RF to Bits

Figure 1 – AESA Antenna with Visible TRM Antennas

In this article, Mike Barrick (Business Dev Manager, NI) looks at the development and testing of Active Electronically Scanned Arrays (AESAs) and their Transmit/Receive Modules (TRMs). The article explores the transition from analog TRMs to digital TRMs (DTRMs) and highlights the challenges and advances in testing these components. Barrick emphasizes the critical role of NI's PXI hardware in providing a comprehensive and scalable test solution that meets the unique requirements of mixed-signal DTRMs. This highlights how modern radar technology achieves high performance and the sophisticated testing required to ensure reliability and efficiency.

AESAs and TRMs

The first scanned phased arrays used a single transmitter and receiver connected to all antenna elements through phase shifters. Today’s Active Electronically Scanned Arrays (AESAs) use many solid-state Transmit/Receive Modules (TRMs), each connected to an antenna element. Figure 1 shows an example of a deployed AESA with TRM antennas visible on the face of the antenna.

Re-architecting of the AESA with TRMs was made possible by advancements in semiconductor technology in the 80s, including MESFETs and JFETs for transmit power amplification, and GaAs for low-noise receive amplification. Over time, the size, cost, and power consumption of TRMs have been reduced, with the potential for future commercial applications.

The Relationship of Radar Beamwidth to Array Size and Number of TRMs

One of the primary factors affecting radar system performance is the beamwidth of the antenna. Assuming that the antenna element used with a TRM is a half-wave dipole, this would spread the transmitted energy over a 78 deg beamwidth, with similar performance on receive. While this might be acceptable for gross detection of aircraft or other objects, this broad beamwidth would mean that radar returns are received from a wide range of “targets” and “clutter” within the beamwidth including undesired aircraft, foliage, and other elements. Narrower beam widths would be more beneficial for higher performance.

Increasing the AESA size using more TRM simultaneously achieves narrower beamwidth and higher gain. Beamwidth is inversely proportional to array size, while gain is directly proportional to array size. As a result, larger arrays with increasing numbers of TRMs would provide higher performance with the ability to pick out single targets at longer ranges and reject the effects of clutter. Although large arrays may be desired for increased radar performance, it’s easy to see that there are limitations on size based on limited available “real estate”, as well as total cost for multiple TRMs (including tests).

Definition and Types of TRMs

A TRM provides a range of functionality in a radar system including:

• High Power Amplification (HPA) for the transmitter
• Low-noise amplification (LNA) for the receiver
• Digitally controlled phase shifting to steer the overall AESA
• Digitally controlled attenuators to set power levels in the TRM

Figure 2 shows a high-level block diagram of an analog TRM.

Figure 2 – Block Diagram of an Analog TRM

Newer Digital TRMs (DTRMs) receive digital data from the radar system and convert it back to digital on the receive side. Often, this leverages newer “Direct Sampling” techniques to convert received RF to digital as close to the antenna as possible, enabling radar processing in the DTRM.

Figure 3 shows a high-level block diagram of 2 DTRMs with antennas.

Figure 3 – Block Diagram of 2 DTRMs in an AESA

AESAs configured with DTRMs offer advantages including reduction of module weight and size due to increased digital vs. analog content. However, the evolution of DTRMs also presents new challenges. Foremost among these is how to test DTRMs from “RF to Bits” in the development, verification, and production test phases.

What is Required to Test a TRM?

Analog TRMs have been tested using systems of Vector Network Analyzers (VNAs), Signal Generators (VSGs) and Signal Analyzers (VSAs) since their inception. Depending on the stage in the DTRM development/verification/production cycle, each of these instruments can be used to extract different and varying levels of measurement data to support test objectives.

VNAs fundamentally provide small signal S-parameter measurements such as gain, input match, and output match, and enable calibration of the TRM using a range of transmit and receive gain and phase settings. In addition to CW measurements, some VNAs can also be useful for pulsed measurements, enabling the measurement of semiconductor devices in the PA that are unable to operate at 100% duty cycle.

