Hi Drop in frequency = mass increase. Mass is moving on to the crystal as time goes on.
Since the package (hopefully) is sealed, there are a fairly limited number of ways to do that. 1) The case of the crystal was "more dirty" than the blank when things were sealed, equilibrium is to move to the blank. 2) The part is a welded seal (resistance weld) and they got a bit more energy in the weld than they needed. The spray from the weld is the mass headed to the crystal. 3) They didn't quite get them sealed and they are leaking. That's only going to work if they were vacuum sealed to start with (not likely). 4) The fill gas (assuming no vacuum) is reacting with the plating on the blank (also not likely). 5) The crystals may have a teflon anti-shock sleeve in them. Teflon is good stuff, but it does outgas. Net would be frequency down. Stepping back a bit, let's look at what they are doing. At 850 MHz 3000 Hz is just about 3.5 ppm. Your 1300 Hz shift is about 1.5 ppm. If the crystals are 10 years old, they are running at an aging rate of 0.15 to 0.35 ppm / year. That's pretty good for a radio crystal that likely was specified to be at 0.5 ppm per year. What ever is happening to your crystals, it's very much in the "expected to happen" category. Of course if it's 3,000 Hz past the end of the adjustment range - that's a lot more ppm per year. Back in the days I designed that sort of stuff adjustment range was spec'd as greater than +/- 15 ppm. That's a lot of range. Bob On Feb 14, 2011, at 10:47 PM, Shawn Tayler wrote: > Very interesting Bob Thanks. > > It brings to mind an annoying issue I run into from time to time. > > VCXO 12.8 Mhz used as a reference in communications gear. Most of the gear > is roughly 10 years old and of similar make, both mobile and portable styles. > They all are dropping in operating frequency. Many as far as 3000 hz at 850 > MHz but it does vary with most in the 1300 hz area. The trim adjustment is > made through a programming interface and in some cases we are running out of > adjustment. > > My question is what is the process in the aging if the crystals that causes > this drop in frequency? I understand it may be mechanical but the level if > vibration the radios are exposed to varies greatly and there does not seem to > be a correlation with the frequency shift we are seeing. > > Just thought I start a discussion. I thinks still soy of on topic? > > Shawn > > Sent from my iPhone > > On Feb 14, 2011, at 5:02, Bob Camp <[email protected]> wrote: > >> Hi >> >>> From a number of radios I've torn crystals out of and looked at: >> >> 1) The blank is broken / cracked. Most likely the radio got dropped >> somewhere along the line. Just like that stone hit on your windshield (on >> two cars at the moment), the crack can start small and propagate. Time from >> impact to failure might be years. >> >> 2) Cement fractures. Except in pressure mounts, the blank is held in the >> holder (clips) with cement. Normally this both holds the part and makes >> electrical connection. Some epoxies shrink with time eventually they shrink >> enough to crack. Big enough crack at the wrong place = no electrical contact. >> >> 3) Plating adhesion. Metal is deposited on the blank using a thin film >> process. If everything is nice and clean, and it's the right metal it sticks >> very nicely. If it's the wrong metal or stuff is dirty - not so much. Add to >> that (possibly high) drive and the metal comes off. Loose enough metal and >> it stops working. >> >> 4) Air in the package. On high performance / low frequency crystals (think 5 >> MHz 3rd), the parts run in vacuum. Let in air and the resistance goes up a >> lot. High resistance = no works. >> >> 5) Broken mounts. If you put enough vibration on a crystal for long enough, >> at the right frequency, you break the mounts. Often this is right where the >> mount hits the post coming up from the base. Happens mostly in mobile gear. >> You can do it with random vibe, but much easier to do with a sine at the >> right frequency. >> >> 6) Contamination. Put a fine layer of crud on the surface of a crystal and >> you can stop it dead. It acts as a mechanical damper, especially at low >> drive levels (like startup). >> >> --------------- >> >> Stuff that you see in papers: >> >> 7) Micro fractures. The machining process that makes the blank bashes away >> at the blank on a microscopic level. Stone to windshield on a micro scale. >> >> That's no where near a complete list, but it's a start. I have seen no >> convincing evidence of a wear out mechanism in a quartz resonator. Quartz is >> pretty sturdy stuff. There are lots of examples of parts older than I am >> still out there running. They do age on and on, they don't seem to die >> except when they are deliberately killed. >> >> Bob >> >> On Feb 13, 2011, at 10:26 PM, Bill Hawkins wrote: >> >>> Group, >>> >>> Jim Garland on the [email protected] list asked about crystals: >>> "A 22.5MHz crystal (HC-5 case) in my homebrew receiver, built about forty >>> years ago, no longer oscillates. It seems to be purely an age-related >>> problem. >>> It is in a standard solid state circuit which bandswitches six crystals, and >>> the other five work just fine. I wonder what causes a crystal to stop >>> working, and whether it is possible to repair them? I've "repaired" dead >>> 100kHz calibrator crystals, and hamband crystals in FT-243 cases, by >>> cleaning off the brass pressure plates, but am not sure if one can do this >>> on thin high crystals. As I recall, the metal electrodes are evaporated onto >>> the sides of the element. 73, Jim W8ZR" >>> >>> One of the replies was: >>> "Broken families, drugs, drink... the normal, I suppose. John K5MO" >>> >>> Scott Robinson asked: "Receiver crystals aren't getting beaten up by high >>> power, >>> but something has killed a lot of them in my R-390A and Drake R-4A. >>> Curiously yours, Scott" >>> >>> And Roy Morgan asked: >>> "I have a 1960's frequency standard from a Nike site: the Sulzer Oscillator >>> and would like to find tech into on it." >>> >>> Any help appreciated. >>> >>> Bill Hawkins >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
