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#71
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Curmudgeon,
Since you are using a "Forged Header" ) .. . . Will Anybody Listen ? ho, Ho. HO ! - Makes One Wonder ~ RHF .. .. = = = curmudg@eon (Curmudgeon) wrote in message = = = ... On 15 Jun 2004 07:43:12 -0500, Dan wrote: On Tue, 15 Jun 2004 05:44:35 GMT, Telamon wrote: In article , Jon Noring wrote: I'm the one who started and cross-posted the related topics (of building a tube-based AM receiver) to the three newsgroups, including rec.radio.shortwave. Nevertheless, I believe the threads are sufficiently on-topic to r.r.s. to not warrant some pro-active effort to try to stop. I've sent a complaint to your news provider. We will see if they think the same way you do. I just sent two complaints to about Telamon trying to disrupt an on-topic discussion. I suggest everyone involved in this thread do the same. Include a complete, abusive or threatening messsage from Telamon including headers. They're not hard to find. Dan Grundig S800, S650, S700, YB400, YB550PE Degen DE1102, Kaito KA1102 Drake R8, Radio Shack DX-440 E. H. Scott 23 tube Imperial Allwave in Tasman cabinet (1936) I just sent 50 complaints to . Heh heh. That'll get their attention about the troll. .. |
#73
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In article ,
Dan wrote: On Wed, 16 Jun 2004 02:34:08 GMT, Telamon wrote: Some people just don't know how to play nice. No, some people don't, do you? Generally yes but you don't. -- Telamon Ventura, California |
#74
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"Steven Swift" wrote in message ... Hey, don't worry about the rrs gripers. We've discussed detectors there before, and we will again. Take a look at RDH4 page 1073. Even with extreme design precautions you can't get much better than about 2% (see page 1080-1081). This guy calculates 0.4% distortion at 50% modulation and 1.6% distortion at 100% modulation. http://www.amwindow.org/tech/htm/diodedistortion.htm In a typical diode detector test circuit he measured 0.32% distortion at 50% modulation and 2% distortion at 100% modulation. To work, a diode detector has to "cut-off" for 1/2 the carrier cycle. This requires that the diode always go through the "knee". A tube diode has a voltage to the three-halves characteristic. A semiconductor diode has an exponential characteries. Expand into a Taylor series, and look at the first couple of terms. Distortion! Diodes as "multipliers" can be made better. How would a precision rectifier do? I'll look for the analysis. BTW, if you trickle current in a tube diode and keep the signal small, you have built a "square-law" detector. It's been my impression that the "knee" area of the curve is the "square-law" area of the curve. Again, it's my impression that the crystal set guys use foreward bias to get their detectors out of the "square-law" area of the diode's curve in order to maximize sensitivity and minimize distortion. Not a diode detector. You can also see this in the Taylor series. Steve Why isn't a forward biased diode not a diode detector? Frank Dresser |
#75
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Why isn't a forward biased diode not a diode detector? Frank Dresser It can be a detector. The germanium diodes can have a tiny current to keep about 0.25 volts across them even with no signal. the presence of a carrier with modulation or no modulation will cause a ripple voltage into a cap, just like the signal at a power supply rectifier. The ripple voltage is created by a small % of the 455kHz signal cycle charging the C1 of the filter. The amplitude of the carrier voltage varies at a slow speed of audio, and the ripple voltage stays the same value, and the detector audio signal closely follows the shape of the modulation, ie, the audio is recovered linearly. If you don't have any idle current in the diode, and drive the diode off the end of a grounded IFT coil, then the ripple voltage varies a lot at low signal, when the R discharging the C1 of the filter has very little voltage across it. So low level signals are very distorted by cut off distortion on the audio cycles. Patrick Turner. |
#76
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"John Byrns" wrote in message ... The analysis on this web page is complete nonsense, at least for the type of diode detector we are discussing here. We are concerned with High-Fidelity envelope detectors, while the web page analyzes a perfect square law detector. It starts the analysis of by giving the complete diode equation, but then quickly says we will forget that and consider the diode to be a perfect square law device, and not only that, but that it will be used in some sort of unspecified circuit that maintains the perfect square law response for the complete detector. I didn't check all the math after the perfect square law assumption was made, but I will assume he got it all correct. This type of analysis may have some application to crystal set design, but not to the type of detectors we are discussing. He did compare his results with the results from a test circuit, but I could find no indication of the signal level he made the measurements at, perhaps I missed that. Even though the test circuit did include an RC network type load as used