The evolution of cellular communication standards from 2G to 3G to LTE as well as the increase in frequency bands has impacted how a mobile handset is designed. To support the various standards and differences in licensed frequency bands across the globe, RF designs in handsets are getting more complicated. By Teh Boon Poh, RF/Microwave Application Consultant, Keysight Technologies
Handset manufacturers have to squeeze more RF signal chains and RF front-end components into the already space-scarce phone board. To reduce component count, size and cost, the front-end power amplifier solutions for mobile phones have evolved from discrete single-band, single-standard solutions to integrated modules that support multi-radio standards and multi-frequency bands.
As a result, test solutions for handset Power Amplifier Modules (PAM) have also been modified to cater to the increasingly complicated Multi-Mode Multi-Band (MMMB) amplifier module test. This highly integrated module will require more stringent RF interference and leakage tests to ensure efficient and reliable system performance. Additionally, the new method for controlling RF front-end devices, which is MIPI RF Front-End (RFFE) will require a test system upgrade.
In view of these market driven dynamics, a flexible, scalable and reconfigurable modular test system is of advantage to meet these changing test requirements in the manufacturing environment.
Test Setup For A Discrete Power Amplifier Module
Typical manufacturing test parameters for handset Power Amplifiers Module (PAM) are Gain, Adjacent Channel Leakage Ratio (ACLR), DC power consumption through the measurement of total current of Icc and Leakage DC Current of control pins of Ictl. The other common parameter is Power Added Efficiency (PAE), which is calculated based on the measured Gain and DC total current Icc.
The test signals used to test these power amplifier modules are digital modulation signals, which have to conform to cellular standards such as GSM/EDG, WCDMA, TDSCDMA or LTE TDD/FDD. The specified performance of the power amplifier (Gain, ACLR, PAE) is set at a certain specific RF output power level. Therefore, iterations of these procedures, which is called power servo are required to tune the power amplifier output power to the specified power level prior to measurement.
A basic PXI modular test system is configured with a Vector Signal Generator (VSG), a Vector Signal Analyser (VSA) and a Source Measurement Unit (SMU). Typically, the VSG output digital modulated RF test signals into the input of the PAM under test, and the VSA measures the power amplifier output channel power. The delta between the measured power by the VSA and specified target power level is then feedback to the VSG to adjust the VSG RF power output as a power servo routine. This iteration goes on until the specified target amplifier output power level is reached by the measurement result of the VSA. As this RF output power level is reached, the ACPR and power consumption are measured.
These measurement steps are looped for each of the targeted power levels (High, Medium, Low) and for the different RF frequency test points. In this test situation, the speed of the VSG and VSA is an important element as it contributes significantly to the overall total test time of the PAM.
Most RF amplifier module manufacturers have also included Error Vector Magnitude (EVM) measurement at each of the targeted power level in manufacturing test to ensure that their products meet the stringent quality for the mobile phone manufacturers.
Test Setup For A Multi-Mode Multi-Band PAM With MIPI RF Front-End Control
Multi-Mode Multi-Band (MMMB) PAM consists of multiple power amplifiers with multiple cellular standards and frequency bands. Each standard requires a separate test with a standard signal format and a measurement setup according each standard. A single MMMB packaged PAM Integrated Circuit (IC) has multiple input and output pins. Therefore a switch matrix is added to allow the selection of a particular input and output pin at a time.
The complexity of multiple RF chains in a handset board has led to the introduction of MIPI RF Front-End (RFFE) interface control to unify the RF front-end component control. This standard has enabled simpler bus architecture and lesser pin count in the MMMB PAM IC.
The need for MIPI RFFE interface controls calls for a high speed Digital Input Output (DIO) to emulate an RFFE to control the PAM during manufacturing test. This could be easily accomplished by adding a modular PXIe high speed DIO module to the PXI system.
Test Setup For A Power Amplifier Duplexer
Handset power amplifier manufacturers have the tendency to incorporate duplexers into power amplifiers for numerous reasons, including simpler handset RF board designs to lesser interconnects between ICs. The Power Amplifier/Duplexer or PAD is the result of integration of power amplifier and duplexer into a single RFIC component for more compact designs. Highly integrated PAD may have more than ten duplexers to support the versatility in Cellular LTE bands. This is to ensure minimum interference between bands as noise and isolation tests are absolutely necessary.
With PAD, tests like filter pass band insertion loss test, which is to ensure minimum signal loss, and rejection band isolation test are crucial to ensure no RF leakage into an adjacent RF band. The third order harmonic test has become a critical test for PAD manufacturing. This can be seen in the case of a PAD that supports up to five or more LTE bands. The third order harmonic test will need a VSA to operate higher than 6GHz (Refer to Table 1 for Fundamental bands), which is the current cut-off frequency of a typical low band VSA receiver. Hence, a manufacturing test system upgrade to adopt a higher frequency VSA is now a necessity.
Depending on the number of integrated duplexers, a two port s-parameter measurement or higher will be required to optimise the measurement throughput of the test system. In this case, having the flexibility of adding a PXIe modular Network Analyser will enable this optimisation. The user can choose to use a single module of a two ports Network Analyser, or scale up to a higher number of ports by adding more PXIe Network Analyser modules.
The flexibility of a modular system approach makes it an ideal test solution for power amplifier modular manufacturing. New modular instruments can be plug-in to the existing system to match added test requirements as new cellular or wireless standard evolves.
In addition, RFIC component manufacturers will benefit from the high throughput modular PXIe, which shortens test time, and the ease of re-configurability for modular based system. The scalability feature in a modular system also allows further test enhancement that caters for more complicated test requirements such digital pre-distortion and envelope tracking.