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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.
- Fall '12