*2.1.1 Low-noise amplifiers (LNA)*

*UWB Technology - Circuits and Systems*

**2. Front-end systems and their circuits**

excessive noise content or distorting the RF signal.

digital section and the selected antenna.

of which is described in Section 3.

*Antenna front end (AFE) schematic diagram.*

**2.1 Antenna front end**

and conversion.

in this book.

A possible way to cluster these functions is divide them in front-end or backend functionalities. The prior are typically connected to the radiating element, implementing low-noise or high-power amplification and some form of signal routing and phase and amplitude modulation. Instead, microwave back-ends are connected to the ADC and therefore provide all the functionalities so the RF signal can be profitably delivered to the digital section. Such functions are typically more complex functions such as extraction of signal characteristics, frequency generation

The two sub-system and the relevant circuits will be discussed in the following. In this chapter, we will not describe signal filtering, being this an extremely extensive topic, excellently covered by Matthaei et al. work [1] and other chapters

As briefly described in Section 1, a microwave front-end system is directly connected to the radiating element. In most cases, these subsystems provide dual mode operation: receive and transmit mode. In receive mode, the incoming RF signal is very weak and its power needs to be amplified to an adequate level, without adding

In transmit mode, the outgoing RF signal has to be raised to the highest possible value in order to guarantee an adequate transmission level. In this case too, distortion must be limited in order to preserve the information carried by the RF signal. Signal routing is often necessary to implement the desired RF path between the

Antenna front ends (AFE) are employed to condition the received signal coming from the antenna port to make it usable for the following sub-systems, an example

The main functions of an AFE circuit are: low-noise amplification, protection against strong interference, signal routing when multiple I/O ports are present, and partitioning into sub-bands if needed. **Figure 1** depicts a simplified schematic.

The first, leftmost, section of the AFE contains the protection and signal routing function. The protection against strong interfering signals is accomplished using a limiting circuit commonly realized through a shunt diode. The limiter has to be the first circuit in order to protect the following components from strong interference signals that might damage sensitive circuitry. Next, there is a signal routing section

**20**

**Figure 1.**

Low-noise amplifiers are an omnipresent component in any microwave receiving system. The LNA's role is to increase the power of the input signal, usually very low especially in long-distance communications, without adding an excessive noise contribution that would make the signal unmanageable by the following stages. The LNA's key characteristics are its gain (G) and noise figure (NF). Secondary, but still important parameters are linearity, power consumption and port matching.

#### **Figure 2.**

*Antenna front end (AFE) typical RF performance: gain (dashed) and NF (solid). Operating BW is 0.5–18 GHz.*

All these parameters are influenced by on the maximum operating frequency, bandwidth and semiconductor technology. Understandably, performance tends to degrade as the frequency and bandwidth increase. Ideally, G should be high while NF should be the lowest possible. Below 10 GHz, 30 dB gain and less than 1.0 dB NF are suitable numbers, reachable when the circuits are realized in III–V compound semiconductors[2]. Above 10 GHz, some degradation has to be accepted in terms of greater NF and smaller gain.

The impact of the LNA's NF and G on system performance can be estimated using Friis' well-known formula that computes the system's cascade noise figure as a function of each stages' NF and G. An important consequence of this formula is that the overall NF of a radio receiver is primarily established by the NF of its first amplifying stage.

Subsequent stages have a weaker effect on signal-to-noise ratio. For this reason, the first stage amplifier in a receiver should be the LNA. Otherwise, as in **Figure 1**, the trade-off between NF and robustness (protection against strong interference) must be accepted.

Regarding the semiconductor, GaAs represents an interesting trade-off between performance and technology readiness level. GaN is slightly less performing, in terms of NF, but has the benefit of handling much more power, making it suitable in receivers where the presence of strong signals is foreseen.

An LNA must satisfy linear, noise, power and intermodulation requirements. Often linear and noise performance require opposite design choices, i.e. matching for noise or matching for gain. Simultaneously satisfying requirements often in contrast between them is not simple at all.

