[jrblasde]

After some further research, I'm onto a new idea I'd like to test out. I saw a few different styles of IF can rebuilds designed to fit onto printed circuit boards that can be fit into the can of the gutted IF transformer. In one design, a guy used two side-by-side axial-lead inductors for the two coupled coils. In another design, a guy had two stacked PCBs, each with a radial-lead inductor. The two PCBs were stacked so that the bottom one has the inductor facing upward, and the top PCB has the inductor mounted on the underside to face the inductor on the lower PCB. In either design, the inductors are fixed. A fixed capacitor is then added in parallel to each inductor, with a capacitance of less than but close to the value needed for the proper resonant frequency. And finally, a trimmer cap is added in series to increase the capacitance until resonance is achieved at the desired frequency--in my case, 9.1 MHz.

    This sounds like an interesting experiment! After all, what is a transformer, if not two coupled inductors? I think I'd like to try the radial-lead inductor method. I will start by sizing the inductors. Looking on Mouser's website, for example, I am reviewing the data sheet for a 33 μH inductor with a minimum self resonant frequency of 9.0 MHz. Next, I need to select the capacitor for the LC resonant circuit. If f = 1/( 2*pi*sqrt(L*C) ), then that means C = ( 1/( 2*pi*f*sqrt(L) ) )^2. Thus, I need a total capacitance of 3.045 pF. I could use a 2.7pF fixed capacitor, and add a 1 pF trimmer for adjustment.

    The only thing I will need to sort out is the bandwidth and coupling of these two resonant circuits. I should probably start out on proto-board to refine the Q value. The bandwidth of an FM station is 200 kHz (with 15kHz dedicated to audio), so I will need to find the loaded Q value of these resonant circuits to then determine the critical coupling.

    Any thoughts? Has anyone tried to redesign an IF transformer with much success?