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Hi OM,
This goes into the intricacies of how forced propositions do not yield a forceful argument. On Tue, 01 Mar 2005 18:06:18 GMT, gwhite wrote: You don't know the output impedance because you don't have a way of determining it by swinging the output full-scale. This is more properly an admission from you, than a projected inability upon us. You may not know how, but this does not prevent me from expressing a value that is suitably accurate. Now, within the field of measurement, no statement is accurate without an expression of its range of error. However, in this regard accuracy is still a remote issue as you offer nothing of practical consideration and have failed to respond to a simple example to provide context. Richard Harrison, , KB5WZI, has in this sense already done the heavy lifting with: From the specifications page also, the power reguirement is TX: 18A 13.8V DC. It`s a linear amplifier. Only 40% efficiency. The designer probably was more interested in low harmonics than efficiency. The final by itself only takes part of the 18A ao its efficiency is more than 40%. continuing.... Even for class A, large signals will/can have rail to rail swing. This marks an artificial imposition not required to respond to the spirit of the topic. Such swings are not necessary. The device will not be linear for large swings: sinusoidal input swing will not result in a sinusoidal output swing. This is immaterial to impedance and is a set-up of another artificial imposition: the Thevenin Model (which was specifically dismissed). Hence we are into a cascade of impositions. But "impedance" is a sinusoidal (s-domain) concept. This is baloney cut thick. S Domains (?) are at best a modern contrivance to model well behaved small signal devices. Their utility follow theory, they do not drive theory. So how can you define an impedance--a sinusoidal concept--when the waveform is not sinusoidal for an inputted sine wave? There are no sine waves in nature, so by this contortion of logic from above there are no s-domains (?). Why are there no sine waves in nature? Because nature is bounded by the Big Bang (a discontinuity) at one end, and has yet to fulfill its infinite extent. In other words, tedious appeals to artificial impositions of purity fail at the gate for their sheer collapse of internal logic. This kind of stuff appeals to arm-chair theorists who find themselves impotent to perform. The point is that the output impedance is time dependent ("causes" the non-sinusoid output for sinusoid drive), which rather makes the concept questionable. As I wrote earlier, one might decide to consider a time averaged impedance, but I'm not clear on what the utility would be. Classic performance anxiety. Engineers learn to live with limitation and to express results and sources of error so that others can judge merit. Priests are better suited with mulling over these issues of ambiguity. There is no "presumption." Linear parameters and theorems totally ignore practical limitations--this is a fact and you can look it up in just about any text on circuit analysis. Knowledge limited. There are many suitable texts that offer a wider spectrum of discussion that are fully capable of answering these issues. However, it is made worse that most of this stuff is derivable from first principles and no recourse to vaster libraries is actually needed. The simple linear model is perfectly okay for small signal devices. It isn't okay for large signal devices. And yet there is no substantive illustration to prove this ambiguous point. What constitutes small, and what demarcates large? Such nebulous thinking clouds the obvious observation that the full range of devices themselves operate on only one principle. What is limited is the human component of their perception, not the physical reality of their operation. The faulty choice of models (S Parameters) is not the fault of either Physics or the devices when they diverge from the crutch of calculation against the wrong mathematical expression. In any case, load pull equipment does not make the pretense of defining output impedance of an active large signal device. It does say what the load needs to be to acquire maximum power out of the device. This is simply the statement from a lack of experience. Thevenins and conjugate matching (for maximum power transfer) are explicitly linear small signal device models. Their use in RF PA output design is a misapplication. These statements are drawn from thin air. So to return to a common question that seems to defy 2 out of 3 analysis (and many demurred along the way) - A simple test of a practical situation with a practical Amateur grade transistor model 100W transmitter commonly available for more than 20-30 years now: 1. Presuming CW mode into a "matched load" (any definition will do); Any definition won't do, and for this discussion the specific "won't do" is using conjugate matching which is a small signal (linear) model. Given the failure to provide any discussion for either or any form of matching suggests a lack fluency in any of them. *You* brought up Thevenins and armchair philosophy regarding it, not me. I rejected it as an unnecessary filigree, but I notice in the quotes above that you readily embraced it as a necessary imposition. I said Thevenins was irrelevent, and now you appear to agree with me. Ken effectively brought up conjugate matching, not me. This compounded with the denial of Thevenin is quickly closing the available matching mechanisms. If it is not about Thevenin, and it is not about Conjugation, then I am willing to wait to hear what it IS about. ....But not really. I have little faith that the difference is appreciated nor how many ways a match may be accomplished or for what ends. The original comment I was challenging was: "...the antenna works as an impedance mathcing network that matches the output stages impedance to the radiation resistance." I am always suspicious of how a quoted claim is couched by the rebutter (cut and paste from the original is always available and citing the link to the complete contextual post is hardly Herculean). However, responding to the bald statement, I find nothing objectionable about it. I simply wanted to make it clear that the "matching" done was not an issue of "output impedance" per se. It is an issue of how the transistor is to be loaded to extract maximum ouput power. Again, a presumption not brought to the table. It may follow as a consequence, but it is not a necessary condition. Our questioner who started this thread is undoubtedly interested in the outcome in terms of maximum radiation for a limited power - it is a chain of causality that is a forced step matching issue from the battery to the ęther. This is a first principle of successful production engineering. 73's Richard Clark, KB7QHC |
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