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John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: 7. It has been suggested that using a 2 MHz IF frequency would allow wider bandwidth than the standard 455 kHz IF frequency. I fail to see why this should be true. Because for the same Q value, the pass band would be 4 times wider Where is it written that the same loaded Q must be used for both filters? If you can change the center frequency, why can't you change the loaded Q? The lower the Q, the more IFTs required for a given amount of pass band and attenuationout of band. Patrick, you are missing the point, the issue was the merits of a 2.0 MHz IF frequency vs. a 455 kHz IF frequency with respect to bandwidth/selectivity, my point was that for the sort of bandwidths we are talking about for audio, a 455 kHz IF can provide virtually identical "pass band and attenuation out of band" with exactly the same number of IFTs as a 2.0 MHz IF frequency. The loaded Qs result from the design specifications in both cases, and are what they are. Sure. But the same Q would give wider BW at 2 MHz. I have not ever done this, so I guess at what the final response could be. There may be architectural advantages to using one or the other IF frequency in a radio, but so far only the bandwidth/selectivity has been mentioned and in that regard an IF of 2.0 MHz offers no significant advantage over a 455 kHz IF for the reception of the full audio bandwidth. I supect it might, and one article in Wireless World refered to using 10.7 MHz. To illustrate this consider the example of the following calculated response curves for both a 455 kHz IFT and a 2.0 MHz IFT: One One 455 kHz IFT 2.0 MHz IFT Q = 15.167 Q = 66.667 Fc-60 kHz -24.30 dB -24.18 dB Fc-50 kHz -21.22 dB -21.09 dB Fc-40 kHz -17.56 dB -17.42 dB Fc-30 kHz -13.22 dB -13.07 dB Fc-20 kHz -8.72 dB -8.60 dB Fc-15 kHz -7.09 dB -7.02 dB Fc-10 kHz -6.27 dB -6.24 dB Fc-05 kHz -6.04 dB -6.04 dB Fc kHz -6.02 dB -6.02 dB Fc+05 kHz -6.03 dB -6.03 dB Fc+10 kHz -6.19 dB -6.22 dB Fc+15 kHz -6.86 dB -6.96 dB Fc+20 kHz -8.34 dB -8.50 dB Fc+30 kHz -12.75 dB -12.95 dB Fc+40 kHz -17.15 dB -17.32 dB Fc+50 kHz -20.88 dB -21.01 dB Fc+60 kHz -24.01 dB -24.12 dB The only advantage the 2.0 MHz IFT shows is marginally better symetry of responce about the ceter frequency, the response of the two IFTs is virtually identical. The equality in performance depends on a large Q difference, with 544 kHz Q much lower than 2MHz Q to get the same BW. The Q of a typical 455 kHz IFT is higher than you have indicated, because the impedance of the LC circuit at Fo is required to be high to suit pentode loading, and to get high gain. If the Q was real low, and hence the Fo impedance, you would probably need 3 IFTs. I have never tried 3 very damped IFTs. Within reason, for bandwidths typical of audio receivers, you should be able to build a filter at 455 kHz that has effectively the same response as a 2 MHz filter. There is no need to throw out the 455 kHz IF just to get wide bandwidth. Its difficult to make a 455kHz typical old IFT produce a nice flat topped 20 kHz wide BW. Its either pointy nosed, undecoupled, or flat topped, critical coupled, or over critical or rabbit eared. I have tried all that. So you have tried all that and rejected the "pointy nosed", "flat topped", and "rabbit eared" response curves. I am left to wonder what sort of response curve you were looking for? Why not settle for a nice "flat topped" response curve and be done with it? I didn't say I had rejected the flat topped critical coupled IF response. Then what did you say? You said you had "tried all that" but now it appears that you were telling a little fib and hadn't actually tried a 455 kHz IF designed to produce the desired response. What I said was what I said. You are confused. Build a radio with 2MHz and measure it, maybe it works better. Just don't knock the idea before trying it, or condemn the idea with postulations about what might be. These things must be tried and measured, to really know. Patrick Turner. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#2
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In article , Patrick Turner
wrote: John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: Patrick, you are missing the point, the issue was the merits of a 2.0 MHz IF frequency vs. a 455 kHz IF frequency with respect to bandwidth/selectivity, my point was that for the sort of bandwidths we are talking about for audio, a 455 kHz IF can provide virtually identical "pass band and attenuation out of band" with exactly the same number of IFTs as a 2.0 MHz IF frequency. The loaded Qs result from the design specifications in both cases, and are what they are. Sure. But the same Q would give wider BW at 2 MHz. I have not ever done this, so I guess at what the final response could be. But so what, I thought we were talking about IFs for audio here, not video IFs? For an audio receiver I would think at the most we would want a 40 kHz bandwidth, more likely 30 kHz, or even 20 kHz in the US where the FCC effectively limits the audio bandwidth to 10 kHz? What exactly do you see as the advantage of a 2.0 MHz IF in an AM broadcast receiver? The way I see it both 455 kHz IFs and 2.0 MHz IFs can be built with the bandwidth necessary for High Fidelity AM audio reception. The stage gains will be virtually identical for both the 455 kHz IFs and 2.0 MHz IFs of similar bandwidth, with the exact stage gain depending somewhat on design choices and practicalities. The wideband 455 kHz IF will have lower stage gain than a normal narrow 455 kHz IF, but the 2.0 MHz IF also suffers from lower stage gain. The wideband 455 kHz IF has the advantage that standard RF front-end components like tuning capacitors and oscillator coils can be used, while the 