spice3 simulation of VCO

Basically, because your VCO model is not modeled, as such, in the AC domain. AC analysis doesn't do that sort of 'stuff'!

Charlie

Sure it does, Charlie. Just do a loop gain (.AC) analysis.

Best performed using my LoopGain technique, as described on my website ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Thank you both for your reply. I will try the loop gain analysis...

Make sure you use enough steps per octave/decade/whatever in your .AC analysis. Otherwise you'll completely sail right over the resonances of the crystal.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

I tried the .AC analysis on VCO, the output is still zero. However, if some noise is added to the VCO, then I can observe an nonzero output of the VCO in AC analysis. Does this mean that the VCO is not oscilating at all? if so, why I can observe an oscilltion in .TRAN analysis?

Thank you

Do you know how to do a .AC analysis? Maybe post your schematic to

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Isn't . AC analysis like the following?

..AC dec No. fstar fstop

I couldn't find .

Go to my website and E-mail me a schematic, PDF preferred but an LTspice listing will also work, and I'll take a look.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.