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#11
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You're right. The author is wrong.
Roy Lewallen, W7EL Clifton T. Sharp Jr. wrote: I'm not sure I was clear on my question. The only nonlinearity I want in my detector diode is the noncontinuous function | Vin when Vin = 0.000000 micronanofemtoattovolts Vout = | | 0 when Vin 0.000000 micronanofemtoattovolts ...while the author seems to be saying that the nonlinearity which exists just above the barrier voltage is essential to detection. He does say, "The slope must be steeper (or shallower) at higher voltages and shallower (or steeper) at lower voltages than at the quiescent operating point." Given my perfect diode and (let's say) a 0.25V peak signal, I contend that my output envelope will be an exact reproduction of the input envelope; but (see his graph #2) his required nonlinearity will seriously distort the output envelope. He seems to be saying that without that, you can't demodulate the signal. I assert that without that, I demodulated it better than he did. |
#12
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"Clifton T. Sharp Jr." wrote in message ... I'm not sure I was clear on my question. The only nonlinearity I want in my detector diode is the noncontinuous function | Vin when Vin = 0.000000 micronanofemtoattovolts Vout = | | 0 when Vin 0.000000 micronanofemtoattovolts ...while the author seems to be saying that the nonlinearity which exists just above the barrier voltage is essential to detection. He does say, "The slope must be steeper (or shallower) at higher voltages and shallower (or steeper) at lower voltages than at the quiescent operating point." Am I missing the contradiction? Say you have a quiescent operating point of 0V. An ideal diode would have an infinately high slope above that point and a 0 slope below that point. Or vice-versa. That seems to fit the given statement. Real world diodes have differing ideal q-points and variable slopes. And most of the other articles seem to be aimed at getting the most out of such diodes. Given my perfect diode and (let's say) a 0.25V peak signal, I contend that my output envelope will be an exact reproduction of the input envelope; but (see his graph #2) his required nonlinearity will seriously distort the output envelope. I think your perfect diode analysis is correct. I think the only nonlinearity the author requires is around a q-point. I don't get the impression the author requires a diode to have a variable slope characteristic at other voltages. He seems to be saying that without that, you can't demodulate the signal. I assert that without that, I demodulated it better than he did. The author's last revision was on 07/2/2003, so he must still have an interest. I suppose you could send him an e-mail. The address is the first item in article #1. -- Frank Dresser |
#13
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"Clifton T. Sharp Jr." wrote in message ... I'm not sure I was clear on my question. The only nonlinearity I want in my detector diode is the noncontinuous function | Vin when Vin = 0.000000 micronanofemtoattovolts Vout = | | 0 when Vin 0.000000 micronanofemtoattovolts ...while the author seems to be saying that the nonlinearity which exists just above the barrier voltage is essential to detection. He does say, "The slope must be steeper (or shallower) at higher voltages and shallower (or steeper) at lower voltages than at the quiescent operating point." Am I missing the contradiction? Say you have a quiescent operating point of 0V. An ideal diode would have an infinately high slope above that point and a 0 slope below that point. Or vice-versa. That seems to fit the given statement. Real world diodes have differing ideal q-points and variable slopes. And most of the other articles seem to be aimed at getting the most out of such diodes. Given my perfect diode and (let's say) a 0.25V peak signal, I contend that my output envelope will be an exact reproduction of the input envelope; but (see his graph #2) his required nonlinearity will seriously distort the output envelope. I think your perfect diode analysis is correct. I think the only nonlinearity the author requires is around a q-point. I don't get the impression the author requires a diode to have a variable slope characteristic at other voltages. He seems to be saying that without that, you can't demodulate the signal. I assert that without that, I demodulated it better than he did. The author's last revision was on 07/2/2003, so he must still have an interest. I suppose you could send him an e-mail. The address is the first item in article #1. -- Frank Dresser |
#14
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"Clifton T. Sharp Jr." wrote in message ...
