|
AM recption notes.
I found this web page while looking for a nifty audio filter I found
last year. At the very least it gives food for thought. http://www.radiointel.com/phil/phils_radio_tuning_tricks.pdf Terry |
AM recption notes.
|
AM recption notes.
Steve wrote: wrote: I found this web page while looking for a nifty audio filter I found last year. At the very least it gives food for thought. http://www.radiointel.com/phil/phils_radio_tuning_tricks.pdf Terry There are some helpful hints here, though I'm puzzled by the gripe this fellow has with synchronous detection. I personally wouldn't spend a lot of dough on any receiver that lacked it. I think a lot of people are sour on synchronous detection because they've bought and/or used a receiver that had a very poor implementation of it. When sync detection is done right, as it is on Drake's R8B, then it will sometimes do more to aid reception than all of these hints put together. But the good news is that we don't have to choose between synchronous detection and this fellow's helpful hints. We can have all of these tools at our disposal. Steve My first expousre to SAM was with an AOR7030+. My first impression was that "great". That lead me down a long path toward building my own SAM+ outboard detector. Along the way I had a chance to test a Kiwa, MAOP and the Sherwood SE-3. Both are good and both are much better then the AOR. I tried several designs and settled on a variation of Pete's AD607 SAM. Again at first I was very impressed. However after using it for several months the glammor started to dull. For better then 99% of my AM listening, which in truth is a very small part of my SWL activity as I am into utility, I ofund the 3 OpAmp simple "improved detector" to be the equal or the match for any of the SAMs I had on hand. I was not able to compare the MAP or SE-3 for any extened period and while both are great units, they do have differences. On balance I like the sound quality of the MAP over the SE3, but I liked teh SE3's ability to track rapid, deep fades, espcially with nearby AM signals. I had my feet held to the fire for my posts about Dallas Lankfords observations about the utility of a sharp LP audio filter that chops every thing above ~3 or 4KHz. Exact break point depends on individual choice. Now that I have a R8B, I must admit that this is the best implentation of a SAM I have yet experienced. However in many situations it too can benefit from the AF LP filter. Perhaps I expected too much from SAM detectors. They aren't magic and are far from perfect. The home.worldnet.att.net/~wa1sov/technical/sync_det.html page is no longer up, but Peter C. McNulty, WA1SOV, offered an additional OpAmp fitler design that was reported to allow the AD607 to fully reject unwanted signals via I and Q difference and summing. I don;t have my printed copy in front of me so I can't relate the math. The design appeard to be valid, and several other EEs agreed with his reasoning. However I was never able to get that part of the design to work. I wrote if off in part to the 50Hz minimum tuning step the R2000 offers. His design is clearly worthy of study and I suspect that for someone with more time it could be a winner. At this point I am most interested in reducing the local RF noise, improving my antennas and getting to understand how to get the most from my phaser. Oh, and I am still after the Holly Grail" of a perfect 12V audio amp. Terry |
AM recption notes.
|
AM recption notes.
On Thu, 30 Nov 2006 20:59:40 GMT, Bart Bailey wrote:
In ups.com posted on 30 Nov 2006 06:06:29 -0800, Steve wrote: Begin wrote: I found this web page while looking for a nifty audio filter I found last year. At the very least it gives food for thought. http://www.radiointel.com/phil/phils_radio_tuning_tricks.pdf Terry There are some helpful hints here, though I'm puzzled by the gripe this fellow has with synchronous detection. I personally wouldn't spend a lot of dough on any receiver that lacked it. I think a lot of people are sour on synchronous detection because they've bought and/or used a receiver that had a very poor implementation of it. When sync detection is done right, as it is on Drake's R8B, then it will sometimes do more to aid reception than all of these hints put together. But the good news is that we don't have to choose between synchronous detection and this fellow's helpful hints. We can have all of these tools at our disposal. Steve At least the guy gave a thumbs up to the Drake designed SAM in the E1, although it wasn't referred to in his comments about pass band tuning. I use both when conditions are rough, as is sometimes the case with KGO (810) at night, the narrowest 2.3kc IF bandwidth filter combined with a PBT shift of +1.2kc and a USB sync, which allows complete rejection of the local XESPN (800) and shifts the audio to a more balanced sound. The use of wider bandwidth IF filters work fine with SAM on easy conditions like Radio Australia but in a crowd the extra bandwidth tends to allow more of an AGC reduction from the then included but undesirable signals. This is especially true if I'm trying to pick off KKOB (770) from the side of the local powerhouse KFMB (760). FWIW: Local is San Diego and the target stations are; KGO (810) - San Francisco CA KKOB (770) - Albuquerque NM The other local interference station XESPN (800) is in Tijuana MX KFMB 760 S9+15 KKOB 770 S9+10 KKOH 780 S9+15 KABC 790 (out of night pattern) XESPN 800 S9+12 KGO 810 S9+15 All coming in beautifully (except KABC) R8B PreAmp Off 50' Random Wire Lang Station, CA |
AM recption notes.
