Editing Superheterodyne receiver (section)

=== Local oscillator and mixer ===

The signal is then fed into a circuit where it is mixed with a sine wave from a variable frequency oscillator known as the [[local oscillator]] (LO). The mixer uses a non-linear component to produce both sum and difference [[Beat (acoustics)#Mathematics and physics of beat tones|beat frequency]] signals,<ref name="AOE_2006"/> each one containing the [[modulation]] in the desired signal. The output of the mixer may include the original RF signal at ''f''<sub>RF</sub>, the local oscillator signal at ''f''<sub>LO</sub>, and the two new heterodyne frequencies ''f''<sub>RF</sub>&nbsp;+&nbsp;''f''<sub>LO</sub> and ''f''<sub>RF</sub>&nbsp;&minus;&nbsp;''f''<sub>LO</sub>. The mixer may inadvertently produce additional frequencies such as third- and higher-order intermodulation products. Ideally, the IF [[bandpass filter]] removes all but the desired IF signal at ''f''<sub>IF</sub>. The IF signal contains the original modulation (transmitted information) that the received radio signal had at ''f''<sub>RF</sub>.

The frequency of the local oscillator ''f''<sub>LO</sub> is set so the desired reception radio frequency ''f''<sub>RF</sub> mixes to ''f''<sub>IF</sub>. There are two choices for the local oscillator frequency because of the correspondence between positive and negative frequencies. If the local oscillator frequency is less than the desired reception frequency, it is called ''low-side injection'' (''f''<sub>IF</sub> = ''f''<sub>RF</sub> − ''f''<sub>LO</sub>); if the local oscillator is higher, then it is called ''high-side injection'' (''f''<sub>IF</sub> = ''f''<sub>LO</sub> − ''f''<sub>RF</sub>).<!-- only cases for difference mixing given -->

The mixer will process not only the desired input signal at f<sub>RF</sub>, but also all signals present at its inputs. There will be many mixer products (heterodynes).<!-- ignoring higher order products --> Most other signals produced by the mixer (such as due to stations at nearby frequencies) can be [[Filter (signal processing)|filtered]] out in the IF [[tuned amplifier]];<!-- dynamic range issues --> that gives the superheterodyne receiver its superior performance. However, if ''f''<sub>LO</sub> is set to ''f''<sub>RF</sub>&nbsp;+&nbsp;''f''<sub>IF</sub>, then an incoming radio signal at ''f''<sub>LO</sub>&nbsp;+&nbsp;''f''<sub>IF</sub> will ''also'' produce a heterodyne at ''f''<sub>IF</sub>; the frequency  ''f''<sub>LO</sub>&nbsp;+&nbsp;''f''<sub>IF</sub> is called the ''image frequency'' and must be rejected by the tuned circuits in the RF stage. The image frequency is 2&nbsp;''f''<sub>IF</sub> higher (or lower) than the desired frequency ''f''<sub>RF</sub>, so employing a higher IF frequency ''f''<sub>IF</sub> increases the receiver's ''image rejection'' without requiring additional selectivity in the RF stage.

To suppress the unwanted image, the tuning of the RF stage and the LO may need to "track" each other. In some cases, a narrow-band receiver can have a fixed tuned RF amplifier. In that case, only the local oscillator frequency is changed. In most cases, a receiver's input band is wider than its IF center frequency. For example, a typical AM broadcast band receiver covers 510&nbsp;kHz to 1655&nbsp;kHz (a roughly 1160&nbsp;kHz input band) with a 455&nbsp;kHz IF frequency; an FM broadcast band receiver covers 88&nbsp;MHz to 108&nbsp;MHz band with a 10.7&nbsp;MHz IF frequency. In that situation, the RF amplifier must be tuned so the IF amplifier does not see two stations at the same time. If the AM broadcast band receiver LO were set at 1200&nbsp;kHz, it would see stations at both 745&nbsp;kHz (1200&minus;455&nbsp;kHz) and 1655&nbsp;kHz. Consequently, the RF stage must be designed so that any stations that are twice the IF frequency away are significantly attenuated. The tracking can be done with a multi-section variable capacitor or some [[varactor]]s driven by a common control voltage. An RF amplifier may have tuned circuits at both its input and its output, so three or more tuned circuits may be tracked. In practice, the RF and LO frequencies need to track closely but not perfectly.<ref name="Terman_1943"/><ref name="Rohde-Bucher_1988"/>

In the days of [[vacuum tube|tube (valve)]] electronics, it was common for superheterodyne receivers to combine the functions of the local oscillator and the mixer in a single tube, leading to a savings in power, size, and especially cost.  A single [[pentagrid converter]] tube would oscillate and also provide signal amplification as well as frequency mixing.<ref name="Langford-Smith_1940"/>

The mixer tube or transistor is sometimes called the ''first detector'', while the demodulator that extracts the modulation from the IF signal is called the ''second detector''.<ref>{{cite book|first=Ralph S.|last=Carson|title=Radio Communications Concepts: Analog|publisher=Wiley|location=New York|year=1990|isbn=978-0-47162-169-0|page=326}}</ref>  In a dual-conversion superhet there are two mixers, so the demodulator is called the ''third detector''.