in a peak detector, if he made the measurements at low levels in the square law region of the diode, the capacitor would not cause the diode to act as a linear peak detector. The whole analysis on this web page is too simplistic and is irrelevant to the subject at hand. Regards, John Byrns He came up with actual numbers, which is more than most do. Anyway, I also noticed that there was no mention of the actual voltages the detector was being driven at. As far as the square law stuff goes, Terman says a the distortion of a true square law detector will be m/4. So 80% modulaton will result in 20% distortion. He might have derived that number, I don't remember. I do remember the bigger point, that is, that operation in the square law region is to be minimized for AM radio detectors. Although I do remember reading that any part of a diodes curve can be characterized as part of a parabola. I don't know if that's really true or not, or if I'm actually remembering it correctly. But such an assumption works fine with the usual rules of diode detectors. Run the diode at a reasonably high voltage to minimize operation below the knee area of the curve. Run the diode into a reasonably high resistance to minimize the effects of the variation in the straighter part of the curve. Keep the DC and AC resistances in balance. Frank Dresser |
#77
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In article ,
"Frank Dresser" wrote: "John Byrns" wrote in message ... The analysis on this web page is complete nonsense, at least for the type of diode detector we are discussing here. We are concerned with High-Fidelity envelope detectors, while the web page analyzes a perfect square law detector. It starts the analysis of by giving the complete diode equation, but then quickly says we will forget that and consider the diode to be a perfect square law device, and not only that, but that it will be used in some sort of unspecified circuit that maintains the perfect square law response for the complete detector. I didn't check all the math after the perfect square law assumption was made, but I will assume he got it all correct. This type of analysis may have some application to crystal set design, but not to the type of detectors we are discussing. He did compare his results with the results from a test circuit, but I could find no indication of the signal level he made the measurements at, perhaps I missed that. Even though the test circuit did include an RC network type load as used in a peak detector, if he made the measurements at low levels in the square law region of the diode, the capacitor would not cause the diode to act as a linear peak detector. The whole analysis on this web page is too simplistic and is irrelevant to the subject at hand. Regards, John Byrns He came up with actual numbers, which is more than most do. Anyway, I also noticed that there was no mention of the actual voltages the detector was being driven at. As far as the square law stuff goes, Terman says a the distortion of a true square law detector will be m/4. So 80% modulaton will result in 20% distortion. He might have derived that number, I don't remember. Yes, that's exactly what I thought, given the analysis methodology he seemed to be using on the web page, the distortion seemed way too low to me. You have inspired me to take a closer look and see exactly what he did, and where he went wrong, or if I have just misinterpreted his methodology. I will report back in a few days time. I do remember the bigger point, that is, that operation in the square law region is to be minimized for AM radio detectors. Exactly, which is why I said that the apparent square law analysis given on the web page was "complete nonsense" in the context of a High-Fidelity AM receiver. Although I do remember reading that any part of a diodes curve can be characterized as part of a parabola. Yes, I think that is essentially correct. I don't know if that's really true or not, or if I'm actually remembering it correctly. But such an assumption works fine with the usual rules of diode detectors. Run the diode at a reasonably high voltage to minimize operation below the knee area of the curve. Run the diode into a reasonably high resistance to minimize the effects of the variation in the straighter part of the curve. Keep the DC and AC resistances in balance. Some people say there is no "knee" in the diode curve, which follows from your observation "that any part of a diodes curve can be characterized as part of a parabola." There is one more thing that contributes to linear operation of a diode detector, and that is the peak hold capacitor. As long as the capacitor charges to the peak envelope voltage, the shape of the diode curve getting there doesn't matter much, whereas if you take the output of the raw diode and average it by putting it through a low pass filter, then the curvature of the diode characteristic greatly affects the linearity of the output. Of course even with the peak hold capacitor there are still problems at very low signal levels, and also with high negative modulation, and the peak hold capacitor does introduce problems of its own like tangential clipping when the modulation at high frequencies is high. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#78