) writes:
wrote: I found this web page while looking for a nifty audio filter I found last year. At the very least it gives food for thought. http://www.radiointel.com/phil/phils_radio_tuning_tricks.pdf Terry Eh, the author confuses DSB and AM. I wouldn't put much faith in his/her analysis. Huh? IN what way? I glanced at it and maybe missed something, but DSB is AM. And he certainly says it at the outset, and when he's talking about the components he's talking about 2 sidebands and a carrier. Now, "DSB" often has fallen into the meaning of "DSB with no carrier", but technically one should specifically define that there is no carrier. Some of the problem with AM reception discussion is that it was defined in a certain set of terms, for decades and then even for beginners up till recent times (and maybe even today). So they'd define AM as a signal that is amplitude modulated, and that sets things up for the vision that the carrier amplitude goes up and down. Then when SSB became commonplace, instead of going back from the beginning and redefining it all, a separate set of definitions gets tacked on. This leaves people thinking they AM and SSB are two different things, when they are basically the same. Then when discussion of "low distortion AM detectors" comes along, it isn't even clear what people are talking about. Because one is not using a certain type of detector for AM (ie 2 sidebands with a carrier), and a different type for SSB. The talk of "Amplitude Modulation" invokes a vision of a detector that is following the voltage variations of the signal. But that's not the case at all. The carrier mixes with the sideband in the "envelope detector" and that beating is what brings the modulation back down to "baseband". It's just not a good mixer. Listen to an SSB signal without a carrier or BFO. That's the sound of the envelope varying according to the modulating signal, and there's no way to make sense of it without a carrier. No differing loads on the dioded detector, no precision half wave detector (with the diode in a feedback loop), no forward biasing of the diode, can ever make up for the lack of carrier. The carrier of an AM signal is needed to beat with the sidebands and get it back to audio. If the carrier fades in comparison with the sidebands, you start hearing things like that SSB with an "envelope detector", because the carrier is no longer strong enough to mix the sidebands down to audio, and the "envelope detector" is actually following the envelope of the signal. The basic concept of demodulation is no different whether the signal is AM (with carrier), DSB (with no carrier) or SSB (with no carrier). They all need the carrier, or a locally synthesized equivalent, to beat the sideband(s) down to audio. If things were spoken of that way from the beginning, then there'd be less of a leap to the "synchronous detector". No only would a universal set of concepts be applied to all modes, but the point of a synchronous modulator would become clear. A single diode is a lousy mixer. On the other hand, since the carrier of an AM signal comes in with the sidebands, there's no reason for having a second and isolated input for that carrier. But, long ago, people would mess with "exalted carrier reception", which would be the first step up from those "envelope detectors". They'd turn on the Q-multiplier, which had a narrow peak but a wide skirt, and that would boost the incoming carrier in reference to the sidebands, so there was a stronger carrier feeding into the "mixer". It seems that only when SSB came along, and there were design reasons to go to better mixers for the demodulation, that two input mixers started being used, commonly called "product detectors". There were design reasons for going to those, but the basic concept of a locally generated carrier did not require anything more than the single diode "envelope detector". Indeed, the concept had been there back in the days of regen receivers, and every superhet that could be used for CW had a BFO that would feed into the "envelope detector", to give to provide a beat with the incoming signal. I should point out that when the synchronous detector was described in CQ magazine in the late fifties, the actual mixers were single diodes. But once you had product detectors, that opened things up. The notion of boosting the incoming carrier for better mixing action became more clear. I've said before, there was an article in QST about an advanced receiver in the fifties, and it had two parallel IF chains. One wide for voice, the other narrow for CW. But, it also allowed the output of the narrow chain to feed the product detector, and there was the "quasi-synchronous detector" before anyone came up with the name. For AM (with carrier), you had two choices. You could strip off the extra sideband and carrier, then the incoming signal was the same as an SSB signal, and then demodulate it as an SSB signal. This saw a lot of useage in the sixties, when SSB only ham rigs hit the market, and yet AM was