IF transformer coupling factor...

Hi,

I would like to know why coupling factor for IF transformer affects the
centre frequency. I hv a circuit in PSpice with two IF transformers...
and when I reduce the coupling factor I get the required centre
frequency which is 455KHz...

for both IF transformers

The inductor values are

L1 = 680uH
L2 = 97uH

and original coupling factor is 0.9

but after simulations the centre frequency obtained is 451KHz

when the coupling factor is reduced to 0.75 I get the required centre
frequency....

anyone with an answer...

John