It depends on Mutual Coupling Coefficient (0.99xx?) and primary inductance even if perfectly balanced windings.
- This coupling k controls the "Leakage Inductance" in each winding.
- Shorting one winding and measuring the other gives you the other winding leakage inductance
Since the primary and secondary inductance cancels out with perfect matched winding , it will still have a leakage inductance from imperfect coupling that results in an impedance acting like a LPF at some high frequency.
f-3dB 45 deg breakpoint will be;
k = coupling factor
L = 10 mH, C = 1 uf L = 10 uH, C = 1 uF L = 10 uH, C = 1 nF
k f-3dB 45 deg f-3dB f-3dB
=== ======= ======= =======
0.9 200 Hz 200 kHz 200 MHz
0.99 2000 Hz 2 MHz N/A
0.999 20 kHz 20 MHz
Now changing the turns ratio
0.999 3.3 kHz @ 1 : 1.1 ratio or 1 : 0.9
This is like taking a long magnet wire of some 10nH/cm and folding in half then twisting it and using the end points. So the mutual coupling is high and primary secondary inductance cancels out but the leakage inductance is much lower.
The results are different for a resistive load where the impedance matching is possible on primary and secondary sides and then the null inductor has a wider BW.
- For an RF directional coupling made from a hybrid balanced transformer like those used in 3dB TV splitters for 75 Ohm cable to give isolation between the 2 ports with matched impedances.