Louis,
Exactly you cannot span a distance with a conductor without incurring inductance. Therefore conductor be it a metal trace on chip, PCB trace , or bondwire will have self inductance. Two conductors will have both self and mutual inductance.
Designing circuits at higher frequencies requires a comfortable understanding of electromagnetic.
Rfmagic, I have an interesting experience with your 25uM rule of thumb on inductive coupling.
Several years ago I was introduced by a colleague the notion of a coupled tank filter.
But before I go into this and observation that I witnessed a few years prior:
I was to design an IC with a few PLLs and LC tank VCO's which could operate simultaneously at the same frequency. to experiment I had an existing IC modified so that the two vcos could both be operating at the same frequency. One of the VCO's was controlled by a PLL the other was free running and controlled by an external voltage source. My goal was to characterize the spurs associated with closely tuned resonant circuits. when I observed the first intersting artifact associated with what is called coupled tanks dtermine if the distance (1mm in this case) was sufficient enough to be safe for this application. while measuring spurs I was looking at phase noise of the PLL driven VCO and then for some reason I switched the input of the spectrum analyzer to the voltage controlled VCO. 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. As the charge leaked out of the open loop VCO control line the VCO center frequency started dropping steadily until it was near the Locked VCO frequency until its frequency suddenly snapped to the locked PLL frequency, for a short time and during this time the the open loop VCO phase noise improved while the PLL locked VCO phase noise degraded around 3dB. It remained locked on the same frequency as the PLL locked VCO frequency for several seconds until it finally got released and continued the reduction in frequency trajectory that it was on before being captured by the PLL locked VCO.
coupled tanks....
if two tank circuits are tuned to the same frequency and have a high enough Q then the coupling between them can be sufficiently coupled with even the weakest k factor. in fact if the the Q is around 100 or more and the tanks are identical then a coupling factor near 0.01 will be enough to transmit power at almost no loss. If you have access to a simulator such as cadence or ADS try it out for yourself. Take a port connected to a LC tank withe high Q then replicate this circuit and change only the port number then see what the coupling factor is needed to sufficiently couple the power delivered from one tank to the other, you will be surprised at the result. You will likely see a butterworth filter delivering perfect power with suprisingly little coupling.
its real..
This is not a novel concept, ( maybe to you or I ) but if you crack open a satellite tv LNB case and take a look at the PCB board in it traces are not physically connected instead there are tiny resonant circuits that make up the spaced apart the act as filters. These structures are alive and well in the world.
so my point is this... understand the nature of the medium your design in and rules of thumb are either irrelevant or un-encompassing.
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