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5 SuperconductorMicroelectronics: A Digital RF Technology for Software Radios Darren K. Brock HYPRES, Inc. When the concepts of ‘pure’ software radio 1 were first introduced they were immediately recognizable to those who had worked on military systems at very low frequencies – at a carrier frequency of 16 kHz, A/D conversion of the RF carrier directly at the antenna was becoming feasible in the 1970s. However, only 10 years ago the prospect of ‘pure’ software radio implementations for commercial wireless systems, operating at carrier frequencies of 2 GHz and beyond, was seen as being decades away. Recent progress in the field of super- conductor microelectronics has, however, been both remarkably rapid and significant, such that superconducting data conversion and DSP devices capable of operation at such frequen- cies have now been successfully demonstrated. The commercialization of this technology promises to be a key enabler of ‘pure’ software radio architectures for both commercial and military wireless systems. This chapter provides a description of the underlying technology and its potential in both commercial and defense wireless systems. The fundamentals of the tech- nology have now been proven; the pace of commercialization will depend upon investment. 5.1 Introduction The speed and flexibility enabled by superconductor microelectronics seems well matched to the goals of proposed software radio architectures. For the purposes of this work, we will only 1 ‘Pure’ software radio, as distinct from ‘pragmatic’ software radio, incorporates the concept of signal digitization at the antenna. Such concepts are introduced in Chapter 1 by Walter Tuttlebee in Software Defined Radio: Origins, Drivers and International Perspectives, Tuttlebee, W. (Ed.), John Wiley & Sons, Chichester, 2002. Chapter 2 by Wayne Bonser in that volume also provides excellent background on the defense motivations, systems, and experi- ence of software radio alluded to later in this chapter. Software Defined Radio Edited by Walter Tuttlebee Copyright q 2002 John Wiley & Sons, Ltd ISBNs: 0-470-84318-7 (Hardback); 0-470-84600-3 (Electronic) examine the field of low temperature superconductors – specifically niobium (Nb), which has a critical temperature (T c ) of 9.23 K. Digital niobium circuits are operated between 4.2 and 5 K and generally employ an IC fabrication process for implementing the rapid single flux quantum (RSFQ) logic family [1]. The kinds of circuits we assume can be made in this technology are data converters and digital signal processing (DSP) type circuits. This assumption is based on the fact that there have been many groups, in both industry and academia, which have successfully demonstrated these types of circuit. However, for current purposes, we will take the liberty to assume that such chips can be made with greater complexity than yet achieved and can be manufactured with a reasonable yield. Fabrication techniques, discussed later, are not dissimilar from those used today for silicon ICs, so this is not an unreasonable assumption, although timescales for such a scenario will be investment dependent. Given these caveats, we can begin a discussion of the application of such super- conductor circuits to both commercial wireless and defense communications. The title of this chapter refers to an approach called ‘digital RF’. By this we mean that the superconductor logic gates will directly process digital signals at RF or multi-GHz frequen- cies. This might be Mbps digital data modulated on an RF carrier, or it might be a digital Gigabit data stream of samples from an analog-to-digital converter (ADC). In the following sections, we show examples, on both the receive and transmit sides, of how such RSFQ circuits can benefit: (1) a CDMA-type base station for commercial wireless; and (2) military systems with their varied and disparate requirements of frequency, bandwidth, and protocol. It is the performance characteristics of RSFQ that can enable the kind of flexible, high data rate applications that are being talked about as third and fourth generation, 3G and 4G, wireless services. The need to embrace legacy systems, while building in flexibility, in defense applications is an even more formidable task. However, such ideal ‘software radios’ may only be possible if implemented using a digital RF architecture with superconductors. 