still common. People needed a means of demodulating the AM signals, and that worked. While I think it got discussed in the fifties as a better means of AM demodulation, nobody in the sixties was talking that way. It was just a means of demodulating AM signals when there was no means of doing so. (Not only did the SSB-only receivers have narrow IF filters, but often there was no way of turning off the BFO, and the product detectors were a type of mixer that required having that second signal at the second input; without it, you'd get little or no output even with an AM signal that brought it's own carrier.) But if you didn't want to do that, you had to deal with getting the "locally generated carrier" in the right place. Not just so it wouldn't beat against the incoming carrier (when it was strong enough) but if it wasn't placed in the right place, the sidebands would not be translated back to audio in the same places. (So if you sent a 1KHz tone, and the "locally generated carrier" was not right in the middle between those sidebands, one sideband would translate down so that 1KHz tone was 1010Hz while the other would translated down to 990Hz, which would obviously clash with each other.) That's what the "synchronous" bit is about. It's about putting the same sort of BFO that you'd use with SSB (which would feed the same sort of product detector used for SSB), with the addition of circuitry to synchronize the BFO with the carrier of the incoming AM Signal. If the discussion had started with the AM detector as a mixer, then there'd be little magic about "synchronous detectors". The whole process is simply about getting the "carrier" strong in reference to the sidebands, so good mixing happens in the detector. The "synchronous" bit is only a secondary thing, a need because you want the locally generated carrier in the right place. There have always been various means of getting better mixing action at the demodulator. But the important thing has always been about doing that. Michael |
AM recption notes.
Michael Black wrote: ) writes: wrote: I found this web page while looking for a nifty audio filter I found last year. At the very least it gives food for thought. http://www.radiointel.com/phil/phils_radio_tuning_tricks.pdf Terry Eh, the author confuses DSB and AM. I wouldn't put much faith in his/her analysis. Huh? IN what way? I glanced at it and maybe missed something, but DSB is AM. And he certainly says it at the outset, and when he's talking about the components he's talking about 2 sidebands and a carrier. Now, "DSB" often has fallen into the meaning of "DSB with no carrier", but technically one should specifically define that there is no carrier. DSB never has a carrier. There is no such thing as DSB and DSB without a carrier, just DSB. Think of a mixer. If the signal are zero mean, you get DSB. If there is a DC offset on the modulating signal, you get AM. Some of the problem with AM reception discussion is that it was defined in a certain set of terms, for decades and then even for beginners up till recent times (and maybe even today). So they'd define AM as a signal that is amplitude modulated, and that sets things up for the vision that the carrier amplitude goes up and down. Then when SSB became commonplace, instead of going back from the beginning and redefining it all, a separate set of definitions gets tacked on. This leaves people thinking they AM and SSB are two different things, when they are basically the same. No, AM and SSB are different. Then when discussion of "low distortion AM detectors" comes along, it isn't even clear what people are talking about. Because one is not using a certain type of detector for AM (ie 2 sidebands with a carrier), and a different type for SSB. The talk of "Amplitude Modulation" invokes a vision of a detector that is following the voltage variations of the signal. But that's not the case at all. The carrier mixes with the sideband in the "envelope detector" and that beating is what brings the modulation back down to "baseband". It's just not a good mixer. Listen to an SSB signal without a carrier or BFO. That's the sound of the envelope varying according to the modulating signal, and there's no way to make sense of it without a carrier. No differing loads on the dioded detector, no precision half wave detector (with the diode in a feedback loop), no forward biasing of the diode, can ever make up for the lack of carrier. And when did I comment about SSB? The carrier of an AM signal is needed to beat with the sidebands and get it back to audio. If the carrier fades in comparison with the sidebands, you start hearing things like that SSB with an "envelope detector", because the carrier is no longer strong enough to mix the sidebands down to audio, and the "envelope detector" is actually following the envelope of the signal. The basic concept of demodulation is no different whether the signal is AM (with carrier), DSB (with no carrier) or SSB (with no carrier). They all need the carrier, or a locally