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In article , Joe McElvenney
writes: Has anyone a pointer to the RF output impedance of the 74HC series gates (at about 10MHz)? I have a few inverters/buffers left over in a package and thought of using them stacked to drive an output port via a simple matching network. I've seen this done before and have simulated it myself but would like it spelled out. Fairchild's application note AN-393 is an informative one from the standpoint of _transient_ switching analysis. Analysis or simulation for impedance matching, presuming matching at a single frequency, leans towards _steady_state_ analysis. "Impedance matching" of very fast transition time rectangular waveforms must be done over a very wide bandwidth to insure fidelity of the rectangular waveform at the far end of the transmission line. Digital devices are characterized under near-total On or Off current source drivers...working into very non-linear active device loads. To get into the theory of a narrowband source impedance condition is possible. Some of that has already been done with the "Class E" amplifiers using certain SCR devices at 40 meters, such as several examples at Cal Tech, Pasadena, CA, groups. A search on Class E material might turn up something that would fit your application. Class C tube or transistor operation can be considered a sort of "soft switching" condition similar to On-Off current sources but that is complicated by the fact that the On state of the active device varies in current carrying capability depending on a narrowband load... hence the "soft switching" label. Triode to pentode vacuum tubes were never very good switches in On-Off operation compared to transistors.over the very wide bandwidths necessary for fast-transient digital waveforms. While it might be an interesting intellectual exercise in analysis, it will take considerable time. Far more time than geting up a small breadboard test circuit and TRYING IT OUT. Nearly all of the "74" (or "54") family components are relatively cheap...small-scale integrated circuit packages cost less than a dollar each in single quantities. Len Anderson retired (from regular hours) electronic engineer person |
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