A recent thread on rec.radio.shortwave began with the merits of two inexpensive digital radios, and progressed through radio engineering details. User "dunxuk" started this thread with the subject, "Degen DE11 vs Tecsun PL200, any opinions?" The first few replies compared the new DE11 to the quirky but much-celebrated DE1103 (KA1103).
Count Floyd started to push the conversation into a different direction: "Damn, you would think in 2006 that single-conversion radios were not even being made! I have an old S-38 and an HE-10, both single conversions, but they were made in the '40's and '50's! They still pull in great DX on the BCB though and the tubes keep my room warm in the winter!"
Telamon had this excellent follow-up: "That's because the age of the design has nothing to do with whether it is 1, 2, or 3 conversion. There were double and triple conversion receivers back then. What determines the number of conversions is cost."
Then we heard from Frank Dresser: "The only real problem with single conversion is image rejection, and older
higher quality single conversion radios obtained increased image rejection with additional tuned RF circuits. ... For the same image rejection at high frequencies, a high IF converted to a lower IF will be cheaper than that lower IF with multiple tuned RF stages. However, conversion stages are noisier and potentially less linear than amplifier stages."
Frank's post continued with more detail: "If anyone ever manages to design an inexpensive narrow bandwidth filter with
an excellent shape factor at 45 Mhz or so, we can pretty much say goodby[sic] to the multiple conversion radio. Skip all those troublesome conversion stages! Exept there might still be a few radios with extra conversion stages. If so, there will surely be somebody saying "I can't believe they're still using that old 20th century technology!!" Of course, the manufacturer might be doing it to distribute gain at different frequencies in order to avoid oscillation. But that's whole 'nother bit of radio design
Matt Weber replied to Telamon: "You can make a fairly respectable single conversion receiver, but there are a couple things you need to do to get reasonable image rejection, and selectivity. Raise the IF frequency to several Mhz, and put a tuned rf amp in front of the mixer. That usually required a crystal filter for good shape characteristics and good selectivity."
Frank Dresser replied to this post as well, and ended with: "Making single conversion radios with good image rejection is certainly possible, but it isn't simple."