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  1. #1
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    Simulating Cross Coupled VCO

    As part of an assignment for my RF course I've been asked to pick an element of a specific RF circuit from a research paper given to us and do some simple simulations of that device.
    The part I chose was the Voltage Controller Oscillator from a PLL. Our task is pretty vague, but essentially we must simply create a circuit in PSPICE/Cadence etc and simulate it to show functionality (in my case that it oscillates!).

    I've been trawling through the interwebz researching the various types of VCO out there. Most info just shows simplified schematics and very few hard figures/details.

    Attached is a screenshot of my PSPICE schematic for a cross coupled, LC tank, VCO (although in this case to keep it simple I just put a fixed cap in rather than a varactor). The problem is that after running transient analysis I only see a 12V DC voltage at the OUTP node. I'm sure I'm doing something very simple wrong, but if anybody had some pointers for getting myself an oscillation I'd be very happy.

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  2. #2
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    Re: Simulating Cross Coupled VCO

    One thing of importance is whether the drain tank inductors are truely separate and isolated as shown in PSPICE diagram or a single tapped inductor having cross coupling.



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  3. #3
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    Re: Simulating Cross Coupled VCO

    Does it oscillate ?? To check this, you should verify the negatif impedance which is seen toward active part of your VCO.
    In fact, there are few methods(open loop gain,oscillator driving point impedance and phase etc) to see the that the oscillation conditions are satisfied or not but first step is to check negative impedance.



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