synthesized equivalent, to beat the sideband(s) down to audio. If things were spoken of that way from the beginning, then there'd be less of a leap to the "synchronous detector". No only would a universal set of concepts be applied to all modes, but the point of a synchronous modulator would become clear. A single diode is a lousy mixer. On the other hand, since the carrier of an AM signal comes in with the sidebands, there's no reason for having a second and isolated input for that carrier. But, long ago, people would mess with "exalted carrier reception", which would be the first step up from those "envelope detectors". They'd turn on the Q-multiplier, which had a narrow peak but a wide skirt, and that would boost the incoming carrier in reference to the sidebands, so there was a stronger carrier feeding into the "mixer". It seems that only when SSB came along, and there were design reasons to go to better mixers for the demodulation, that two input mixers started being used, commonly called "product detectors". There were design reasons for going to those, but the basic concept of a locally generated carrier did not require anything more than the single diode "envelope detector". Indeed, the concept had been there back in the days of regen receivers, and every superhet that could be used for CW had a BFO that would feed into the "envelope detector", to give to provide a beat with the incoming signal. I should point out that when the synchronous detector was described in CQ magazine in the late fifties, the actual mixers were single diodes. But once you had product detectors, that opened things up. The notion of boosting the incoming carrier for better mixing action became more clear. I've said before, there was an article in QST about an advanced receiver in the fifties, and it had two parallel IF chains. One wide for voice, the other narrow for CW. But, it also allowed the output of the narrow chain to feed the product detector, and there was the "quasi-synchronous detector" before anyone came up with the name. For AM (with carrier), you had two choices. You could strip off the extra sideband and carrier, then the incoming signal was the same as an SSB signal, and then demodulate it as an SSB signal. This saw a lot of useage in the sixties, when SSB only ham rigs hit the market, and yet AM was still common. People needed a means of demodulating the AM signals, and that worked. While I think it got discussed in the fifties as a better means of AM demodulation, nobody in the sixties was talking that way. It was just a means of demodulating AM signals when there was no means of doing so. (Not only did the SSB-only receivers have narrow IF filters, but often there was no way of turning off the BFO, and the product detectors were a type of mixer that required having that second signal at the second input; without it, you'd get little or no output even with an AM signal that brought it's own carrier.) But if you didn't want to do that, you had to deal with getting the "locally generated carrier" in the right place. Not just so it wouldn't beat against the incoming carrier (when it was strong enough) but if it wasn't placed in the right place, the sidebands would not be translated back to audio in the same places. (So if you sent a 1KHz tone, and the "locally generated carrier" was not right in the middle between those sidebands, one sideband would translate down so that 1KHz tone was 1010Hz while the other would translated down to 990Hz, which would obviously clash with each other.) That's what the "synchronous" bit is about. It's about putting the same sort of BFO that you'd use with SSB (which would feed the same sort of product detector used for SSB), with the addition of circuitry to synchronize the BFO with the carrier of the incoming AM Signal. If the discussion had started with the AM detector as a mixer, then there'd be little magic about "synchronous detectors". The whole process is simply about getting the "carrier" strong in reference to the sidebands, so good mixing happens in the detector. The "synchronous" bit is only a secondary thing, a need because you want the locally generated carrier in the right place. There have always been various means of getting better mixing action at the demodulator. But the important thing has always been about doing that. Michael Christ all mighty, what is with the verbal diarrhea? You are trying to analyze modulation by looking at demodulation. This is wrong thinking. You analyze modulation by looking at modulators. AM: One mixer. Feed it a carrier and signal. The carrier is zero mean. The signal has a DC bias sufficient that the signal always remains positive. Congrats, you gave birth to AM. DSB: One mixer: Feed it zero mean carrier and signal. Out pops DSB. SSB: Slightly more complicated since it involves Hilbert transformers and quadrature mixers. I can do an explanation, but it might take a paragraph. My original comment still stands. The author of the paper confused AM and DSB. |
AM recption notes.
