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Monday, June 6, 2016

Top 3 Methods of Creating a (90?) Quadrature Phase Shift

The key to IQ mixers and their possible applications and underpinnings as a phase modulator and phase detector is the quadrature phase shift, for the IF and LO side for a single sideband mixer or an image reject, and at the LO side for an IQ mixer. When talking about the quadrature phase shift, it’s important to keep in mind that a time shift is not the same as a phase shift. In the area of RF, optics and microwaves, this is probably one of the most difficult concepts to grasp. Creating more useful and flexible quadrature phase shifts can help reach the goal of an ideal single sideband mixer/image reject/broadband IQ, and can be achieved by the use of different techniques.

The following techniques are ranked from worst to best, with quality being defined as amplitude and phase balance besting bandwidth. The correct technique for your application depends on whether you are trying to make a radar scanner, designing a communications chip for a cell phone, or building a demonstration system in your lab.

3: Lange Coupler
Pros: Compatible with MMIC, quasi-planar, wideband
Cons: Low power handling, requires air bridge or wire bonds, more complicated to design

The most commonly used device employed in balanced technologies such as MMIC balanced amplifiers is the Lange coupler is the most common device used in MMIC balanced amplifiers and other balanced technologies. Generally, unless gaps between the two edge-coupled micro-strip lines are small, planar couplers are very weakly coupled. However, if you are working with quasi-two dimensional structures, the Lange coupler is a great choice overall.

2: Digital Phase Splitter
Pro: Excellent amplitude and phase balance, ultra-broadband
Con: Low power, requires 2x LO frequency, no analog mode

It is hard to top a digital phase splitter in your quest for a perfect quadrature phase shift. While this is not a very good quadrature splitter device in general, it works well to create an LO drive for an IQ mixer, which is used frequently for LO clock generation with low frequency silicon RFICs.

1: 3 dB Quadrature Hybrid Coupler
Pros: High power handling, phase and amplitude balance, multi-octave bandwidth, suitable for data
Cons: Difficult to design and fabricate, difficult to integrate, large

A hybrid combiner like Werlatone's Broadband 3 dB High Power 90 Degree Hybrid Coupler is the gold standard for quadrature signal generation. It ca be used on a single sideband mixer or on the data side of an image rejects in order to achieve a better than 20sB rejection, it has excellent balance in both phase and amplitude, and can operate across broad bandwidths such as 2 to 26 GHz. An added benefit is that they can handle 20 watts of CW power or more, due to their construction.

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