2.0 MHz IF will require special RF components. What exactly are the advantages of a 2.0 MHz IF from a selectivity/bandwidth point of view? There may be architectural advantages to using one or the other IF frequency in a radio, but so far only the bandwidth/selectivity has been mentioned and in that regard an IF of 2.0 MHz offers no significant advantage over a 455 kHz IF for the reception of the full audio bandwidth. I supect it might, and one article in Wireless World refered to using 10.7 MHz. Certainly a high IF frequency will have advantages in image response, but if the bandwidth is the same, the audio quality should be similar. What exactly did Wireless World say was so great about using a 10.7 MHz IF for a MW AM receiver? Wireless World is a hobbyist magazine and all their authors are not necessarily up to speed, although in the old days they often did have articles by people who knew what they were talking about with respect to radios. I suspect that the reason Wireless World might have used a 10.7 MHz IF in a MW AM broadcast receiver is because it was an easy way for a hobbyist, who both doesn't have a clue what he is doing, and doesn't have the necessary test equipment, to get a super wide bandwidth. To illustrate this consider the example of the following calculated response curves for both a 455 kHz IFT and a 2.0 MHz IFT: The only advantage the 2.0 MHz IFT shows is marginally better symetry of responce about the ceter frequency, the response of the two IFTs is virtually identical. The equality in performance depends on a large Q difference, with 544 kHz Q much lower than 2MHz Q to get the same BW. Yes, although I have some reservations about the use of the term "Q", that is obvious, but so what, what difference does it make? The Q of a typical 455 kHz IFT is higher than you have indicated, because the impedance of the LC circuit at Fo is required to be high to suit pentode loading, and to get high gain. You also are going to sacrifice stage gain in the same way with a 2.0 MHz IF, so this is no more of a problem for the wideband 455 kHz IF than for the 2.0 MHz IF. If the Q was real low, and hence the Fo impedance, you would probably need 3 IFTs. This is a consequence of the wide bandwidth, not the IF frequency, the problem is identical at 2.0 MHz. I have never tried 3 very damped IFTs. The fact that you haven't tried something doesn't prove anything one way or the other. Also, what does "damped" mean in this context? I would have to do some research, but I suspect that "damping" is more related to filter bandwidth than to the center frequency, and both filters are aiming for the same bandwidth. What I said was what I said. You are confused. Maybe, in what way are you suggesting I am confused? I would suggest to you that you don't understand how to design an IF filter, and don't understand what can be done at 455 kHz. Build a radio with 2MHz and measure it, maybe it works better. You are the 2.0 MHz IF advocate not me, you still haven't suggested any reason why it might work better from a bandwidth/selectivity standpoint? Just don't knock the idea before trying it, or condemn the idea with postulations about what might be. I'm not, I know it would work, what I don't understand is what the advantages are over a 455 kHz IF of the same bandwidth? You are not explaining yourself, cite some concrete facts. These things must be tried and measured, to really know. While I can't claim to have designed the filter I used, I have actually built a transistor superhetrodyne AM tuner using a 455 kHz block filter with a 30 kHz IF bandwidth. Will the 2.0 MHz IF work better than this? Have you tried a properly designed wideband 455 kHz IF filter to see how it worked? The filter I used came out of a 2-way land mobile radio and I think it was about an 8 pole filter. Back in the old days of land mobile here in the US, wider channels with greater bandwidth were used than are used today. Over time the channels were squeezed down to accommodate additional channels in the same space, and block filters of several different bandwidths were available to suit the changing allocations and operating frequencies. I have also built wideband single frequency TRF receivers using modified double tuned IF transformers. So what it boils down to is that you haven't tried a wideband 455 kHz filter while I have, and I haven't tried a 2.0 MHz IF filter, which you may or may not have done. I at least have cited some concrete facts about IF filters, while you have only muttered about Q, without indicating how it actually relates to the problem. I am not a "filter jock" (tm) but I think it is generally desirable that the Q of the components used in a filter be high, especially when we get beyond simple double tuned transformers. What you are calling Q is more related to how the filter is terminated, which is a different matter than the Q of the components that make up the filter. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#3
