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