50ppm crystal:  cheap.  50ppm resistor:  hella expensive.  Why? - Page 1
Forum section: electronics / beginners

author: JoeN
author_rank: Frequent Contributor
timestamp: November 10, 2014, 08:14:10 pm
content: Is this because crystals are innately high precision devices or that high precision resistors are innately low volume products or something else entirely? Resistors, by their nature, are far simpler devices than crystals and everything else being equal I would expect their manufacturing costs to be less, not more.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 10, 2014, 08:40:15 pm
content: Quote from: JoeN on November 10, 2014, 08:14:10 pmThatQuoteAnd that too.

author: daveshah
author_rank: Unknown
timestamp: November 10, 2014, 09:31:30 pm
content: There is high demand for precision frequency references in communications (almost all wireless systems have relatively tight frequency and timing requirements, and some wired systems too), but precision voltages/currents/resistances are only needed in a few niche applications (i.e. measurement devices).

author: Macbeth
author_rank: Super Contributor
timestamp: November 11, 2014, 12:06:27 am
content: What about making your own high accuracy resistor network using resistance wire? Of course it's for hand made one-off stuff and you would need a good reference, maybe an old fashioned Wheatstone bridge and galvanometer, but it's certainly do-able I think?

author: timb
author_rank: Super Contributor
timestamp: November 11, 2014, 01:53:52 am
content: High accuracy resistors are cheap and easy to make. Digi-Key can even laser trim Vishay precision resistors to any value when you order, even single quantities.Getting low drift resistors is another story. You can pay upwards of $50 for a single sub-1ppm resistor.Sent from my Smartphone

author: ve7xen
author_rank: Super Contributor
timestamp: November 11, 2014, 02:32:51 am
content: Fundamentally, it seems that time is by several orders of magnitude the easiest property to measure. I don't think that the reasons for that are well understood, and may be caught up in the arrow of time problem.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 11, 2014, 03:27:46 am
content: Not so mysterious: time can be divided into discrete units, and therefore measured quite easily.Voltage can, too: using quantum Josephson junctions, voltage can be quantized into nV steps; or more accurately, converted in a direct fashion into GHz frequencies.Current, at least in a sufficiently small (quantum scale) system, is quantized, and easily measured as such; in a small superconducting loop, the current flow / magnetization (same thing, since they're directly proportional) is quantized because only discrete pairs of electrons can be flowing around the loop (or put another way, the ensemble wave functions have to makeAww hell... that might be an awesome analogy...So, physicists love wave mechanics and transmission lines. If a system can be expressed in that way, it's very powerful, because a huge toolkit becomes available. It's not always a very clean system ("deep" gravity waves are dispersive and nonlinear, for instance), but it's something.So, normally in a metallic crystal, you have electrons zooming through the lattice. Occasionally they bump off atoms or impurities, and this is more likely to occur at higher temperature (the atoms are bumping around more). If you imagine an electron not as a particle but a sort of presence, a pinching of the local wave function, then you get the feeling that, as an electron propagates, it has a sort of wake, which pulls on atoms (electron-phonon interaction), and occasionally gets scattered by their thermal motion, or other abberations (like impurities). Which is all consistent with metallic conduction. But normally, the velocities of those waves are all over the place, thermally scattered.Suppose the lattice were shaped in such a way that it had one resonant velocity, plus or minus: well then, you'd have a transmission line, which permits waves at only one velocity, and you'd get propagation and reflection (from mismatch) and quantization of waves (a finite sized crystal is a resonator of some sort). You might still have losses, but perhaps that could be reduced arbitrarily or through some other mechanism. As it turns out, Cooper pairs are just the phenomenon needed: it seems the spins of electrons traveling in coherent pairs cancel out in suitable materials, at suitable temperatures, that they don't disturb the lattice but skate right on through instead.Aaaanyway, I digress... Suffice it to say, there are a number of phenomena that can be measured with discrete accuracy (i.e., as exact integer numbers rather than real numbers with inevitable error), but that are very difficult to do in most situations due to uncontrolled circumstances (most notably temperature, which scrambles readings so that even relatively moderate (~10^9), let alone huge (~10^23) integers apparently get scrambled into "real number plus noise" figures.Tim

author: coppice
author_rank: Super Contributor
timestamp: November 11, 2014, 04:01:44 am
content: Do you mean 50ppm/C or 50ppm error (i.e. a 0.005%) resistor? People seem to be replying to both interpretations.I think the main reason is probably that few people care. If there were a mass market for 0.005% resistors, with a temperature coefficient good enough to maintain that accuracy over a reasonable temperature range, maybe cheap parts would be developed. As it is, 0.1% resistors are high volume and cheap. Anything with a tighter tolerance rapidly becomes a niche product and that's the greatest enemy of cheap.

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 11, 2014, 06:07:42 am
content: Quote from: ve7xen on November 11, 2014, 02:32:51 amIt is certainly the easiest type of measurement to distribute from standard sources to labs around the world. We used to transfer time signals via terrestrial radio; that was pretty accurate, but still suffered a bit due to path length changes with changes in the ionosphere. The GPS system allows much higher accuracy. With GPS, a very stable oscillator can be continuously monitored and calibrated. GPS disciplined oscillators are relatively cheap, and offer measurements directly traceable to standards. You can't distribute standard kilograms or ohms to thousands of labs around the world with that kind of precision nearly as easily or inexpensively.That doesn't directly answer the original question about why free-running crystal oscillators are able to cheaply achieve higher accuracy than similarly priced resistors, but it does illustrate that ultra high precision time references are easily distributed worldwide by radio waves via satellite. We have no similar mechanism to distribute standard high precision resistance measurements.