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John Byrns wrote: What exactly are the advantages of a 2.0 MHz IF from a selectivity/bandwidth point of view? Both pros and cons to this John. Since the bandwidth is a percentage of the center frequency, the shape of the bandwidth will change based on the distance from the center frequency (as a percentage.) Assuming just for the moment +/- 5 KHz. At 455 KHz that's about 1% above and below. At 2 MHz, that's now only .25% above and below. As you get further from the center frequency, percentage wise, the shape of the curve as it transitions from inside to outside of the band pass is going to look different at the upper frequency than it does at the lower frequency. Certainly a high IF frequency will have advantages in image response, but if the bandwidth is the same, the audio quality should be similar. With the notable exception of the difference in shape of the roll off above and below the center frequency. In the world of designing filters (and overall system performance) this is called group delay. A shorter, perhaps more recognizable term would be linear phase shift over the entire band pass of the filter. Wireless World is a hobbyist magazine and all their authors are not necessarily up to speed They were under the same constraints as the Weekly World News. "If it wasn't true, they couldn't print it." Note smiley face here. ;-) Yes, although I have some reservations about the use of the term "Q", that is obvious, but so what, what difference does it make? Back to the original comments about Q. In a perfect world, it would only be a matter of the LC ratio setting the bandwidth of a tuned circuit. Of courses, there are other things that get in the way to reduce the overall Q of a circuit. Nasty little things like the series resistance of the coils, dielectric losses in both the coil forms and capacitor insulation material. Back to the original "ideal" values of Q. 15 at 455 KHz and 67 at 2 MHz. It is physically "more challenging" to get higher Q at a higher frequency. All of the various losses of the components tend to get in the way. Wire losses, dielectric losses and any losses of the ferrite used in the core materials. Also, what does "damped" mean in this context? I would have to do some research, but I suspect that "damping" is more related to filter bandwidth than to the center frequency, and both filters are aiming for the same bandwidth. "Damped" means adding some form of resistance across the reactive components of a circuit. As an example, if you were to assemble a nice 455 KHz IF transformer and found that the bandwidth was too narrow, a fast method of widening it would be to place parallel resistors across the windings. Another point about "damped" is that if a tuned circuit has too high a Q, a sudden transient will tend to make it oscillate. In communications receivers, this is obvious that a signal sounds more like you're ringing a bell, than simply turning a tone on and off. (Kind of like using the sustain pedal on a piano.) Maybe, in what way are you suggesting I am confused? I would suggest to you that you don't understand how to design an IF filter, and don't understand what can be done at 455 kHz. Lets not go in that direction. An IF transformer is simply a two pole butter worth filter. That it can have different input and output impedance just makes it really convenient for taking the source from a plate and connecting it to a grid for a load. By definition, a butter worth filter has a smooth curve with only one peak (in the middle.) And the shape (steepness) of the band pass is related to the overall Q of the circuit. The next type of filter, would be Chebychev, This is no more than a "predistorted" butter worth filter network. By allowing a certain amount of ripple in the pass band, the shape of the rejection can be made sharper. The obvious trade off is the amount of distortion to the signal within the pass band. A simple example of this would be stagger tuned IF coils. Two or more peaks, and a dip (or dips), ripple, in the middle. While I can't claim to have designed the filter I used, I have actually built a transistor superhetrodyne AM tuner using a 455 kHz block filter with a 30 kHz IF bandwidth. Will the 2.0 MHz IF work better than this? Have you tried a properly designed wideband 455 kHz IF filter to see how it worked? The filter I used came out of a 2-way land mobile radio and I think it was about an 8 pole filter. The point you've probably overlooked in land mobile operations is that it was NEVER designed as a "hi-fi" system. There's a reason for the term "voice grade." Having as much a 3 dB of ripple in a band pass filter is meaningless especially when the filter is in the midst of a limiting IF strip for FM recovery, and on AM demodulation. What really matters here is limiting the bandwidth of the received signal to ONLY include that of the wanted (in channel) information and none of the unwanted (adjacent channel) information to get to the discriminator. So what it boils down to is that you haven't tried a wideband 455 kHz filter while I have, and I haven't tried a 2.0 MHz IF filter, which you may or may not have done. I at least have cited some concrete facts about IF filters, while you have only muttered about Q, without indicating how it actually relates to the problem. I am not a "filter jock" (tm) but I think it is generally desirable that the Q of the components used in a filter be high, especially when we get beyond simple double tuned transformers. What you are calling Q is more related to how the filter is terminated, which is a different matter than the Q of the components that make up the filter. You should take the time to read up on "filter jockeying" John. You're making a lot of incorrect assumptions on how they work. The primary requirement on the Q of individual components in filter design is only such that their value of Q be high enough to not materially effect the overall Q of the circuit. As an example, (and without getting into cryogenic treatments and styrofoam cups) A speaker system sounds better through 25 feet of #12 AWG wire than it does through 25 feet of #18 AWG wire. And that's strictly due to the resistive loss of the wire in comparison to the losses in the actually speaker design and implementation. I had a electronics instructor in college that would show you "The secret of electronics" that he kept hidden, and locked, inside a small jewelry box if you "caught on" during his course. With some fanfare, he would slowly open the box and you would see an inductor, a resistor and a capacitor. And it's really just that simple. What gets complicated is when you forget that all three items have hidden values of the others contained within them. (I.e. the difference between practical and theoretical parts.) Jeff -- "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." Benjamin Franklin "A life lived in fear is a life half lived." Tara Morice as Fran, from the movie "Strictly Ballroom" |