5.1.1 Superconductivity and the Josephson Effect We begin by briefly reviewing the phenomenon of superconductivity [2]. Although most readers will be familiar with the trait that superconductors exhibit zero resistance when cooled below a critical transition temperature (T c ), fewer may recall the second (and perhaps even more remarkable) feature – superconductors can contain magnetic flux only in certain discrete quantities. Called ‘flux quantization’, and illustrated in Figure 5.1, this behavior can be exploited to construct a variety of circuits that have no dual in the semiconductor realm. If a closed section of superconductor material is subjected to a magnetic field, screening currents will orient themselves such that the flux threading the closed section is quantized. This amount of magnetic flux F threading the loop is given by integrating the normal component of the incident field B over the area A of the closed section F ¼ R BdA ¼ nF 0 where n is an integer and F 0 ¼ h=2e ø 2:07 £ 10 215 Webers (We) is called the ‘flux quan- tum’ or ‘fluxon’. In order to create digital circuits, an active superconductor component is needed – the Josephson junction (JJ). As shown in Figure 5.2, a JJ consists of two Nb electrodes separated by a thin insulator (typically Al 2 O 3 ). Denoted in a circuit diagram by a cross, the JJ’s principal parameter is its critical current I c . When a bias current I , I c is applied from base to counter electrode, the device exhibits no resistance. However, when I . I c is applied, the JJ becomes briefly resistive. The time scale of this event is dictated by the capacitance of the thin Software Defined Radio: Enabling Technologies128 insulator. For a junction of 3 £ 3 mm, this is about 1 ps. For a junction of 0.3 £ 0.3 mm, this is about 0.1 ps. A design consideration for Josephson junctions in RSFQ circuits is that they be sufficiently damped to prevent hysteresis upon exceeding the critical current, so that the junction quickly Superconductor Microelectronics: A Digital RF Technology for Software Radios 129 Figure 5.2 Josephson junction and SQUID configurations Figure 5.1 The phenomena of superconductivity: zero resistance and magnetic flux quantization returns to the zero voltage state. As we show below, this rapid voltage pulse corresponds to a single flux quantum F 0 , and forms the basis for RSFQ logic. As illustrated in Figure 5.3, this is generally analyzed in terms of a shunted junction model, in which the ideal Josephson junction of capacitance C is shunted with a linear resistance R [3]. The junction itself can be characterized as a nonlinear inductor of magnitude given by the Josephson inductance L J ¼ F 0 /2 p I c . Such a parallel network has two characteristic times, RC and L J /R. If the former time is larger, the junction is underdamped; in the other limit, it is overdamped. If we embed the resistively shunted junction of Figure 5.3 into the closed superconductor loop of Figure 5.1, we obtain a superconducting quantum interference device (SQUID), illustrated in Figure 5.2. The inductive loop provides the quantization of magnetic flux, and the junctions provide a switching mechanism for loading and unloading flux into and out of the loop. This SQUID configuration, known for many years now, is the basis of all superconductor electronics. 5.1.2 Established Applications of Superconductors In the past few years, superconductor microelectronics has started to emerge into the commer- cial arena from university and industry R&D laboratories, providing unsurpassed performance characteristics [4]. Superconductor magnetoencephalography (MEG) systems for imaging the human brain are commercially manufactured by several companies and over a hundred of these systems are in use today. The extreme sensitivity of these instruments allows diagnostic medical data to be gleaned from neuron dipole moments down to a few nA-m 2 . Software Defined Radio: Enabling Technologies130 Figure 5.3 Hysteretic and nonhysteretic Josephson junction behaviors Even the Systeme Internationale (SI) unit of the Volt is defined by a superconductor integrated circuit. HYPRES, Inc. (Elmsford, NY) currently offers commercial products based on superconductor integrated circuits, packaged with a small mechanical refrigerator to provide temperature regulation, allowing this standard volt to be reproduced anywhere in the world with quantum mechanical accuracy. Simply put, these are applications that cannot be performed with any other technology; therefore the motivation to accept the unique character of cryogenic operation is strong. As a consequence, these applications have driven the state of the art in cryopackaging to the point where all cryogenics have become invisible to users of such products. 