**2.2 Transmit/receive modules**

Transmit/receive modules (TRM) are the key building block of most telecommunication apparatus, Radars and many other Electronic Systems. Their role is to process the RF signal in both operating modes: transmit and receive. Typically, a TRM operates in a half-duplex manner, i.e. in a certain instant it is either in receive mode or in transmit mode, therefore processing either a received signal or a signal to be transmitted. A possible schematic diagram of a T/R module is reported in **Figure 7**; a TRM is always connected in some way to a radiating element (Ant).

In order to keep the dimension of the TRM as small as possible, some circuits are involved in both transmit and receive mode and therefore need to process the signal independently form the port at which it arrives. Referring to **Figure 7**, such components are the switches (SWT), the attenuator (ATN) and the phase shifter (PHS). Incidentally, the latter is required in systems that perform beam steering and can be avoided elsewhere. The attenuator instead can be inserted for multiple purposes: it can be used to prevent strong RF signals leaking to the following circuits or to obtain beam amplitude tailoring, in phased arrays.

Critical components are the LNA, already described, and the high-power amplifier (HPA) described in the following section. Another critical component is the switch connected to the antenna port. The key feature of this element is to show very low losses. High losses would entail an unacceptable degradation of both received and transmitted signal. In the past, for high frequency applications, this element was often a bulky ferrite circulator. With the advent of GaN semiconductor, well-known for its superior power handling capabilities, MMIC technology has become the standard. Finally, the gain control section in the receive path is used to attenuate strong incoming signals. It is seldom used in transmit mode, since in most application the goal is to transmit as much RF power as possible.

**25**

bandwidths anyway.

*Schematic diagram of a TRM.*

**Figure 6.**

**Figure 7.**

*linear performance at 12 GHz (right).*

TRM for EW [8].

*UWB Circuits and Sub-Systems for Aerospace, Defence and Security Applications*

In Electronic Warfare systems, which notoriously manage UWB signal to contrast different emitters, the maximum to minimum operating frequency of the TRM can be as high as 6-to-1. Higher ratios became unfeasible since the increase in the BW would be obtained to detriment of performance, and, in any case, it could be impractical since it is very challenging to design directional antennas having wider

*UWB 2–18 GHz GaN switch measured insertion loss and isolation over the full operating BW (left) and non-*

For other aerospace and avionics applications, the operating BW is typically

The typical output power is around 5 W (37 dBm) while the RX gain is on average 20 dB. Such performance was accomplished through Multifunction chips and ASIC component integration in new multi-layers technology (Roger 4003/Cu/ FR4) were adopted in order to reduce cost, space, production life cycle and increase integration level. High output power in the transmit mode was achieved using a 4 W

**Figure 8** reports the key-parameters of a highly-integrated GaAs-based compact

The HPA is the key component of any microwave transmission systems, and its

Its role is to boost the transmitted signal's power without adding undesirable signals generated by distortion. At the same time, the HPA should be efficient, in terms of its capability of transforming to the power provided by the DC supply RF power. From a technology point of view, they come in at least two variants: vacuum

performance may have a huge impact on the final system architecture.

20–40% the value of the central operating frequency.

wideband amplifier and by minimizing circulator loss.

*2.2.1 High-power amplifiers (HPA)*

*DOI: http://dx.doi.org/10.5772/intechopen.87095*

**Figure 5.** *Schematic circuit topology (left) and micro-photograph (right) of an UWB 2–18 GHz GaN switch.*

*UWB Circuits and Sub-Systems for Aerospace, Defence and Security Applications DOI: http://dx.doi.org/10.5772/intechopen.87095*

**Figure 6.**

*UWB Technology - Circuits and Systems*

L2/R2, while the two compensated FETs are Q1 and Q 2.

nology for specific high-end applications.

obtain beam amplitude tailoring, in phased arrays.

application the goal is to transmit as much RF power as possible.