John R. Strohm wrote: "Clifton T. Sharp Jr." wrote... (http://uweb.superlink.net/bhtongue/7diodeCv/7diodeCv.html). In other words, the slope of the V/I curve must change as a function of applied Voltage. The slope must be steeper (or shallower) at higher voltages and shallower (or steeper) at lower voltages than at the quiescent operating point. I don't see how a changing slope during forward conduction could do anything other than distort the demodulated waveform, especially on tiny signals. Think about the V-I curve on an ideal diode. Observe that, if the diode is reverse-biased, no conduction takes place. In principle, you can crank the reverse voltage up as high as you like, and STILL no conduction takes place. (In practice, you run into avalanche and zener breakdown.) If the diode is forward-biased, the diode conducts like crazy. There is a CORNER in the curve, i.e. a change in the slope of the V/I curve, at NOMINALLY V=0V. (Actually, V=0.7V for silicon, V=0.3V for germanium, V=1.7V for red LEDs, about 2.2V for yellow, about 2.4V for green, and so on and so forth...) I'm not sure I was clear on my question. The only nonlinearity I want in my detector diode is the noncontinuous function | Vin when Vin = 0.000000 micronanofemtoattovolts Vout = | | 0 when Vin 0.000000 micronanofemtoattovolts ...while the author seems to be saying that the nonlinearity which exists just above the barrier voltage is essential to detection. He does say, "The slope must be steeper (or shallower) at higher voltages and shallower (or steeper) at lower voltages than at the quiescent operating point." Given my perfect diode and (let's say) a 0.25V peak signal, I contend that my output envelope will be an exact reproduction of the input envelope; but (see his graph #2) his required nonlinearity will seriously distort the output envelope. He seems to be saying that without that, you can't demodulate the signal. I assert that without that, I demodulated it better than he did. I think you and the author are saying the same thing. Consider that in your ideal diode case, the quiescent operating point is zero volts. Above that, the slope of your diode is infinte (zero ohms) and below that it's zero (infinite ohms). The author is suggesting that you do NOT need it to be so sharp, and in fact, I find that I can detect RF down to below 100 microvolts or so with a Schottky diode designed for zero-bias detector service, though the output voltage is very tiny down there. If I could buy one of your perfect diodes, 100uV (RMS) of RF would give me around 141uV of output, instead of the sub-microvolt I do get. The low output is the result, of course, of the fact that the diode dynamic resistance at -140uV is only very slightly different from its dynamic resistance at +140uV. In a real diode, the change in slope is gradual with no sharp corners, but if you could find a diode which looked like, say, 1000 ohms in the reverse direction for all voltages and 1000.1 ohms in the forward direction, it would still work as a detector, albeit a poor one. Cheers, Tom |
#15
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"Clifton T. Sharp Jr." wrote in message ...
John R. Strohm wrote: "Clifton T. Sharp Jr." wrote... (http://uweb.superlink.net/bhtongue/7diodeCv/7diodeCv.html). In other words, the slope of the V/I curve must change as a function of applied Voltage. The slope must be steeper (or shallower) at higher voltages and shallower (or steeper) at lower voltages than at the quiescent operating point. I don't see how a changing slope during forward conduction could do anything other than distort the demodulated waveform, especially on tiny signals. Think about the V-I curve on an ideal diode. Observe that, if the diode is reverse-biased, no conduction takes place. In principle, you can crank the reverse voltage up as high as you like, and STILL no conduction takes place. (In practice, you run into avalanche and zener breakdown.) If the diode is forward-biased, the diode conducts like crazy. There is a CORNER in the curve, i.e. a change in the slope of the V/I curve, at NOMINALLY V=0V. (Actually, V=0.7V for silicon, V=0.3V for germanium, V=1.7V for red LEDs, about 2.2V for yellow, about 2.4V for green, and so on and so forth...) I'm not sure I was clear on my question. The only nonlinearity I want in my detector diode is the noncontinuous function | Vin when Vin = 0.000000 micronanofemtoattovolts Vout = | | 0 when Vin 0.000000 micronanofemtoattovolts ...while the author seems to be saying that the nonlinearity which exists just above the barrier voltage is essential to detection. He does say, "The slope must be steeper (or shallower) at higher voltages and shallower (or steeper) at lower voltages than