Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

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

Status
Not open for further replies.

ZengLei

Full Member level 1
Joined
Jan 24, 2006
Messages
99
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Location
WuHan China PR
Activity points
2,143
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!
 

ZengLei

Full Member level 1
Joined
Jan 24, 2006
Messages
99
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Location
WuHan China PR
Activity points
2,143
.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!
 

RDRyan

Full Member level 2
Joined
May 24, 2007
Messages
133
Helped
20
Reputation
40
Reaction score
3
Trophy points
1,298
Location
China
Activity points
1,976
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
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top