LTC4125 resonant freq and foreign object detection scheme

Hi ADI Expert

I have few questions about LTC4125 and hope if you can provide me some suggestion

Application: 

 I use a LTC4125 as the Tx to transmit power to a  LTC4126 for a 50mA battery charger, the air gap is around 3~4mm and should be fixed, Ith is set at 0.6A

1. I picked the TX resonant freq  around 157.5Khz (L:6.8uH , C: 0.15uF), RX 170Khz (16.5uF C:0.056uF)

I got the resonant freq around 165Khz  as the below pic1 when there is no RX closed to TX, and the Optimum power search is stopped because of VFB>Vin (pic2)

however if a RX is presented, the resonant freq is up to 200Khz when the exit condition is found and power starts delivering from TX to RX  (pic3)

I am not sure if the behavior is correct? (will Resonant freq change quite a lot when RX is closed to Tx ?)

pic1                                                                                                          

  

Pic2

  

Pic3

2. According to the fig11 of the datasheet ,when a foreign object is presented, the resonant freq is increased with lower Vtank amplitude 

however it's not what I found when I put a Aluminum foil on the top of my TX coil(please see pic4), the freq is even lower than when TX/RX is matched and the Vtank is higher

so I couldn't use the FTH pin to setup a fault for foreign objects. (I am now using IMON with a BJT to cause NTC fault to stop power delivering)

but still, I would like to know why the freq did not rise as I expected. I am not sure if the material would be a problem (Copper is used in the test of the datasheet)

                                                                                                     

    

Pic4

 

any suggestion is appreciated

thank you

Ken