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In article ,
(John Byrns) wrote: In article , "Frank Dresser" wrote: "John Byrns" wrote in message ... The whole analysis on this web page is too simplistic and is irrelevant to the subject at hand. He came up with actual numbers, which is more than most do. Anyway, I also noticed that there was no mention of the actual voltages the detector was being driven at. As far as the square law stuff goes, Terman says a the distortion of a true square law detector will be m/4. So 80% modulaton will result in 20% distortion. He might have derived that number, I don't remember. Yes, that's exactly what I thought, given the analysis methodology he seemed to be using on the web page, the distortion seemed way too low to me. You have inspired me to take a closer look and see exactly what he did, and where he went wrong, or if I have just misinterpreted his methodology. I will report back in a few days time. OK, I have taken a closer look at the analysis on the web page at this URL: http://www.amwindow.org/tech/htm/diodedistortion.htm and it is more screwed up than I thought. The analysis starts with the Shockley diode equation, and then the exponential power series equivalent to the Shockley equation is stated as equation #2. At this point the author "examines" the second power component of the equation, not quite making it clear that is all he is going to examine, and will base the entire analysis on only the second power component of the diode characteristic. An equation for the output of a square law diode is given as equation #4, which is derived by squaring the equation representing a carrier AM modulated by a single tone. Equation #4 actually represents the V/I characteristic of a square law diode, and does not necessarily represent the output of such a diode, but we will accept it as such for the purposes of this analysis. After considerable mathematical manipulation and six more equations the author comes to the final diode output in equation #9, and after low pass filtering to eliminate the carrier and carrier terms in the output he comes to equation #10 which represents the demodulated signal output from the detector. The author's equation #10 is: (10) m(t) = (m**2)/4 + m*cos wmt + (m**2)/8[cos 2wmt] The authors derivation of equation #9 from equation #4 was too convoluted for me to easily follow, so I did my own derivation which required only two intermediate steps rather than the 5 steps the author required, my result for equation #10 was: (10) m(t) = 1/2 + (m**2)/4 + m*sin wmt - (m**2)/4[cos 2wmt] Neglecting the sin in place of cos for the main modulation term, and the sign on the second harmonic term, we notice that the author lost the DC term somewhere, and his second harmonic term is half of mine with an 8 in the denominator rather than the 4 I derived. These differences could be due to errors in my derivation, which often happen on the first pass, but considering that my distortion result, discussed next, is the same as Terman's, it seems likely that 4 is the correct value for the denominator of the second harmonic term. I plan to eventually try to plow through the authors derivation of equation #9 to see where he made his errors. The error in the denominator would only account for a factor of two in the distortion percentage, but he compounds the error when he calculates distortion as power ratio rather than a voltage ratio which I believe is conventional. Taking the ratio between the amplitude of the fundamental, m*cos wmt, and the amplitude of the second harmonic, (m**2)/8[cos 2wmt] and squaring the author comes up with his equation #12 for percent distortion: (12) THD (%) = (((m**2)/8)**2)/(m**2)) * 100, or ((m**2)/64) *100 which yields 1% distortion at 80% modulation and 1.5625% distortion at 100% modulation. My version of equation #12, based on the ratio of the fundamental, m*sin wmt, and the amplitude of the second harmonic, (m**2)/4[cos 2wmt] becomes: (12) THD (%) = ((m**2)/4)/m) * 100, or (m/4) *100 which yields 20% distortion at 80% modulation and 25% distortion at 100% modulation. These results show distortion more than an order of magnitude greater than the distortion figures calculated on the web page. Note that my result is in agreement with Terman's result at 80% modulation, as quoted above, which leads me to suspect that I didn't make any serious mathematical errors in my derivation. The errors in the web page author's analysis stem from two sources, first the amplitude of the second harmonic term is too small by a factor of two due to an error or some sort in the derivation of the equation. The second cause of the error is due to the fact that the web page author expresses distortion as a power ration rather than the conventional voltage ratio. Now of course all this is for a perfect square law detector, which does not apply to what we have been talking about, which is a peak envelope detector. The peak envelope detector which is considerably more difficult to analyze, which probably explains why the author of the web page didn't even try, and some authorities have gone so far as to say the problem is so complex that it is basically intractable to rigorous mathematical analysis. I hope I didn't make too many typos in this, please let me know if I did so I can correct them when I post the results of my analysis of exactly where the author's derivation went wrong. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#79
