I had this crazy idea to attempt to build a direct conversion receiver for
regular old AM (i.e., two sidebands, carrier included). It eventually
occurred to me, however, that there's the very significant problem of
synchronizing the LO to the incoming RF carrier. In diagrams I've seen,
normally a PLL is used at the IF frequency (455kHz being common for AM
receivers, of course) to lock the two together. Is there anything
comparable one can do with direct conversion? It seems that if your LO is
sync'd with the incoming carrier, you'll have a very large DC component (the
AM carrier!)... if not, however, you still might get some DC that's been
aliased and while this will (should) always be less than the DC component
when you're locked, it gives no indication of which way your LO needs to
move to achieve lock.

So apparently what I'm really asking is... can one build a phase detector
that works at DC? Or does an AM direct conversion receiver necessarily
require an IF strip?

Thinking out loud,
---Joel Kolstad