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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 |
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