5.1.3 Emerging Applications – Software Defined Radio As the ‘cryophobia’ associated with superconductor microelectronics is overcome, the range of possible applications continues to widen. In communications, dispersion-free, ultra-high Q superconductor microwave filters for cellular base stations are today offered from several companies in the United States, Europe, and Japan. Close to a thousand such units have been purchased and installed around the United States, with orders pending from major carriers in Europe. The use of superconductor material allows the very high Qstobe maintained, while microminiaturizing the overall filter size. The ultra-sharp filter ‘skirts’ that result enable increased channel selectivity and, with a cooled LNA, yield increased sensitivity as well. Recently, wireless telephony has been shifting from voice/narrowband data to wideband data, along with demands for significant increases in capacity. These have become the industry’s major drivers, with the major obstacles becoming air interface compatibility and bandwidth allocation. An increasingly embraced solution to surmount these obstacles lies in the concepts of software radio [5]. However, realization of software radio systems Superconductor Microelectronics: A Digital RF Technology for Software Radios 131 Table 5.1 Demonstrated RSFQ digital circuit performance Circuit type Circuit metric(s) Circuit type Circuit metric(s) Toggle flip-flop 144 GHz 2-bit counter 120 GHz 4-bit shift register 66 GHz l-kbit shift register 19 GHz 6-bit flash ADC 3 ENOB a at 20 GHz 6-bit transient digitizer with 6 £ 32 bit on-chip memory buffer 16 GS/s 14-bit high-resolution ADC (2 MHz) 14 ENOB and 2100 dBc SFDR b 18-bit DAC Fully functional at low speed 1:8 demultiplexor (synchronous) 20 Gb/s 1:2 demultiplexor (asynchronous) 95 Gb/s 1-bit half-adder 23 GHz 2-bit full-adder 13 GHz 8 £ N bit serial multiplier 16 GHz 14-bit digital comb filter 20 GHz 128-bit autocorrelator 16 GHz Time-to-digital converter 31 GHz a ENOB, effective number of bits. presents a host of challenges – chief among them the unprecedented requirement on analog- to-digital converter (ADC) performance [6]. This is the area where superconductor micro- electronics represents an emerging solution. With demonstrated ADC, DAC, and DSP components, this technology may well become a key enabling technology for software radio [7]. Table 5.1 summarizes the performance already achieved with such superconduct- ing devices to date. Unlike commercial systems, which are primarily cost/performance- driven, defense applications tend to be primarily performance driven, with cost as a close second. In addition, military radio requirements are far more demanding than those for commercial systems. 5.2 Rapid Single Flux Quantum Digital Logic By now the reader may be wondering why a digital superconductor IC technology has not already been established. In fact, there were two large digital superconductor programs – one that ran at IBM from 1969 to 1983 and another in Japan from 1981 to 1990. Rather than relying directly on quantized bundles of magnetic flux as bits, those efforts (and others at the time) attempted to use the voltage state of the JJ as a ‘1’ and the superconducting state as a ‘0’. Many fully functional circuits were demonstrated, culminating with a 1 GHz 4-bit micro- processor by NEC [8]. However, it was this choice of logic convention which ultimately led to the conclusion of the program. A reset effect called ‘punchthrough’ limited the speed of operation to just a few GHz. In contrast, very large scale integration (VLSI) RSFQ circuits should operate up to 250 GHz. Power consumption was another issue. A typical latching gate dissipated about 3 pW. Although this sounds small, RSFQ technology dissipates only one tenth of this, at 0.3 pW/gate. The need to distribute an AC power supply was also a problem and made timing issues extremely complex. 