Mixer (RF)

Also known as: mixer, RF mixer, frequency mixer

A mixer is a three-port nonlinear or switching device that multiplies an incoming radio-frequency signal by a local-oscillator tone to shift it to a new frequency.¹ Multiplying two sinusoids produces components at the sum and difference of their frequencies, so a mixer converts a signal up or down in frequency without changing the information it carries — the heart of every superheterodyne receiver.

A mixer multiplies RF by the LO, producing sum and difference frequencies; a filter keeps one (the intermediate frequency) and rejects the other.

Overview

Mixers realise the heterodyne principle: combining two frequencies to create new ones at their sum and difference. In a receiver the difference product is usually kept as the intermediate frequency (downconversion); in a transmitter the mixer moves baseband up to the carrier (upconversion). Because the process is linear in the information but nonlinear in frequency, the modulation on the RF signal is transferred intact to the new frequency.

How it works

Multiplying two cosines uses the identity cos(A)·cos(B) = ½[cos(A−B) + cos(A+B)], which is exactly the sum-and-difference behaviour a mixer exploits. Real mixers approximate multiplication in one of several ways:

Diode (passive) mixers switch one or more diodes on and off at the LO rate. A single-diode mixer is simple but leaks the LO and RF to the output; balanced and double-balanced designs (e.g. a diode ring) cancel those leakage terms, giving good port-to-port isolation and rejecting even-order products.
Active mixers such as the Gilbert cell use transistors to both multiply and add gain, so they can have conversion gain instead of the ~6–7 dB conversion loss typical of passive diode mixers.

Because mixing keeps both the sum and difference, a filter after the mixer selects the wanted product. The unwanted one — plus the LO feedthrough — must be suppressed.

Variants

Downconverting vs upconverting — the same device, distinguished by whether the wanted output is below or above the input.
Single-balanced / double-balanced — increasing port isolation and spur suppression.
Image-reject and I/Q mixers — pairs of mixers fed 90° apart cancel the image response in hardware, the basis of quadrature and zero-IF architectures.
Subharmonic mixers — driven at half the LO frequency, useful at high frequencies.

Relevance to SDR

Every superheterodyne front end contains at least one mixer, and it is the source of two classic problems SDR users must manage. First, the image frequency: a signal spaced from the LO by the same difference as the wanted signal — but on the opposite side — also lands on the IF, so it must be rejected by a preselector filter or an image-reject/quadrature design. See image frequency. Second, because mixers are inherently nonlinear, strong signals generate intermodulation products and spurs; a mixer’s third-order intercept point sets how gracefully it handles crowded bands.

Most consumer SDRs — RTL-SDR tuners, SDRplay, Airspy — implement mixing inside their tuner chips (often as quadrature/zero-IF mixers), so the “mixer” is a block on a die rather than a part you handle. GopherTrunk operates entirely on the digital I/Q stream those chips produce; it performs further frequency translation in software with a numerically controlled oscillator and a digital down-converter — mathematically the same multiply-and-filter operation, done in DSP rather than in a physical mixer.

Sources

Frequency mixer — Wikipedia, on mixer operation, balanced topologies, conversion loss, and image response. ↩