In article .com,
"Steve" wrote: wrote: I found this web page while looking for a nifty audio filter I found last year. At the very least it gives food for thought. http://www.radiointel.com/phil/phils_radio_tuning_tricks.pdf Terry There are some helpful hints here, though I'm puzzled by the gripe this fellow has with synchronous detection. I personally wouldn't spend a lot of dough on any receiver that lacked it. I think a lot of people are sour on synchronous detection because they've bought and/or used a receiver that had a very poor implementation of it. When sync detection is done right, as it is on Drake's R8B, then it will sometimes do more to aid reception than all of these hints put together. But the good news is that we don't have to choose between synchronous detection and this fellow's helpful hints. We can have all of these tools at our disposal. Some of it is regional reception patterns, ax grinding, ignorance, and some of it is just plain nut case thinking focusing on the negative aspects because the sync circuit in radios are not perfect and ignoring the good performance that results most of the time. Listening to AMBCB in the car where I don't have a sync detector and home where I do sure makes me wish I had it in the car. I'm not talking about some small improvement I'm talking huge. Without sync you can have a strong but completely unintelligible signal for a few seconds to a few minutes a lot of the time in the evenings on any station not right in town or more than 50 miles away or in other words most stations. Usually the distortion is more like a few seconds length on SW but whether you miss a few words or miss whole paragraphs of the conversation it is very annoying. It depends on conditions of course but when you have selective fading turning on the sync detector makes an absolutely huge difference in reception even with the Sony 7600 portable. And of course if you have side band selectable sync detection with another station or local noise source generating interference to one side of a station you want to receive usually results in near 100% rejection of the offender by selecting the opposite side band. It's as close to a magic improvement in reception you are going to get on a radio. Again this results in a huge non-arguable difference in reception quality. The improvement in reception most radios have with sync detection is huge not small and so the improvement is not open to argument. People that argue about it are being stupid. People can write anything on Usenet or on a web page and a lot of it is crapola. -- Telamon Ventura, California |
AM recption notes.
In article ,
"Brenda Ann" wrote: "Michael Black" wrote in message ... ) writes: wrote: I found this web page while looking for a nifty audio filter I found last year. At the very least it gives food for thought. http://www.radiointel.com/phil/phils_radio_tuning_tricks.pdf Terry Eh, the author confuses DSB and AM. I wouldn't put much faith in his/her analysis. Huh? IN what way? I glanced at it and maybe missed something, but DSB is AM. And he certainly says it at the outset, and when he's talking about the components he's talking about 2 sidebands and a carrier. Now, "DSB" often has fallen into the meaning of "DSB with no carrier", but technically one should specifically define that there is no carrier. Some of the problem with AM reception discussion is that it was defined in a certain set of terms, for decades and then even for beginners up till recent times (and maybe even today). So they'd define AM as a signal that is amplitude modulated, and that sets things up for the vision that the carrier amplitude goes up and down. Then when SSB became commonplace, instead of going back from the beginning and redefining it all, a separate set of definitions gets tacked on. This leaves people thinking they AM and SSB are two different things, when they are basically the same. Then when discussion of "low distortion AM detectors" comes along, it isn't even clear what people are talking about. Because one is not using a certain type of detector for AM (ie 2 sidebands with a carrier), and a different type for SSB. The talk of "Amplitude Modulation" invokes a vision of a detector that is following the voltage variations of the signal. But that's not the case at all. The carrier mixes with the sideband in the "envelope detector" and that beating is what brings the modulation back down to "baseband". It's just not a good mixer. Listen to an SSB signal without a carrier or BFO. That's the sound of the envelope varying according to the modulating signal, and there's no way to make sense of it without a carrier. No differing loads on the dioded detector, no precision half wave detector (with the diode in a feedback loop), no forward biasing of the diode, can ever make up for the lack of carrier. The carrier of an AM signal is needed to beat with the sidebands and get it back to audio. If the carrier fades in comparison with the sidebands, you start hearing things like that SSB with an "envelope detector", because the carrier is no longer strong enough to mix the sidebands down to audio, and the "envelope detector" is actually following the envelope of the signal. The basic concept of demodulation is no different whether the signal is AM (with carrier), DSB (with no carrier) or SSB (with no carrier). They