Hi Guys,
I am trying to make a Pierce Oscillator and was going through the Paper by Eric Vittoz " High-Performance Crystal Oscillator Circuits: Theory and Application ".
In the Paper he was Plotting the Locus of Complex Plane Impedance Zc( by varying the Gm from 0 to +infinity) and -Zm the motional branch of the crystal oscillator and where the locus intersect he mentioned that corresponds to a particular gm namely gm,crit (at point A) at which our net series resistance goes to zero. so point above that gm and gm till Point B gives us the net negative real part as a result oscillator begin to start up and we will see the Oscillations.
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So was trying to see that behavior for my Design . I have chosen a crystal oscillator at around 80MHz with Motional Parameters as Rm=86.47 Lm=27.87mH Cm=142aF Co=3.5pF Q=162k.
So I have also Plotted the complex Plane Impedance using MATLAB for different values of C1 and C2.
Like here if C1 and C2 =1pf No matter whatever the gm you may take no oscillations would occur.
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Similarly for C1=C2=10pf we can see that Locus crosses the -zm(p) so here we can have the oscillations .
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So the Locus which I have made is for the Lossless case when Z1, Z2, and Z3 are lossless only Capacitive in nature . ( I know in my design there would be some losses because of ro of Mosfets and some parasitic capacitance) so for that I am taking sufficient margin i.e not operating close to 6.3mA/V My net Gm=gmn+gmp is around 26mA/V. But Still I am not able to see any oscillations here. Here I have used CMOS inverter as linear amplifier by using a feedback resistor of 1Mega Ohms. Both NMOS and PMOS are in region 2(Saturation) but still no luck.
https://iiitaphyd-my.sharepoint.com/:t:/g/personal/ashish_papreja_research_iiit_ac_in/Ef-Sujakt-lIuOGl5qbydRQBTB34X1l4wdC4ESDjHrIJDw?e=9gmBc1
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I also tried to see what is the negative Real part I will see from the Simulator if I remove the Zm branch from it and placed isin of 1A( AC magnitude)and plotted the Voltage across it the real part comes very low around -20 ohms at 80MHz frequency. Then just for sake of checking, I reduced the Rm to 10 ohms in my earlier testbench shown in fig just above. to see if I get any oscillations now. Then also There were no oscillations.
Testbench for Seeing the Negative real part of impedance by removing the Zm branch and placing an Isin of ac mag 1A and seeing Real(V) .
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Can you guys help me with what am I doing wrong here?
Thanks in advance.
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