Developing a Reconfigurable RF Transceiver Lab Kit

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"The reconfigurable RF transceiver lab kit provides an efficient interactive learning lab kit to gain RF transceiver knowledge. It is just like LEGOs because we can dissemble and reconfigure the transceiver structure to illustrate the RF functions beneath the transceiver. This bridges the gap of co-designing software and hardware of an RF transceiver for my future job."

- 陳為暘 - Wei-Yang Chen, 國立中正大學電機工程所

The Challenge:
Illustrating the interaction of an RF transceiver between the transceiver system and its building blocks, providing an understanding of how the building block characteristics combine to determine the system performance, and designing a software-defined measurement suitable for RF training and education.

The Solution:
Using NI LabVIEW software, an NI PXIe-5641R RIO IF transceiver, and the specially-developed RF module to form a reconfigurable RF transceiver lab-kit.

Author(s):
陳為暘 - Wei-Yang Chen - 國立中正大學電機工程所
黃明輝 - Ming-Hwai Hwang - 國立中正大學電機工程所
張盛富教授 - Prof. Sheng-Fuh Chang - 國立中正大學電機工程所
呂佩諭 - Pei-Yu Lyu - 國立中正大學電機工程所

Current textbooks introduce radio frequency (RF) systems as single stand-alone systems instead of systems that can be disassembled to give students a deeper understanding of the interaction between each subcircuit and the entire system. Thus, we first need to effectively illustrate the interaction between all the subcircuits and the entire RF system. The current hand-on measurement practice in an electrical circuit lab uses a vector signal generator, vector signal analyzer, network analyzer, or frequency spectrum analyzer. These professional devices are necessary for research and development, but they are expensive for college teaching and corporation in-job training. Therefore, we also need to design an effective measurement set-up suitable for RF training and education.

For our system, we incorporate NI LabVIEW system design software, an NI PXIe-5641R RIO IF transceiver, and the RF transceiver modules designed by our team, which can be flexibly grouped to form a reconfigurable RF transceiver lab-kit. In the design of this lab-kit, we built the basic RF circuit into a single building block (like a LEGO® block), and each block includes unique functions. Based on the target application, students can reconfigure the building blocks to build up the specialized RF transceivers. Then use LabVIEW to observe the individual building-block characteristics and how their characteristics combine to determine the system performance.

Using Modular Hardware for Reconfigurability

An RF transceiver system can be realized with different transmit and receive architectures. This lab-kit was developed for quadrature heterodyne architecture, which is widely used in current and emerging wireless and mobile communication systems. Figure 1a shows the architecture schematic diagram.  Students can become overwhelmed when seeing an entire system, and they don’t know how the system interacts with its subcircuits. Therefore, we designed each subcircuit of the system into an individual building block, such as the antenna, RF filter, RF low-noise amplifier (LNA), RF power amplifier, and RF oscillator blocks (Figure 1b). As with LEGO blocks, this design provides students with imagination and innovation. Students can arrange different characteristics of blocks to form a transceiver architecture with distinct system performance, which is called reconfigurability. For example, LNAs using different noise figures, local oscillators with different phase noises, power amplifiers of different saturation, and filters with different selectivity, will create transceivers with different functions.

Figure 1. (a) The RF transceiver quadrature heterodyne transceiver architecture, (b) photographs of the circuit buidling blocks

Circuit Simulation With AWR Microwave Office

The dual-band bandpass filter and the concurrent dual-band amplifier are used as exemplary design for active circuits (circuit simulation) and passive circuits (electromagnetic simulation).

Dual-Band Bandpass Filter

This 0.8/2.6 GHz dual-band bandpass filter circuit was designed with the concept of stepped-impedance resonator to realize the dual-band bandpass feature and was built with FR4 board material. Figure 2a shows the microwave office circuit simulation using full EM simulation. Fig 2b shows the simulation and measurement of S parameters, with the first pass-band ranging from 791 MHz to 821 MHz and the second passband from 2570 MHz to 2690 MHz, respectively. The simulation and measurement agree excellently well.

Figure 2. Dual-band bandpass filter, (a)AWR Microwave Office simulation schematic,  (b) simulation and measurement results

Concurrent Dual-Band LNA

The Philips BFG425W transistor were used as the amplifying transistors for the concurrent dual-band LNA (Figure 3a). The matching circuit uses dual-band matching structure to optimize the amplifiers’ concurrent dual-band amplifying performance. Figure 3b shows the use of AWR circuit simulation software for the concurrent dual-band LNA design. The operating frequency bands are 790 MHz to 920 MHz and 2570 to 2690 MHz, respectively, with the input and output losses larger than 10 dB, and the gain larger than 20 dB, as shown in Figure 3c.

