[mikethedruid]

Look at the schematic diagram for an IF transformer. You will see that both the primary and secondary are LC tuned circuits, with an inductor in parallel with a condenser. The inductor and condenser are in parallel and form what is called a "tank" circuit. It will resonate most efficiently at one particular frequency. In aligning a superheterodyne radio one tunes each side of the IF transformer so that it is peaked exactly at the desired IF frequency. This maximizes the signal at that signal being passed through the transformer, and cuts back the signals at all frequencies above and below the IF frequency. A single intermediate frequency is used because one can design an amplifier to be most efficient at one particular chosen frequency. If you design it for a broad range of frequencies it cannot be designed to be as efficient. In many superheterodyne sets there is only one stage of intermediate frequency amplification used because even this one stage can amplify the incoming radio signals enough so that the sensitivity of the receiver is sufficient. In some receivers, however, there can be a second stage of IF amplification which tunes the radio even more sharply, giving even greater selectivity, and rejecting adjacent signals.

The intermediate frequency is achieved by mixing the incoming radio signal tuned by the antenna circuit with a frequency generated by the local oscillator, which is above (or below) the incoming radio frequency by exactly the desired intermediate frequency. The two signals "beat" against each other and produce other signals which are the oscillator frequency above, and also below the incoming radio signal. One can use either of these as an intermediate frequency, but most often the lower is used. It is then amplified as explained above.

In some expensive "communications receivers" a second local oscillator is used, and mixed with the amplified IF frequency resulting in a second IF frequency. This greatly improves selectivity and rejects the false signal above (or below) the proper tuned signal. These are known as "dual conversion" sets. In some sets a third conversion is used to cancel both the image above and below the proper signal. These are known as "triple conversion sets," and are only used on VERY expensive communications receivers which have great selectivity and sensitivity. Almost all the antique radios we find ourselves restoring will be single conversion sets, Single conversion provides quite adequate selectivity for most listening purposes.

I hope this answers your questions.