What is a Triple Balanced RF Mixer

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Although single balanced mixers may often sufice for some applications, where higher performance levels are needed, double balanced mixers may be used, but the highest levels of performance are provided by triple balanced mixers.

These mixers have the RF input, local oscillator input and the out all balanced and this results in significant performance improvements.

Triple balanced mixer basics

The key point about triple balanced mixers is that instead of one or two ports being balanced as in the case of the single and double balanced mixers, all three ports are balanced.

A triple-balanced mixer is effectively made from two double balanced mixers and as a result it is sometimes called a doubly double balanced mixer.

It utilises many more electronic components having two diode bridges or quads, with a total of eight junctions. Power splitter at the RF and LO microwave baluns feed the mixer structure, and this allows for both of the diode quads to be coupled.

This allows for the IF signal to be available at two separate isolated terminals, that typically exhibit very large bandwidths compared to other mixer architectures.

The improved isolation provided by the triple balanced mixer provides for much higher levels of spurious signal, intermodulation distortion suppression.

The improvement in performance needs to be offset against the fact that they need higher levels of LO drive, and of course the increased complexity and electronic component count result in increased cost.

The basic concept or block diagram of a typical triple balanced mixer is given below:

This architecture of this type of mixer provides better isolation and suppression of spurious and intermodulation products than the double balanced mixer.

Triple balanced mixers also have a wide IF bandwidth. Triple-balanced mixers are ideal for applications where wideband signals need to be translated from one frequency range to another with minimal intermodulation products.

Triple balanced mixer advantages

The additional circuitry used for the triple balanced mixers compared tot he double or single balanced mixers results in a number of improvements in their performance and the performance of the circuits that use them.

• Superior Isolation:   Excellent isolation between the local oscillator (LO) and RF input ports, minimizing unwanted signal leakage.

• Enhanced Linearity:   Improved linearity, especially at higher input signal levels.

• Reduced Spurious Signals:   Significantly lower levels of spurious signals, such as intermodulation products.

• Higher Dynamic Range:   The ability to handle a wider range of input signal levels without distortion.

There are some disadvantages to the use of a triple balanced mixer

• High LO drive:   A higher level of local oscillator drive is required for the triple balanced mixer compared to a double or single balanced mixer.

• Cost:   The cost of these mixers is higher than other types because of the additional circuitry and high cost elements required withint he overall mixer.

Triple balanced mixers are not nearly as widely used as double balanced mixers - they are less widely known and their cost can be such that they are less widely used.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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