#4
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John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: Patrick, you are missing the point, the issue was the merits of a 2.0 MHz IF frequency vs. a 455 kHz IF frequency with respect to bandwidth/selectivity, my point was that for the sort of bandwidths we are talking about for audio, a 455 kHz IF can provide virtually identical "pass band and attenuation out of band" with exactly the same number of IFTs as a 2.0 MHz IF frequency. The loaded Qs result from the design specifications in both cases, and are what they are. Sure. But the same Q would give wider BW at 2 MHz. I have not ever done this, so I guess at what the final response could be. But so what, I thought we were talking about IFs for audio here, not video IFs? For an audio receiver I would think at the most we would want a 40 kHz bandwidth, more likely 30 kHz, or even 20 kHz in the US where the FCC effectively limits the audio bandwidth to 10 kHz? What exactly do you see as the advantage of a 2.0 MHz IF in an AM broadcast receiver? Its a lot easier to get a wider pass band of 30 kHz with 2MHz IFTs than with 455 kHz IFTs. Try it some time, and then you'll know. The way I see it both 455 kHz IFs and 2.0 MHz IFs can be built with the bandwidth necessary for High Fidelity AM audio reception. The stage gains will be virtually identical for both the 455 kHz IFs and 2.0 MHz IFs of similar bandwidth, with the exact stage gain depending somewhat on design choices and practicalities. The wideband 455 kHz IF will have lower stage gain than a normal narrow 455 kHz IF, but the 2.0 MHz IF also suffers from lower stage gain. The wideband 455 kHz IF has the advantage that standard RF front-end components like tuning capacitors and oscillator coils can be used, while the 2.0 MHz IF will require special RF components. What exactly are the advantages of a 2.0 MHz IF from a selectivity/bandwidth point of view? I suspect 3 x 2MHz IFTs would be easier to get a flat topped pass band and sufficient steep roll off just outside the band. I also suspect any old 455 kHz IFTs could easily have about 3/4 of their turns removed, and retain the same caps of 250pF. For 250pF, to get 455 kHz, one needs 0.48 mH For 250pF, and 2 MHz, one needs 0.025 mH. To reduce L by 20 times, the turns would need reducing by a factor of 1/4.47. Thus the DCR would fall, and Q could rise. I have used ex IFT windings with turns removed for high Q RF input coils on my reciever, to get the range of tuning required between 500 and 1750 kHz with a 20 pF to 360 pF tuning gang. The ferrite slug is retained. The wire is litz wire, with low RF resistance, hence it gives a high Q, but for 2 mHz, solid round wire would probably be OK, like in 4.5 MHz TV IFTs and 10.7 MHz FM IFTs. HF IFTs are easier to wind than 455 kHz. There may be architectural advantages to using one or the other IF frequency in a radio, but so far only the bandwidth/selectivity has been mentioned and in that regard an IF of 2.0 MHz offers no significant advantage over a 455 kHz IF for the reception of the full audio bandwidth. I supect it might, and one article in Wireless World refered to using 10.7 MHz. Certainly a high IF frequency will have advantages in image response, but if the bandwidth is the same, the audio quality should be similar. What exactly did Wireless World say was so great about using a 10.7 MHz IF for a MW AM receiver? Wide AF response was easily achieved. Wireless World is a hobbyist magazine and all their authors are not necessarily up to speed, although in the old days they often did have articles by people who knew what they were talking about with respect to radios. I differ. WW and what it became, Electronics World wasn't just an amateur's magazine. It had cutting edge articles about electronics from 1917 onwards, and I suggest you park yourself beside a pile of all the old copies and have a good read. Most of the info was only comprehensible by very well university educated professionals, or intellectuals, and most ideas were backed up with mathematical proofs which nearly all the general public couldn't understand. I suspect that the reason Wireless World might have used a 10.7 MHz IF in a MW AM broadcast receiver is because it was an easy way for a hobbyist, who both doesn't have a clue what he is doing, and doesn't have the necessary test equipment, to get a super wide bandwidth. I leave you to your suppositions. To illustrate this consider the example of the following calculated response curves for both a 455 kHz IFT and a 2.0 MHz IFT: The only advantage the 2.0 MHz IFT shows is marginally better symetry of responce about the ceter frequency, the response of the two IFTs is virtually identical. The equality in performance depends on a large Q difference, with 544 kHz Q much lower than 2MHz Q to get the same BW. Yes, although I have some reservations about the use of the term "Q", that is obvious, but so what, what difference does it make? Build a receiver, and find out. The Q of a typical 455 kHz IFT is higher than you have indicated, because the impedance of the LC circuit at Fo is required to be high to suit pentode loading, and to get high gain. You also are going to sacrifice stage gain in the same way with a 2.0 MHz IF, so this is no more of a problem for the wideband 455 kHz IF than for the 2.0 MHz IF. Use more stages if stage gain is low. The EA design used 3 IFTs, with two j-fet IF amps, with