# Why does a foster seeley circuit need transformer coupling?

Having read most of the online references to this circuit and performed some simulations, I still don't have a firm grasp on how this circuit functions.

Here's a schematic to refresh (possibly painful) memories:

and here is one of the better intuitive explanations (in my opinion) of how the circuit works.

If you simulate this circuit and make a tight coupling between the primary and secondary of the "transformer", the circuit will not work as expected. If the coupling is tight, the top of the secondary will be in-phase with the primary, as you would expect. But, as the explanation above describes it, you want the secondary windings acting like inductors so that there is a 90 degree phase shift at the top and bottom of the secondary with respect to the centre tap (at the resonant frequency).

So, if my understanding is correct and the secondary winding halves are acting like inductors to the current arriving from the top of the primary tank (via C at RF) why does this circuit need transformer coupling at all?

Anybody care to confirm or reject my speculation in the comment section? Anybody - sob!

• Signal inversion? Sep 1, 2017 at 3:32
• My feeling is that this has to do with coupled inductors and resonant induction. I think the two tanks need to be magnetically coupled enough to produce resonant ringing in the secondary, but not enough to overpower the RF current arriving to the centre tap. The resonance in the secondary ensures no phase shift occurs to the arriving CT primary signal - but, you do get the 90 degree shift at inductor ends. Am I on the right track?? Sep 1, 2017 at 6:43
• I think I understand it now. With loose coupling the secondary tank is free to act more inductively and can develop a phase shift above and below resonance. That is, currents in the secondary can be phase shifted relative to voltages in the primary. This is in contrast to a typical transformer with tight coupling where that phase shift wouldn't occur (I'm speculating!). The primary voltage arriving at the centre tap then acts as a kind of reference against which the phase shifted voltages top and bottom are compared. Sep 1, 2017 at 9:04