FET mixer and Image reject mixer basics | difference between

This section of RF Mixer tutorial covers following sub topics:
RF Mixer tutorial   RF Up converter design using RF mixer   Basics   Vendors   Balanced mixers   FET & Image reject mixers  

This page on FET mixer vs image reject mixer mention basics of RF mixer types and dictate difference between FET mixer and Image reject mixer.

FET mixer

FET mixer

The double balanced FET mixers are developed for both active and passive applications.

Active FET mixer type is based on Gilbert architecture. They use biased semiconductor devices. They work with low LO power levels. They provide conversion gain at decreased linearity in comparison to passive type.

Passive FET mixer type usually use FET quads in ring shape. They provide good linearity. The disadvantages are requirements of high LO power level and delivers higher conversion loss. This mixer type is similar to diode based double balanced mixer. The difference between these two is that FET mixer has 6 terminals as shown in figure-1 while double balanced diode mixer has 4 terminals.

FET Mixers are available with junction-FET devices as well as MOSFET devices.

Image reject mixer

image reject mixer

It is realized as inter-connection of pair of two balanced mixers. Image reject mixers are useful where image and RF frequency bands overlap or image frequency is very close to the RF frequency which need to be rejected using RF filter.

The LO ports of balanced mixers used here are being driven in phase, but the signals applied at RF ports will have 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 as image rejection is better with higher LO. Moreover image rejection depends strongly on the phase mismatch. As depicted in the figure-2, they use parallel RF inputs and phase shifted LO. As shown IF outputs are combined using 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 mixer compare to other RF mixer types.

Merits and demerits of Image Reject Mixer

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

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