Calculation formula related to ic8

[loannguyen]

Hello,
I am studying Hammerhead Pulse Induction Metal Detector circuit ver. Rev D1.

I don't know how to determine the capacitor values.
In ic8a the capacitor and resistor design is like an active low pass filter (I'm not sure) but I want to know the specific conditions (cutoff frequency... um, or something like that) to be able to calculate the values of the capacitors and resistors.
And in ic8b, I don't know how to calculate the capacitor values.
Finally, I want to ask about how you determine the Av of each opamp, what do you base on to determine the Av?

[Carl-NC]

IC8a (along with IC7a,b) form a sampling integrator which demodulates the signal to baseband. The baseband bandwidth of a target being swept over is typically 5-20Hz so that's where you want the bandwidth. HH1 uses 16Hz. IC8b is just a gain stage with an adjustable threshold offset. It operates on the same baseband so the same thing applies. HH1 uses 3.4Hz, I don't know why I set it that low. The gains are set as high as you can get away with (noise, overload) but should include an adjustment (R35) to reduce if needed.

Hammerhead is intended as a learning platform so if you build it with pin sockets you can easily replace the Rs and Cs and see how different gains and bandwidths behave.