Multi-DUT RF Conditioning Design Considerations
By Robin Irwin, Aeroflex
Multi-DUT testing is becoming a standard requirement for cellular and connectivity manufacturers to increase test utilization. For both the test and system engineer, this presents test challenges and exposes the importance of RF specifications when designing a system.
Often this requires a good understanding of the capability and specifications of the RF conditioning element in the system.
What is a Multi-DUT System?
There are many terms used by engineers, test vendors and chipset vendors. Often the terms are important as they mean different things, sometimes describing the overall strategy being adopted, others referring to specific functions or parts of the system. Terms commonly used include multi-up testing, multi-DUT testing, parallel test, instrument sharing, multi-channel testing, RF conditioning, RF routing/distribution, device multiplexing, ping-pong test—the list could go on. The feature article in Microwave Product Digest, November 2013  “Flexible Approaches to Multi-Device Testing of Mobile Terminals” provided a background on the key elements to a test system and the options by which they can be used in a variety of multi-DUT scenarios. As a quick recap, these were some general test system elements that were introduced:
• RF test resource—for most applications, this refers to an individual signal generator or signal analyzer. Multiple test resources exist to form a multi-DUT test system.
• RF channel—a combination of signal analyzer and signal generator. In certain test scenarios for multi-DUT, a number of RF channels may be replicated.
• RF conditioning—the RF routing and interconnection between the RF test resource(s) and (often multiple) DUTs and antenna port(s).
• RF fixturing—additional RF routing, device handling or external equipment, which is considered external to the test equipment and necessary for production flow.
The term multi-DUT test refers to testing of multiple devices, encompassing a range of potential scenarios. In effect, it refers to the intention of the test engineer to use the RF test resources as efficiently as possible to test multiple devices in the production flow. This is highly likely to require RF conditioning to enable this to happen, and may involve multiple RF channels as part of the system. We wish to focus on the RF conditioning.
Multi-DUT/Multi-Antenna RF Conditioning
For years, RF conditioning modules have provided integrated test solutions—multiple DUT antenna ports, multiple access technologies (e.g. TDMA, FDMA), MIMO, diversity requirements and interference tests—where the RF conditioning typically is comprised of of switching and combining elements.
Virtually all modern devices consist of multiple antennas. For example, the use of MIMO in connectivity and its increasing use in cellular. This places new demands on testing for calibration and verification test, and in turn the capability of the RF conditioning unit. A good example of this is test time in calibration of cellular chipsets which have both primary and diversity receive chains. Calibration of each receiver at a time increases test time if the RF conditioning unit can only switch between chains. A flexible RF conditioning unit is one that can provide that isolation between the two chains as required, but optionally split the downlink signal to both chains and accurately level the single signal generator input. This enables calibration of both receivers at the same time in an effort to reduce test time. Conversely, that ability to switch to each receiver in turn might be the preferred method for testing receivers on a MIMO WLAN device, ensuring that there are no crosstalk issues on the board.
Multi-DUT is probably not new to the manufacturing test engineer or system engineer who uses modular equipment and has to specify how best to interface with devices. It might well be a new approach for a test engineer who has been using cellular or connectivity one-box-testers (OBTs) and who sees the interfacing with the test equipment as part of the RF fixturing.
One-Box Testers vs Modular PXI?
RF conditioning for many OBTs is integrated as part of the test equipment. It is therefore taken for granted by many test engineers that the RF conditioning provided is capable of the demands of the application the instrument is targeted for. In terms of specifications, the RF conditioning element in the system is often specified as part of the overall system. Equally, being integrated as part of an instrument sometimes restricts flexibility as the choice of OBT as a more general RF test platform, which can reduce lifespan.
A middle ground now, more often adopted by test vendors, is to use the modularity of PXI, but to package it into a system configuration as an OBT/box solution. This can often look like a compromise in flexibility given the use of PXI behind the scenes. The flexibility of such an approach depends on how easy it is to replace components in service for reconfiguration.
Also, whether the software architecture permits an easy change and new adoption of the RF conditioning test components. By way of example, many tier-one cellular vendors and companies invest heavily in RF conditioning design in conjunction with RF fixturing. They implement a multi-DUT technique or multi-antenna setup for deployment in volume. RF conditioning PXI modules designed for any application use where the specifications fit with the task at hand can be replaced/changed to fit evolving needs. It is worth understanding what those design considerations are.
