September 28, 2021
System designers are always looking for simple solutions for their complex system designs. Well, look no further for RF front-end receiver solutions in the defense, aerospace, and 5G wireless infrastructure realm. This blog post is a practical guide to reducing design complexity while meeting those tough noise figure requirements for 5G infrastructure, defense, and aerospace applications.
Many RF front-end (RFFE) systems are unique, but the receivers are similar in many ways. In general, RF sensitivity is a key specification of all wireless radio receivers. The RF receiver's ability to pick up the required level of radio signals while ignoring the unwanted ones will enable it to operate more effectively within its application.
Some methods of measuring RF sensitivity of a receiver are:
Power amplifier (PA) and Low Noise Amplifier (LNA) technologies generally have little issue amplifying signals within a receiver. Instead, the limiting factor tends to be restricting noise, as noise masks the wanted signal. Two critical performance considerations are receiver sensitivity and SNR for wireless communications, radars, instrumentation, satellites, and others.
In terms of receiver noise, it is the first stage or LNA and any loss that comes after it — as this is critical in determining the overall performance of the entire radio receiver. Optimizing the SNR and NF of the LNA performance improves the whole receiver. Moreover, this performance must be optimized for the entire system bandwidth.
In the 5G, defense, and aerospace arenas, the bandwidths of LNA's and other system components are increasing to achieve higher levels of data capacity required to handle today's applications. This bandwidth increase means noise level optimization must accommodate the same bandwidth area. This is obviously difficult, but it must be achieved to meet today's capacity and throughput requirements with a high level of receiver sensitivity.
Network densification is a must to implement 5G effectively. Increasing the number of access points per area and implementing more transmitters and receivers at each access point boosts densification. This boost improves the overall capacity and throughput of the wireless network, and with the use of high dynamic range transceivers with better sensitivity, these systems are making 5G possible. More base stations and access points per area also change the RF front-end requirements (RFFE). It reduces the transmit power required since the average distance from the User Equipment (UE) to the base station is shorter. In addition, more antennas are being added to these access points to help increase spatial streams improving capacity and signal reliability.
Multiple-input multiple-output (MIMO) has been added to increase signal reliability further to improve uplink system capacity. Increasing the spatial streams using many antennas and MIMO improves the SNR, which is good because advanced radio systems like 5G require higher SNR to support higher data rates.
Many 4G LTE systems have already moved to 5G. These systems have massive-MIMO capability, which is an extension beyond traditional MIMO, offering a much higher number of antennas, like 32, 64, 128 and more antenna arrays on base station antenna systems. These massive-MIMO antennas help focus energy to improve throughput and efficiency of the network. These 5G networks also have very high bandwidth capability. For example, Frequency Range FR1 (410 MHz – 7125 MHz) can have transmission bandwidths up to 100 MHz. Therefore, LNA designers are creating very wide-band LNA's to accommodate several 5G band RF chains, allowing for easier product design. To achieve these wide-band capabilities, the LNA must have superior noise figure and EVM capabilities across the entire bandwidth. Additionally, they need to be small because these RFFE components now reside on the antenna at the top of the tower.
Figure 1: Components of an RF Front-End
Because these components are generally at the top of the base station towers, they require high power handling capabilities. They must survive a high input power strike and, if struck, need to not only survive but must recover and begin operation again very quickly. Therefore, components such as LNAs, which are first in the chain after the receiver input switch, need to have input power handling capabilities of 20 dBm or greater to meet this task.
A lot is happening in the defense and aerospace RFFE realm as well. Especially in the military radar, satcom, electronic warfare communications and digital receivers. Below are some basic block diagrams. As you can see from the numerous module designs embedded, there is a clear push for smaller form-factors, lighter weight, and more highly integrated products with both receive and transmit chains in one package like in the 5G applications. As one would expect, these features are just as appealing to the defense and aerospace arena and aligned with the SWaP – size, weight, and power – objective.
Figure 2: Examples of RFFE's used in defense and aerospace
Receiver products in defense and aerospace (D&A) not only require high power capability for best-in-class amplification, but they also require the ability to survive extreme conditions like those in the infrastructure area. But the survivability function is usually needed at much higher input levels in the kilowatts range with resistance to jamming. This is required in primarily military, aerospace radar, and military communication applications, where electronic countermeasures (ECMs) might be used to overwhelm a receiver as a defense tactic.
Therefore, survivability and resistance to ECMs, like radio jamming, need to endure high power strikes. If struck with a high power on the input, they will need to live through it and recover back to communications quickly. These devices must also operate over larger bandwidths than previously done.