Luckily enough, many design strategies have been described, s dome of which dating back to the 1970s [3] up to more recent ones [4]. In the latter, a comprehensive design strategy that simultaneously accounts for linear and noise requirements is presented. Most of these strategies have a limited bandwidth since feedback is computed at the central design frequency. In [5] a survey of GaAs LNAs operating at very different frequencies (from 5 to more than 100 GHz) is presented. The most suitable design technique is indicated depending on the LNA's operating frequency.

On the other hand, there are other deign topologies, mainly distributed, that are capable of obtaining UWB performance. **Figure 3** depicts the circuit schematic and micro-photo of decade bandit LNA operating between 2 and 18 GHz [6].

The LNA depicted in **Figure 3**, demonstrates 23 dB typical gain and 4 dB typical NF over the entire 2–18 GHZ BW. Another interesting feature is its capability of withstanding high input power signals, demonstrated up to 10 W RF continuous wave. Gain and noise figure of the LNA reported in [6] is plotted in **Figure 4**,

#### **Figure 3.**

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

**23**

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

demonstrating the LNA's capability to obtain more than 20 dB over a very wide

Switching circuits in microwave system are used to implement signal routing therefore performing path selection. Usually they have one common input port and

The switching device can be either a diode or a Field Effect Transistor (FET). As usual, each possibility has its pros and cons, and will be discussed in the following. The diode switch has better loss performance; it can be fractions of dB even at tenths of GHz. On the contrary, FET switches are quite *lossy* and very easily reach 1–2 dB insertion loss even below 10 GHz. Apart from this very important param-

Firstly, the FET is voltage controlled and does not dissipate any DC power thanks to the very high impedance of the gate terminal. On the contrary, diodes require a large current to achieve their low loss state, and therefore some DC power is dissipated across the diode. Secondly, the FET has faster switching time, i.e. the time required to select a different output once the appropriate external command has been received. The switching time in FET switches can be as low as a few nanoseconds. Diode switches may require tenths of nanoseconds to change their state since the direction of the bias current needs to be reversed, and this is not immediate considering the stray capacitances in the control section and the diode itself. Finally, FET switches are more robust and linear, especially if realized in wide band-gap semiconductors as GaN. They can tolerate up to tenths of Watts, while the switch

Isolation, i.e. the unwanted leakage to an unselected path, is another important parameter in switches. However, this performance mainly depends on the selected

The frequency behaviour of the two technologies is comparable, especially when small gate length transistors are employed. In both cases, diodes and FETs, accept-

Consequently, the choice between FET and diode switches, should be carried

Several UWB switching topologies have been proposed and validated. Typically, an inductor is inserted in the switching circuit to resonate the diode's or FET's OFFstate parasitic capacitance [7]. A resistor is also inserted, in this way a more uniform behaviour is obtained over a larger operating BW. The schematic applying this

N possible output ports, and only one can be selected at a certain instant.

operating BW. In the same condition, the NF averages at 3 dB.

*UWB 2–18 GHz GaN distributed LNA simulated vs. measured gain (left) and NF (right).*

eter, all other aspects tend to be in favour of the FET.

diodes seldom survives incident powers above a few Watts.

able performance up to 50 GHz, and even beyond, are achievable.

out considering losses, power handling and switching time requirements.

switch topology, rather than the selected technology.

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

*2.1.2 Switching circuits*

**Figure 4.**

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

**Figure 4.** *UWB 2–18 GHz GaN distributed LNA simulated vs. measured gain (left) and NF (right).*

demonstrating the LNA's capability to obtain more than 20 dB over a very wide operating BW. In the same condition, the NF averages at 3 dB.

### *2.1.2 Switching circuits*

*UWB Technology - Circuits and Systems*

greater NF and smaller gain.

amplifying stage.

must be accepted.