at the quiescent operating point." Given my perfect diode and (let's say) a 0.25V peak signal, I contend that my output envelope will be an exact reproduction of the input envelope; but (see his graph #2) his required nonlinearity will seriously distort the output envelope. He seems to be saying that without that, you can't demodulate the signal. I assert that without that, I demodulated it better than he did. I think you and the author are saying the same thing. Consider that in your ideal diode case, the quiescent operating point is zero volts. Above that, the slope of your diode is infinte (zero ohms) and below that it's zero (infinite ohms). The author is suggesting that you do NOT need it to be so sharp, and in fact, I find that I can detect RF down to below 100 microvolts or so with a Schottky diode designed for zero-bias detector service, though the output voltage is very tiny down there. If I could buy one of your perfect diodes, 100uV (RMS) of RF would give me around 141uV of output, instead of the sub-microvolt I do get. The low output is the result, of course, of the fact that the diode dynamic resistance at -140uV is only very slightly different from its dynamic resistance at +140uV. In a real diode, the change in slope is gradual with no sharp corners, but if you could find a diode which looked like, say, 1000 ohms in the reverse direction for all voltages and 1000.1 ohms in the forward direction, it would still work as a detector, albeit a poor one. Cheers, Tom |
#16
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Tom Bruhns wrote:
In a real diode, the change in slope is gradual with no sharp corners, but if you could find a diode which looked like, say, 1000 ohms in the reverse direction for all voltages and 1000.1 ohms in the forward direction, it would still work as a detector, albeit a poor one. Son, that's called a Selenium rectifier. Goddam newbies. Rob |
#17
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Tom Bruhns wrote:
In a real diode, the change in slope is gradual with no sharp corners, but if you could find a diode which looked like, say, 1000 ohms in the reverse direction for all voltages and 1000.1 ohms in the forward direction, it would still work as a detector, albeit a poor one. Son, that's called a Selenium rectifier. Goddam newbies. Rob |
#18
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Rob Judd wrote in message ...
Tom Bruhns wrote: In a real diode, the change in slope is gradual with no sharp corners, but if you could find a diode which looked like, say, 1000 ohms in the reverse direction for all voltages and 1000.1 ohms in the forward direction, it would still work as a detector, albeit a poor one. Son, that's called a Selenium rectifier. Goddam newbies. Now if you'd mentioned copper oxide, I might have been mildly impressed... ;-) Seems like there were some iron based ones, too. And then there's the coherer... Not all early detectors were insensitive, however. Have a look at the Marconi detector which uses a moving magnetic band. Seems like we had a thread here about it a few years ago, and there was a nice article about it in EW+WW. All the seleniums I ever used ('cept for the shorted ones) worked a whole lot better than 10001:10000! Cheers, Tom |
#19
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Rob Judd wrote in message ...
Tom Bruhns wrote: In a real diode, the change in slope is gradual with no sharp corners, but if you could find a diode which looked like, say, 1000 ohms in the reverse direction for all voltages and 1000.1 ohms in the forward direction, it would still work as a detector, albeit a poor one. Son, that's called a Selenium rectifier. Goddam newbies. Now if you'd mentioned copper oxide, I might have been mildly impressed... ;-) Seems like there were some iron based ones, too. And then there's the coherer... Not all early detectors were insensitive, however. Have a look at the Marconi detector which uses a moving magnetic band. Seems like we had a thread here about it a few years ago, and there was a nice article about it in EW+WW. All the seleniums I ever used ('cept for the shorted ones) worked a whole lot better than 10001:10000! Cheers, Tom |
#20
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Tom Bruhns wrote:
All the seleniums I ever used ('cept for the shorted ones) worked a whole lot better than 10001:10000! All the seleniums I noticed (in TV service, that is) didn't... which leads us to that unforgettable smell from a failed one. Ghufph. -- The function of an asshole is to emit quantities of crap. Spammers do a very good job of that. However, I do object to my inbox being a spammer's toilet bowl. -- Walter Dnes |
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