[time-nuts] Linear voltage regulator hints... --> WHY?

[Bob Stewart]

Hi Charles,
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?
   
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."  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.

Best regards,

Charles





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