Having dealt with crystal filter procurement for over 40 years, there are lots of mechanical issues as alluded to below. For crystals in the 5 to 10 MHz range, the edges have to be beveled so the blank is convex. Cleanliness is very important since it affects the plating of the electrodes. I would say over four decades, the Q for a narrow CW bandwidth crystal filter went from around 80K to 300K. Cost was always an issue for use in a filter that an amateur radio operator would be willing to pay for.
The less the crystal has to be tweaked onto frequency the better. Solder seal isn't as good as cold welded cans since contamination is much worse when the can is soldered shut. If stringent loss requirements were paramount, sometimes the can is back filled with helium instead of nitrogen. For crystals that can be subjected to strong signals, as in radio roofing filters, intermodulation in the crystal is an important factor. A filter that in the past might have been made with $0.35 crystals out of China, is likely not consistently good enough today. The filter has a major impact on performance of a radio that has a dynamic range over 100 dB. Driving a crystal filter at even -20 dBm, let alone higher, can drive contaminates off the crystal blank for a short time. Intermodulation performance can be affected for 10s of minutes after "IMDing" a crystal. A really good crystal for a filter can cost more than $5.00 in production quantities, compared to those that cost far less than a dollar. Rob, NC0B -----Original Message----- From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Attila Kinali Sent: Sunday, June 26, 2016 3:42 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Quartz Crystal Motional Movement On Sat, 25 Jun 2016 21:30:30 -0400 Bob Camp <kb...@n1k.org> wrote: > Every paper I have ever read on the intrinsic Q of quartz makes the > claim that Q * F is a constant ( Q goes up as frequency goes down). Yes, same here. But sofar I have not seen any theoretical proof of that. But then, I have not been looking into those papers that deal with that, as it would open a big can of wurms (aka a lot of theory to learn to even understand the most basic stuff). On Sat, 25 Jun 2016 18:17:58 -0700 Hal Murray <hmur...@megapathdsl.net> wrote: > > The highest Q I remember seeing were BVA's that reached 2e6 to 3e6 @ > > 5MHz > > What determines the Q of a crystal? Is it atomic level impurities? > Crystal defects? ... This is a very good question, but unfortunately, I do not have an answer. It seems (with a big "I am not sure" disclaimer) that modern solid state physics, or rather the sub-part that deals with crystals, has good predictive power of what the piezo electric properties of a certain quartz crystal cut will be. But I do not know how accurate these predictions really are, whether they apply to any other piezo-electric material (there is a lot of research going on in this field) and what limits the predictions accuracy. >From what I have understood, the bigest contributors to Q are: * overtone number * the cut angle * the form of the crystal (aka energy trapping) and placement of the holder * the placement of the electrodes and how they are applied Crystal defects seem to be either only a minor factor, or they can be kept out of the crystal good enough that they don't matter. The only references I have seen that deal with this were those from around WWII that mention twining. Impurities of crystals do not seem to have any large effect on Q but determine long term stability and radiation hardness. > How has that changed over time? Is there a Moore's law for crystals? I am not aware of any study of the change of Q of crystal oscillators over time. Only of those that study the frequency stability. And even those do not give a lot of information what the factors and the size of influnce they have. It's mostly "we built this design and measured it for 3 years" kind of stuff. > How does the quality of crystals used for timing compare to the > crystals used for semiconductors? Biiiiig difference. Modern crystals for use in semiconductor devices are optimized for size. 2.5x2mm is today kind of a standard size for high frequency oscillators, but by far not the smallest. 32kHz tuning forks can be even smaller (at least in one dimension). These crystal are a rectangular piece of quartz that is supported on its corners and usally of constant thicknes. This means that a lot of oscillation energy is present at the holder contacts, which in turn means it's being damped, ie the Q is being lowered. For these designs, the stability of quartz crystals is so high anyways, that some loss of Q is easy to tolerate. For timing crystals, you want the crystal to be as large as possible (less influence of holder, higher power) and lens shaped (trapping the energy in the center of the crystal while keeping the holder "out of the way"). These differneces in design lead to Q factors in the range of a couple 1000 to a few 10'000 for the cheap crystals, while the timing/low noise crystals seem to be usually in the range of a few 100'000 to 1'000'000. > Are there any other economically significant uses of high quality crystals? Anywhere where you need low close in noise and high stability. Cell phone base stations are probably one of the largest markets these days. But also telecomunication equipment in general seems to have a need for stable low noise oscillators. Attila Kinali -- Malek's Law: Any simple idea will be worded in the most complicated way. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- If this email is spam, report it to https://support.onlymyemail.com/view/report_spam/ODExMjI6MTg5NTEwNTcwNjpyb2JAbmMwYi5jb206ZGVsaXZlcmVk _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