10 Multi-DUT RF Conditioning Design Considerations
Adoption of parallel multi-DUT techniques is much easier when the test vendor has addressed many of the critical performance considerations for making this work across many devices in a modular fashion. Many of these specifications are appropriate for enabling repeatable, high yield, high volume cellular and connectivity testing.
Ten key factors designed into off-the-shelf RF conditioning units for multi-DUT or parallel multi-DUT testing should include the following:
1. Wide and uninterrupted frequency coverage—an uninterrupted range covering cellular and connectivity technologies is essential for testing modern devices. This extends the usable life of the RF conditioning element. It allows for test platforms to be reconfigurable to address a mix of products and technologies. A recent example in cellular where this has been evident is with the introduction of new LTE bands. For connectivity, WLAN in new licensed and unlicensed spectrum, such as 802.11af and 802.11ah.
2. Passive or active conditioning—choices may exist where the RF conditioning contains active components that can be controlled, such as gain and attenuation. An independent multi-stage gain setting for forward and reverse paths provides flexibility for adjusting each DUT test port.
3. Path loss equalization—path loss through each path to the specific device will be different.
Path loss equalization is essential to provide an accurate power level from the signal generator to all the devices for synchronization and for receiver measurements (such as RSSI and sensitivity). This is achieved by balancing losses in an active RF conditioning unit, something that external components may not address. A relative error between DUT ports of <0.25 dB would be a guide.
4. 50-ohm termination—for cellular calibration in particular, chipsets are including more capability inside them to do internal calibration. For internal calibration, no test equipment interaction is required with RF test resources. However, it is common in a multi-DUT scenario to connect devices before the test resources become free to test the device in question. Isolating paths with a test port terminated to 50 ohms is important for the effectiveness of the internal calibration prior to RF test.
5. List mode—particularly where RF conditioning is active (such as gain stages) and where multi-DUT test scenarios are more complex, having the ability to accurately control and sequence operation is essential. For example, routing list addresses using a PXI backplane and having separate source and receiver channels for independent control of downlink and uplink operation.
6. Input power handling—an RF conditioning unit which can handle high power transmissions makes it more useable across different technologies, especially where signal types exhibit high peak to average power levels. +34 dBm or higher CW input is a good guide.
7. Port isolation—essential to avoid cross talk between devices. Using common external off-the-shelf components may not provide a sufficiently high enough isolation. Adding attenuators can help but adds to the system loss. Isolation is also required between RF input and RF output ports which connect to the RF test resources in the test equipment.
8. Temperature stability—an active RF conditioning unit can monitor and mitigate the effects of temperature drift on a production line. This can be achieved by using calibration data inside the conditioning unit to provide compensation for gain modes/levels to receive channels.
External components attempting to achieve the same functionality often cannot be as tightly integrated/controlled. This contributes to repeatability of test in general.
9. Switching Time—it is important that the multi-DUT RF interface from the test equipment to the devices themselves is not a bottleneck. This is in terms of both the time taken to control (through a programming interface) and the physical switching itself. For most cellular and connectivity applications, a switching time of <100 us is a guide.
10. Level settling time—finally, switching time should be considered alongside settling time. Level settling to within 0.1 dB of final path loss would be a suitable target with respect to the previous switching time of <100 us.
Figure 2 summarizes some of the key design criteria mentioned above with some recommended specifications for adequately testing modern cellular devices in a production context for high yield.
Securing investment in time and effort in system designs can be reduced with modular test hardware and RF conditioning which is designed for a variety of applications and technologies. Furthermore, chipset specific multi-DUT off-the-shelf test vendor solutions provide those key specifications by design within modular and customizable test architecture, ready for volume deployment.
 “Flexible Approaches to Multi-Device Testing of Mobile Terminals,” Microwave Product Digest, November 2013.
 Aeroflex PXI Maestro http://ats.aeroflex.com/modular-instrumentation-pxi-products/pxi-systems-solutions/pxi-software-products/pxi-maestro
 Extending Test Capabilities of the PXI 3000 Test Solution with 3060 Series RF Combiner Modules http://ats.aeroflex.com/support/technical-support/product-support-documents/3060-app-notes/3060-app-note
 Aeroflex PXI 3066 Multi-Way Active Combiner Data Sheet http://ats.aeroflex.com/support/technical-support/product-support-documents/3060-data-sheets/3060-data-sheet
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