quite heavily damped 455 kHz IF coils. If the Q was real low, and hence the Fo impedance, you would probably need 3 IFTs. This is a consequence of the wide bandwidth, not the IF frequency, the problem is identical at 2.0 MHz. I have never tried 3 very damped IFTs. The fact that you haven't tried something doesn't prove anything one way or the other. It means that what you or I am saying may not include all the facts about the subject. Build and measure will give the facts. Also, what does "damped" mean in this context? Strapping resistance across the LC tuned circuit to reduce the Q. The rate of attenuation just either side of the pass band becomes much less, so more IF stages must be used. I would have to do some research, but I suspect that "damping" is more related to filter bandwidth than to the center frequency, and both filters are aiming for the same bandwidth. Damping reduces Q, and increases BW. But it also reduces Z at Fo, thus reducing gain in an amp which must be a current source, like a pentode or j-fet, to realise the best selectivity for the LC circuit. What I said was what I said. You are confused. Maybe, in what way are you suggesting I am confused? I would suggest to you that you don't understand how to design an IF filter, and don't understand what can be done at 455 kHz. I know enough about IFT design, after having built my own radio. Build a radio with 2MHz and measure it, maybe it works better. You are the 2.0 MHz IF advocate not me, you still haven't suggested any reason why it might work better from a bandwidth/selectivity standpoint? I refuse to repeat myself any further. Just don't knock the idea before trying it, or condemn the idea with postulations about what might be. I'm not, I know it would work, what I don't understand is what the advantages are over a 455 kHz IF of the same bandwidth? You are not explaining yourself, cite some concrete facts. I have already stated that for a given Q, the pass band for a 2MHz IFT is naturally a lot wider than for a 455 kHz IFT. Put it this way, if you make IFTs of 100 kHz, then its all the harder to get a flat topped bandpass response which is 20 kHz wide, with high sloped skirt response each side. These things must be tried and measured, to really know. While I can't claim to have designed the filter I used, I have actually built a transistor superhetrodyne AM tuner using a 455 kHz block filter with a 30 kHz IF bandwidth. Ceramic filters are another way to achieve the same bandpass filter that the IFT could do. But they were never used in tube sets for the BCB. Will the 2.0 MHz IF work better than this? I suspect yes, but getting a 2 MHz cermic filter with 30 kHz of BW might be unobtainium. Have you tried a properly designed wideband 455 kHz IF filter to see how it worked? Yes, and trying to squeeze 20 kHz of flat topped BW was difficult with stock IFTs. I have already said what my solution was, to use a variable distance coils and some damping on IFT no1, which allowed me to have only 2 IFTs, and 1 IF amp, a 6BX6, fixed bias, for low thd IF amplification. The filter I used came out of a 2-way land mobile radio and I think it was about an 8 pole filter. Back in the old days of land mobile here in the US, wider channels with greater bandwidth were used than are used today. Over time the channels were squeezed down to accommodate additional channels in the same space, and block filters of several different bandwidths were available to suit the changing allocations and operating frequencies. I have also built wideband single frequency TRF receivers using modified double tuned IF transformers. One of the Electronics Australia kit designs I have used a two stage TRF design with highish Q LC, with stagger tuning at the low F part of the band. This utilised having mutual capacitive coupling of the Ls in their earthy ends to ground via one common 0.1 uF. I couldn't easily reproduce the nice response curves of the kit set, and it was not good enough to give selectivity between locals here where I wanted to hear a 300 watt station which was only 45 kHz away from a 5,000 watt station. But otherwise, the TRF was a fine performer. So what it boils down to is that you haven't tried a wideband 455 kHz filter while I have, and I haven't tried a 2.0 MHz IF filter, which you may or may not have done. I have tried getting 455 kHz IFTs to go wider, but I was dissapointed with overall results, because I'd have needed 3 IFTs, and lots of damping. I got 10 kHz of audio BW at low thd using simple methods of damping, sliding IFT1 coils closer, and some RC boosting of audio HF. I thus achieved the use of tubes, good AF BW, and excellent local station selectivity, which allowed me to hear my wanted 300 watt station without the 5,000 watt station able to be heard even though it is only 45 kHz away. I at least have cited some concrete facts about IF filters, while you have only muttered about Q, without indicating how it actually relates to the problem. I am not a "filter jock" (tm) but I think it is generally desirable that the Q of the components used in a filter be high, especially when we get beyond simple double tuned transformers. What you are calling Q is more related to how the filter is terminated, which is a different matter than the Q of the components that make up the filter. I leave you to wonder the full content of my mutterings, and I do hope you spend some time soon in your shack with a soldering iron and response meter. The other advantage of a 2 MHz IF is that the filtering of RF from the recovered audio is easier, because the C value is less, and the filter used has less effect on recovered audio at 10 kHz, and at high amplitudes. But don't let me mention it, I know you'd be aware of it already. Patrick Turner. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#5
