RF Converters Deliver on the Promise of True Wideband Radio
Converters that can directly synthesize signals in the radio frequency range (RF converters) have matured and are revolutionizing conventional radio design. With the ability to digitize and synthesize instantaneous signal bandwidths up to 2 GHz to 3 GHz, RF converters can now deliver on the promise of providing true wideband radios, allowing radio designers to greatly simplify hardware design and well support software reconfiguration capabilities, which is impossible for conventional radio designs.
In today’s article we explore how advances in RF converter technology are making new data acquisition systems and wideband radios possible, and discuss the feasibility of software configuration.
Every radio designer faces a design trade-off between performance within the signal bandwidth and power consumption. How the radio designer meets this constraint determines the size and weight of the radio and fundamentally affects the location of the radio, including buildings, towers, poles, underground vehicles, packages, pockets, ears, or glasses. Each radio location has an amount of power available that is commensurate with its location. For example, the power available on a building or tower is likely to be higher than the power available from a smartphone in your pocket or a Bluetooth headset in your ear. In all cases, one fundamental truth holds true: the less power a radio requires and the more throughput it can support per unit of power, the smaller and lighter the radio. This fact has huge implications and has been the driving force behind many innovations in the communications electronics industry for many years.
Figure 1. RF converters enable wideband radios to deliver data-hungry services such as video streaming and gaming.
Semiconductor companies are packing more functionality and higher performance into devices of the same or smaller size, and devices using these devices are fulfilling the promise of being smaller, more functional, or lighter (or in some cases all three). The smaller, lighter, and more functional devices are better, allowing them to be placed in locations that were previously impossible due to other constraints. For example, a device that previously required a building can now be placed on a tower because of the reduced footprint; a radio unit that was previously placed on a tower can be reduced to a unit placed on a pole if it is lightweight enough; a unit that was previously heavy enough to be carried in a vehicle can now be placed in a backpack.
Today's environment is full of traditional devices that need to be placed on buildings, towers, poles and vehicles. Driven by the need for people around the world to connect with each other, engineers designed equipment using the components available at the time to meet the challenge, which has led to the ubiquitous communication environment we have today. We can talk, send messages, instant message, transfer photos, download, upload and browse anytime and anywhere through a variety of different networks (including mobile networks, wireless LANs, ad hoc short-range wireless networks, etc.). All of this is connected to a broadband wired network, and the data is transmitted by RF cables and ultimately transmitted through optical fibers.
Enhanced video experience
Multiple studies have shown that the demand for data is expected to continue to grow over the next decade. This is driven by a seemingly insatiable appetite for more data-rich content, which requires greater bandwidth. For example, cable and fiber-to-the-home operators continue to compete in home broadband services by offering higher-speed connections and more HDTV channels. Ultra-high-definition (UHD or 4k definition) TV requires more than twice the capacity of HDTV, with channel bandwidth requirements exceeding those used today.
In addition, immersive video including virtual reality (VR), as well as games with multi-dimensional freedom and 3D effects (180° or panoramic vision, etc.), all using 4k UHD TV, require up to 1 Gb of bandwidth per user, which is far beyond the already demanding requirements of simple 4k UHD TV broadcasting and streaming. Online gaming requires the network to provide symmetrical data bandwidth because latency is critical, which drives the development of wider bandwidth upstream transmission capabilities. This demand for wider upstream capabilities, in turn, has prompted equipment manufacturers to upgrade their designs to achieve symmetrical wide bandwidth transmission.
The power of today’s RF converters is critical to enabling advancements in the transmission of such rich video content. They must be able to deliver large dynamic range signals while also requiring excellent spurious performance to support the use of higher order modulation schemes such as 256-QAM, 1024-QAM, and 4k-QAM. Installed coaxial cable equipment and distribution amplifiers have limited bandwidths of 1.2 GHz to 1.7 GHz, and these higher order modulation methods must be used to improve the spectral efficiency of each channel. Higher performance in headend transmission equipment can extend the life of the installed base, ease capital budget constraints, and support multiple service operators (MSOs) with a longer window to upgrade their equipment and transmission systems.
