FET Mixer vs. Image Reject Mixer: A Comprehensive Comparison

Mixers are essential components in RF systems, with FET mixers and image reject mixers serving unique purposes. This guide compares their working principles, advantages, and limitations, helping you choose the right mixer for your application. Learn how these mixers tackle challenges like signal distortion and image frequency rejection.

FET Mixer

Image alt: FET mixer

Double-balanced FET mixers are developed for both active and passive applications. Active FET mixer types are based on Gilbert architecture. They use biased semiconductor devices and work with low LO power levels. They provide conversion gain at decreased linearity compared to passive types.

Passive FET mixer types usually use FET quads in a ring shape. They provide good linearity, but the disadvantages are the requirements of a high LO power level and higher conversion loss. This mixer type is similar to a diode-based double-balanced mixer. The difference is that the FET mixer has 6 terminals (as shown in Figure-1), while the double-balanced diode mixer has 4 terminals. FET Mixers are available with junction-FET devices as well as MOSFET devices.

Image Reject Mixer

Image alt: image reject mixer

An image reject mixer is realized as an interconnection of a pair of two balanced mixers. Image reject mixers are useful where image and RF frequency bands overlap, or the image frequency is very close to the RF frequency, which needs to be rejected using an RF filter.

The LO ports of balanced mixers used here are driven in phase, but the signals applied at the RF ports will have a phase difference of 90 degrees. A 90-degree hybrid is used to separate out image and RF frequencies. These mixers are used in RF circuit design because image rejection is better with higher LO power. Moreover, image rejection depends strongly on the phase mismatch.

As depicted in Figure-2, they use parallel RF inputs and a phase-shifted LO. As shown, IF outputs are combined using a quadrature hybrid. The wanted signal (i.e., USB) and image frequencies (i.e., LSB) are provided separately on the two individual ports for use.

Following are the features/characteristics of Image reject mixers compared to other RF mixer types.

Merits and Demerits of Image Reject Mixer

• Image Reject Mixers provide image rejection of about 20dB.
• These mixers are useful for low IF frequencies.
• The disadvantages of image mixers are a complex design and the need for more current for operation.
• Another demerit of the image reject mixer is when two amplifiers are added on the RF path before the RF mixers. In this situation, image noise generated by the two RF amplifiers cannot be rejected.

Conclusion

FET mixers and image reject mixers cater to distinct needs in RF design. While FET mixers are versatile, image reject mixers offer superior performance in eliminating unwanted frequencies. Understanding their differences ensures informed decisions for optimized system performance.