what is the advantage of a direct conversion receiver

what is the advantage of a direct conversion receiver ( the kind with a local oscillator and a "zero frequency IF") over a tuned RF receiver, or by that I mean a receiver that doesn't use a LO?

George

A direct conversion receiver can be smaller and less expensive than a standard super heterodyne tuner. Note they both use LO, one for the direct conversion, and at least two for the superhet.

But without the IF section how does it have more sensitivty or selectivity than a simple tuned RF receiver?

The smaller size and lower cost don't come for free :)

Superhet tuners are used when performance is paramount.

But I'm trying to get a comparison between Direct conversion (with a LO) and a tuned RF, not a direct conversion and a superhet. Is the LO / mixer is the direct conversion receiver nothing more than a beat frequency oscillator that is just a way to demodulate the rf? If so, is it really that much better than a diode detector?

I see, when you said tuned RF, I was thinking a traditional (superhet) tuner. Could you clarify what you mean by tuned RF? Thanks.

If I am not mistaken, a TRF receiver is just a tuner (and sometimes an amp followed by another tuner with the same resonant frequency and another amp, or perhaps several such circuits), followed by a detector and then an audio amp and speaker

Direct downconversion is just simple and cheap. You don't have to worry about image frequencies, and you can set the channel bandwidth easily using baseband filters instead of IF bandpass filters.

But if you only want to do simple envelope/magnitude detection (like with a diode detector), then direct downconversion probably doesn't offer much benefit. To recover the envelope you need to sample both I/Q channels and calculate it.

I am sorry, I am not familiar with this terminology. What is I/Q channels?

Perhaps this will help. http://en.wikipedia.org/wiki/Quadrature_phase