Multi-band, multi-mode testing
Today’s smartphones are a far cry from traditional cell phones as more and more features are integrated. Many features have radios associated with them, so there may be five to seven or even more frequency bands in current mobile devices. When smartphones are manufactured, each radio must be tested, which brings new challenges to manufacturers of multi-mode communication testers. Although the test volume increases with the number of radios, fast testing is still required to reduce the cost of testing. Considering the size and cost of the tester, it becomes impractical to build different radio hardware for each radio in the mobile device. As more frequency bands are opened or proposed for mobile services, the challenge of testing the increasing number of radios in mobile devices increases.
RF converters are well suited to this challenge. Whether it is a transmitter or a receiver, RF converters provide flexibility that conventional radios cannot achieve. Wideband RF converters can simultaneously capture and directly synthesize signals in each frequency band, thereby supporting the simultaneous testing of multiple radios in mobile devices. Using the channel selectors built into the RF DAC and RF ADC, multiple radio signals can be efficiently processed in the converter. For example, Figure 2 shows that each RF DAC has three sub-channel processing units, which can combine signals from three different frequency bands, and then use a digitally controlled oscillator (NCO) for digital up-conversion, and then convert them to RF signals by the RF DAC.
Figure 2. Example of an RF DAC with a channel selector.
In other market segments, such as test equipment for the aerospace and defense market, there is an increasing demand for wideband test solutions for pulsed radar and military communications. Due to the large number and types of radars, electronic intelligence, electronic warfare equipment, and communications equipment that need to be tested, test equipment manufacturers must create a flexible instrument with a rich feature set. For example, arbitrary waveform generators must be able to create a variety of signals, including linear frequency modulated pulse signals, phase coherent signals, and modulated signals of various output frequencies and bandwidths. The measurement equipment must be equally powerful to receive these signals when testing the exciter or transmitter. RF converters can serve such applications well by enabling direct RF synthesis and measurement at RF frequencies. In some cases, this can eliminate the need for up-conversion or down-conversion, and in other cases, reduce the number of frequency conversions. Hardware is simplified, and size, weight, and power requirements are reduced. The addition of digital features such as channelizers, interpolators, NCOs, and synthesizers allows efficient signal processing on specialized low-power CMOS technologies.
Software Defined Radio
RF converters are one of the key factors in software-defined radio.
RF converters can directly synthesize and capture radio frequencies within a bandwidth of several GHz, digitally implementing up-conversion or down-conversion functions, so that the entire up-conversion or down-conversion stage is no longer required, and the radio architecture is simplified.
Removing the analog frequency conversion stage and the associated mixers, LO synthesizers, and filters can reduce the size, weight, and power consumption (SWaP) of the radio, making the radio adaptable to more application scenarios and using a smaller power supply. This technology makes the radio small and light enough to be handheld, vehicle-mounted, or installed in various airborne assets such as aircraft, helicopters, and unmanned aerial vehicles (UAVs).
Figure 3. RF converter-driven software-defined radio enables cross-platform interconnect communications.
In addition to enabling better cross-platform communications, radio hardware built with RF converters has the potential to be multi-functional, multi-mode, and multi-band. RF converters are now capable of reaching lower radar bands and higher bands in the near future, making the concept of a single device that can be used as both a radar and a tactical communications link a reality. Such a device has clear advantages in field repair, upgrades, procurement procedures, and costs.
The ability to directly synthesize and capture radar frequencies makes RF converters ideal for phased array radar systems. Direct RF converter synthesis and capture reduces much of the conventional radio hardware, making the individual signal chains smaller and lighter. This allows many of these radios to be integrated into a smaller space. Smaller arrays and units suitable for shipborne arrays or ground-based phased arrays, as well as for signals intelligence operations, can achieve a smaller SWaP.
