My final project for MIT's 6.013 (Electromagnetics and Applications) class was a radiofrequency vector-matrix multiplier. I along with my teammates, Ishaan Vohra and Dominic Stewart-Guido, wrote a paper documenting our work and presented in front of a panel of industry leaders.
Our setup consisted of a vector network analyzer (the NanoVNA), 2 Wilkinson power dividers, and 4 voltage variable attenuators (the AT-110TR).
The driving principal is that an attenuator multiplies an incoming signal from our VNA by some factor to represent it as a variable we will call A. A second attenuator will multiply this value by another variable factor to output a signal representing A*B. These results will be interpreted at the end when the signal is terminated and read as a transmission coefficient by the VNA. To make this calculator more practical, we used Wilkinson power dividers and combiners to allow for calculations in the form of A*B+C*D, which are practical for dot products and matrix-vector operations.
A diagram depicting our setup is shown below as a general concept on paper and in ADS (Advanced Design System), as well as it's implementation in reality.
This system was modularized as shown above to make sure we could deliver a minimum viable product in case we ran out of time. Impedance-matching allowed us to use the modules while minimizing unwanted reflections. All components were selected to be 50 ohms and connected with 50 ohm TEM transmission lines. Our custom PCBs for the attenuators were also designed with traces to match this.
According to the attenuator's datasheet, it operates best with coplanar waveguides at 900MHz or 1.8GHz. To manufacture traces thin enough to have a 50 ohm impedance while not being too long, I decided to use the Bantam PCB mill. This was capable of milling traces up to 1/64 of an inch. Using TxLine, I calculated the dimensions to create a 50 ohm microstrip line in ADS.
An external DC circuit was also made to control the variable attenuators with buffered poteniometers and a 7v power supply.
