Multiple Conversion Receivers

[Electro11]

If I were to design a receiver for signals with carrier frequencies in the band from 15MHz to 65MHz and bandwidth 15kHz using an IF filter with center frequency of 455kHz, what range of frequencies would my local oscillator require in order to be capable of generating?

 

[BrianG]

The frequency of the local oscillator does not determine the bandwidth, therefore the tuning range of the L.O. is the fundamental range, 15-65MHz, plus 455kHz. The bandwidth is determined entirely by the requency response of the I.F. filter/amplifier.

Note that due to the relatively poor "Q" of tuned L-C circuits of receiver front-ends at these sort of frequencies, most modern receiver designs use double-conversion techniques to ensure best selectivity.

 

[ulix]

With a such design, you'll get mirror signals. It means: Every received signal will appear on two places on your dial. Mixing products will be your local oscillator plus and minus IF (455 kHz). Frontal selectivity is not enough narrow to separate those two signals.
Much better results are reached using standard 10.7 MHz filters and detector circuits, there are lot of such filters, for wide band or narrow band FM on the market. NBFM filters have typicaly 16 KHz bandwidth. But I recomand you to read and learn some basic principles of receiving technology before you start to build something realy useful.
Good luck! ulix +