I can see the time domain response but i want to see the output in the frequency domain
the circiut image and hspice program are shown bellow.
did i use the red line correctly to see the frequency domain output of oscillator?
**ring oscillator using differential stage**
.lib 'C:\synopsys\Hspice_G-2012.06-SP1\ADDED LIB by me\mos.35u\logs355v.l' TT
That's not a reasonable way to use AC analysis for oscillator circuit analysis. You don't see an output signal in AC analysis because there's no input signal.
As FvM mentioned you can see the oscillations in AC analysis. Try applying Barkhausen stability criterion to the circuit.
So, perform a setup by which you get loop gain and the phase information and you can tell whether it's oscillating or not.
P.S,
For the ckt, I see the differential ring oscillator has 3 stages. Double check the theory of diff. ring oscillators because odd no. of stages won't oscillate I guess (not sure though).
I think, odd number of inverters is just O.K. Negative DC gain is necessary to prevent latching into a non-oscillating state. Circuit phase shift will allow to fulfill the oscillation condiction if the gain is suffcicient at the same time.
thanks, FvM and BMR.
odd number of differential stage will oscillate and as i have said before the circuit is oscillating and i can see the time domain output. my problem is that i want to see the variation in oscillating frequency with temperature, so i should plot a graph in which the vertical axis is frequency and the horizontal axis is temperature, that is my main goal.
You can't derive the exact oscillation frequency from AC analysis for > 1 gain margin because it depends on large signal behaviour. The best way would be an autmated frequency measurement in a set of parameterized transient analyses.
i read your answer and thanks for your answers but unfortunately i don't understand them ! can you tell me what should i exactly to do ? maybe you means i must take the simulation result into matlab? i thought maybe i can use an transient analysis for each temperature and then i take the transient result into the matlab and then i use FFT to see the frequency of oscillator in each temperature.