Phased arrays have been used in radar applications for many decades. Recent trends are driving their adoption into other applications such as Electronic Warfare (EW), satellite systems, and even 5G communications. There are several new component technologies that are driving this migration: multiple transmit/receive (T/R) modules on a chip, higher-performance PCB laminates, and the acceptance of GaN as a power amplifier (PA) semiconductor process.
Characterizing a T/R module places a high demand on a test system’s performance and flexibility. The test system needs to support a variety of test modes while maintaining accuracy and constantly improving throughput. In many cases, it must measure high output power while at the same time delivering very low, highly accurate stimulus power. The system must provide inter-modulation measurements on pulsed signals and also on frequency-converting devices under test (DUTs). As a rule, noise figure and/or spectral measurements are also required. T/R modules with separate TX and RX paths and an antenna connector call for three-port measurements and also typically require two LO signals.
A vector network analyzer (VNA) extension unit can support a wide range of measurements in a compact, individually configurable design. It allows the user to configure the VNA for customizable signal conditioning and switching and allows the VNA to specifically provide a suite of measurements on active DUTs under manual or automatic system control. These measurements can be made in both the forward and reverse directions, all with a single connection to the DUT.
This article provides an overview of the basic building blocks of a phased array system and discusses how the nature of their design leads to a challenging test environment. It shows how a VNA extension unit (the R&S ZVAX TRM) performs this wide range of T/R module measurements with a single connection.
Phased Array Overview
The physically stationary phased array antenna beam is steered by changing the relative amplitude and phase to each radiating element. This steering works on both transmit and receive. By changing the relative amplitude and phase across the beam, we can steer the beam and reduce the side lobes of the resulting beam pattern (Figure 1). On the transmit side, sidelobes are usually unwanted radiations of energy in unwanted directions. On the receive side, they allow signals into the receiver from unwanted directions when the system is in receive mode.
Figure 2 shows the major blocks in a phased array system. These blocks are similar for either a communications system or a radar/EW system. The left block shows the Receiver-Exciter if the system is a radar or a modem if it is a communications system. This block generates the signals used for transmitting and houses the receivers in receive mode.
The next block represents the beam-forming network, which is responsible for routing the transmitted and received signals to the many T/R modules and radiators in the system. The beamforming network has a small number of (or one) input signals and many output signals. The beamformer drives the next block, which are the system’s T/R modules. These modules are responsible for controlling the amplitude and phase of the transmitted and received signals to the individual elements of the antenna.
Drilling down to the individual building blocks of the system, let’s start with the analog beamforming network (Figure 3). These are usually made of passive structures like splitters and combiners. The structures are configured as a distribution network by combining these into the desired topology for the specific system. They are usually manufactured out of a multilayer PCB, where the individual passive structures can be printed and interconnected using strip-line and micro-strip structures.
The beamforming networks need to be tested for insertion loss and insertion phase across their many paths. The performance requirements are usually quite tight — typically, measurement tolerances of 0.1 dB and a few degrees. It becomes difficult to hold this uncertainty as the frequency increases and becomes very difficult as the frequencies increase above 8 GHz. Multi-port VNAs are the measurement tool of choice to characterize these networks.
Figure 4 shows an example block diagram of a T/R module. The upper leg is for transmitting while the lower leg is used for receiving. There are input and output switches to change from transmit or receive modes. The amplitude weighting and phase shifting is done using the variable gain amplifiers and phase shifters in each leg. These are controlled digitally and can be varied over a large value of states. There is also a PA used for transmit and an LNA used for receive. These components also need to be characterized for noise figure, compression, intermodulation, and linearity.
In R&D, each T/R module design needs to be tested and characterized over each one of these states. For many designs, each T/R module may also require testing in production. Combining the large number of T/R modules with many potential gain/phase states drives the need for improving test throughput times.
The comprehensive characterization of T/R modules and active DUTs requires test equipment that supports diverse test scenarios. For example, test equipment must be able to provide and handle both very high and very low powers without any modifications to the test setup or perform intermodulation and group delay measurements on converters with an embedded local oscillator (LO).
The extension unit adds additional signal conditioning to the network analyzer. This offers several economic and technical benefits. If several network analyzers are available to perform various tasks, not all analyzers need to be equipped with all options. This means that fewer options are needed for the network analyzer itself, thus reducing investment costs. The extension unit offers another advantage by providing additional protection for the network analyzer during measurements, as its high-power front end may be used instead of the VNA ports.
Adding signal conditioning to the network analyzer offers yet another benefit: measurements requiring no signal conditioning can be carried out using just the standalone VNA, providing uncompromised network analyzer performance, i.e. with the instrument’s superior dynamic range, sensitivity, and stability.
