[time-nuts] Linear voltage regulator hints... --> WHY?
bob at evoria.net
Fri Dec 12 00:54:46 EST 2014
I hope you don't mind if I throw my two cents in, as this began as a question about my GPSDO project. We had a thermal drift problem that Dan traced to the PWM to EFC interface and resolved. The question to the list was whether there was a regulator package that had a built-in reference with good thermal performance. Somehow the thread went off on all the tangents, which can be good. In the process it became clear that there weren't any regulators that would fit our needs, so we would have to go with a reference and op-amp etc. So, now we just need to decide whether to use a pass transistor or a controllable regulator. The budget will probably result in a 25 or 50 cent pass transistor, a good(ish) op-amp and a reference that's multi-purposed for the board's other needs (ADC, RC integrator, etc). But it's certainly good to see what the other options are just in case. And if the project stays a two-off, then there's plenty of leeway to use better parts here and there if the pinouts are the same.
Bob - AE6RV
From: Charles Steinmetz <csteinmetz at yandex.com>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Thursday, December 11, 2014 11:28 PM
Subject: Re: [time-nuts] Linear voltage regulator hints... --> WHY?
>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." 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? 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.
>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. 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? 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.
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.
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