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question about the resonant freq of LTC4125 with LTC4126

Hi ADIExpert

I have a system using LTC4125 as a wireless TX and LTC4126 as a wireless RX

I have a question about the resonant freq discrepancy between RX and TX

is there rule or constraint for the RX LC and TX LC resonant freq?

This was my test flow 

1. first a TX coil:6.8uH, Tx cap: 0.1uF, (calculated resonant freq: 193kHz) is tested.

this is the auto resonant freq (around 203Khz) measured without any RX attached

2. then, I put a RX coil: 13.3uH, RX cap: 0.039uF( calculated resonant freq: 221kHz) on the TX

I measured the waveform (freq:226K) as below with power transferring from TX to RX without problem 

3. Finally,  I tried to reduced the  RX resnoant freq to match Tx's one by changing the cap from  0.039 to 0.047uF

but the circuit doesn't work, the auto resonant freq seems could not be found (TX freq has gone beyond the max resonant freq)

May I have any advice why this happened?

also, if I keep the cap 0.039u, the resonant freq is 10% higher than the TX setting. if there is any issue may cause or just lower efficiency?

thank you