5.2.1 Circuit Characteristics 5.2.1.1 Circuit Structures In RSFQ circuits, it is not a static voltage level, but the presence or absence of quantized magnetic flux (fluxons) that represents information bits. The basic RSFQ structure is a super- conducting ring that contains one Josephson junction plus a resistive shunt outside it (see Figure 5.4). Suppose a current is already a circulating around the loop, supporting one fluxon. At a certain critical current level (about 100 mA for typical designs), additional DC current across the loop causes the fluxon to be ejected, with the Josephson junction acting as a briefly opened exit. Rather than use the escaping flux directly, RSFQ relies on the fact that the movement of a fluxon into or out of this loop induces a very short voltage pulse (known as an ‘SFQ pulse’, for single flux quantum) across the junction. If the Josephson junction were a square, 1 mmona side, this SFQ pulse would be <1 ps long and 2 mV in amplitude. The SFQ pulses become narrower and greater in amplitude as the junctions decrease in area but, because their magnetic flux is quantized, the voltage–time product of the pulse always remains the same: 2 mV-ps, i.e. 2 £ 10 215 Wb. The energy consumed each time an SFQ pulse passes through a junction is just the circulating current of about 100 mA times the amount of flux F 0 , or only ~2 £ 10 219 J. These SFQ pulses are used to form RSFQ digital logic gates composed of only a few basic Software Defined Radio: Enabling Technologies132 circuit structures. These building blocks allow the generation, transfer, storage, and condi- tional routing, or ‘switching’, of SFQ pulses. Shown in Figure 5.5, the three basic structures include an active transmission stage (JTL or Josephson transmission line), the storage loop, and the decision making pair (or comparator). 5.2.1.2 Fabrication and Packaging RSFQ integrated circuits are made with standard semiconductor manufacturing equip- ment; however, there are many fewer mask layers (typically about ten) and the actual processing involves much less complex depositions [9,10]. Because RSFQ logic is an all- Superconductor Microelectronics: A Digital RF Technology for Software Radios 133 Figure 5.4 Physical realization of a resistively shunted Josephson junction Figure 5.5 The three basic structures of RSFQ logic thin-film technology, there are no doping profiles to calculate, no high temperature drive- ins, no epitaxial growths, or chemical vapor depositions. These differences are expected to translate directly into reduced costs in the large scale manufacture of RSFQ electro- nics. Architectures containing both front end analog circuitry, as well as digital processing blocks, are fundamental SDR requirements. This configuration presents extraordinary diffi- culties for semiconductors, due to ‘crosstalk’–problems of interference between the analog and digital sections of the same chip. Because of the unique reliance on single quanta of magnetic flux to convey information, RSFQ are inherently more immune to this sort of crosstalk. The RSFQ technology also has a clear path to extend performance. Unlike semiconductor devices, the speed of RSFQ ICs comes from inherent physical phenomena, not ultra-small scaling. This means that existing lithography techniques can be employed and, more impor- tantly, existing equipment can fabricate circuitry that surpasses conventional limits of perfor- mance. Because RSFQ logic uses the lossless ballistic transmission of digital data fluxons on microstriplines near the speed of light, the wire up nightmare that silicon designers face is substantially reduced. This scenario also allows the full speed potential of individual gates to be realized. Other features of this technology that make it suitable for growth into the traditional market include its compatibility with existing IC packaging techniques. These include compatibility with optical (fiber) signal input and output, a maturing multichip module (MCM) technology with multi-Gb/s digital data transfer between chips, and simple interface circuits to convert to and from both ECL logic and CMOS logic levels. 5.2.2 Example RSFQ Logic Gate – RS Flip Flop To transfer SFQ pulses as information bits, a clock may be used to provide a steady stream of timing pulses (one per clock cycle), such that the presence of a data pulse within the clock cycle denotes a logic (1), while the absence of one denotes a logic (0). Combinations of Josephson junctions can then be interconnected to achieve SFQ pulse fan-in and fan-out and create a variety of logic structures. Although all common binary logic primitives (like AND, Software Defined Radio: Enabling Technologies134 Figure 5.6 Basic building blocks of an RSFQ gate OR, or XOR) can be fashioned, it is often more convenient to create gate macros directly rather than from lower logic primitives. This technique maximizes speed and minimizes junction count. The operation of an RSFQ reset–set flipflop gate provides a simple example (see Figure 5.6). If a set pulse arrives, J1 transmits it into the quantizing inductance loop, where it becomes trapped as a circulating current – the 1 state. This current biases J3, so that when a clock/reset pulse arrives at the gate, it causes J3 to transmit the stored fluxon to the output, thus resetting the flipflop to the 0 state. Alternatively, if no set pulse input has occurred during the clock period and a clock/reset pulse arrives, the unbiased J3 cannot transmit the pulse and J2 is forced to let the fluxon escape the circuit, so no pulse appears at the output. 