*Schematic circuit topology (left) and micro-photograph (right) of an UWB 2–18 GHz GaN switch.*

**2.2 Transmit/receive modules**

technique is depicted in **Figure 5** (left) together with its physical implementation (right). The two compensating inductors and resistors are labelled with L1/ R1 and

The UWB switch shows an insertion loss lower than 2.2 dB, an isolation higher than 25 dB, and a power handling capability better than 38.5 dBm at the 1 dB compression point in the entire bandwidth. The SPDT's key performance is plotted in **Figure 6**. GaN-HEMT technology therefore demonstrates a good level of maturity for microwave power switch applications and as such is becoming the reference tech-

Transmit/receive modules (TRM) are the key building block of most telecommunication apparatus, Radars and many other Electronic Systems. Their role is to process the RF signal in both operating modes: transmit and receive. Typically, a TRM operates in a half-duplex manner, i.e. in a certain instant it is either in receive mode or in transmit mode, therefore processing either a received signal or a signal to be transmitted. A possible schematic diagram of a T/R module is reported in **Figure 7**; a TRM is always connected in some way to a radiating element (Ant).

In order to keep the dimension of the TRM as small as possible, some circuits are involved in both transmit and receive mode and therefore need to process the signal independently form the port at which it arrives. Referring to **Figure 7**, such components are the switches (SWT), the attenuator (ATN) and the phase shifter (PHS). Incidentally, the latter is required in systems that perform beam steering and can be avoided elsewhere. The attenuator instead can be inserted for multiple purposes: it can be used to prevent strong RF signals leaking to the following circuits or to

Critical components are the LNA, already described, and the high-power amplifier (HPA) described in the following section. Another critical component is the switch connected to the antenna port. The key feature of this element is to show very low losses. High losses would entail an unacceptable degradation of both received and transmitted signal. In the past, for high frequency applications, this element was often a bulky ferrite circulator. With the advent of GaN semiconductor, well-known for its superior power handling capabilities, MMIC technology has become the standard. Finally, the gain control section in the receive path is used to attenuate strong incoming signals. It is seldom used in transmit mode, since in most

**24**

**Figure 5.**

*UWB 2–18 GHz GaN switch measured insertion loss and isolation over the full operating BW (left) and nonlinear performance at 12 GHz (right).*

In Electronic Warfare systems, which notoriously manage UWB signal to contrast different emitters, the maximum to minimum operating frequency of the TRM can be as high as 6-to-1. Higher ratios became unfeasible since the increase in the BW would be obtained to detriment of performance, and, in any case, it could be impractical since it is very challenging to design directional antennas having wider bandwidths anyway.

For other aerospace and avionics applications, the operating BW is typically 20–40% the value of the central operating frequency.

**Figure 8** reports the key-parameters of a highly-integrated GaAs-based compact TRM for EW [8].

The typical output power is around 5 W (37 dBm) while the RX gain is on average 20 dB. Such performance was accomplished through Multifunction chips and ASIC component integration in new multi-layers technology (Roger 4003/Cu/ FR4) were adopted in order to reduce cost, space, production life cycle and increase integration level. High output power in the transmit mode was achieved using a 4 W wideband amplifier and by minimizing circulator loss.

#### *2.2.1 High-power amplifiers (HPA)*

The HPA is the key component of any microwave transmission systems, and its performance may have a huge impact on the final system architecture.

Its role is to boost the transmitted signal's power without adding undesirable signals generated by distortion. At the same time, the HPA should be efficient, in terms of its capability of transforming to the power provided by the DC supply RF power. From a technology point of view, they come in at least two variants: vacuum

**Figure 8.**

*Key performance of an UWB 6–18 GHz GaAs TRM output power in TX (top) and gain in RX (bottom).*

tubes and solid state circuits. With the advance of semiconductor technologies, vacuum tubes are becoming a legacy product. Nonetheless, they still provide a valuable solution when the power to be transmitted is in the order of tenths of kilowatts. Typically this requirement is related with a few avionic and spaceborne applications. The advantages of solid state device in terms of ruggedness, size, reliability, performance and cost are such that, whenever a solid state alternative becomes accessible, it quickly becomes adopted by the System Engineering team.