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In article , Patrick Turner
wrote: John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: There may be architectural advantages to using one or the other IF frequency in a radio, but so far only the bandwidth/selectivity has been mentioned and in that regard an IF of 2.0 MHz offers no significant advantage over a 455 kHz IF for the reception of the full audio bandwidth. I supect it might, and one article in Wireless World refered to using 10.7 MHz. Certainly a high IF frequency will have advantages in image response, but if the bandwidth is the same, the audio quality should be similar. What exactly did Wireless World say was so great about using a 10.7 MHz IF for a MW AM receiver? Wide AF response was easily achieved. Wireless World is a hobbyist magazine and all their authors are not necessarily up to speed, although in the old days they often did have articles by people who knew what they were talking about with respect to radios. I differ. WW and what it became, Electronics World wasn't just an amateur's magazine. It had cutting edge articles about electronics from 1917 onwards, and I suggest you park yourself beside a pile of all the old copies and have a good read. Most of the info was only comprehensible by very well university educated professionals, or intellectuals, and most ideas were backed up with mathematical proofs which nearly all the general public couldn't understand. I am reasonably familiar with Wireless World, I have 3 & 1/2 of those copy paper boxes full of old issues from the 1930's through the 1950's. I would estimate that I have at least half the issues from that period whcih was probably the golden age of AM receiver technology. I have to take serious exception to your characterization of the "mathematical proofs" included in their articles. There may have been the odd article with some mathematical depth, but those were few and far between. The math presented seems to have been just enough to go over the head of the average reader, but was hardly complex enough to be "only comprehensible by very well university educated professionals, or intellectuals". I suspect this light weight approach just slightly above the level of the man in the street was carefully calculated to impress the average reader without putting the material at a level where he couldn't understand it at all. That is not to say that they didn't have many excellent authors who knew all the math, but it is a serious stretch to imply that they included any real mathematical depth, they included only enough to look impressive to the untutored reader. I suspect that the reason Wireless World might have used a 10.7 MHz IF in a MW AM broadcast receiver is because it was an easy way for a hobbyist, who both doesn't have a clue what he is doing, and doesn't have the necessary test equipment, to get a super wide bandwidth. I leave you to your suppositions. OK, but for all practical purposes my "supposition" seems to be identical with your statement above that "Wide AF response was easily achieved", which I take to be a quote from the actual Wireless World article? Damping reduces Q, and increases BW. But it also reduces Z at Fo, thus reducing gain in an amp which must be a current source, like a pentode or j-fet, to realise the best selectivity for the LC circuit. This is a half truth, what matters is that the filter is correctly terminated, not that pentode, triode or whatever drives it. As far as stage gain goes, increasing the frequency from 455 kHz to 2.0 MHz is likely to decrease the gain by a similar amount to widening the 455 kHz filter to the same bandwidth as the 2.0 MHz filter. What I said was what I said. You are confused. Maybe, in what way are you suggesting I am confused? I would suggest to you that you don't understand how to design an IF filter, and don't understand what can be done at 455 kHz. I know enough about IFT design, after having built my own radio. That isn't clear at all, you seem to be obsessed with "Q", and hardly if ever mention "k", and how it relates to "Q" in determining the characteristics of an IFT. You occasionally mention "critical" coupling but haven't tied that concept in with the "Q" and "k" of an IFT, nor have you mentioned the related concept of "transitional" coupling. I would expect to hear more mention of these concepts from someone who knows "enough about IFT design". Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#6
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John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: There may be architectural advantages to using one or the other IF frequency in a radio, but so far only the bandwidth/selectivity has been mentioned and in that regard an IF of 2.0 MHz offers no significant advantage over a 455 kHz IF for the reception of the full audio bandwidth. I supect it might, and one article in Wireless World refered to using 10.7 MHz. Certainly a high IF frequency will have advantages in image response, but if the bandwidth is the same, the audio quality should be similar. What exactly did Wireless World say was so great about using a 10.7 MHz IF for a MW AM receiver? Wide AF response was easily achieved. Wireless World is a hobbyist magazine and all their authors are not necessarily up to speed, although in the old days they often did have articles by people who knew what they were talking about with respect to radios. I differ. WW and what it became, Electronics World wasn't just an amateur's magazine. It had cutting edge articles about electronics from 1917 onwards, and I suggest you park yourself beside a pile of all the old copies and have a good read. Most of the info was only comprehensible by very well university educated professionals, or intellectuals, and most ideas were