To move the desired radio stations signal into the

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Unformatted text preview: tricky to do in hardware for a good bandpass filter. Instead, what’s done is to build an excellent bandpass filter whose passband is centered on a fixed pair of frequencies ±fI , referred to as the intermediate frequency or IF. For AM radio, the standard IF frequency is fI = 455 kHz. This filter passes frequencies in the range fI ± 5 kHz and −fI ± 5 kHz, and effectively block out other frequencies. It can also include the amplification necessary to raise the signal level from the microvolts at the antenna to the higher levels needed to drive downstream electronics. To move the desired radio station’s signal into the passband of the IF filter/amplifier, we use the hetero­ dyning idea that we’ve seen several times, i.e., multiply the signal at the antenna by a sinusoidal signal of appropriate frequency, fo kHz. (In AM radio jargon, this multiplication is called mixing.) To tune in differ­ ent stations, we just change the frequency of this sinusoidal signal, which is easy to do using a controllable oscillator. The common practice is to pick fo > fc ; this is referred to as super-heterodyning....
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This document was uploaded on 03/17/2014 for the course ELECTRICAL 6.02 at MIT.

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