Since its first appearance in R&D labs at the beginning of the new millennium, GaN has travelled a long way, and has now become the standard semiconductor, even in ADS systems, where reliability and process repeatability is a main concern. The advantages of GaN, over other III–V semiconductor, for high-power and highfrequency systems, reside on its capability to deliver a high amount of RF power in a small footprint, with little or none thermal management issues. Especially the last feature, make GaN attractive for ADS applications, often operating in harsh thermos-mechanical environments.

MMIC GaN HPAs are capable of delivering hundreds of Watts at low microwave frequency (<5 GHz), tenths of watts at microwave frequencies (5–20 GHz), and some Watts even at millimetre-wave.

**27**

**Figure 9.**

*Multi-stage down-converter topology.*

*UWB Circuits and Sub-Systems for Aerospace, Defence and Security Applications*

Scientific literature focusing on the design of HPAs is practically infinite, and here we will give an extremely short hint on some design topologies. Usually HPA are synthesized by combing, at the HPAs output, the power provided by some FETs (where some is usually a power of 2). Moreover, suitable techniques can be applied to increase the output power and efficiency [9]. These techniques rely on synthesizing the output impedance to respect an optimum condition both at the operating frequency and also at its higher order harmonics. As you can imagine this can be

If a very large BW is sought, then other circuit topologies become handy. For example, distributed amplification is well known for its UWB frequency response. Simultaneously, a *cascode* transistor topology can be applied [10], increasing even

As briefly described in Section 1, a microwave back-end system is responsible for delivering the RF signal—or better its information content—to the ADC and consequently the DSP unit or to the low-frequency (often referred to as VIDEO)

Typically, this is accomplished through frequency conversion, when the DSP performs A/D sampling, or by performing some manipulation on the RF signal so its power and/or frequency component can be determined by the subsequent stages.

UWB downconverters and up-converters usually require multi-stage conversion plan since a single frequency conversion would not be able to eliminate all over spurs or leakage of the Local Oscillator (LO) signal. In fact, a very large sweep of LO frequency would be needed to down-convert the required portion of the large input RF BW into the smaller Intermediate Frequency (IF) BW. Therefore, at some point, there will be inevitably a strong intermodulation product or harmonic of LO that would fall in the IF BW. To overcome this issue a multi-stage frequency conversion

Here we will discuss in detail the down-converter architecture, but similar

The first stage of the schematic depicted in **Figure 9**, is the filter bank, so the UWB signal RF IN signal is split into smaller adjacent sub-bands. Typically, each sub-band is less than an octave, and consequently the number of filters depends on

topology, depicted in **Figure 9**, is advisable when the RF BW is large.

assumptions and design goals hold for the up-converter.

*DOI: http://dx.doi.org/10.5772/intechopen.87095*

complicated, especially in wideband applications.

**3. Back-end systems and their circuits**

more the amplifier's BW.

analogue stages.

**3.1 Frequency conversion**

*UWB Circuits and Sub-Systems for Aerospace, Defence and Security Applications DOI: http://dx.doi.org/10.5772/intechopen.87095*

Scientific literature focusing on the design of HPAs is practically infinite, and here we will give an extremely short hint on some design topologies. Usually HPA are synthesized by combing, at the HPAs output, the power provided by some FETs (where some is usually a power of 2). Moreover, suitable techniques can be applied to increase the output power and efficiency [9]. These techniques rely on synthesizing the output impedance to respect an optimum condition both at the operating frequency and also at its higher order harmonics. As you can imagine this can be complicated, especially in wideband applications.

If a very large BW is sought, then other circuit topologies become handy. For example, distributed amplification is well known for its UWB frequency response. Simultaneously, a *cascode* transistor topology can be applied [10], increasing even more the amplifier's BW.