all need the carrier, or a locally synthesized equivalent, to beat the sideband(s) down to audio. If things were spoken of that way from the beginning, then there'd be less of a leap to the "synchronous detector". No only would a universal set of concepts be applied to all modes, but the point of a synchronous modulator would become clear. A single diode is a lousy mixer. On the other hand, since the carrier of an AM signal comes in with the sidebands, there's no reason for having a second and isolated input for that carrier. But, long ago, people would mess with "exalted carrier reception", which would be the first step up from those "envelope detectors". They'd turn on the Q-multiplier, which had a narrow peak but a wide skirt, and that would boost the incoming carrier in reference to the sidebands, so there was a stronger carrier feeding into the "mixer". It seems that only when SSB came along, and there were design reasons to go to better mixers for the demodulation, that two input mixers started being used, commonly called "product detectors". There were design reasons for going to those, but the basic concept of a locally generated carrier did not require anything more than the single diode "envelope detector". Indeed, the concept had been there back in the days of regen receivers, and every superhet that could be used for CW had a BFO that would feed into the "envelope detector", to give to provide a beat with the incoming signal. I should point out that when the synchronous detector was described in CQ magazine in the late fifties, the actual mixers were single diodes. But once you had product detectors, that opened things up. The notion of boosting the incoming carrier for better mixing action became more clear. I've said before, there was an article in QST about an advanced receiver in the fifties, and it had two parallel IF chains. One wide for voice, the other narrow for CW. But, it also allowed the output of the narrow chain to feed the product detector, and there was the "quasi-synchronous detector" before anyone came up with the name. For AM (with carrier), you had two choices. You could strip off the extra sideband and carrier, then the incoming signal was the same as an SSB signal, and then demodulate it as an SSB signal. This saw a lot of useage in the sixties, when SSB only ham rigs hit the market, and yet AM was still common. People needed a means of demodulating the AM signals, and that worked. While I think it got discussed in the fifties as a better means of AM demodulation, nobody in the sixties was talking that way. It was just a means of demodulating AM signals when there was no means of doing so. (Not only did the SSB-only receivers have narrow IF filters, but often there was no way of turning off the BFO, and the product detectors were a type of mixer that required having that second signal at the second input; without it, you'd get little or no output even with an AM signal that brought it's own carrier.) But if you didn't want to do that, you had to deal with getting the "locally generated carrier" in the right place. Not just so it wouldn't beat against the incoming carrier (when it was strong enough) but if it wasn't placed in the right place, the sidebands would not be translated back to audio in the same places. (So if you sent a 1KHz tone, and the "locally generated carrier" was not right in the middle between those sidebands, one sideband would translate down so that 1KHz tone was 1010Hz while the other would translated down to 990Hz, which would obviously clash with each other.) That's what the "synchronous" bit is about. It's about putting the same sort of BFO that you'd use with SSB (which would feed the same sort of product detector used for SSB), with the addition of circuitry to synchronize the BFO with the carrier of the incoming AM Signal. If the discussion had started with the AM detector as a mixer, then there'd be little magic about "synchronous detectors". The whole process is simply about getting the "carrier" strong in reference to the sidebands, so good mixing happens in the detector. The "synchronous" bit is only a secondary thing, a need because you want the locally generated carrier in the right place. There have always been various means of getting better mixing action at the demodulator. But the important thing has always been about doing that. Michael Damn good explanation, Michael. And 100% accurate. He always writes good posts. -- Telamon Ventura, California |
AM recption notes.
On Fri, 01 Dec 2006 08:43:43 GMT, Bart Bailey wrote:
Interesting that KABC (790) isn't heard in your area. Not as strong as KNX (1070) but definitely good copy here. What do you hear at 1MHz? KCEO - Escondido or KOMO - Seattle You're between them. I can only get a co-channel mix here in town but if I go up to Mt Laguna, about 40 miles east, then the loop separates them nicely. Also on occasions there's an unknown co-channel MX station under KGO that the loop can't separate from here. Haven't tried it on the hill yet. I can hear KABC, plus another station and they fight with each other and the KABC skywave. KOMO 1000 is always there, almost like fringe groundwave. I think it may be ducting along the coast. There's a ''TIS'' station at UCLA on 810; more of a problem than the Mexican. |
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