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OK, I have taken a closer look at the analysis on the web page at this URL: http://www.amwindow.org/tech/htm/diodedistortion.htm and it is more screwed up than I thought. snip a vastly complex and incomprehensible disputation of the largely incomprehensible text and formulae at http://www.amwindow.org/tech/htm/diodedistortion.htm About all we want is low distortion detection, and it matters noe that we cannot follow all this mathematical analysis. There is no mention of the output voltages measured with respect to the % of modulation. But anyway, a table at the conclusion of the article gives the thd at various %m, :- Modulation Index (%) THD (%) 10 1.02 25 0.08 50 0.32 100 2.0 150 6.3 Table 2 - Measured Total Harmonic Distortion Versus Modulation Index But we dunno what the output voltages are, and no doubt the thd results would be very different if the output voltage was 10v instead of say 1v at 10% modulation, especially with a solid state diode. From the test circuit shown, there is no bias current flow in the diode to keep it turned on even without an RF signal to demodulate. This would also reduce thd. Nobody needs to know math involved with diode detectors to get much lower thd than is realised in most old fashioned and attrocious tube detector stages in conventional AM radios. Patrick Turner |
#80
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In article , Patrick Turner
wrote: OK, I have taken a closer look at the analysis on the web page at this URL: http://www.amwindow.org/tech/htm/diodedistortion.htm and it is more screwed up than I thought. snip a vastly complex and incomprehensible disputation of the largely incomprehensible text and formulae at http://www.amwindow.org/tech/htm/diodedistortion.htm About all we want is low distortion detection, and it matters noe that we cannot follow all this mathematical analysis. Indeed, my original point was simply that the analysis on that web page, which had been mentioned in this thread as being somehow relevant, was actually totally irrelevant because it dealt with a square law detector, not a linear diode peak envelope detector as is commonly used in High-Fidelity AM receivers. It was then pointed out in this thread that the conclusion of the web page did not agree with Treman's calculations for the square law detector. My "incomprehensible disputation" was simply to tie up the loose ends and show where the web page went wrong on its square law detector analysis, which would still have been irrelevant to High-Fidelity designs even if it had been done correctly. There is no mention of the output voltages measured with respect to the % of modulation. I pointed out that very fact in my first post about this web page, that no details were given of the operational under which the experimental results were measured. With respect to the square law detector analysis, the voltage level doesn't matter, square law is square law irrespective of the carrier level, so the distortion doesn't change with signal level in an ideal square law detector, it only changes with the modulation percentage. From the test circuit shown, there is no bias current flow in the diode to keep it turned on even without an RF signal to demodulate. This would also reduce thd. You have still haven't enlightened us with some concrete information about how much, if at all, your biased diode detector really helps reduce the distortion of the diode peak envelope detector. I haven't looked at biased diodes as AM detectors myself, although I am given to understand that the proper bias can reduce the distortion of a diode peak envelope detector, but I am also given to understand that the proper bias is dependent on signal level, which requires a complex circuit to cause the bias to maintain the proper relationship to the signal level. Although I haven't seen it mentioned, I would assume that a very tight AGC circuit would also serve to allow a fixed bias to be applied to the diode. I would think that if a simple bias scheme such as yours really significantly helped lower the detector distortion, we would have seen more implementations of this idea in high quality receivers over the years. There have certainly been plenty of expensive AM receivers built over the years, that didn't skimp on the parts count, where an extra resistor or two, to bias the diode wouldn't break the bank. That is not to say that I haven't seen cheap transistor radios that had biased detectors, but it never seemed to be actively pursued in the better AM receivers of the tube era. You could better make your point if you posted a couple of graphs for distortion vs. signal level for a diode detector, with and without bias, and for several modulation levels, maybe 80% and 100%. Nobody needs to know math involved with diode detectors to get much lower thd than is realised in most old fashioned and attrocious tube detector stages in conventional AM radios. Well you are probably right about that, but for a completely different reason than you have in mind. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
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