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On Fri, May 2, 2025 at 09:02 PM, Cheng Fei Phung wrote:
I see you added a simple RLC series resonant circuit to represent a crystal.? Do you know that it is an appropriate model?? Does this oscillator circuit depend on series resonance, or parallel?? (I don't know; I am just asking.)? Are the electrical characteristics OK to model the motional parameters of the crystal? ?
Glancing at the IEEE article, it looks like they used a much more complex electrical model for the crystal, with not one or two but three series resonant RLC circuits in parallel with each other, plus three additional capacitors.? Your equivalent circuit is much simpler.? Is the simpler model adequate?
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The .tran statement on your schematic is ".tran 0 10 0 100u startup", so it calls for a Maximum Timestep of 100 us.? With a timestep of 100 us, it can simulate signals with frequencies up to about 5 kHz.? But your crystal model's series resonance is around 1.1 MHz.? To simulate a circuit oscillating at 1.1 MHz requires a timiestep that is smaller than 500 ns.? There is a chance that you made the Maximum Timestep so large that LTspice is incapable of seeing any sort of oscillation around 1 MHz.
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This is where simulations can become very difficult.? With a timestep smaller than 500 ns, it can take a very long time to simulate 10 seconds or 1000 seconds.
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Your N channel transistors are still upside-down, and the Bulk pins of many of the transistors are likely connected wrongly.? Also you have zero values for PS, PD, AS, and AD, which is technically incorrect, but I don't know how much difference it makes.? As you probably know already, I1 should not be there, but LTspice removes it.
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I suspect that your DC voltages are not right.? The negative supply voltage for the first three stages (node V1) is at -1.5 V which might be right.? But the regulated voltage for the output amplifier (node VN) is +1.5 V.? Is that right?? I think it powers the output amplifier with the wrong polarity.? Are you powering that section correctly?
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There is no DC path from node Q1 to ground, which extends to nodes N004 and N005.? LTspice tells you that the nodes float, and it "corrects" for it by adding GFLOAT to one of the nodes.? I think this won't affect the performance of this circuit, but I'm not sure.? GFLOAT will lower the Q of the resonant circuit a little.
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Note I have not read the IEEE article.
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Andy
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