Figure 3. Concurrent Dual-Band LNAs, (a) circuit structure, (b) AWR Microwave Office simulation schematic, and (c) simulation results.

Circuit Layout and Packaging With NI Ultiboard

When working on the RF layout, the routing and bending of transmission lines, via-hole grounding, and power-line ripple bypassing must be carefully taken into account since their induced parasitics considerably affect the circuit performance. All the subcircuits were packaged inside an aluminum alloy case. The RF I/O subminiature version-A connectors (SMA) were assembled on both sides of the metal housing and a 2.54 dual 10-pin I/O control port was placed on the top cover. For operating convenience during the experiment, all the circuit board size was set to 5x5 cm2. We used the NI Ultiboard software for circuit layout design. The element library in Ultiboard provided many device packaging forms, and more geometric plotting features compared to other circuit layout software, which made it flexible and convenient for RF circuit design. Further development of the more diversified geometric shapes and higher integration with AWR’s simulation software, will bring more powerful assistance to the RF circuit design. Figure 4a and Figure 4b show the transmitter’s power amplifier circuit layout in Ultiboard, and Figure 4c shows a photograph of the implemented circuit.

Figure 4. Power amplifier with NI Ultiboard, (a) circuit layout schematic, (b) 3D schematic, (c) circuit photograph

Virtual Measurement: $20K USD Virtual Instrumentation Finishing the Job of $200K USD Physical Instruments

As described above, we performed circuit design, simulation, and layout, and organized all the subcircuits to realize any desired RF transceiver functions. Then we faced the problem of measurement. Typically, people used the equipment set-up from a vector signal generator, vector signal analyzer, network analyzer, or frequency spectrum analyzer. These precision instruments were best used for research and development. They were be over-equipped if used for teaching or lab training. Therefore, we developed an innovative, economic measurement system to fill the gap in the current market, where no such measuring system exists. We found that the NI PCI-5640R transceiver and the NI PXIe-5641R transceiver met the needs for these purposes. In particular, the PXI transceiver module, used in collaboration with a standalone cabinet and controller, can be scaled to become high-end RF transceiver systems for the MIMO multi-antenna RF system measurement.

We incorporated the LabVIEW software, RIO IF transceiver modules, and the RF transceiver modules which could be flexibly grouped to accomplish a reconfigurable RF transceiver system, as shown in Figure 5. LabVIEW controlled the variables of the modulator signals, and then the NI PXI-5641R transmitted the chosen modulator signals. We adopted the sample programs, provided by NI, when we started developing the LabVIEW interface, but in practice we needed different software windows for signal transmission and analysis. We further integrated all the sample programs to a single interface (Figure 5). We not only made internal improvements to each sample program, but we also enriched and simplified the interface so that students could intuitively and clearly see the parameters and make adjustments. Currently, this reconfigurable RF transceiver system teaching kit, in collaboration with the PXI system, can be used to develop a dozen different lab teaching kits. In the future, by using the software coding flexibility of LabVIEW, students can even develop applications to meet different communication protocols and standards to satisfy the needs of wireless communication courses.

The PXI system, together with the cabinet and the controller, successfully replaced the vector signal generators and vector signal analyzers and improved our original system, largely reducing the teaching costs. The LabVIEW GUI is intuitive and fast for program coding. LabVIEW software also provides convenience for design, which helps us successfully integrate the other features.

Figure 5. Reconfigurable RF Transceiver Lab Kit (RF LEGO)

Demonstration of System Reconfigurability

Noise Figure Reconfigurability

Using the noise figure (in dB) of the system as an example, from the literature, we know that NF is related to signal-to-noise ratio (SNR). A lower NF of a receiver means a better SNR of the signal. Below is the calculation formula of an RF receiver’s noise performance:

In this formula, F is the noise factor (=10NF/10), G is the available power gain, and n denotes the order of circuit stage. Using this formula, we can tell that the receiver’s noise factor is dominated by the first LNA. Therefore, we designed the first LNA stage with different noise figures but maintaining same gain, named as the noise-reconfigurable amplifier, as shown in Figure 6a, where four different noise figure values were designed. Now, based on the formula (1), we can obtain the changes of the RF receiver’s overall noise factor for the first LNA with four NFs. Take quadrature amplitude modulation (QAM) as an example. A bad SNR will cause each constellation of the constellation diagram to diffuse. Figure 6b shows the measurement result of the impact of noise changes on the system. This clearly shows the changes of its 16-QAM signal under different NFs, agreeing with the theory.

