Do I understand correctly that even 2 identical components (same brand and part number) can perform not quite as well as the other? In such a case, what's the way to get the 'ideal' component? Just keep buying them and testing them to see which one gives the desired values?
In some circuits that is correct. The only way is to test many devices. Components are designed to meet certain minimum specifications and certain tollorances. The circuits are designed so they work within the devices parameters. Sometimes when looking for the maximum performance you will have to subistute several devices to get the best performance but that is seldom necessary. When the first big (12 foot ) dishes were being sold for tv reception a very low noise high gain amplifer was needed. Some contained
2 transisitors with the same part number but maybe with a 1- or -2 after it. The devices were selected for the lowest noise figuer for the first device. I am not sure now, but not too long ago the microprocessors were tested for the speed they could operate at without missing an instruction and were priced at the speed they operated at. They were then marked and sold for the speed they were reliable at. That is why you could overclock a computer , especially if you raised the voltage on them a tenth of a volt or so. The processor may or may not burn out sooner, but if it lasts 5 years instead of 10 or more years and you are changing the computer every 2 to 4 years it will not make any differance.
True
Perhaps this analogy will help, but my analogies are always dangerous.
Think about a recipe that needs eggs in it. You go to the store and there are stacks of eggs. Every carton has a size label on it. But the eggs in every carton are slighly different, no two are EXACTLY the same weight, down to the million'th of an ounce, even if the eggs are in the same carton or two cartons have the same label on them.
Fortunately, your recipe probably won't explode if you have an egg that isn't exactly the right size, there is flexibility for a little variation built into the recipe. But some recipes need more accuracy than others, and you or they plan for this.
The same is true for most components. It is a very rare item that comes only and exactly in exactly perfectly completely precise sizes. Actually at the moment I cannot think of one that I could go buy.
So most products that are built from items are supposed to have some built in flexibility for the variation. In fact many circuits that are built are specifically designed knowing that there will be this variation and thus compensate for it.
As you say, it certainly is possible to keep buying and testing parts until you find the desired values. This is actually done in some cases, for example where a circuit needs a pair of parts that are as close to identical as possible. So your idea is used. But lots of other cases it is easier or cheaper to be flexible.
Does that help?
This is true. Most of the time, though, the differences are so slight as to not matter.
I'm guessing that when we're talking about mass-produced components, such as diodes, transistors, resistors, etc., the larger the number of your production run, the more identical the parts will be toward one another. But if you're making only small quantities of something, chances are the differences will be more noticeable.
I sometimes run into power-control circuits in which output transistors have to be "matched"; that is, you don't want one transistor carrying more of the load than the other. I personally don't like this and consider it poor circuit design. It shouldn't have to be that critical. What's worse, having to order a large batch of replacement power transistors and being able to match up only a few out of a couple of dozen runs into money. It's especially aggravating when you see different logos on different transistors; each manufacturer has its own special way of making parts, and you can bet that they won't match exactly with the same part made by somebody else.
Fortunately, there are not that many instances in which "matching" is really critical. RF circuits can be, though, particularly when you're dealing with a balanced modulator; if the diodes aren't matched up just right, you may or may not be able to get it to balance.
From my experience, the best way to get "matching" parts is to order a large batch from the same manufacturer, hopefully from the same production run.
You have to quantify what "ideal" is.
If you cannot quantify the qualities you need, then you're not in the right business.
It's sort of like the fact that there's nobody in the world who is exactly six feet tall.
Tim.
component datasheets give the tollerance spreads. I guess that you have to just design with that in mind. If you are testing for a too narrow performance spectrum you will probably be beaten by father time and global warming as some , if not all material caracteristics will change in time, temp, pressure etc.... Although it is a special case, I had some frustrating time sorting through crystals to get frequencies close to ideal.
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