5.2.3 RSFQ Data Converters 5.2.3.1 Analog to Digital Converters The quantum precise periodic transfer function of the superconducting quantum interfer- ence device (SQUID) makes superconductor circuits an excellent choice for data conversion from a continuous time to a discrete time format [11]. Figure 5.7 shows a block diagram and a chip photo of an RSFQ ‘high resolution’ ADC based on a phase modulation/demodulation architecture [12]. This superconductor ADC design is especially linear, because the quantization thresholds are set by a ratio of fundamental physical constants (h/2e) in the SQUID in the front end. This leads to an enhanced spurious free dynamic range (SFDR) in comparison to semiconductor ADCs, whose thresholds are set by the matching of device characteristics. Common perfor- mance metrics for ADCs are the SINAD and the SFDR. 2 SINAD is a signal-to-noise and Superconductor Microelectronics: A Digital RF Technology for Software Radios 135 Figure 5.7 Phase modulation/demodulation ADC: block diagram and chip photograph 2 These metrics are described generally in Chapter 2, and, in the context of ADCs, in Chapter 4. distortion measurement, and represents the dynamic range of the signal with respect to all digitization artefacts. SFDR is the spurious free dynamic range, reflecting the linearity of the ADC process by indicating the ratio of signal to the highest spurious signal in the Nyquist band. Demonstrated performance for the first HYPRES RSFQ ADC (shown in Table 5.2) is a SINAD of 58.2 dB (9.4 effective bits) and an SFDR of 278.7 dBc at 100 MS/s Nyquist rate sampling. The same chip also provides 14.5 effective bits (a SINAD of 89.1 dB) with an SFDR of 2100 dBc for a DC to 2.3 MHz band at 5.5 MS/s. The circuit consists of two major parts: a differential code, front end quantizer and a digital decimation low pass filter. The front end is composed of an analog phase modulator and a digital phase demodulator. The phase modulator consists of a single-junction SQUID, biased by a DC voltage from a special voltage source, which is stabilized by an internal clock frequency. The phase demodulator consists of a time-interleaved bank of race arbiters (SYNC) followed by a thermometer to binary encoder (DEC). In order to obtain a binary differential code from the thermometer code outputs of the synchronizer bank, the encoder block adds up these outputs and subtracts N/2 each clock period. The differential code from the output of the front end is passed to a digital decimation low pass filter (DSP), which uses a standard cascaded integrator comb (CIC) architecture with two integration stages. The first integration stage restores the signal from the differential code, and the second one provides first-order low pass filtering. The dynamic resolution, or effective number of bits (ENOB), of this ADC is determined by the input signal bandwidth (BW), the internal clock frequency f clk , and the number of synchro- nizer channels N and is given [13] by ENOB ¼ log 2 ðNf clk = p BWÞ 1 1=2 log 2 ðf clk =2BWÞð1Þ The first term in this formula accounts for a slew rate limit (i.e. limited by the signal derivative), while the second one comes from standard oversampling gain. Here, the BW is assumed to be half the output sampling rate (i.e. at the Nyquist limit). Therefore, (1) gives a bandwidth-to-resolution trade-off ratio of 1.5 bits per octave, as expected for a first-order oversampling ADC. 5.2.3.2 Digital-to-Analog Converters A number of different so-called ‘programmable Josephson voltage standards’ have been proposed [14,15]. Each of these designs consists of a superconductor digital-to-analog converter (DAC) based on the properties of flux quantization. When a quantized SFQ Software Defined Radio: Enabling Technologies136 Table 5.2 Demonstrated RSFQ ADC performance Sample rate MS/s ENOB bits SINAD dB SFDR dBc Input MHz 100 9.4 58.2 278.7 DC to 50 25 12.6 77.8 291.0 DC to 10 5.5 14.5 89.1 2100 DC to 2.3 [...]