For an FM discriminator of the type John is describing, all you need is a frequency-dependent phase shift and a phase detector to turn that into a voltage change.
You can do it with an RC lowpass, even, and RC lowpasses don't have any delay at all. (One way to see this is that you need a time machine to undo a time delay, but an RC highpass can undo the effect of an RC lowpass.)
Cheers
Phil Hobbs