Interfaces for Frontend Module Test

Production tests on multiport DUTs, such as frontend modules for smartphones, are increasing in complexity. In addition to the traditional RF measurements, T&M equipment must also handle new features and developments, such as DUT configuration via GPIO and handler I/O interfaces, and via the MIPI RFFE interface. Test equipment vendors are meeting the challenge with new hardware and firmware features. By Volker Herrmann and Tanja Menzel.

Today’s mobile radio standards are becoming ever more complex, covering a growing range of functions. This is also reflected by the frontend modules (FEMs) used in smartphones, where the number of integrated components, such as low-noise amplifiers and filters, is on the rise. This diversity also has an impact on T&M equipment, leading to an increase in the number and types of RF measurements required and in the number of ports to be analysed. FEMs with 16, 20, 23 or more ports are now typical.

New front interfaces are being been developed to meet these challenges to reduce setup complexity and accelerate the test process.

MIPI RFFE interface – a necessity for front-end modules

A standardised interface is a key requirement for FEMs in order to ensure interoperability with other components in a mobile device. For example, other mobile phone components must be able to address the frequency selection filters in a frontend module in order to utilise the more than 12 mobile radio bands required, together with standard services such as WLAN and GPS. A working group within the MIPI Alliance developed the MIPI RF frontend (RFFE) interface for this purpose. The MIPI Alliance is a global, non-profit organisation bringing together a number of companies with the objective of defining interface standards for the components used in mobile phones. The MIPI Alliance views its role as supplemental to existing organisations, such as 3GPP. It is organised into several working groups that specify the individual interfaces on a mobile phone. One of these is the RFFE working group, whose frontend interface standard has already been adopted by leading test equipment vendors.

Increased number of ports

FEMs with 16 or more ports are now typical VNA solutions when more than 4 ports are needed. In general there are two possible ways to generate a multiport VNA system: one is to extend the number of ports with switch matrix solutions, the other is to use a true multiport system. Usually a switch matrix solution is good if one needs to extend an existing VNA. But one needs to be aware that complex control software is needed in order to control a switch matrix. By using a switch matrix from the same vendor as the VNA, this is usually already taken care of. For the most comfort, one should use a switch matrix which is full crossbar. In this case there are no restrictions on which ports can be measured in combination with other ports. These solutions are also quite attractive price wise, but this is at the cost of RF performance.

To get the fastest solutions on the market and the best RF performance one needs to look into true multiport VNAs. The increased performance is gained by truly adding more VNA ports and not just adding switches to the existing ports. These additional ports are usually equipped with extra shielding thus increasing the isolation in between the different ports. Furthermore those ports come equipped with additional receivers and less attenuation between the port and the generator signal and thus offer the capability of measuring all ports at the same time and a higher output power. This decreases the measurement time and supports measurements at levels of around 10 dBm and more.

With these different possibilities for doing multiport VNA measurements the best flexibility is given to cover all needs that one might encounter when performing measurements on FEMs.

Integrated interface card in place of external modules

Integrating the new MIPI RFFE interface card with a network analyser makes it possible to program MIPI RFFE interfaces on FEMs directly from the analyser firmware. This significantly reduces the complexity of test setups compared with solutions using external modules. If an external MIPI RFFE control board were used, for example connected to a controller via USB, it would take considerable effort both to address the control board and to synchronise it with the test system and the DUT.

Making interface card functions available as SCPI commands, in addition to the analyser GUI, allows programming of the MIPI RFFE and the GPIO interfaces both for a channel and for a frequency segment. Voltages of the GPIOs can be set independently of one another and also set automatically via a sequencer at the start of the sweep. This functionality reduces the need for an external power supply as DUTs with a supply voltage of up to 10 V and 100 mA can be supplied via the interface card.

With the sequencer, FEM programming can be incorporated into the sweep sequence and adapted as required for the individual frequency segments. The segmented sweep function makes it possible to measure a variety of parameters, including insertion loss, isolation or reflection, in a single sweep for various configurations of the FEM. This results in efficient FEM characterisation.

Where the MIPI RFFE interface card can also be used to read the contents of the registers of a frontend module, this information can be used to improve testing, particularly in production applications. For example, a frontend module ID can be read and subsequently made available during the production process. By reading the registers, the FEM programming can also be verified by checking that the registers were set correctly.

In addition to this functionality, the newest integrated MIPI RFFE interface cards come with the ability to also perform current and voltage measurements. This feature eliminates the need for an extra current- or voltmeter and thus reduces the number of required equipment further. The range can be chosen, in dependence of the used pin, for a range from -2µA to +2µA up to – 100mA to +100mA for current measurements. For voltage measurements a range of up to +10Vs can be chosen.

Software and hardware developments like these will continue to evolve, taking the pain out of production test for increasingly complex multiport devices.

Rohde & Schwarz

By Volker Hermann and Tanja Menzel

Volker Herrmann, RF & Microwave Senior Application Engineer Volker Herrmann received a Dipl. Ing. (FH) degree from the Mannheim University of Applied Sciences in 2000. In 2010, he joined Rohde & Schwarz as an application engineer supporting vector networks analysers. Previous positions include application engineer and product manager for wireless products at semiconductor companies.

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