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February 21st 04, 07:47 PM
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In article , Paul Burridge
writes: On 20 Feb 2004 21:53:26 GMT, (Avery Fineman) wrote: A diode doubler using a toroid transformer, pair of diodes and a tuned circuit in the output works fine right off the paper pad and slide-rule (or calculator) numbers. Typically the source is a distorted sinewave Is the type of distortion critical? How about a clipped/clamped sinewave? Yes and no. :-) A quantitative answer isn't possible since the waveform must be described accurately in shape (or spectrum analyzed) in order to determine the harmonic content. Suffice to say that a square wave cannot be used with a passive diode doubler; all the energy is contained in the short transition times and that is rarely enough to be worth it. If an untuned oscillator output is to be doubled, that scoped waveform will quite probably look distorted. Such is quite likely to be a good harmonic content source for a passive diode doubler. The obvious alternate is to tune the oscillator output to the second (or third) harmonic right off... :-) I mentioned diode doublers because (1) they are passive; (2) they are relatively broadband; (3) common legacy fast diodes such as 1N914 and 1N4148 can work in that application beyond 20 MHz; (4) they work with cylindrical-shape coils also but toroidals forms make the whole circuit physically smaller. If the source's impedance is too high to handle a passive diode multiplier, then an active-device multiplier is a better choice. [at this point it is a promotional insert time to publicize ARRL publications of "tried and proven circuits" provided one copies ALL the parts of the circuit exactly as shown to be tried and proven...:-) ] The original thread question was general enough that the number of variables would fill a shopping cart. Quantitative answers to such questions aren't possible. At best, only suggestions of a general nature can be the answers. Digital logic off-the-shelf is excellent for making things right off the paper design because they work with two stable states with very high transition times; stay within the rise, fall, and propagation times and fan-out rules and it should work right off the scratchpad. Analog circuits are a whole new game with different rules and a large number of unknowns even if some detailed specs are available. For one-of-a-kind homebrew applications of analog multipliers, I'd say it was time for experimental bench cut-and-try work first. A paper analysis is going to take TIME even if the smarts are there. Empirical data derivation (cut-and-try) is quick, much quicker than the paper chase. I say empirical since the supply voltages may be different than some book example, few have instruments for measuring source and load impedances or spectral content and power level of the source. The simpler the prototype-idea circuit, the easier it is to make a stock kind of circuit on the bench and probably characterize it over a wide frequency range and, possibly, with varying supply rail voltages and power levels. Heh...a LOT of production circuits were engineered that way even though the companies who made it came along after and made them look like seven wonders of the world in PR literature later. They were after _reproducible_ circuits in _their_ systems, not as shining textbook examples. Some passive component values may have been selected to reduce the overall type-of-parts count by using "common" values needed in other circuits. That's perfectly acceptible as long as a circuit works and can be reproduced...at a profit. :-) Can I answer your original question? Not really. Think of a passive diode doubler as a full-wave rectifier. Those take a fundamental sine and "double it over" (negative swing made positive through trans- former) to make two half-sine pulses of the same polarity for each full AC cycle. There's a lot of "second harmonic" in that rectifier output...which makes for easier filtering since the ripple voltage frequency is twice what it would be for a half-wave rectifier. Using fast legacy diodes at a much higher frequency turns out to be the same sort of thing. Unlike a rectifier circuit, the output of the doubler can be tuned to that second harmonic (a no-no for most power supply rectifiers) to get the most output. You could use a clipped sinewave input, but why and where is the clipping done and what extra circuits or components are needed to justify that? I've only outlined SOME of the mental questions each brewer has to make for themselves. Concentration is needed for application. Len Anderson retired (from regular hours) electronic engineer person |
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