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I have question about specifically FM radio receiver that use the heterodyne principle:

How Intermediate Frequency, that's fixed in one frequency (10.7 Mhz), works in FM receivers if FM radios need frequency modulation to pass a useful signal?

Does the mixer with local oscillator in a FM receiver pass a bandwidth or just one frequency?

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2 Answers 2

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The incoming radio signal isn't mixed down to exactly 10.7 MHz, that would be pointless. Rather, the incoming signal (which varies in frequency) is mixed with a stable local oscillator; the result is a signal that has the same frequency variations but centered around 10.7 MHz.

This intermediate frequency is filtered through either tuned LC circuits or a ceramic resonator, but these circuits are not so narrow-banded that the frequency deviation is lost. The frequency swing of commercial FM stations is +- 75 kHz anyway, or 0.075 MHz, less than 1% of the IF frequency.

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  • \$\begingroup\$ Sure? I belive it's +/-75kHz. Also is IM a typo for IF? \$\endgroup\$
    – carloc
    Mar 5, 2017 at 16:11
  • \$\begingroup\$ @carloc You are correct, it's 150 kHz total. Text corrected. \$\endgroup\$
    – JvO
    Mar 5, 2017 at 17:05
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The whole bandwidth of the fm signal is mixed down to the intermediate frequency. Just simple multiplication of the radio signal and the local oscillator achieves this.

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