[time-nuts] Linear voltage regulator hints... --> WHY?
kb8tq at n1k.org
Fri Dec 12 08:09:36 EST 2014
> On Dec 12, 2014, at 12:28 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> Bob wrote:
>> Separate from the analysis of the voltage on the OCXO, there is another part to this:
>> Ok, so why am I harping on the "need" for all this from a system standpoint ?
> We've been around this track a time or two before, me frustrated with your "make it just good enough" philosophy and you with my "always do the best you can" philosophy. We're not likely to persuade each other, or even influence anybody else, but I think it is worth going around at least one more time.
>> 1) Adding stuff to a design that does not make it measurably better is simply a waste of money.
> Preliminary nit: I agree that any "improvement" that does not make something measurably better is of no value. Indeed, it is no improvement at all. But you didn't mean literally "not measurably better" -- you meant "not better for the task at hand.”
In the case at hand, the task is a GPSDO with a frequency vs temperature issue. The issue is coming from the OCXO and not the reference. Improving the reference will (in this case) have no impact on the problem.
> A digital caliper reading to 0.0001" is "measurably better" than a ruler graduated in 1/32 inch, although the difference is not important if one is measuring the thickness of a 2x4 for framing a house. But some day you may want to measure something besides a 2x4....
> On to the substance:
> "Do the best you can" isn't necessarily about adding anything to a design. It's about carefully determining an error budget and developing a design that meets the budget. Of course, you can set the design goals for each subsystem so that the overall system should juuuust work if everything else is perfect, or so that the system should work under most conditions, or so you'll never have to consider whether that subsystem might be contributing anything significant to the system errors. If the latter is no more difficult and no more expensive than either of the former, why WOULDN'T you design it that way?
It is *rare* that an improvement does not impact cost or complexity. It most certainly is not the case in this situation.
> I was taught many years ago that "good thinking doesn't cost any more than bad thinking," and I have generally found that to be true. Meaning, it is frequently the case that "the best you can do" is no more difficult and no more expensive than doing something less, it just takes better thinking and a more accurate analysis. Whenever that is the case, which IME is very often, doing less is, IMO, a design fault.
> Most often, it's a matter of, "Why ground that capacitor there? Over here would be better," or "Why use a noninverting amplifier? If you use an inverting amplifier, the HF rolloff can continue beyond unity gain," or something similar.
> Note, also, that many of the people asking questions on the list do not seem to have a thorough design specification for their project, and may not even know what all they will use a gizmo for. Settling for what a list pundit might think is "good enough" for the person's needs (e.g., residual phase noise floor ~ -150dB and reverse isolation of ~ 40dB for a buffer amplifier) may turn out to be inadequate when the person acquires some better oscillators and a DMTD setup and needs -175dB and 90dB. If they do the best they can the first time, they may not have to re-do it later.
But - rather than looking at the system and it’s needs, we spin off to “improvements”. Inevitably the result is a -175 db solution to a -145 db problem.
>> 2) Others read these threads and decide "maybe I need to do that..".
>> 3) Still others look at this and decide "If I need to do that, I'm not even going to start". That's not good either.
> Again, neither one is a problem if doing the best one can is no more difficult and no more expensive than doing something less.
Except that in the actual example case at hand it very much is more expensive and more difficult.
> If someone has already done the good thinking and suggests a workable approach, and all you have to do is a sanity check to implement the idea (perhaps even improving on the design), again -- why WOULDN'T you?
That’s not what’s being done here. The example case is not following the course you are talking about at all.
> There is always someone handy who is quick to point out all of the other ways to do things, so the person contemplating the project can evaluate the different approaches for himself.
> Sometimes, of course, going the next step up the "best you can" ladder involves an expensive part (e.g., silicon-on-sapphire semiconductors), or a much more complex design, or some use restriction (must be submerged in liquid nitrogen). In that case, one must think very carefully about the error budget and determine if that step is really necessary. But the vast majority of the time, we do not face that situation IME.
For a very few people the limit may indeed be liquid helium. There is a *much* larger group who are turned off at a far lower cost or complexity point.
> The bottom line is: There is no virtue in doing "just enough," certainly not in the case of amateur projects that will not be manufactured in large numbers for slim profit (where every millipence must be saved, if the accountants are to be believed -- often, they shouldn't be, but that's another topic entirely). Never apologize for doing better than "just enough," as long as doing so does not cause collateral problems.
> To me, that is the art of design -- knowing that the finished gizmo is the best I could make at the time and with the resources available.
> In philosophy-of-design circles, one sometimes hears that "a race car should be designed so that everything is totally spent as it crosses the finish line -- the engine should explode, the transmission should break, and all four tires should blow out simultaneously. Anything that is still working was, by definition, overdesigned." Aside from the obvious hyperbole, I think the underlying point is plain wrong. I know I admire the designers, whoever they were, when someone pulls a submarine off the ocean floor after 70 years and the batteries still have a charge and the gauges and radios still work.
> Finally, one not-so-obvious point about amateur designs. Sometimes, something is a true one-off -- there will never be another made to that design. In that case, some design requirements can be relaxed. You can use trimmer caps or resistors where you would prefer not to in a commercial design, for example, and you may use disfavored logic kludges to work around timing problems. But then there are designs that you will publish or otherwise share -- and these, I suggest, need to be even more bulletproof than commercial designs, since you are not in control of the construction, parts choices, etc. that others who follow your lead will make. Yes, you can make disclaimers and suggest where the sensitive bits are, but for the design to be truly useful to others, you need to pay attention to all that and design as many of the traps out as you possibly can -- which can be much harder than designing something to work properly when it is made in a factory under your supervision.
The issue is not “do people go overboard”, of course they do. Everybody does. Turning that by it’s self into a virtue is a mistake. In 99.99% of the real world cases, cost and complexity will go up and reliability will thus go down. The result will not be better, it will be worse. The best design that achieves the goal is simplest design. That’s every bit as true in the basement as it is in volume production.
Your comments do not address the other side of what I’ve been trying to convey:
Going overboard with no analysis is *not* a good way to do any of this. Even after getting the system specs and design constraints for this example, we do not bring that into the discussion. We get into long (and very well written) defenses of complexity for it’s own sake. We don’t get into analysis. The takeaway to the casual reader is that complexity is the goal and that analysis is an un-important part of the process that only optional comes into it. There are a *lot* of people reading the list who could execute a simple design. There are far fewer who can properly execute a very complex one. The focus on complexity for it’s own sake does have impact.
Are we really that far apart - not really. We each are talking about two sides of the same coin. The real world is a messy place. Analysis often takes a back seat to the “fun of doing something”. That’s not to say it should though …
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