM antenna, and aim it in the direction which gives the best results. This may be "aimed towards the desired station" (increasing its relative strength), or "aimed at an angle away from the undesired station" (to put the interfering station in a "null" in the antenna's reception pattern). ** Sure - an antenna rotator with a high gain Yagi on top is what every home needs. Pure ********. The most common FM antenna is the TV antenna on the roof or maybe a dipole parked inside the roof cavity. .... Phil |
#54
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How can the same FM station appear at two different spots on the dial?
On Wednesday, October 29, 2014 10:23:21 PM UTC-4, micky wrote:
On Wed, 29 Oct 2014 22:09:28 -0400, micky wrote: ** I keep buying radios from the 60's and 70's at hamfests, looking for one that will get 88.1, 88.5 and 101.1 Not 101.1. 90.1, C-Span radio, which I guess I've lost interest in. It's boring as all get out during the committee hearings, and the 7AM program used to be great, but it's been discovered by the wackos. I can't stand C-span though they have some good programming. The problem is that fake synthesized brass keyboard they use for the theme music. How hard would it have been to buy a CD of a real brass ensemble playing it? That piece has been recorded many times. |
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