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Thank.
I am designing a simple crystal oscillator with a simple invertor with resistor feedback.
I would like to know the setup for hspice simulation of the negative resistance.
I try to simulate it by bias the 2 terminals with proper dc and the simulate it by ac analysis, but the result seem not to be correct.
How can I know my circuit negative resistance by simulation?
i have not done that, but razavi's book "CMOS analog design" has mentioned that a paper may have talked about negative resistance at the end of Chapter "frequency characterics of amp" you may have a look.
1> add all parasitic loading on X1,X2, including parallel capacitor in crystal model(without L,Rs,Cs);
2> Hspice:
add ac between X1,X2
.ac
.net x1,x2
zin(r) zin(i)
3> |zin(r)| should bigger than Rs
I guess butterfish tell you to suppress your crystal into your netlist (of course you don't have it in your schematic)
Between the nodes called X1 and X2 (where you will put your crystal) and keeping your external capacitors, you put an AC voltage equal to 1 on your schematic and your impedance is equal to 1/i where i is the current flowing into the AC voltage source.
To butterfish
I don't understand how this simulation is accurate because your don't explain how we must to set the circuit on DC.
The Operating point defines the transitors propreties
.net defined the two ports for impendence ananlyze.
MLR67 is right, you can use Vac/Iac replace this comand.
but I means only add crystal's Cp into simulation netlist, not all of the crystal model.
Hi MLR67,
X1,X2 have its own dc opration point, isn't it?
the ac source can be fed between X1,X2 though a 1F capacitor.
I had tried to simulate neg resistance by adding a sin current source with our target freq. and working current(without the crystal model)
it is found to be around 250ohm
But I think the simulated resistance is not correct.
as when I try to increase the ESR of the crystal model to 200ohm,
crystal oscilator is not osc. :O
I agree with you , off course theChrystal needs to be remove from the netlist for the negative resistance measurement.
Concerning the DC operating point , because we are in positive feedback (oscillator and by the way negative impedance ) I don't understand how we can get a stable DC operating point and not a random one.
As you can see David get some discrepancies.
For 32K oscillator, the negtive R should be about -??KOhm, the ESR of crystal is about 30~100KOhm.
I'm not sure where is the problem because I can't see your simulation circuit.
Hi, MLR67,
I'm not sure about what's your mean of DC point.
The DC point I mean is dc voltage of X1,X2 when there is no ac signal in.
If the Amplifier is a simple inverter with a feedback resister,
the DC point will be about: V(X1)=V(X2)=VDD/2 , do you agree?
I had tried to simulate neg resistance by adding a sin current source with our target freq. and working current(without the crystal model)
it is found to be around 250ohm
But I think the simulated resistance is not correct.
as when I try to increase the ESR of the crystal model to 200ohm,
crystal oscilator is not osc. :O
According to your model, we have to set the frequency and current for the AC sine current source.
I don't understand what's the working current?
Can u explain?
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