[time-nuts] questions on uncompensated crystal

[drfrank.stellmach at freenet.de]

   Hi Stan,

   Your question is really quite interesting, as high precision by high
   cost is easy to achieve, but the real art is to do it by low cost. In
   this sense, I would like to give you some really practical hints.
   The company's division I'm working for, produces automotive
   electronics, i.e. instrument clusters with a "precise" clock, i.e. < 1
   min /month deviation over a temperature range of -40 to +85°C, giving
   +/-50ppm overall. We have production volumes of 100k-300k/yr. per
   application, or several 10Million applications for all of our
   customers. High and longtermed reliability / quality in production and
   field and ridiculously low costs are mandatory. We cannot use external
   oscillator circuitry, hardware trimming, TCXO or temperature
   compensating methods for those reasons.

   We use embedded controllers; the oscillator consists of an internal
   CMOS inverter, 2 ceramic caps 5-10% and an AT-cut, fundamental crystal
   with clock trimming by software correction. SMD components are
   mandatory, also for cost reason, so I recommend an AT51-GW (gullwing)
   type crystal, which is an AT51 / CS10 / HC49 (supplier dependant) case
   with bended leads and an additional metal clamp for mounting.  Other
   cases with plastic plate underneath are more expensive.

   Price for the crystal should be less than 20 Cents, depending on
   further requirements.

   I highly recommend NDK ( [1]www.ndk.com), they produce the quartz
   blanks on their own, and also design nice OCXO's , so they have a lot
   of technological experience.
   Vectron (formerly Corning, formerly Telequarz) or Kinseki as 2nd
   sources, also have automotive experience.
   Ask for tighter automotive specs, but lower price, as ours depend also
   on our automotive quality requirements (e.g. AEC Q200 and more plus
   tough warranty regulations).

   27MHz can be realized by fundamental crystal, but it's on edge of
   technology because the crystal is pretty thin already. Therefore, a
   tighter specification is critical and costly, so use PLL design with
   9MHz or less instead!

   We often have discussions between the (digitally minded) designers of
   the inverter and the crystal supplier: For us, an additional critical
   aspect for series production are oscillation start margin (which is
   very crudely checked by our hardware designers) and ESR dips over
   temperature. Crystal quality (e.g. residual silver particules from
   electrodes) vs. input/output impedance etc. of inverter are to be
   observed.

   Due to cost and practical reasons, we also do not deal around with
   simulations or excessive measurements. I also confirm
   , that SPICE simulations etc. are not applicable here.

   Instead, we use cost free service/knowledge of suppliers. We send a
   sample board to NDK, they measure the circuitry and propose values of
   C1, C2 and the nominal center frequency of the crystal they will
   deliver. Perhaps they know your type of inverter already, as they have
   a huge database. Takes about 4-6 weeks.

   It's not possible to choose other parameters freely anyhow, in
   contradiction what you assumed, as nominal frequency, case / crystal
   blanket size and oscillator determine the overall parameters!

   Typical specification for a 16MHz crystal over automotive temp. range
   of -40..+85...+125°C are:

   +/- 20...50ppm freq. deviation at room temperature, 50µW
   additionally 20..50ppm over temperature (20 ppm will double the price
   compared to 50ppm)
   50µW-1mW max. power
   5ppm/yr. aging
   ESR and Dips < 80 Ohm over temp.

   I think that you have to do some qualifications of your product on
   your own to safeguard your requirements.
   Your initial precision and also the long term aging requirements are
   hardly to be achieved by design or specification only.
   On the other hand, it depends on your or your purchasings' negotiation
   skills, what your supplier is willing to specify, as you for sure need
   tighter specs.

   Best regards
   Dr. Frank Stellmach, Germany

References

   1. http://www.ndk.com/