Image Reject / Rejection Mixer

- overview of the essential details of the Image Reject or Image Rejection mixer or multiplier used to remove one of the mix products.

Image reject mixers, or image rejection mixers as they are sometimes known are used in a number of RF and microwave applications. Image reject mixers are can be particularly useful in reducing the overall system cost by enabling the complexity of filters to be reduced.

Image reject mixers use phase cancelling techniques and remove one of the two major mixer products from the output of the mixing or multiplication process.


RF mixer image output

In any RF mixing or multiplication process two signals are produced, namely the sum and difference signals:

Output signals   =   (f1 + f2)   &   (f1-f2)


This can be seen diagrammatically:

RF mixing or multiplication products

RF mixing or multiplication products

Of the two products from a mixer, normally only one is required. Often the unwanted one will fall well outside the required bandwidths and can often be removed very easily. However in some applications this is not the case and the unwanted or image product can be close to the wanted signal and can require complicated filtering to remove it sufficiently. There may also be other instances where the image signal may cause a problem.


Image reject mixer basics

In order to be able to operate, image reject mixers, image rejection mixers utilise the phasing techniques to cancel out the unwanted mix products. To achieve this an image reject mixer utilises two balanced mixers and the quadrature (90°) hybrids as shown below.

The two balanced mixers within the image reject mixer are driven in quadrature by the RF signal. The LO drive to each mixer is in-phase and the IF output is combined in quadrature.

Image reject mixer configuration

Image reject mixer configuration

It is not possible to achieve perfect cancellation in a real image reject mixer - the mixers must be identical and as must the amplitude balance and phase shift of all the quadrature and in-phase power splitters.<.p>

Often integrated or modular image reejct mixers are employed as these offer the best performance. However it is possible to achieve reasonable results with one manufactured from discrete components - levels of image rejection of around 20 dB should be obtainable using this approach. Using commercially available units levels of 35 to 40 dB should be obtained.


Image reject mixer considerations and performance

There are a number of considerations to be taken into account when using an image reject mixer.

  • Conversion loss:   The conversion loss of an image rejection mixer will be higher than that of a standard mixer as the overall loss will need to include that of the quadrature hybrids, power splitters, etc. The additional loss introduced by the these components will need to be added into the overall equation. However the level of loss is still normally acceptable - typical figures expected may be around 8 - 10 dB.
  • Level of image rejection levels:   Typical levels of image rejection will vary according toth e product. However typical levels of image rejection that may be achieved are often in the region of 25 - 35 dB.
  • Frequency dependence:   The level of image rejection obtained with an image reject mixer is largely determined by the amplitude and phase balance within the image rejection mixer circuitry. These parameters are frequency dependent to a degree and therefore the performance of an imager reject mixer will also be frequency dependent.

Image rejection mixers are not nearly as widely used straight mixers. However they can provide significant advantages in some applications saving considerable complexity and costs. As such image reject mixers are a useful tool in the RF designers toolbox.

By Ian Poole


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