AndyS wrote:

Andy writes:
This is the eighth response to your question. All of the others
are accurate. HOWEVER, there is one technical problem with
a homodyne that outweighs the previous resonses...

The L.O, has to be EXACTLY the same frequency as the incoming
signal to provide proper demodulation......

With a superhet, depending on the modulation, you can be
many Khz in error ( for AM) or a few dozen off ( for voice SSB).
With homodyne, you may have to process a psuedo-baseband signal which
has frequency components you don't want..... that is a bitch....

In order to achieve the LO to be EXACTLY the same
frequency (phase doesn't matter since you can use I and Q),
it is necessary to achieve frequency lock. That requires a
much higher S/N than a simple superhet with a detector...
So you lose sensitivity. In some systems , you can lock
to a remote carrier, but you aren't really talking about those
methods, I don't. think....

So, the major technical problem is not SIMPLICITY or
BANDWIDTH or NUMBER OF STAGES...,.. it is
how to obtain an LO of the correct frequency.... All the
other problems are simple compared to this.... for most systems...

Andy W4OAH

PS I welcome dissent and would like very much to learn
if anything I have said is in error......

I believe that what you said is true, but only for AM. What other kind
of modulation requires better frequency accuracy from a direct
conversion receiver LO than from a superhet LO? I believe that only a
small fraction of today's amateurs are interested in AM reception, but
of course it's the bread and butter of the SWL and BCL.

I've build direct conversion receivers for many years. Their simplicity
is particularly evident when you compare a transceiver having a direct
conversion receiver with one having a superhet receiver -- to be honest,
be sure to include all the extra filtering necessary with the superhet.
The single biggest disadvantage to DC receivers, in my opinion, is the
difficulty of making a good AGC, particularly in conjunction with narrow
audio bandwidth. And they do have their own set of potential problems,
such as unintended AM demodulation, the effects of LO leakage and
radiation, and the difficulties in making a clean, stable, high gain
audio amplifier. But all can be overcome once one understands the causes
of the problems. All but perhaps the last one will be present in a
digital version, too, so a casually designed and/or built one is likely
to be a poor performer.

Roy Lewallen, W7EL