I will briefly explain why some receivers are double and even triple conversion and why that has nothing to do with selectivity. The superheterodyne design mixes a local oscillator with the desired frequency to make the first IF or conversion. You can either feed the local oscillator in to the mixer below the desired frequency which is called low-side injection or you can feed it in above the desired frequency for high-side injection. If, for example, you have a receiver tuned to 28 MHZ with a 455 KHZ first IF, the local oscillator can either be 28.455 MHZ or 27.545 MHZ and you still hear 28 MHZ. Now for the bad news. Let's say your 28 MHZ receiver uses low-side injection. The mixer is built to allow 28 MHZ in from the antenna by means of a tuned tank circuit set to 28.0 MHZ exactly. There is, however, a nasty fly in the ointment. Remember studying about the Q factor in tuned circuits? That is the way the quality of a tuned circuit is measured and the technique is to sweep a signal through the pass band of the tuned circuit and determine how far off frequency you have to be for the circuit to be down 3 DB from what it is in the center. One of the laws of physics is that in a tuned circuit, the Q factor is a percentage of the frequency so a LC tuned circuit at 1.8 MHZ will be rather sharp compared to the pass band of a voice signal and it will not pass much more than one AM signal which is not too bad. By the time you get to 28 MHZ, however, that tuned circuit may pass frequencies between 26 and 29 MHZ fairly well. So, what's wrong with that? Everything! You have a 455 KHZ first IF. You are listening to 28.0 MHZ by injecting a 27.545 MHZ signal in to the mixer so here's what else happens. If you didn't have that tuned circuit in the mixer to select 28.0 MHZ, you would still hear 28.0 MHZ and you could also hear 27.545 - 455 KHZ or 27.09 MHZ which is right in the Citizens' Band. In other words, that is the potential with every superheterodyne receiver. The first mixer converts the local oscillator + the desired frequency to the IF and it can also convert the local oscillator minus some other frequency to that same IF. That other frequency is the image. okay, let's put back that tuned circuit in the mixer. If we tuned the receiver down to the AM broadcast band, it's not too bad. We hear pretty much only what we wanted to hear. Now, let's go back to ten meters and 28.0 MHZ. If you are listening in AM mode, you should maybe hear a few CW signals as popping sounds but there's something else. There is a strong image of whatever CB channel is at 27.085. That darn tuned circuit in the mixer is worthless by the time you get to ten meters or VHF. Okay. That's a single-conversion receiver. Let's make it double-conversion by putting a second oscillator and mixer at the 455 KHZ IF. The second conversion oscillator is at 355 KHZ so our second IF is at 100 KHZ. This would work just fine except that we still have that crappy front end in which the images from the CB channels are all over ten meters. We will get a little more selectivity because we combine the selectivity of the 455 KHZ if and the 100 KHZ second IF but the receiver is still a piece of junk. Now, let's make a decent receiver. The first mixer mixes a local oscillator that is maybe 70 MHZ higher than the desired frequency with that frequency to produce a first IF of 70 MHZ. The difference between the image frequency and the desired frequency is now 140 MHZ which is much, much easier to build a tuned circuit for. If you put a crystal filter at the 70 MHZ first IF that has a fabulous Q factor, then your second conversion mixer doesn't have to try to filter everything as much. You end up with good selectivity and virtually no images over the whole tuning range. There are a lot more problems in building a good receiver and I don't even begin to know them all, but I hope this helps in the discussion as to whether single conversion or double conversion receivers are best. There are some good ones of both kinds but generally a double-conversion receiver with a high first IF beats the pants off of a single-conversion receiver with a fairly low-frequency first IF. Everything I have said here is generalities and some of the older gear used lower first IF's with higher-Q multiple tuned circuits in the first mixer and achieved beautiful results. Martin McCormick WB5AGZ Stillwater, OK Systems Engineer OSU Information Technology Department Telecommunications Services Group