backed up with mathematical proofs which nearly all the general public couldn't understand. I am reasonably familiar with Wireless World, I have 3 & 1/2 of those copy paper boxes full of old issues from the 1930's through the 1950's. I would estimate that I have at least half the issues from that period whcih was probably the golden age of AM receiver technology. I have to take serious exception to your characterization of the "mathematical proofs" included in their articles. There may have been the odd article with some mathematical depth, but those were few and far between. The math presented seems to have been just enough to go over the head of the average reader, but was hardly complex enough to be "only comprehensible by very well university educated professionals, or intellectuals". I never learnt any electronics maths at high school. I was told that if I went into a career as an electronics design engineer, I'd have to be far better at maths. I agree, many articles don't have much maths, but a lot do, and all one can do is read between the lines of incomprehensible maths. By the mid 1970s, there were many young bright mainly british stars who showed off their mathematical abilities, most probably to appear to be stars, and top of the bloomin heap, and to foster good future employment with major electronics firms. The readers' letters section daown the back of the mag had the arguments between engineers who couldn't agree. Plenty of that alright. The internet changed all that, along with everyone trying to keep progress secret as possible, and only for eyes of the financial backers. I suspect this light weight approach just slightly above the level of the man in the street was carefully calculated to impress the average reader without putting the material at a level where he couldn't understand it at all. That is not to say that they didn't have many excellent authors who knew all the math, but it is a serious stretch to imply that they included any real mathematical depth, they included only enough to look impressive to the untutored reader. Well, there was a pile of stuff I couldn't understand. I just got the general idea and built stuff, and got very nice results as good as anyone with all that math ability would. I suspect that the reason Wireless World might have used a 10.7 MHz IF in a MW AM broadcast receiver is because it was an easy way for a hobbyist, who both doesn't have a clue what he is doing, and doesn't have the necessary test equipment, to get a super wide bandwidth. I leave you to your suppositions. OK, but for all practical purposes my "supposition" seems to be identical with your statement above that "Wide AF response was easily achieved", which I take to be a quote from the actual Wireless World article? Not my quote of WW. But basically, one of the aims was expressed to mean thy same as I said. Damping reduces Q, and increases BW. But it also reduces Z at Fo, thus reducing gain in an amp which must be a current source, like a pentode or j-fet, to realise the best selectivity for the LC circuit. This is a half truth, what matters is that the filter is correctly terminated, not that pentode, triode or whatever drives it. Well most IFTs made for tube radios would perform abysmally is driven with triode amps with Ra = say 10k. This would over damp the LC circuit in most cases. Try damping an ordinary radio's IFTs with 100K, then 47k and finally 22k for each of the 4 IF coils in a set. Tell me what you find. As far as stage gain goes, increasing the frequency from 455 kHz to 2.0 MHz is likely to decrease the gain by a similar amount to widening the 455 kHz filter to the same bandwidth as the 2.0 MHz filter. Use 3 x IFTs, and an extra stage of IF amplification. I still reckon the 2MHz will work, and when I have time, I'll try the idea, and tell everyone about it. But to know any earlier, try it out for yourself. What I said was what I said. You are confused. Maybe, in what way are you suggesting I am confused? I would suggest to you that you don't understand how to design an IF filter, and don't understand what can be done at 455 kHz. I know enough about IFT design, after having built my own radio. That isn't clear at all, you seem to be obsessed with "Q", and hardly if ever mention "k", and how it relates to "Q" in determining the characteristics of an IFT. You occasionally mention "critical" coupling but haven't tied that concept in with the "Q" and "k" of an IFT, nor have you mentioned the related concept of "transitional" coupling. I would expect to hear more mention of these concepts from someone who knows "enough about IFT design". I don't need to use k to confuse everyone. An IFT is a simple RF transformer operating at a fixed F. The magnetic lines of force from one coil react to transfer some power from a primary LC to a secondary LC. The coupling and insertion loss is whatever you are gonna get. The looser the coupling, ie, the further apart the coils, the sharper is the nose shape of the two circuits. Let's assume you have a current source, ie, high impedance signal source, or generator for the primary. Assume the output from the sec goes to a high impedance load, like the grid of a pentode tube, with little miller capacitance. The load of the sec LC is transfered to the pri, depending on the closeness of coupling. Far apart gives a large insertion loss, and lowest RL for the pri signal source, but the response shows the attenuation is twice that of a single LC as you move away from the centre F. Then as you bring the coils together, the insertion loss and load value reduces, and the response suddenly becomes flat topped, but the attenuation out of the pass band is still twice that of a single circuit. Then with coils even closer, the insertion loss is low, but there are two peaks in the response, but outside the two peaks the response remains twice that of a single LC circuit. k isn't needed to be considered since we are dealing practically with what you get when you use LC circuits arranged as they are in IFTs. We simply wanna know what happens. Its also spelled out in numerous old radio books, and there are maths for those inclined. The beauty of audio and radio engineering is that a lot of it can be done using only very basic maths, and following well known practices and precautions. It isn't as complex as rocket science. We don't have to worry if we shoot some dude off into space, and find that our equations were wrong, and he spends eternity orbitting Mars with no way back. Patrick Turner. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
