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# Why we should add 2 additional C to help Crystal oscillate??

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#### ZengLei

##### Full Member level 1
qzp100k vs qzs100k

Hi Looking at the circuit

Why we usually add another two addtional capacitors connect to the two terminals
of a crystal ? And how we choose the value of the two capcacitors ?We also could see in the pic that there is a very large resistor(1 mega Ohm)in parallel with the crystal,what's it used for?

I checked the model of crystal,i found something very interesting as below:

* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
* 100khz frequency standard, DT cut, parallel resonant, Q=20000
* calibration capacitance = 5pf
.subckt QZP100K 1 2
*
lqz 1 11 lmod 588.873289
.model lmod ind(tc2=3.5e-8)
cs 11 12 4.30423941e-015
rqz 12 2 18.5k
cp 1 2 1.72169576e-012
.ends

* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
* 100khz frequency standard, DT cut, series resonant, Q=20000
.subckt QZS100K 1 2
*
lqz 1 11 lmod 588.873289
.model lmod ind(tc2=3.5e-8)
cs 11 12 4.30148495e-015
rqz 12 2 18.5k
cp 1 2 1.72059398e-012
.ends

o,both the frequency of the two crystal is 100k,but what's the differences?I got confused!

.subckt crystal

I checked the model of crystal,i found something very interesting as below:

* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
* 100khz frequency standard, DT cut, parallel resonant, Q=20000
* calibration capacitance = 5pf
.subckt QZP100K 1 2
*
lqz 1 11 lmod 588.873289
.model lmod ind(tc2=3.5e-
cs 11 12 4.30423941e-015
rqz 12 2 18.5k
cp 1 2 1.72169576e-012
.ends

* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
* 100khz frequency standard, DT cut, series resonant, Q=20000
.subckt QZS100K 1 2
*
lqz 1 11 lmod 588.873289
.model lmod ind(tc2=3.5e-
cs 11 12 4.30148495e-015
rqz 12 2 18.5k
cp 1 2 1.72059398e-012
.ends

o,both the frequency of the two crystal is 100k,and the symbol are also same,but what's the differences?I got confused!

crystal lqz

I think one of the capacitor are used for phase shift. the very large resistor in parallel is used to set the bias of the inverter, and keeps it in amplifer condition.
The circuit of QZP100K and QZS100K maybe it's the same. From the subcircuit, we can see the node and the parameter value are nearly the same.

Ryan

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