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We know that a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal.

  • But what do we mean by 'local'?
  • How is 'local oscillator' different from normal oscillator?
  • Why can't a normal oscillator, like an LC-tank circuit or an opamp circuit, be used in a heterodyne receiver?
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    \$\begingroup\$ The local oscillator is the one "local to", i.e., part of, your radio--as opposed to the remote oscillator used by the transmitter. \$\endgroup\$ – Hearth Jan 18 at 15:14
  • \$\begingroup\$ An oscillator with an LC tank circuit can be a fine local oscillator in a heterodyne receiver. An opamp RC circuit will tend to be way too drifty and noisy, but you could try. \$\endgroup\$ – TimWescott Jan 18 at 19:00
  • \$\begingroup\$ Heterodyning usually cares about the phase-noise producing adjacent closein energy on either side of the carrier. Leeson's Equation provides a rule-of-thumb model of phasenoise from various sources. \$\endgroup\$ – analogsystemsrf Jan 19 at 4:08
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You got that wrong: The "local" in oscillator doesn't describe the kind of oscillator used. It could just be an LC-tank, it could be a crystal-derived oscillator, it could be something synthesized from a reference clock or something recovered from the data stream received:

The "local" in oscillator refers to the fact that it's what the mixer uses locally to mix down or up, as opposed to the oscillator at the other end of the communication, which simply isn't the same oscillator.

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    \$\begingroup\$ You could also contrast it with an external oscillator - like a 10MHz reference input. \$\endgroup\$ – CramerTV Jan 19 at 0:35
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Think of frequencies. The local oscillator generates a local frequency, that is only used by your mixer as the intermediate frequency. Whereas the transmitter frequency is not staying on your local board but flying through the air as radio waves. The transmitter and the receiver does not need to use the same intermediate frequency, but could use different local oscillators.

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LO to an RF Engineer means a "local" sine wave signal or "within the device" (or Radio) used for heterodyning or RF mixing.

schematic

simulate this circuit – Schematic created using CircuitLab

The LO could be fixed or a synth or a VCO with PLL to tune RF to the fixed IF filter. It may be used as an up- or down-converter.

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