The pairing of VSG with VNA provides additional measurement capability that will be required for some stages of testing. These instruments enable the test engineer to excite the TRM with a customizable waveform and measure the signal on the output. A wide range of potential measurements are enabled using these instruments including Noise Figure (NF), Adjacent Channel Power (ACP), Error Vector Magnitude (EVM), Third Order Intercept (TOI), Power Added Efficiency (PAE), and others.

As described in the previous section, DTRMs no longer have an analog input/output for testing, so traditional instruments are of limited utility. New solutions matching the mixed signal nature of DTRMs are required, with the capability to make new measurements that are analogous to VNAs, VSGs, and VSAs.

New Measurements for DTRMs

The major change in the transition from Analog TRMs to DTRMs is the replacement of the RF port from the radar with a digital serial and/or parallel port. Measurements that are similar to traditional measurements are still required, but new means of emulating digital signals from the radar are needed.

While traditional VNA RF in/RF out measurements are no longer available for DTRMs, analogous measurements with Digital in/RF out and RF in/Digital out are now possible. Assuming that digital data is supplied to the DTRM to activate a specific RF frequency/phase/amplitude output and sequenced to the next RF state, swept measurements similar to VNA S₂₁ measurements can be constructed. Likewise, assuming that RF at a specific RF state is supplied to the DTRM and sequenced to the next RF state, swept measurements similar to VNA S₁₂ measurements are possible. If desired, output match (S₂₂) could also be measured using traditional RF techniques.

Traditional VSG/VSA measurements could be emulated using similar techniques. Analogs to traditional TX measurements such as Power, ACP, EVM, TOI, PAE, and others could be constructed using the appropriate digital data into the DTRM and measuring RF using a VSA. Likewise, analogs to traditional RX measurements such as BER/BLER and NF could be constructed using the appropriate RF state and data stream into the DTRM and measuring data on the digital side.

The Value of PXI for the DTRM Test

Today’s modular measurement solutions enable the user to configure multi-function instruments in a single chassis, matching the mixed signal Digital+RF requirements for DTRMs. A combination of serial and parallel data modules along with RF modules meets unique test requirements for the mixed signal digital/RF nature of the DTRM. When paired with CW and pulsed power supply modules, the result is a compact solution offering a unique set of measurements fitting DTRM test requirements.

PCI eXtensions for Instrumentation (PXI) is the leading modular measurement solution in the market, and a wide range of modules matching DTRM test needs are available. National Instruments (now NI, an Emerson company) founded PXI in 1997 and has developed a wide range of modular instruments matching industry requirements since that time. Relative to DTRM test requirements, this includes:

• Vector Signal Transceiver (VST) modules combining both VSG and VSA functionality in a small form factor
• VNA modules including the newest version, that enable combined VST and VNA measurements on a single set of ports
• Source Measurement Unit (SMU) modules to provide CW and pulsed power to the System Under Test (SUT)
• Tight timing synchronization of all modules in the chassis using the PXI backplane

The new PXIe-5842 VST instrument is a highly capable instrument capable of RF measurements from 30 MHz to 26.5/54 GHz. Options for 2 and 4 GHz of Instantaneous Bandwidth (IBW) exceed typical bandwidths used for modern radars. Measurement algorithms are available to accelerate the implementation of customizable measurements including NF, ACP, EVM, TOI, PAE, and other parameters.

The optional addition of the PXIe-5633 module enables measurement of DTRM output match (S₂₂) as needed.

When paired with additional NI PXI modules for High-Speed Serial (HSS), digital control, and power supply, unique single-chassis solutions can be assembled fitting complex measurement requirements for mixed-signal digital/RF DTRMs.

Conclusion

Modern AESA-based radars use a large number of TRMs to achieve high performance with narrow beam widths and high gain. Each of the TRMs in the AESA must be tested in development, verification, and production to verify performance in design and guarantee performance on the battlefield.

Test solutions for legacy analog TRMs have consisted of VNAs, VSGs, and VSAs, but newer DTRMs require evolved capability to transmit and receive digital data at the same time as RF measurements. NI’s PXI instrument portfolio has the unique ability to meet the measurement requirements of DTRMs, enabling the user to build a custom mixed-signal instrument using available VST and HSS modular instruments with power supplies and VNAs to match overall test requirements.