Figure 6. A wireless receiver’s NF Impact on signal quality, (a) noise-reconfigurable amplifier circuit block, (b) the constellation diagram 16-QAM signal under different NFs.

Beamforming Lab: Seeing the Radiation Beam

In a wireless transceiver system, the radiation pattern is also an indispensable parameter. In this lab, we can connect a beamformer to the reconfigurable RF transmitter’s antenna to form a 1 x 4 phase array antenna system. This allows the antenna’s electromagnetic wave output to have a higher directivity. We designed a power detection module (Figure 7) for a simple antenna pattern measurement platform so students can quickly ‘see’ the radiation pattern of the transceiver system. When different electromagnetic wave powers are received at the antenna end, we can determine the strength of the antenna’s receiving signal by the number of lit LEDs. The number of lit bulbs out of all 16  LEDs represents the power detection module’s receiving power. Its receiving power resolution is -2 dB, and it is capable of detecting power ranging from -52 to -22 dBm.

Figure 8a and Figure 8b show the experiment results, in which the smaller diagram at the bottom right corner is the ideal antenna pattern of this beamformer. Through this lab, students can quickly observe the beamformed pattern in a phase-array antenna system.

Figure 7. Power detection module

Figure 8. An illustration of measuring antenna pattern by using the power detection module .

Conclusion

We developed a reconfigurable RF transceiver lab-kit and related textbooks. The RF transceiver lab-kit includes an NI PXIe-5641R RIO IF transceiver, and the specially-developed RF module.  It has three breakthroughs:

  1. System Reconfigurability—Incorporating reconfigurability feature into each subcircuit of the system, such as noise reconfigurability and saturation reconfigurability, reduced the difficulty of changing circuits for different parameters. Users can receive instant feedback while changing each parameter and understand how the subcircuit characteristics combine to determine the system performance.
  2. Circuit Reconfigurability—When users operate the system, they can regroup or replace the subcircuits in the system freely, like playing with LEGO blocks, and observe how they impact on the system performance.
  3. Virtual Instrument—By integrating the NI IF-frequency transceiver interface card with LabVIEW, we reduces the complexities in the conventional system measurement setup and cut down the costs of lab kit proliferation.

The table below shows how this lab kit compares to the conventional RF system:

 

Professional vector signal measurement set up

Reconfigurable RF Transceiver Module

Equipments

Vector signal generator, vector signal analyzer

NI LabVIEW software, an NI PXIe-5641R RIO IF transceiver,

Cost

~$200,000 USD

~$20,000 USD

Setup Environment

Complicated

Simple

Knowledge delivery effectness

Each equipment provides its own display. No combined dsipay.

Good. The GUI gives a clear integrated information display.

Software Support

Needs to be controlled individually.

All are from NI and controlled by LabVIEW.

Measurement capability

Can measure full functions of individual subcircuits and transceiver system.

Can measure the partial function of subcircuit and full functions of transceiver system

Operation

Needs to be set up individually.

Simple. Different parameters are controlled by LabVIEW.

Table 1. Teaching performance comparison between this Lab and the conventional teaching kit

Additional Materials

Figure 9a shows the measurement environment setup using the conventional measurement method. The conventional method uses a vector signal generator, vector signal analyzer, network analyzer, or frequency spectrum analyzer, which induces high costs for schools or vocational training institutes. It  also wastes the powerful functions these tools provide. Using this teaching kit (Figure 9 b) only requires reconfigurable RF transceivers, a PXI module (with an NI PXIe-5641R), and a monitor.

Figure 9. (a) Conventional RF system measurement setup (b) Measurement using PXI Modules in collaboration with this lab kit

Contact information
Professor Sheng-Fuh Chang
Department of Electrical Engineering,
National Chung-Cheng University
168, University Rd., Ming-Hsiung Chia-Yi, Taiwan, R. O. C.
TEL: +886-5-2720411 ext. 33218
E-Mail: ieesfc@ccu.edu.tw

PhD. Student: Wei-Yang Chen
Wireless Communication Lab. 543,
Department of Electrical Engineering,
National Chung-Cheng University
168, University Rd., Ming-Hsiung Chia-Yi, Taiwan, R. O. C.
Tel: +886-5-2720411 ext. 23243,
Fax: +886-5-2720862
E-mail: iyoweiyoung@gmail.com

Author Information:
陳為暘 - Wei-Yang Chen
國立中正大學電機工程所
Taiwan
iyoweiyoung@gamil.com

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