... everywhere in the world (even the United States) TDMA systems require one radio per frequency channel, so, in order to support a large number of simultaneous callers, one requires an equally large number of radios Because of the wideband ADCs possible with superconductors, one can use an approach which requires only a single radio, directly at the RF carrier frequency, to digitize all the calls at... noise by using a stage of the cooling platform 4 John Ralston, for example, persuasively argues such a case in Chapter 4 of Software Defined Radio: Origins, Drivers and International Perspectives, Tuttlebee, W (Ed.), John Wiley & Sons, Chichester, 2002 Software Defined Radio: Enabling Technologies 142 5.4.2.1 Receiver Noise Temperature Calculation 5 When examined at the system level of the complete receiver... voice and data transfer Already, most wireless systems are implemented digitally, and Superconductor Microelectronics: A Digital RF Technology for Software Radios 141 there has been growing prediction that the industry is evolving towards a ‘software radio implementation 4 This involves moving the point of digitization (the point where the transition from a modulated analog carrier is transformed into... the capabilities described in the previous sections, one can begin to imagine how almost all the functions required by a digital RF transceiver could be integrated monolithically into a radio on a chip’– specifically, a radio that performs all the functions in the process of converting voice or data information to and from a modulated RF signal (see Figure 5.12) Nominally, such functions include: processing... superconductor electronics, one can directly digitize each 6 For information on JTIDS and LINK 16, see Chapter 2 by Bonser in Software Defined Radio: Origins, Drivers and International Perspectives, Tuttlebee, W (Ed.), John Wiley & Sons, Chichester, 2002 148 Software Defined Radio: Enabling Technologies band at RF – i.e no analog mixing This produces digital I and Q outputs of each of the three bands or any... radio, promises to revolutionize wireless systems in the coming decades, with initial applications anticipated in the relatively near future for base station and defense applications Acknowledgements Special thanks to my colleagues who contributed text, figures, and/or assisted with proofreading, including Dr Deepnaryan Gupta, Dr Oleg A Mukhanov, Dr Alan M Kadin, and Jack Rosa 150 Software Defined Radio: ... generation’, IEEE Spectrum, Vol 37, 2000, pp 40–46 [5] Mitola, J ‘Software radio architecture evolution: foundations, technology trade-offs, and architecture implications’, IEICE Transactions in Communications, E83-B, 2001, pp 1165–1173 [6] Salkintzis, A.K., Nie, H and Mathiopoulos, P.T., ‘ADC and DSP challenges in the development of software radio base stations’, IEEE Personal Communications, Vol 6, 1999, pp... Dt ! Dt/a R ! Ra C ! C/a 2 L!L N ! Na 2 P ! Pa 2 a Scaling down by a factor of a , in regime where shunt resistance is still needed Superconductor Microelectronics: A Digital RF Technology for Software Radios fc < 1 75 GHz ; < 10t a½mmŠ 139 ð2Þ where again this scaling relation should continue only down to a scale of 0.3 mm, for a maximum limiting VLSI clock speed of about 250 GHz These scaling factors... Systems – the Myth and the Reality The Myth: low noise receivers are not useful in interference limited systems, because thermal noise is small compared to interference from other users 144 Software Defined Radio: Enabling Technologies The Reality: low noise receivers are useful in interference limited systems, because they allow a reduction in power for all users, permitting higher information capacity (i.e... network optimization algorithms that greatly benefit from the ability to perform more complicated handoffs in densely covered sectors Superconductor Microelectronics: A Digital RF Technology for Software Radios Figure 5.11 145 Coverage range of conventional vs superconductor receivers 5.4.4 High Power Amplifier Linearization The high power amplifiers (HPAs) of any base station transmitter system consume . proposed software radio architectures. For the purposes of this work, we will only 1 ‘Pure’ software radio, as distinct from ‘pragmatic’ software radio, incorporates. of software radio [5]. However, realization of software radio systems Superconductor Microelectronics: A Digital RF Technology for Software Radios 131 Table