rtc crystal open-loop simulation

When I simulate real-time clock crystal's open-loop with ac, there are two cases as attachment:
1)The BLUE result, it uses a ~15meg resisitor as the feedback for setting inverter's operating point. This result is what I needed.
2)The PINK result. it uses several switches in series(a switch is a nmos and a pmos in parallel) as the feedback for setting inverter's operating point and this switch is about 15meg, too. The result is not reasonable.

Why? I only find case 2) when temperature is 120 degree. The other situations such as SS/0, SS/60, FF/0, FF/60, TT/0 and TT/60 are all case 1).

Thanks!
Trashbox

Dear Sir,

Experiment with blue curve is more similar to our real life experience.
Here we assume your RTC want to work at low power region (likely < 10uA)

Meanwhile, resistance of mos switch seems over estimated.
mos switch of 15MEG is rarely found.
We need to know more detail about how you come up with MOS resistance estimation.

jcpu said:

Dear Sir,
Experiment with blue curve is more similar to our real life experience.
Here we assume your RTC want to work at low power region (likely < 10uA)

Click to expand...

Yes.The spec. is RTC power <3uA

Meanwhile, resistance of mos switch seems over estimated.
mos switch of 15MEG is rarely found.

We need to know more detail about how you come up with MOS resistance estimation.

Click to expand...

Suppose the oscillation waveform peak-peak value is 3v, the current through this feedback resistor is about 3V/15e6 ohm=0.2uA

I simulated it as following:
VDD=3.0v
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MOS in series
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VSS=0v
and probe the current through MOS so that the resistance is 3.0v/Current.

Thanks!

Added after 3 hours 47 minutes:

Hi jcpu!
I've solve this issue. It is because leakage current which lead to inverter's operationt point voltage shift.
Thanks for your comments.

jcpu said:

Dear Sir,

Experiment with blue curve is more similar to our real life experience.
Here we assume your RTC want to work at low power region (likely < 10uA)

Meanwhile, resistance of mos switch seems over estimated.
mos switch of 15MEG is rarely found.
We need to know more detail about how you come up with MOS resistance estimation.

Click to expand...