All these parameters are influenced by on the maximum operating frequency, bandwidth and semiconductor technology. Understandably, performance tends to degrade as the frequency and bandwidth increase. Ideally, G should be high while NF should be the lowest possible. Below 10 GHz, 30 dB gain and less than 1.0 dB NF are suitable numbers, reachable when the circuits are realized in III–V compound semiconductors[2]. Above 10 GHz, some degradation has to be accepted in terms of

The impact of the LNA's NF and G on system performance can be estimated using Friis' well-known formula that computes the system's cascade noise figure as a function of each stages' NF and G. An important consequence of this formula is that the overall NF of a radio receiver is primarily established by the NF of its first

Subsequent stages have a weaker effect on signal-to-noise ratio. For this reason, the first stage amplifier in a receiver should be the LNA. Otherwise, as in **Figure 1**, the trade-off between NF and robustness (protection against strong interference)

Regarding the semiconductor, GaAs represents an interesting trade-off between

An LNA must satisfy linear, noise, power and intermodulation requirements. Often linear and noise performance require opposite design choices, i.e. matching for noise or matching for gain. Simultaneously satisfying requirements often in

Luckily enough, many design strategies have been described, s dome of which dating back to the 1970s [3] up to more recent ones [4]. In the latter, a comprehensive design strategy that simultaneously accounts for linear and noise requirements is presented. Most of these strategies have a limited bandwidth since feedback is computed at the central design frequency. In [5] a survey of GaAs LNAs operating at very different frequencies (from 5 to more than 100 GHz) is presented. The most suitable design technique is indicated depending on the LNA's operating frequency. On the other hand, there are other deign topologies, mainly distributed, that are capable of obtaining UWB performance. **Figure 3** depicts the circuit schematic and

The LNA depicted in **Figure 3**, demonstrates 23 dB typical gain and 4 dB typical NF over the entire 2–18 GHZ BW. Another interesting feature is its capability of withstanding high input power signals, demonstrated up to 10 W RF continuous wave. Gain and noise figure of the LNA reported in [6] is plotted in **Figure 4**,

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

micro-photo of decade bandit LNA operating between 2 and 18 GHz [6].

performance and technology readiness level. GaN is slightly less performing, in terms of NF, but has the benefit of handling much more power, making it suitable in

receivers where the presence of strong signals is foreseen.

contrast between them is not simple at all.

**22**

**Figure 3.**

*LNA.*

Switching circuits in microwave system are used to implement signal routing therefore performing path selection. Usually they have one common input port and N possible output ports, and only one can be selected at a certain instant.

The switching device can be either a diode or a Field Effect Transistor (FET). As usual, each possibility has its pros and cons, and will be discussed in the following. The diode switch has better loss performance; it can be fractions of dB even at tenths of GHz. On the contrary, FET switches are quite *lossy* and very easily reach 1–2 dB insertion loss even below 10 GHz. Apart from this very important parameter, all other aspects tend to be in favour of the FET.

Firstly, the FET is voltage controlled and does not dissipate any DC power thanks to the very high impedance of the gate terminal. On the contrary, diodes require a large current to achieve their low loss state, and therefore some DC power is dissipated across the diode. Secondly, the FET has faster switching time, i.e. the time required to select a different output once the appropriate external command has been received. The switching time in FET switches can be as low as a few nanoseconds. Diode switches may require tenths of nanoseconds to change their state since the direction of the bias current needs to be reversed, and this is not immediate considering the stray capacitances in the control section and the diode itself. Finally, FET switches are more robust and linear, especially if realized in wide band-gap semiconductors as GaN. They can tolerate up to tenths of Watts, while the switch diodes seldom survives incident powers above a few Watts.

Isolation, i.e. the unwanted leakage to an unselected path, is another important parameter in switches. However, this performance mainly depends on the selected switch topology, rather than the selected technology.

The frequency behaviour of the two technologies is comparable, especially when small gate length transistors are employed. In both cases, diodes and FETs, acceptable performance up to 50 GHz, and even beyond, are achievable.

Consequently, the choice between FET and diode switches, should be carried out considering losses, power handling and switching time requirements.

Several UWB switching topologies have been proposed and validated. Typically, an inductor is inserted in the switching circuit to resonate the diode's or FET's OFFstate parasitic capacitance [7]. A resistor is also inserted, in this way a more uniform behaviour is obtained over a larger operating BW. The schematic applying this

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

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 technology for specific high-end applications.
