Re: Infinite Impedance detectors.
It is conceivable that the first IF transformer might have a special primary, to provide the load expected by the frequency changer. Too much signal here might cause nonlinearity.
There is more to IFT specification than -3dB frequency. A typical off-the-shelf IFT will probably be slightly undercoupled, to get a good shape combined with ease of adjustment - but of course this assumes a particular load and source impedance. Factory-built radios did not need to use off-the-shelf IFTs so variations are possible.
The infinite impedance detector is nearer to being an ideal envelope detector than a diode detector, but it still suffers from the limitations of an ideal envelope detector. It still can't do negative peaks at 100% modulation.
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