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.
Monday, June 6, 2016
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