Capacitance value for PIC crystal

I once got 'bit' by cheap suppliers that produced poorly ground crystals. talk about spurious oscillation frequencies! more than 1

I liked simulation to show what could happen during Production with full ranges of tolerances, and then redesign to be able to absorb anything the vendors sent, well almost.

Always asked the Engineers, SAY what you're going to do, DO what you said you'd do, and then PROVE you did it. THEN designs woud smoothly transition into Production.

found them!

these models are based upon lower frequency crystal models included in the MicroSim's Library for Crystals representing AT cut

with the inforrmative header:

• Release date: September 1987

• The parameters in this model library were derived from:

• Quartz-crystal timing accuracy

• Electonic Design 2, January 19, 1976; pp. 74-79

• and:

• Electronics Designers' Handbook, Second Edition

• edited by L.J. Giacoletto

• McGraw-Hill Book Co., 1977, pp. 16-16 to 16-31

• Example use: X3 3 4 QZ32768

• Notes:

• 1) Temperature variation of resonant frequency for properly cut

• crystals is quadratic. The coefficient depends on the cut used

• and is implemented in these subcircuits by giving the equivalent

• inductor a temperature coefficient. The AT cut is an exception,

• and has a cubic temperature dependence which is not included in

• these models.

• 2) The values of lqz, cs, and cp do not vary much from one crystal

• to the next. The value of rqz, however, can vary as much as a

• factor of 2 up or down. So the crystal's Q can vary over a

• range of 4 to 1.

• 3) Parallel resonant crystals can be tuned slightly by attaching an

• external capacitor in parallel with the crystal. The crystal's

• frequency is built assuming a value of the external capacitor,

• called the "calibration capacitance". Its value is noted in

• the comment at the head of each parallel resonant crystal model. and includes this one:

• 10Mhz frequency standard, AT cut, parallel resonant, Q=25000

• calibration capacitance = 50pf .subckt QZP10MEG 1 2

• lqz 1 11 2.54647909e-003 cs 11 12 9.96041181e-014 rqz 12 2 6.4 cp 1 2 2.49010295e-011 .ends

the following models I created based upon some samples sent to a friend of mine:

• 20MHz Ethernet crystal for parallel operation

• #1 Crystal

• 20Mhz frequency standard, unknown cut, parallel resonant, Q =??

• calibration capacitance = ?? 50pf

• .SUBCKT QZP20A 1 2

• LQZ 1 11 3.33181E-03 CS 11 12 19.0069E-15 RQZ 12 2 6.45805 CP 1 2 4.65668E-12 .ENDS

• #2 Crystal

• 20Mhz frequency standard, unknown cut, parallel resonant, Q =??

• calibration capacitance = ?? 50pf

• .SUBCKT QZP20B 1 2

• LQZ 1 11 3.26998E-03 CS 11 12 19.3664E-15 RQZ 12 2 7.38786 CP 1 2 4.59108E-12 .ENDS