In the past, D&A digital receivers have been narrowband due to limitations with technology. But this has changed, as new technology advancements in Gallium Arsenide, Gallium Nitride and Silicon have allowed for larger sustainable bandwidths. This has brought an onset of many new defense and aerospace applications and features to existing product.
Many military applications require this wide-band and multiband communications with a low probability of intercept/low probability of detection radar. With the addition of frequency hopping to mitigate signal detection, these incorporate wide bandwidth and spectrum for transmission and reception. These aspects can lead to more noise on the receiver and a reduction of protection. And if a receiver is exposed to high power levels for an extensive amount of time, the components can degrade quickly, suffer performance issues, or die. Therefore, it is imperative designers take the necessary steps to ensure reliability and receiver sensitivity.
Ultimately, each individual application in the above-mentioned areas drives system design and requirements. But, at a high level, some RF front-end requirements remain the same.
The receiver noise performance always starts at the first stage of the RFFE. At the RFFE, the signal levels are at their lowest, and if there is noise on the signal, it is difficult to determine what is noise and what is the incoming signal. As you move beyond the switch, LNA and into driver stages, all the signals will be amplified. Determining what the incoming signal is will be even more difficult. Therefore, it is imperative that noise is minimized in the components prior to and at the LNA. It is essential that the preferred signal is separated from the input noise as early as possible – in the LNA – as this performance is what the entire receiver chain will see.
The best parameter tradeoffs lead to optimized performance
Designers must make crucial tradeoffs between parameters like gain, gain flatness, input/output match, linearity, power consumption and size, all while making certain the LNA is inherently stable. Designers must make certain they balance among these parameters while keeping the system stable and check for stability over the full range of operating conditions.
Low receiver noise figure can indeed translate into improved performance and range, but this is a tradeoff a system designer must make as a better NF may result in diminishing returns of receiver performance. Thus, a nominal improvement in one application may not be worthwhile in another. Qorvo's helpful cascade analysis calculator can assist in providing a start in these system-level design tradeoffs.
Figure 3: Qorvo's cascade calculator on the Design Hub
One important thing to consider in an application like the one shown above in Figure 3 is the ratio between the LNA and any insertion loss following it (i.e., in the above example, this would be the filter). If the filter after the LNA is lossy, the NF will increase. For instance, in the above scenario, if the LNA stage 1 had a Gain of 15 dB, instead of 19 dB, the NF would be 0.47 dB rather than the 0.37 dB as shown. Additionally, if the LNA had a Gain of 19 dB and the second stage filter had an insertion loss of -4.0 dB, the NF would be 0.39 dB, again increasing the NF.
Receiver application and temperature
One obvious approach to reduce input noise is picking the best LNA with the NF figure parameter. Another important consideration for the receiver LNA is its performance over temperature. Temperature plays a significant factor in the Gain flatness over the entire frequency range and the stability of the LNA. Both of these parameters can contribute to a change in NF. Cooling the LNA or front-end with heatsinks or heat-dissipating techniques will improve thermal noise. Matching the design can also help in reducing the temperature and thermal noise of the front-end. Some applications in radio astronomy use cryo-cooling to keep the NF low. Moreover, the stability of the LNA is essential as an unstable LNA can increase system NF.
Every noise source has a comparable noise temperature. Noise temperature is used to describe the noise performance of a device rather than the NF and is mainly used as a system parameter. This makes the input noise temperature concept more meaningful and convenient. It manifests itself in the input of the receiver, where the signal levels are low and is the limiting minimum noise any circuit can achieve at a given temperature. It is also evenly distributed across the entire system spectrum. Thermal noise is also a function of the system bandwidth. Matching the bandwidth to the frequency response and input signal will reduce thermal noise. To assist you in calculating your NF and NF temperature, Qorvo has an online calculator, as shown below.
Figure 4: Qorvo's noise figure temperature calculator on the Design Hub
Limiter & Circulator Impacts on D&A Receivers
As noted earlier, an LNA's high input power capability is important. A method to reduce the possible impact of high input power to the receiver is to add a limiter or circulator at the input. This does help with protection but has a side effect of increasing the noise seen at the LNA. This fix also reduces the sensitivity of the receiver – reducing signal range, throughput and performance. Therefore, if you pick an LNA with very high input power, the limiter or circulator would not be needed, helping the overall receiver performance.
Ultimately, the noise figure and system linearity affect receiver sensitivity. Tradeoffs among several key parameters like gain, matching, linearity and bandwidth must be incurred to achieve optimum receiver sensitivity performance — all while keeping a close eye on interferences, temperature, and the ability to sustain receiver attacks.
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