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In article , Patrick Turner
wrote: John Byrns wrote: In article , Patrick Turner wrote: Damping reduces Q, and increases BW. But it also reduces Z at Fo, thus reducing gain in an amp which must be a current source, like a pentode or j-fet, to realise the best selectivity for the LC circuit. This is a half truth, what matters is that the filter is correctly terminated, not that pentode, triode or whatever drives it. Well most IFTs made for tube radios would perform abysmally is driven with triode amps with Ra = say 10k. This would over damp the LC circuit in most cases. That is true of "IFTs made for tube radios" when they are being used as originally intended, but what happens when we hobbyists modify them for High-Fidelity use by increasing "k" and decreasing the circuit "Q" by adding resistors? In this case since we need external termination resistors anyway, all we need do is connect the resistor between the anode of the triode and the input of the IFT and all will be well, there is no need for a current source to drive the filter, ideal would be a source with just the required termination resistance. There is no reason why the required termination resistance can't be connected between a low impedance source and the input of the filter, it does not have to have one end earthed. Try damping an ordinary radio's IFTs with 100K, then 47k and finally 22k for each of the 4 IF coils in a set. Tell me what you find. Every filter, be it an IFT or something more complex, is designed to be terminated in specified impedances, which may be a specified resistance, an open circuit, or even a short circuit, what matters is that the termination is correct, not that the filter is driven by a pentode. As far as stage gain goes, increasing the frequency from 455 kHz to 2.0 MHz is likely to decrease the gain by a similar amount to widening the 455 kHz filter to the same bandwidth as the 2.0 MHz filter. Use 3 x IFTs, and an extra stage of IF amplification. I still reckon the 2MHz will work, and when I have time, I'll try the idea, and tell everyone about it. But to know any earlier, try it out for yourself. I never said 2.0 MHz wouldn't work, in fact I specifically stated at least once that I thought 2.0 MHz would work. If 2.0 MHz is what floats your boat then that's what you should use, although I notice that you choose to use the traditional 455 kHz in your radio design. What I said was simply that 455 kHz would also work in a wideband High-Fidelity AM radio. What I said was what I said. You are confused. Maybe, in what way are you suggesting I am confused? I would suggest to you that you don't understand how to design an IF filter, and don't understand what can be done at 455 kHz. I know enough about IFT design, after having built my own radio. That isn't clear at all, you seem to be obsessed with "Q", and hardly if ever mention "k", and how it relates to "Q" in determining the characteristics of an IFT. You occasionally mention "critical" coupling but haven't tied that concept in with the "Q" and "k" of an IFT, nor have you mentioned the related concept of "transitional" coupling. I would expect to hear more mention of these concepts from someone who knows "enough about IFT design". I don't need to use k to confuse everyone. An IFT is a simple RF transformer operating at a fixed F. The magnetic lines of force from one coil react to transfer some power from a primary LC to a secondary LC. The coupling and insertion loss is whatever you are gonna get. The looser the coupling, ie, the further apart the coils, the sharper is the nose shape of the two circuits. Let's assume you have a current source, ie, high impedance signal source, or generator for the primary. Assume the output from the sec goes to a high impedance load, like the grid of a pentode tube, with little miller capacitance. The load of the sec LC is transfered to the pri, depending on the closeness of coupling. Far apart gives a large insertion loss, and lowest RL for the pri signal source, but the response shows the attenuation is twice that of a single LC as you move away from the centre F. Then as you bring the coils together, the insertion loss and load value reduces, and the response suddenly becomes flat topped, but the attenuation out of the pass band is still twice that of a single circuit. Then with coils even closer, the insertion loss is low, but there are two peaks in the response, but outside the two peaks the response remains twice that of a single LC circuit. k isn't needed to be considered since we are dealing practically with what you get when you use LC circuits arranged as they are in IFTs. We simply wanna know what happens. If you simply "wanna know what happens" why do you even need to consider "Q"? Your narrative description above sure makes it sound like "k" is important, you just haven't tied "Q" and "k" to the response shapes you describe. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
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