Understanding Equalization When Compensating for High-frequency Roll-off in Your 5G Designs

5G is widely expected to transform how we live our daily lives, with many analysts now predicting that it will be a driver of the next wave of economic growth. The roll-out of this transformational networking technology is currently in its first phase. However, as the deployment begins to enter the second phase – with the rollout of mmWave, today’s RF design challenges will multiply. At the user equipment (UE) level, devices will be required to support multiple radio access technologies (RAT), including not only 5G but also 4G/LTE, 3G, and Wi-Fi, meaning that transceivers will have to meet strict power consumption and size constraints. The propagation characteristics of 5G will require new front-end architectures, with the RF functionality and the antenna being integrated into a single package. To make this possible, advanced techniques such as beamforming will require multiple RF chains and antenna elements to be densely packed in the UE. 

At the infrastructure level, mmWave 5G will be delivered by small cells, with network densification required to support large numbers of connections and the propagation characteristics of the carrier frequencies. Transceiver design in this environment is complicated by the higher signal frequencies, which increase losses and stray capacitance and limit efficiencies and power levels. A large number of antennas and signal paths will lead to significant amounts of inter-carrier interference – which must be compensated. Another problem for designers is achieving a flat frequency response for their RF transceivers over the operating frequency range. Traditionally they have used passive equalizers for this purpose. In this article, we consider the tradeoffs associated with this approach before showing the benefits of instead using a positive slope amplifier in RF designs.

Flattening the Frequency Response 

Meeting the requirements for gain flatness over the entire operating frequency range is a common problem in the design of discrete RF transceivers. Ideally, the gain in the transceiver’s signal path should be flat; however, since each component in the chain has a finite bandwidth, the overall system gain response tends to roll off as frequency increases. This is seen as a negative slope in a graph of gain versus frequency. Achieving a flat gain profile can be challenging for a transceiver operating over a wide frequency range. The simplified receiver signal chain shown in Figure 1 includes a low noise amplifier (LNA) and two RF amplifiers. 

Figure 1 Simplified RF Signal Chain with Typical Roll-off in Frequency Response

The three amplifiers' bandwidth and negative gain characteristics influence the overall system gain and bandwidth. Based on the simplified assumption that all three blocks have the same gain and bandwidth, the overall gain versus frequency profile will be similar to that shown above. In reality, the gain and bandwidth of the individual amplifiers will not be identical, so each of them will contribute to gain errors over the entire frequency range. Designers typically use two standard techniques to compensate for this gain roll-off. The first is to use fixed equalizers in the signal chain to flatten the gain response; this approach adds attenuation, which has a frequency response slope approximately opposite that of the gain slope. The second is to use an amplifier with a positive gain slope over the desired bandwidth.

Fixed Equalizer Limitations

Fixed equalizers help manage negative gain slopes in the RF signal chain because they are available with accurate attenuation slope values. This allows designers to select one with an attenuation slope that matches their system gain slope to produce the desired overall system frequency response. However, using an equalizer brings several trade-offs to consider. Firstly, while they help to flatten the profile of the frequency response, they reduce the gain of the overall signal chain. Secondly, the addition of an equalizer in the RF signal chain negatively impacts the noise figure of an RF receiver. Finally, if equalization is implemented close to a power amplifier (PA), the transmitter output power is reduced. 

Positive Gain Amplifier

For RF systems where the tradeoffs previously considered are a problem, it is preferable to use a gain flattening technique that uses gain compensation instead of attenuation. A positive gain slope amplifier allows the RF receiver to maintain the overall signal chain performance for gain, noise figure, and dynamic range while meeting gain flatness specifications over the entire frequency range. On the transmitter side, this amplifier can flatten the frequency response without reducing the output power of a PA. Figure 2 illustrates the combined gain response of a typical wideband receiver and a positive gain slope amplifier showing the flattening effect and an increase in the overall gain of the composite response.                                  

Figure 2 Using a positive slope amplifier to compensate for roll-off

The CMX90G301 by CML Microcircuits is an example of a positive gain amplifier that can eliminate passive equalizers in various general-purpose wireless RF applications. This low-power monolithic microwave integrated circuit (MMIC) gain block can be easily cascaded in an RF signal chain operating in the 1.4 – 7.1 GHz frequency range to provide a positive gain slope of +1dB and a small signal gain between 14.5 and 15.5dB (Figure 3).

Figure 3 Application circuit for the CMX90G301

The MMIC is highly integrated to minimize component count and board area, and RF ports are matched on-chip to 50 Ω with DC-blocking capacitors. An active bias circuit allows the device to operate over a wide supply voltage of 2.7V to 5V with a typical operating current of 20 mA. This simplifier is based on GaAs pHEMT devices, which enable a combination of low DC power, a low noise figure (2dB), and high gain. An alternative MMIC from CML, the CMX90G302, is available for applications that require even more gain-slope compensation (+2dB).

Figure 4 Gain and Noise Figure for the CMX90G301

Conclusion

A negative gain slope is a common feature of wideband transceiver signal chains. While RF designers can use passive equalizers to compensate for this high-frequency roll-off, these introduce several trade-offs which make this approach unsuitable in some applications. Positive gain slope amplifiers, like the CMX90G301 and CMX90G302, offer an alternative approach that brings many benefits, including smaller solutions size, ease of design, and low noise.