Speed ​​up phased array antenna testing using high-performance digitizers

Engineers have traditionally used vector network analyzers (VNAs) to perform amplitude and phase measurements on the transmit/receive modules in phased array antennas. Recent improvements in manufacturing technology have enabled more flexible and powerful transmit/receive modules to be mass-produced.

However, calibration and characterization become manufacturing bottlenecks because a complete phased array antenna may contain numerous T/R modules and multiple RF output channels from the array components. Calibration of the measured complex arrays can take days or weeks, depending on the amount of data that needs to be collected. To shorten the test time, a completely different approach must be adopted.

For cross-channel phase and amplitude measurements including transmit/receive modules, the test system using the Agilent M9703A provides up to hundreds of synchronous input channels, which can quickly perform many tests simultaneously. At the same time, the Agilent M9703A can provide the bandwidth required for digital conversion of signals such as pulses and linear frequency modulation pulses (chirps) to meet future complex test requirements.

Preface

Additional production test time increases manufacturing costs. For active antenna arrays, a significant amount of production test time is spent on calibration. While engineers are under pressure to reduce manufacturing costs, they also want to increase the flexibility of their test systems to cover a wide range of use cases and ensure that their test platforms can test higher performance arrays with faster frequency switching performance and higher bandwidth.

This article briefly introduces the important issues related to the use and selection of digitizers in phased array antenna test systems. In addition, it provides a solution to increase test speed using the modular Agilent AXIe M9703A (digitizer).

Key Issues Facing Phased Array Antenna Test Engineers

This article addresses the test challenges faced by phased array antenna engineers, including:

The need for fast, wideband, and high-resolution sampling of the IF signal after down-conversion.

Phase coherent sampling is achieved on all input channels to provide relative amplitude and relative phase measurements.

Depending on the test scenario, different trade-offs between sensitivity and analysis bandwidth are required.

First, the antenna signal enters the microwave mixer block, which down-converts the microwave/RF from the antenna array to an IF within the 3 dB analog bandwidth of the digitizer. The choice of the mixer block and the local oscillator depends on the input frequency and the acceptable conversion loss. The carrier frequency ranges from S to Ka band (2 to 40 GHz).

Next, the signal passes through several possible attenuation/gain and filter stages to fully exploit the dynamic range and front-end characteristics of the digitizer.

Finally, the signal is sampled separately by the digitizer. If the required IF bandwidth is much narrower than the digitizer's Nyquist bandwidth of about 800 MHz (1.6 GSa/s), a digital down conversion (DDC) process can be used to tune the full bandwidth analog-to-digital converter output data to the desired narrower frequency analysis band. Using a DDC reduces the overall noise power in the band, thereby improving the signal-to-noise ratio (SNR).

If the noise power is random and evenly distributed throughout the spectrum, the noise power decreases by:

10 x log（BWFin/BWInit ）。

(Table 1) Sampling rate and bandwidth modes of Agilent M9703A-DDC

Using Agilent M9703A to increase calibration speed

The Agilent M9703A digitizer offers several unique features:

Multiple parallel measurements can be performed simultaneously to increase test speed.

Provides greater bandwidth and more simultaneous input channels than any other network analyzer solution on the market.

Provides a variety of measurement settings from narrow to wide frequency spans, as well as optimal sensitivity and resolution.

The digitizer combines eight high-speed, high-resolution channels onto a single converter card, and multiple digitizers can be integrated into an Agilent AXIe chassis. The Agilent M9505A chassis can accommodate five digitizers, providing a total of 40 data collection channels in a desktop test system.

With its 50Ω DC-coupled input, the digitizer can be easily combined with several upstream signal conditioning and down-converter modules to form a total test solution.

The test solution can be adjusted to suit the different power levels of the antenna array components, the selected test solution IF frequency, and the physical form factor requirements. Agilent provides many products and measurement expertise to assist in the most suitable application. To synchronize the system, a variety of external or system reference frequency inputs can be used. The timing and trigger signals provided by the AXIe backplane include:

100 MHz system frequency

100 MHz PCIe frequency

Point-to-point star trigger for embedded system modules (ESM)

Bidirectional point-to-point star trigger

12-channel parallel trigger bus

The Agilent M9703A is equipped with two I/O outputs that provide status signals that allow engineers to optimize test plans to synchronize the trigger of the digitizer with the action that starts the device under test (DUT) in the next test phase. An example is the "Acquisition Running" status signal.

When the collection state changes from "executing" to "idle", the output level changes to inform the engineer that the test system is ready to accept the change of DUT state. To connect the control PC, you can use the wiring to connect the PCIe and ESM (embedded system module) together, or you can use an embedded processor such as Agilent M9536A to control the collection system from one of the AXIe slots.

Figure 1: Example of phased array test scope configuration

Conclusion

Using the Agilent M9703A as the digitizer core of your phased array antenna test solution means you can be sure of the value of increased test speed and productivity by performing multiple measurements simultaneously. Although fast testing usually means lower measurement resolution, using multiple channels of the Agilent M9703A to perform the same test can be achieved by performing multiple tests simultaneously in one AXIe chassis.

In addition, using DDC digital signal processing algorithms, you can trade off between the signal's analysis bandwidth and noise ratio to create a test solution tailored for each test without having to replace the digitizer hardware.