I'm doing a project involving wireless power transfer, where an LED will be turned on via 2 induction coils (tx and rx). The interfacing power circuitry for this has already been figured out (an arduino, transistor, caps, and resistors). What I want to do is transmit the wireless power at a specific frequency though (13.56kHz), so I'm trying to design a narrow band-pass filter that will allow for transmitting at this frequency. My understanding is that the transmitting coil already transmits at it's own frequency, dependent on the capacitors, and its own inductance value, but I'd like to 'fine tune' this to only transmit at 13.56kHz. This is where I think I'd need a bandpass filter, right before the transmitter coil, right?
The problem I'm having involves the filter design itself:
I'm doing my design by calculating component values for a 3rd order filter, using the "maximally flat LPF prototype design" method, then using impedance and frequency scaling to transform to a BPF. My bandwidth is 5%, so 678Hz. The element values I'm using are for "maximally flat time delay LPF prototypes"; so for N = 3, I obtain g1= 1.2550 g2 = 0.5528 g3 = 0.1922 g4 = 1. I'm simulating all this in LT Spice.
My dilemma is that I'm not getting as narrow a pass-band as what I think I need? It seems to be centered around 13.56kHz, but with a wide pass-band. What else can I do to make the bandwidth narrow? Secondly, is this the right method to use in order to have a filter that will allow for a specific transmission frequency?
Lastly, is it better to go for an active filter design method, where you use op-amps in the filter circuit? Or is this mostly for audio applications?
Below is my circuit in LT Spice, and the frequency response.
The element values I'm talking about (g1 to g4) are taken from my lecture notes, which I assume are a standard in theoretical filter design.