On Mon, 14 Nov 2005 04:07:57 GMT, "Frank Dresser"
wrote:


"matt weber" wrote in message
.. .
What breakthrough has made single
conversion so state of the art?


Absolutle nothing, in fact single conversion sucks unless it is an up
conversion, and even then, mixer noise will wipe out reception above
about 10Mhz absent a good tuned RF amplifier in front.


Why would up conversion mixer noise wipe out reception above 10 MHz? How
would the presumed mixer noise problem be fixed by a further conversion?
The bind is in many low end receiver designs, the mixer is also the
local oscillator, so most SW receivers that are variants of the All
America 5 design (and there were many) had very poor performance above
10Mhz or so.

Of course
providing 3-5 Khz selectivity at 40Mhz tends to be a bit challenging.
Q on the order of 10,000......

Single Conversion with a 455khz IF strip doesn't have problems with
bandwidth, but image rejection in the SW bands sucks big time.

That's true enough with inexpensive receivers which relied on a single
(de)tuned circuit for RF selectivity. But the better receivers would
cascade two or more tuned stages, isolated with RF amplifiers.
Actually most interesting design in a single conversion receiver I
think I ever was was in the mid 1960's Squires-Saunders built one with
a tuned RF stage with a Q muliplier on it, so they had a Q of a couple
thousand on the front end and made a killing on the gain as a result
of gain-bandwidth product. Suffices to say that with that sort of
front end selectivity, image rejection wasn't a problem. Obviously
impedance matching with the antenna was crucial to performance, but it
was undoubtedly the best single conversion HF receiver every
commercially built (and had a price tag to match).

Frank Dresser