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Simple crystal oscillator simulation in LTspice

Thread starter #1
I'm attaching an LTspice file that simulates a crystal oscillator with resonant frequency 1MHz, and it works fine. But if I change L1 from 2.5 H to 9.8 mH to simulate a crystal oscillator with resonant frequency 16MHz, it flat-lines (no oscillations). If I replace the CMOS inverter with a custom model for a chip like the 74HCU04, it works. I tried a few different MOSFET's, turning off initial operating point computation (uic), starting with the supply at zero (startup), and starting with different IC's. In every case there is no oscillation. Any tips would be greatly appreciated.
 

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JimB

Super Moderator
Most Helpful Member
#2
I cannot read your simulator file.
(I don't "do" simulators, maybe I should. I digress)

In your circuit, instead of just changing the inductance by a factor 16^2,
try changing the inductance by a factor of 16 and the frequency determining capacitors by a factor of 16.

JimB
 

crutschow

Well-Known Member
Most Helpful Member
#4
You can't use the generic MOSFET models for that high a frequency.
What's wrong with using the 74HCU04 model?
 
#5
Going from henries to millihenries is going to make an enormous difference in circuit Q, which could prevent oscillation. You'll need to find the lumped circuit parameters for an actual 16 MHz crystal, and use those. The lumped inductance value will likely still be more than a henry.
 
Thread starter #6
Thanks crutschow for the feedback. Yes, the 74HCU04 works fine in simulation and on a breadboard. The objective was to implement a simple transistor circuit that works, and to figure out what breaks down in the LTspice simulation. Thanks BobW. Good point that simple scaling may not work and more inductance may be needed.
 

MikeMl

Well-Known Member
Most Helpful Member
#10
Books have been written about how to use Spice to accurately simulate the start-up behavior of crystal oscillators.
 
Thread starter #11
Hello MikeMI, it would be great if you can provide a pointer to your favorite book on this topic. Google searches find far too many examples without analysis, specific parameter values, or a discussion of SPICE/LTspice.
 

MikeMl

Well-Known Member
Most Helpful Member
#12
What I meant to convey is that this topic is so complex, it takes a several hundred page book to explain it...

I just Googled "modelling start-up of crystal oscillator using spice" and got thousands of hits.
 
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