https://www.maximintegrated.com/en/design/technical-documents/app-notes/4/4622.html

It seems they are using an Infinite Gain Multi Feedback Active Bandpass filter similar to what is described here:

https://www.electronics-tutorials.ws/filter/filter_7.html

A few things bother me about the Max circuit and I wanted to see what other folks thought.

First the formula for obtaining the resonant frequency of the receiver from what I can see is

Frequency = 1/(2*pi*sqrt(C1*C2*R1*R2))

I tried plugging the application note values for C1 C2 and R1 R2, I get in the 9.8khz range and not 10khz. Is this right?

Second, this circuit does not seem to have phase shift adjustment tuning to ensure the sender 10khz wave is synchronized with the receiver clock. I know there are techniques to handle this by ANDing the input with 90 degree phase shifted input as well as others, but this particular circuit does not seem to have any of these. Am I missing something?

Thanks again,

Stephan