MegiQ VNAs at Complutense University of Madrid (UCM)
Professor Luis Ángel Tejedor Álvarez, UCM, Spain.
The Complutense University of Madrid (UCM) is the biggest University in Spain and one of the oldest in Europe (founded in 1293). It has a long tradition in science and humanities. However, in Spain, engineering degrees were traditionally taught at polytechnic schools. At UCM, we have an Informatics Faculty, an Electronics department at the Physics Faculty, and other departments and research groups able to teach the basic skills of engineering (maths, physics, etc.). So, we decided to create a new degree in Communications Electronics Engineering.
The degree started in 2012, and it definitely wasn’t the best time to start an adventure. There was no money to hire new teachers, create new infrastructures or buy new equipment. We had computing labs and two electronics laboratories, but they were focused in low frequency electronics. We only counted with a few high-end instruments used in R&D projects, which were not always available. In addition, it was a bit reckless to use these expensive instruments to teach people that are using them for first time.
So, we had a problem: we needed to get several spectrum analyzers and VNAs to setup a new RF laboratory, with a budget of a few thousand euros each year. Looking to the websites of traditional suppliers like Keysight, Rohde & Schwarz, Anritsu or Tektronix was quite depressing, with prices normally higher than 15.000 or 20.000 €. I considered buying second-hand instruments and I remember several suppliers in eBay selling repaired instruments at affordable prices. But they were instruments from the 1980’s and honestly, I didn’t want to make the students come back to the 3½” diskettes and the serial port to take out their data.
I discovered MegiQ VNAs and it was exactly what we needed. A VNA that works like a traditional one, allowing to configure the frequency range, the output power, to make SOLT calibrations and to take quite accurate measurements. It is perfect for teaching! Moreover, it is affordable!
|I discovered MegiQ VNAs and it was exactly what we needed.|
The purpose of the RF laboratory sessions is to get the students familiar with the RF instrumentation, the calibrations, measurements, s2p exportation, procedures, etc. We don’t require a VNA able to measure up to 20 GHz, with a fancy touchscreen, the last version of Windows, an incredible low noise or a wide family of expensive accessories. What we need is just a professional tool, to teach how this kind of instruments work.
We have created 3 practices using MegiQ VNA up to now.
Practice 1: Measurements of Impedances and S parameters with the VNA
In this practice, the students get familiar with the MegiQ VNA and measure different circuits included in the MegiQ Sandbox. First, they configure the instrument, they calibrate it with the UFL cables and then they characterize the low pass, band pass and high pass filters and the attenuator, using the markers to measure bandwidth, insertion and return losses, etc. After that, they perform several impedance measurements, representing the results in the Smith chart. In this way, the students check the geometric places of the short-circuit, the 50 Ω load and the open-circuit in the Smith chart. Finally, they measure the impedance of the LC series and LC parallel circuits, and they have to find the resonance frequency.
Some measurements made by the students:
Students measuring circuits included in the MegiQ VNA Sandbox
Practice 2: Characterization of directional couplers, dividers and switches
In this practice, the students characterize several circuits, commercial or designed at UCM, with SMA connectors. So they start connecting SMA terminated cables and calibrating. Then, they measure the directional couplers (a branch line and a rat race) and the dividers, with and without 50 Ω loads to see the importance of proper terminations. They measure the central frequency, the insertion losses, coupling, isolation, directivity and phase differences between the outputs. Finally, the students measure a SPDT switch implemented with PiN diodes, measuring the insertion losses when the different branches are selected.
Student measuring a RF switch with a MegiQ VNA (in the right) and SMA cables
Practice 3: Balanced structures
In this practice, the students characterize the different components of a balanced structure (considering the connection cables part of the filters), and then they measure the complete response of the structure.
Balanced structure, with directional couplers and two filters
In this way, they see the advantages of the structure: the good matching at all frequencies at port 1, with all the power reflected to port 4.
Balanced structure output 4
Balanced structure test bench
After the successful experience, we have purchased a second VNA0460 and we plan to develop new practices and increase the number of test benches. The number of enrolled students in our new degree is increasing every year, so growing is our next challenge.