What is a Transmit Receive Module?

Last edited: 2024-10-26 16:45:17

A Transmit Receive Module carries out the necessary functions in a phased array just prior to the signal being emitted through the antenna element. It encompasses both transmission and reception capabilities, which is why it’s called a Transmit Receive Module.

A Transmit Receive Module's (TRM) main components are amplifiers, filters, phase shifters, switches, and a circulator. A schematic for a typical TRM is shown in the figure. The TRM can be switched into either be in transmit or receive mode which alters the path of the signal. Some of the functionality of the TRM is used both while transmitting and receiving, namely the phase shifter and attenuator. The phase shifter is what enables the beamforming to scan the beam in different directions while the attenuator is used to control the signal amplitude. Sharing these components between both modes has several benefits. Not only does it reduce the number of active components that would increase the complexity but it also saves weight and volume. While the savings for a single TRM might be low, the total decrease in weight and volume for an array of thousands of TRMs is significant.

Power Amplifiers in TRMs

Inside the TRM there are two main amplifiers which are shown in the figure above. In the receive path sits the low noise amplifier (LNA) that is in charge of amplifying the received signal from the antenna. This very weak signal is greatly affected by noise which is why minimizing the noise that the amplifier adds is of high importance. In addition to the LNA, a receive protection circuit is used to make sure that signals with too high power do not get into the LNA.

In the transmit path sits the HPA. It is responsible for amplifying the signal to the desired power level and ideally provides perfectly linear amplification. However, to maximize the transmitted power a trade off between linearity and output power usually has to be made. For example, having too much output power will start to compress the output signal causing distortions and these are things an HPA designer has to take into account. Furthermore, an HPA is designed to be presented to a specific impedance, both on the input and output ports. Typically both these are set to 50 $\Omega$ by design, requiring the rest of the system to be designed around the same impedance but this varies for different applications.

In the case of an HPA with a 50 $\Omega$ output impedance that is presented to a load differing from the designed one, reflections will occur. These reflections can greatly affect the HPA resulting in reduced power efficiency and gain which can have a severe impact on its performance or potentially even break it. Making sure that the HPA is protected from reflections is therefore of high importance to achieve greater and more stable performance.

The TRM's RF Circulator

Between the TRM and antenna, a circulator is most commonly used. A circulator is a passive component and a type of duplexer, allowing for bidirectional communication of a single path. In practice this means that signals coming from the HPA in the transmit path will be sent to the antenna and signals coming from the antenna will be sent to the receive path, thereby separating the signals without using a switch to select one path. Additionally, it also makes sure that reflections occurring at the antenna do not travel back to the HPA, degrading its performance. While the use of a circulator handles the main problems of connecting the TRM and antenna it also comes with the drawbacks of being expensive and requiring a considerable amount of volume. Another drawback is that while it typically adds less than 1 dB loss of signal power, the total loss of power can be larger when accounting for reflections between the circulator and antenna.

Transmit Receive Modules in Phased Antenna Arrays

The enabling technology behind Active Electronically Scanned Arrays (AESAs) comes from the ability to control hundreds or thousands of TRMs individually. Each TRM connects to an individual antenna element and is collectively controlled by a beamformer that decides how every TRM should behave at all times. This allows an AESA to create multiple directed beams which can be used for multiple tasks at once or to improve one further.

The most common implementation of AESAs is for radar systems. Radar systems based on AESAs have several advantages compared to other technologies such as mechanically scanned arrays (MSAs) and passive electronically scanned arrays (PESAs). Both AESAs and PESAs have quicker scanning speeds than MSAs since they do not require any mechanically moving parts.

Furthermore, the main difference between PESAs and AESAs is the number of TRMs. While a PESA has individual phase shifters, it only has a single TRM compared to AESAs which have one for each element. This gives many advantages such as allowing for the multibeam functionality of AESAs that allow radar systems to track a larger amount of targets. The individual beams can also be used to transmit multiple frequencies at once to disguise their signals which is a common task within electronic warfare and is not possible with PESAs that only can transmit a single beam. Another key feature of having individual TRMs is that even if a few TRMs fail, the functionality of the array remains which adds redundancy to the system.

Beamformer

Connecting to all the TRMs is the beamformer. The beamformer is what distributes and combines the signal between the TRMs in addition to steering the TRM's respective phase shift and amplitude at all times. When in transmission mode, the beamformer takes a modulated waveform and distributes it between the TRMs, and decides which phase shift should be added to each of them along with the amplitude to get the desired beam. In receive mode, the beamformer still controls the TRMs but instead of distributing a waveform, it is responsible for combining the received TRM signals into a single signal. Both in transmit and receive mode, the beamformer's phase control of the TRMs is crucial to get a beam that transmits or receives in the desired direction.