Hi Rfcooltools,

Code:

As I moved the center frequency of the open loop vco closer to the PLL controlled VCO I observed  the the phase noise of the open loop VCO started to improve as it got near the PLL driven VCO.   Just to put it into perspective the two VCO's were physically a millimeter apart.  And yet I was observing this phenomena.  So I I thought it may have something to do with the voltage source.  Then something even more interesting happened.  I set the open loop VCO above the PLL locked VCO (both have a positive KV) and then I disconnected the voltage source from the open loop VCO while I left the PLL controlled VCO locked and running.  

Your experiment is very interesting and i am very much intrigued by your observed results.

1. the node where your VCO's control voltage line was connected to the external voltage source had a large capacitance ?

2. were there any circuitry between your PLL and the VCO (in the intervening 1mm spacing) or was it all just blank silicon except may be metal fill layers for passing DRC density checks.

From your description, it sounds as though that you are ascribing the slow change and the pause in the oscillation frequency of the VCO to coupling between inductors - is that correct?

Most inductors today have a patterned ground shield at the bottom and also a shield on the sidewalls so much so that these layers effectively block all flux lines and very little leakage results. I would have thought that the coupling factor between two inductors would have been very small as a result - would be very interested to know what sort of coupling factors result if you have done any calculations on this

Louis,

What you are asking is difficult to get accurate without a em simulation tool since coils have their own tightly coupled mutual inductance.  As a result the coupling to a second coil will be different than two separate parallel conductors as you correctly stated.  An yes RLCK will not give the coupling between inductors if the inductors are a parameterized cell (pcell).   This is due to the way the LVS sees the inductor as an RLCK ignore region usually defined by an inductor recognition layer defined by the fab in the LVS deck.  

Without an em tool you could do the following:

1. if you have rlck then  flatten both inductors in the layout and remove the inductor recognition layer and place a metal resistor before the pin to keep the LVS from seeing the inductor as a short (without the inductor recognition layer LVS will see the inductor as one piece of metal shorting out two or more pins and thus you will have to trick it into being LVS clean.) .  After all this RLCK will be inaccurate in predicting the inductor values due to limitations in how it approximates coupling, but might give you insight into what the coupling between two inductors might be.  
2. the reason I gave a lengthy discussion on coupled tanks was to explain that coupled resonators at the same frequency of resonance need only week coupling to transfer power.  Why is this important?  because if the resonant frequency of the two tanks are sufficiently far apart then it doesn't matter as much as to what the coupling is between.  Check this by biasing up the entire circuit and placing an AC current through the first of the two inductors.  Where does the AC voltage peak for that inductor?  Remove the AC current source and do the same for the second where does it peak. Then try to get a bounds on what mutual inductance will give you perceptible  performance degradation.  Off frequency resonances will need much stronger coupling to produce undesired results.  

Try the experiments above and report the numbers or wave results and we can see if they can be interpreted further.   Also include Q vs frequency of each inductor as well as inductance vs frequency of each inductor.  and dimensions and spacing of each inductor.  

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