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Lossless taper


Introduction
Combiners are used in a lot of different circuits, this particular style of combiner has been used successfully in several antenna array circuits.  This style of combiner has two particular features that can be advantageous:
  1. It has a built-in approximation to a cosine-style taper without adding loss
  2. It has not dependent on having a power-of-two number of elements
A typical way to build in a cosine taper is to add attenuators and use loss to create the taper.  However, adding 6-10dB of attenuator loss on the edges can impact the noise figure even if the loss is added after an LNA.  By reducing the number of resistive elements, contributions from Johnson–Nyquist noise can be reduced.  A relatively small amount of attenuation could be added to improve the taper.

Another benefit is decoupling the size of the array from hitting a power-of-two number which is convenient for traditional Wilkinson feed structures.  Three-way Wilkinson combiners are sometimes option, but often have performance and fabrication issues.  When physical size is a constraint, this can help decouple the size of the antenna elements and the overall size of the antenna array from a power-of-two corporate feed structure.

Click to expand this image of the 12-way "lossless" taper combiner.

This page will go through the steps to make a six-way combiner using Mason, to highlight the technique and some of the features of Mason.  If desired, additional microstrip lengths could be added to make the combiner end at the antenna element period, but here we will image we are going to connectors.

Layout of the 6-way "lossless" taper combiner

Implementing with Mason
The design file discussed on this page has been attached at the bottom for your convenience.

Frequency and Flags
The first thing we will do is set up the frequency and the flags for this run.  The major point to note is the PADS-ASCII flag has been set, so we will generate a PADS Layout file at the end of the run.


Microstrip Properties
Because we are using microstrip lines, we need to define the microstrip properties using one of the mason_microstrip library components, specifically "Microstrip: Properties".  Here we can set parameters like the dielectric properties and the dielectric height. 


Variables
In order to have a consistent design, we need to use the variable blocks to control the physical properties of the various Wilkinson dividers.  Each variable can be fixed, or can have optimization and statistics parameters associated with them.  Here, we are using the concept of sets to simplify our design process. 

Consider that this problem has two sets of orthogonal design criteria: the performance of the Wilkinsons for match and isolation, and the phase matching of the edge elements (which do not go through as many Wilkinsons) to the center feeds.  It is undesirable to optimize for both at the same times, since the optimizer could play shenanigans by tweaking line widths to adjust phase matching... which can lead the optimizer to a sub-optimal local minimum.

By using sets, we can separate those variables that are associated with match and isolation from those variables that equalize the line lengths.

Calculations
In practice, calculations are often used to make sure the geometry of the design works out.  Here, I am adding some additional line length to the Wilkinson in order to make sure the appropriate gap for the resistor is maintained.  Additional calculations could be used to ensure the Wilkinson feed points ended at the same periodic spacing as the antenna elements.

Optimization
Three optimizers are specified in the optimizer block below; each optimizer is considered a "set".  The first two calculations are associated with the first optimizer (set="1").  The last calculation is associated with the last two optimizers (set="2 3").  In the variable block above, variables have been associated with either the first simplex optimizer, or with the random and second simplex optimizer. 

Therefore, three optimizations will be run.  The first simplex optimizer will allow W_Wilk2, W_Wilk3, L_90deg, and R_Wilk2 to vary, and will use the 20*log10() evaluations defined below to rate the merit of the design.  The second and third optimizers will fix to the best values found of W_Wilk2, W_Wilk3, L_90deg, and R_Wilk2, and will allow l_comp to vary using the arg() evaluation defined below to rate the merit of the design.


Circuit
The full circuit is too large to fit on the web page, but the first stage of the Wilkinson in displayed in order to show what at least part of the circuit looks like in Mason, plus to show how the L_extend calculation is used (in TL62 and TL66). 


Performance
This circuit has been roughly optimized with relatively little effort for decent match and phase balance.  It is by no means optimal, but rather is just meant to show techniques. 


ċ
lossless_taper.dsn
(212k)
Gregory Kiesel,
Jun 24, 2010, 8:58 PM
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