What are RF Mixers?

RF Mixers, also known as frequency mixers, are nonlinear electrical circuits or devices used in radio frequency (RF) and microwave systems to convert signals from one frequency to another. The primary function of an RF mixer is to change the frequency of an input signal. This is achieved by mixing the input signal with a local oscillator (LO) signal. The mixer produces two new frequencies: the sum and the difference of the input and LO frequencies. These are called the intermediate frequencies (IF). Mixers are used in a variety of RF/microwave applications, including military radar, cellular base stations, and more. A mixer can perform both up conversion as well as down conversion of the frequency of given input signal, up conversion is converting the signal to a higher frequency and down conversion is converting it to a lower frequency.

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If the input signal has a frequency FRF and the frequency of the local oscillator is FLO, the mixer output will contain components at FRF + FLO  and FRF - FLO. In concept, the LO signal acts as a gate of the mixer in which the mixer is considered ON when the LO is a large voltage and OFF when it is a small voltage. The LO can only be an input port, while the RF and IF ports can be interchanged between the second input or output. 

There are two main categories of mixers, active and passive mixers

Passive Mixers: These mixers do not require external power to operate. They use diode and other non-linear components. These mixers tend to have higher conversion loss. 

Active Mixers: These mixers use active components like transistors and require external power source. These mixers provide gain, which can amplify the signal, reducing the conversion loss.

Key mixer parameters 

Conversion loss or gain: Measured in dB, conversion gain measures the signal gain in an active mixer, while conversion loss (known also as CL) measures the insertion loss in a passive mixer. Conversion gain is defined as the ratio of the IF output power to the RF input power. For passive mixers, CL is the most important parameter next to the noise figure. It is defined as the difference in power between the input RF power level and the desired output IF frequency power level. Of course, the lower the CL the better. Typical values of conversion loss range between 4.5 to 9 dB. Other losses that may occur are from transmission line losses, balun mismatch, diode series resistance, and mixer imbalance. In addition, the wider the frequency ranges for the three ports, the worse the CL. Click here to learn more.

Isolation: This is defined as the amount of power leakage from one port to another. When isolation is high, the leakage between the ports will be small. There are two main types of isolation in mixers - RF/LO and LO/IF - Click here to learn more about this.

Input intercept point (IIP3): IIP3 is the RF input power at which the output power of the unwanted intermodulation products and the desired IF output would be equal. Click here to learn more.

Spurious: Spurious external signals generate undesired frequencies that may fall into the IF-band. Spur tables provided by manufacturers show the relative amplitudes of each response under given LO drive conditions - Click here to learn more.

Noise figure (NF): Defined as the added noise generated by the mixer and present at the IF output, the noise figure is the second important parameter (CL is the first) for the passive filter. For a passive mixer, the NF is almost equal to the loss.

Dynamic range: This is the signal power range over which a mixer provides useful operation.

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