In 2008, 4RF Communications started development of a new range of radio products to augment their existing point-to-point, long-range wireless link product. Used by oil, gas, and utility companies for monitoring and control applications, the Aprisa SR is a point-to-multipoint Smart Supervisory Control and Data Acquisition (SCADA) radio operating in the 400 to 470-MHz licensed spectrum band with a 12.5-kHz channel size and narrowband 9.6-kbps capacity. The Aprisa SR is designed to address the key concerns facing the industry, such as the need for enhanced security, the need to efficiently handle the increasing complexity of SCADA networks, and the evolution to an IP-based and smart grid infrastructure.
The Aprisa SR’s comprehensive feature set and highly reliable, technically advanced radio platform enable a variety of monitoring and control applications that address current and future requirements. The radio is configurable as a base station, remote station, or repeater for seamless integration into any network topology (Figure 1). It supports a large number of serial and Ethernet interfaces in a single box, and has built-in security features.
4RF Communications required a different test strategy to ensure low unit test costs, high throughput, and good test coverage for the new product features. Therefore, 4RF turned to test and measurement company CPE Systems to design and develop a test system for cost-effective production testing of the radio. The test requirements for the product included component and unpowered testing, device programming, radio signal analysis and calibration, and no operator intervention.
4RF Communications outsourced the test system development due to their limited experience with medium- to high-volume test fixtures and limited internal engineering resources. The development was contracted to CPE Systems, which selected the National Instruments PXI platform coupled with LabVIEW and NI TestStand software to provide the flexible test solution.
Test Development Process
Test targets for the system were:
- Test a board in 5 minutes
- Be suitable for a volume of 3,000 products per month
- Have no operator attendance during the test
- Be operated by nontechnical staff
- Access all test points by test probes on one side of the board
- Include a debug facility
- Capable of expanding for future product variants such as other RF bands and bandwidths
The tests were split into three main areas. First was DC testing for testing component values, supply voltages, and current consumption, and functional testing of the low-voltage shutdown, switch panel, and LED indication. The second area was built-in self-test (BIST) for boot loader and software installation, testing of the RAM and flash, and confirmation of Ethernet address allocation. These tests were programmed into the device and accessed through a command line interface. The third area was RF functional testing and calibration, which was used to test and calibrate the transmitter, receiver, and system functions of the Aprisa SR board.
Due to the product development schedule, the test system was developed in parallel with the product, which entailed five re-spins of the board design. However, due to how the test system was designed and specified with flexibility in mind, these PCB redesigns required only one change to the jig. Using NI hardware and software enhanced the ability to carry out concurrent development.
One of the key constraints for the test system was test time, with an overall target of 5 minutes per board. This target required a significant amount of optimization in the RF calibration algorithms to ensure they operated efficiently. The NI PXIe-5663 vector signal analyzer (VSA) and NI PXIe-5673 vector signal generator (VSG) supported this algorithm optimization process.
The Aprisa SR printed circuit board assembly included RF transmission and reception circuits, and had to be tested out of its enclosure. As such, RF interference and screening had to be included in the jig design. This was achieved using the CAD model of the product housing to machine an RF screening enclosure that formed part of the top plate of the jig (Figure 2). This shielding produced results from the test fixture that were close to those achieved with the board in its enclosure.
The Aprisa SR radio has internal data encryption, which makes it impossible to generate simulated data streams to test the receiver sensitivity. Using the VNA, the radio signal recorded and retransmitted using the VSG at different levels to test the receiver sensitivity with actual data. This methodology meant that the encryption process could change in the future without affecting the test system software.
Developing the RF test system presented challenges in terms of the management processes required to coordinate the project across multiple sites, and in terms of the technical challenges that involved testing a complex RF product at high speeds with screening incorporated in the fixture. The outcome of the development was a custom, yet affordable, test system that tests and supports the manufacturing process of the high-performance SCADA radio product.
This article was written by Stephen Patterson of CPE Systems, Chatswood, New Zealand, using hardware and software from National Instruments, Austin, TX. For more information, Click Here .