The stimulus and measured signals from the network analyzer are fed to the extension unit via the network analyzer’s direct generator/receiver inputs. The signals are modified via the extension unit and either output through its ports or fed through the network analyzer ports. It includes a high-power test set with access to the network analyzer’s generator and receiver paths.
The extension unit allows test engineers to add user-supplied components into both the source and measurement paths. This expands measurement capabilities by inserting amplifiers, attenuators, or even other test instruments into the measurement path. The internal hardware and switching allows this capability to be used while keeping the DUT connected. These elements may be software-controlled and switched in as required.
In addition to the standard features, the extension unit also offers optional pulse modulators, combiners, output amplifiers, and low-noise preamplifiers (LNAs). The individual components are activated via mechanical switches as required for a given measurement setup.
The optional amplifiers may be switched in on the source paths. They may be used to overcome any additional loss in the signal path and increase the output power at the test ports of the extension unit as well as the test ports of the VNA. This allows output power at the ports to be between +5 and +15 dBm, depending on installed options and frequency. Optional LNAs may be added to the extension unit. The LNAs may also act as a pre-amp for noise figure measurements on active DUTs.
The ability to implement complex signal routing by means of a single switch and control platform is another option. The switch and control platform base unit can be a manually operated instrument or it can be controlled via an Ethernet interface. This interface allows connection to a PC for automatic and manual control via a software application.
Figure 5 shows a generic block diagram of a T/R module, with all its functional blocks. The transmit (TX) side is shown in the upper path and the receive (RX) side is shown in the lower path. There are input and output switches that control whether the module is in transmit or receive mode. The amplitude and phase of the TX and RX signals are adjusted by the variable phase shifters and variable gain amplifiers on their respective paths. These adjustments are usually controlled digitally and allow the beam to be steered across the face of the array. Typical systems have hundreds to thousands of these T/R modules in each phased array.
The TX path uses a PA for transmitting while the RX path uses an LNA. Typically, test engineers want to look at the TX S-parameters from port 1 to 2, as shown on the diagram, as well as the RX S-parameters (port 2 to 1). The S-parameters are used to characterize all amplitude and phase settings of the digitally controlled components. The LNA and PA are also characterized for noise figure, intermodulation, 3rd order intercept (3OIP), and compression. The input and output matches are also important. Since T/R modules are often used in radar systems that operate in a pulsed mode, pulsed measurements are typically a requirement.
Single Connection Characterization
With a single connection, the network analyzer and extension unit system can measure and characterize both the forward (TX) and reverse (RX) directions. The receiver characteristics like gain, noise figure, and linearity are measured along with the transmitter characteristics like output power, compression, and intermodulation distortion. Measurements may be made in either direction under swept, CW, multi-tone, or pulsed stimulus. Data is typically collected over all the T/R module states of gain and phase settings. If the VNA has an embedded PC, it can be used to directly control the T/R module directly. This simplifies test setups and reduces test time. Hardware handshaking also allows for buffered data transfer while acquiring data for further speed improvement.
When measuring the RX side of the module, four VNA sources allow two-tone stimulus using the internal combiners of the extension unit. For these receiver tests, the internal preamp can be used to measure the noise figure of the LNA. This setup can be customized with the addition of user-switched inserted components in the source or measurement loops. The stimulus to the module can be either stepped CW, pulsed, or two-tone. The pulsed and two-tone stimulus are usually stepped across the frequency range of the module. The stimulus can also be swept input power.
On the transmit side, high-power couplers are useful when making high-power PA measurements. As with the RX tests, this setup can be customized with the addition of user-inserted components in the source or measurement loops. The transmitter stimulus may also be either stepped CW, pulsed, or two-tone.
Parallel Module Testing
Testing parallel modules or partial modules can be a great way to speed up measurement throughput and reduce the cost of testing. Measurements on two parallel receive chains can be done by using all available ports. Similar to the RX path, the same is possible for testing of transmitter paths in parallel. Adding a simple switch matrix allows for multiple T/R module bi-directional testing. With a single connection, the system can measure the entire suite of T/R module measurements in both transmit and receive mode.
As phased arrays spread into more applications, the need for improved test solutions is increasing. Accurately characterizing T/R modules at a high throughput rate can challenge a test system’s performance and flexibility. By adding user-supplied accessories and instruments to the VNA, almost any combination of tests may be performed.
This article was written by Darren McCarthy, Technical Marketing Manager at Rohde & Schwarz America, Columbia, MD. For more information, visit here .