The Technology Behind RF Converters
One of the key technology advances that has enabled the success of RF converters is the continued scaling of fine-line CMOS processes. As the gate length and feature size of the basic CMOS transistors decrease, digital gates become faster, smaller, and use less power. This allows RF converters with reasonable power and area to integrate a large amount of digital signal processing functionality on chip. Accommodating digital channel selectors, modulators, and software programmable filters is important for building efficient and flexible radios. This more efficient DSP also opens the door to using digital processing to help correct analog imperfections in the converter. On the analog side, each new node provides faster transistors with better matching performance per unit area. These improvements are critical to achieving faster, high-precision converters.
Process technology advances alone are not enough; there are also important architectural improvements that make RF converters possible. The architecture of choice for RF DACs is the current steering DAC architecture. The performance of this type of DAC depends on the matching of the current sources that make up the DAC. Uncalibrated current source matching is proportional to the square root of the current source area7. Matching per unit area improves with each technology node.
However, for high-resolution converters, even at the most advanced nodes and with low enough random mismatch, the current sources can be very large. This large current source makes the converter larger, and worse, the parasitic capacitance of the large current source degrades the high-frequency performance of the DAC. A more attractive solution is to calibrate a smaller current source to achieve the required matching level. This can significantly reduce the additional parasitic effects from the current source, achieving the required linearity performance without compromising high-frequency performance. If performed correctly, this calibration can be highly stable over the entire temperature range, and the calibration can be done once. A stable one-time calibration means that calibration does not need to be run periodically in the background, saving operating power and alleviating the problem of spurious products caused by calibration running in the background.
Another architectural choice that helps ultra-high-speed converters achieve their performance targets is the choice of switch architecture used to steer the DAC current. The traditional dual-switch structure (Figure 4) has several disadvantages when operating at very high speeds9,10. The data driven into the dual switch can remain constant for one to many clock cycles, so the tail node has a data-dependent settling time. If the clock rate is slow enough that this node can settle in one clock cycle, then this is not a problem. However, at very high speeds, this node cannot fully settle in one clock cycle, and the data-dependent settling time will cause distortion in the DAC output.
Figure 4. Example of a dual-switch DAC cell.
If a quad switch is used (Figure 5), the data signals are all zeroed. This results in the tail node voltage being independent of the data input, alleviating the above problem. The quad switch also allows the DAC data to be updated on both edges of the clock. This feature can be used to effectively double the DAC sampling rate without doubling the clock frequency.
Figure 5. Example of a four-switch DAC cell.
Using a carefully designed current source calibration algorithm and a quad-switch current steering cell, combined with today’s fine-line CMOS processes, a high-speed sampling DAC with excellent dynamic range can be designed. This allows for the synthesis of high-quality signals over a wide frequency range. When this wideband DAC is combined with an auxiliary DSP, it becomes a very flexible, high-performance radio transmitter that can be configured to provide signals for all of the different applications mentioned earlier in this article.
Future Radio
Today’s RF converters have already driven fundamental changes in radio architecture design, and in the future, they will drive even greater changes. As process technology continues to advance and RF converter design is further optimized, the impact of RF converters on radio power consumption and size will continue to shrink. These technological advances come at the right time to enable a new generation of radios, such as emerging 5G wireless base station applications (such as massive MIMO), as well as large-scale phased array radar and beamforming applications.
Deep submicron lithography will enable more digital circuits to be placed on the RF converter die, integrating critical compute-intensive functions such as digital pre-distortion (DPD)13 and peak clipping (CFR) algorithms, which will help improve power amplifier efficiency and significantly reduce overall system power consumption. This integration will reduce the pressure on high-energy FPGA logic and move related functions to lower-power dedicated logic. Other possibilities include integrating the RF converter and its digital engine with RF, microwave or millimeter-wave analog devices, further reducing size and simplifying radio design, providing a bits-to-antenna system-level approach to radio design. With RF converters, all kinds of opportunities are emerging.
RF converters are the technology that enables the world to move beyond what is possible.
Thumbs up for RF converters!
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