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What Is a Frequency Mixer?

Geisha A. Legazpi
Geisha A. Legazpi

A frequency mixer is a nonlinear or multiplier electrical circuit that is able to produce continuous sum and difference frequencies of two or more input signals. The device is widely used in telecommunications transmitters and receivers. The mathematical products of sinusoidal signals have additive and subtractive effects on the resulting frequencies. In the common superheterodyne receiver, for instance, the frequency mixer inputs are the radio frequency (RF) signal that is incoming and the local oscillator (LO) signal and the resulting sum or difference of the two inputs is the intermediate frequency (IF). Besides the sum and difference frequencies, there are other outputs in smaller amplitudes including higher-frequency or higher-order harmonics.

Using the IF amplifier stages simplifies the first RF amplifier stages of the receiver. Usually, only one front-end stage needs to be tunable across the receiver band. The rest of the RF amplifiers after the frequency mixer are tuned only to the IF. A special mixer called a subharmonic mixer may input an LO frequency that is half the required frequency, pass through a nonlinear device, and effectively generate an LO at twice the original frequency.

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When vacuum tubes or triodes were used, a grid between the cathode and the anode was able to control the flow of electrons. By proper design, the triode was able to amplify RF signals and to oscillate given the right passive-tuned tank circuits as frequency reference. The triode was developed to have more than one control grid. In early 1900s, the pentagrid converter was used as a frequency mixer where a triode with four additional grids performed the amplifier, oscillator, and mixer functions, resulting in single-vacuum front-end for receivers.

Ring modulation allows the use of transformers and a bridge diode configuration to produce the desired multiplication of the message and the carrier frequency. The result is a frequency spectrum with the message shifted higher in frequency by an amount equal to the carrier frequency. On the receiver side, the product detector may be used to input the modulated carrier and the carrier reference frequency to recover the original message.

A Gilbert cell is a bipolar transistor circuit that makes use of the current gain of direct-coupled bipolar transistor stages to generate the required analog multiplication to produce frequency mixer outputs. The first input signal is fed to the base terminals of two transistors working as a differential pair, while the second input is fed to the base terminals of two differential pairs of transistors with emitters that provide the collector currents for the differential pair for the first input signal. The resulting output is available at the collectors of the differential pairs for the second input signal.

The frequency mixer may also work as a frequency multiplier. If the same signal is fed to the input of a frequency mixer, the output frequencies include integer multiples of the input frequency. The second and third harmonics will be available at the output with the second harmonic having a higher level than the third harmonic.

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