How sorry for "dummy" question. I have difficulty to understand how the AM receiver.

Being a child I was taught that AM receiver resonates only with the radio frequencies it is tuned to, and "passes" only this frequencies to "other" circuits.

Recently I have read in articles that somehow the AM frequencies have the component of carrier frequency and component of side band frequencies. I even can more or less understand the formula derivation of it, which is quite simple. What I have difficulty to understand is: If AM receiver tuned to receive only one frequency, e.g the carrier frequencies, how all this side band frequencies are not getting filtered out by the receiver?

Please be patient all you radio gurus.


1 Answer 1


There is no realizable filter that blocks all frequencies except one.

The fact is that frequencies above and below the tuned frequency are passed but with greater and greater attenuation the farther away from the carrier frequency.

enter image description here

More importantly, the actual information is in the side bands, not the carrier. If the filter only passed the carrier, there would be only the constant amplitude carrier frequency at the output of the filter.

Mathematically, to produce an amplitude modulated carrier requires frequencies above and below the carrier frequency to 'beat' with the carrier.

enter image description here

When the three constant amplitude frequencies, on the left are combined (summed), the result is the amplitude modulated signal on the right. Thus, we must pass not only the carrier but also the side band frequencies to the amplifiers and detector of the receiver.

(Note: There are several types of amplitude modulation but that is beyond the scope of this answer.)

  • $\begingroup$ Anticipating the next question, the same concept of a filter with finite bandwidth applies also to FM (Frequency Modulation). $\endgroup$
    – Edward
    Jul 25, 2014 at 13:45
  • $\begingroup$ Wow, thanks!!! So it is my assumption about ideally tuned receiver which was wrong? The receiver must accept not a single, but a bandwidth of frequencies around the carrier frequency, which then will be demodulated? $\endgroup$
    – Boris
    Jul 25, 2014 at 13:57
  • $\begingroup$ @Boris, that's exactly right. $\endgroup$ Jul 25, 2014 at 18:56

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