Cryo Central - Telecommunications

From the Winter 2004 issue of Cold Facts magazine . . .

The recent M-Calc IV — 4th Industry Assessment workshop discussing military and commercial applications for low-cost cryocoolers, held in November in San Diego, highlighted progress being made in cryogenics as applied in telecommunications.

The reliability and long lifetime of projects now being introduced are extraordinary, making this technology increasingly feasible for both HTS and LTS uses.  Cold Facts contacted some of the leading players in this field and gathered their comments, news of products and developments and a list of resources for further study.

From Daryl Treger, Strategic Analysis, Inc., chair of MCALC IV (treged@sainc.com):  A two-day industry meeting on Military and Commercial Applications for Low-cost Cryocoolers (MCALC) was held in San Diego CA on November 20th and 21st, 2003.  The meeting was sponsored by Strategic Analysis, Inc. (www.sainc.com) in cooperation with DARPA, the Army Night Vision Labs (NVESD), and Dr. Martin Nisenoff of Nisenoff and Associates (retired NRL).  Attendees included representatives from the US and foreign cooler vendors, users and integrators, with presentations discussing their cryocooler needs and programs.

The continuing objective of this workshop is to perform an industry assessment on the current status of low cost, highly reliable cryocoolers and to estimate the current and perceived needs of the cooled electronic communities. ; Emphasis is focused on low cost cryocoolers operating in the temperature ranges below 232K (- 40°C) with attention to the requirements of the user communities on reliability, efficiency, temperature stability, EMI, vibration, audible noise, etc.

During the workshop, there was a series of presentations from the user communities (such as IR cameras, cooling of semiconductor devices and chips, cellular base stations, satellite applications, medical applications and high temperature superconductivity) outlining their projection of cryocoolers needed for present and future generations of equipment, and presentations from a variety of cryocooler vendors outlining what is currently available.  MCALC IV also had additional presentations from some key low temperature applications and cryocooler companies (down to 4K).  The twenty-three invited speaker presentations included twelve cryocooler manufacturers, ten users, and one discussion of the Cryocooler Database sponsored by the University of Twente.  There was also exhibitor space available for cryocoolers vendors to display their products, brochures and technical data.

The Proceedings from MCALC IV is now available on CD.  To view the conference agenda, to find out more information on the MCALC IV Conference, to express interest in MCALC V, to have your name added to the MCALC contact list, or to purchase a Proceedings CD, go to http://www.sainc.com/MCALC4.

From Elie Track, Chief Technology Advisor/Senior Consulting Partner, Hypres, Inc.: Cryocoolers for Digital Wireless Communications – A Significant Emerging Market Recent advances in superconducting electronics and in cryocoolers, combined with developing market needs in wireless communications – commercial as well as military – have combined to create an exciting prospect.  Development programs are currently underway, with the aim to produce unique receivers and transmitters (transceivers) that have the ability to cover the full available spectrum digitally, thus enabling the holy grail of "software radio," i.e., universal and seamless communications across all varying wireless systems, regardless of the frequency or coding protocols they use.  The prospects for new products utilizing cryocoolers for electronics applications is indeed exciting.

Cryocoolers and cryocooling have always been characterized as the key enabling product and technology for superconducting electronics.  Indeed, while a limited number of superconducting electronics applications have been successfully pursued through the use of liquid cryogens, for example SQUID magnetometry in research laboratories and in magnetencephelography (MEG), the "killer apps" have always been thought to require - – in addition to the societal or technological market need – the ability to provide the user with a turnkey system where the cryogenics are fully transparent.  And for superconducting electronics, which require cryogenic temperatures for their operation, this can only be done through the use of closed-cycle refrigerators (CCR), namely cryocoolers.

The recent excitement in the field has been generated from the simultaneous convergence of several factors.  The key development is market driven.  Wireless communications are an exponentially increasing market and this increase has become limited by the availability of useable electromagnetic spectrum and by the hardware capable of using it efficiently.

The holy grail of wireless communications is what is commonly referred to as "software radio," namely a fully digital implementation where signals – which are carried through the airwaves by electromagnetic waves – are changed from analog to digital format immediately after being received by the antenna, and conversely for transmission.  While many systems and wireless phones operate in digital mode today, the actual conversion to/from digital processing occurs typically after a number of analog steps necessitated by the lack of adequately fast and wideband electronics.  Enter digital superconducting microelectronics (SME), which intrinsically has the ability to provide direct digitization of radio-frequency (RF) signals.  SME transceivers, and only SME transceivers, enable the full implementation of software radio, thus becoming ubiquitous in all cellular towers and military communication systems.  However, for acceptance in these demanding markets, a number of conditions have to be satisfied.  Beyond being turnkey systems, long-term reliability must be assured, and in some platforms, maintenance-free operation is required.  In addition, the systems must be rugged, withstanding harsh environments in certain cases, for example when required to be tower-mounted or on-board aircraft.

The need for migration towards digital systems is generally recognized, and programs are actively underway to transfer as much of the system from analog into digital hardware — with the accompanying software and the power of digital signal processing to provide flexibility and interoperability.  In the commercial arena, this has led to bi-mode and tri-mode cellular phones that can successfully operate in different countries.  In the military communications market — a much more demanding field with myriads of systems and different protocols — the same development is under way under the JTRS (Joint Tactical Radio System) program.  This JTRS program is multiphased, addressing varying military platforms in each of its phases.  The first phase, referred to as "Cluster 1," applies to a number of vehicular and rotary aircraft radios.  This will be followed by "AMF" (Airborne, Maritime, and Fixed facilities) addressing another family of military platforms.  In all these implementations, the desired solution is for all-digital software radio, enabled by SME.  Parallel paths with conventional technologies are also pursued, to provide lower risk options, albeit without the full power of all-digital implementation.

On the SME front the implementation requires low temperature superconductors (LTS) because of the need for digital, high density integrated circuits which are inconsistent with the limitations of the high temperature materials (HTS).  HTS however does provide solutions for parts of the systems — and are currently deployed in many commercial systems where they offer the ability to optimize the analog portion of the these systems.  In LTS, Hypres, Inc. is under contract with the Department of Defense (DoD) to develop an all-digital receiver (ADR) as a first step to an all-digital transceiver (ADT).  These are applicable to all military platforms intended in the JTRS program, and will be readily adaptable to the easier field of commercial wireless systems.  Their implementation depends critically on the parallel development of a (family of) compact, reliable, efficient cryocoolers operating in the 4K to 5K temperature range.  Hypres estimates the heat lift required to be in the range between 50 mW and 200 mW at those temperatures with intermediate stages providing ancillary cryocooling for thermal packaging efficiency and for other devices that benefit from a low temperature environment.  Interactions are actively underway between the developers of SME, cryocoolers, and the DoD in planning and implementing these parallel developments, on the way to all-digital software radio solutions, with a vision of seamless, universal, wireless communications.

Related news about Hypres: In an effort to realize a true software-defined radio, Hypres has developed an ultra-high-performance analog-to-digital converter (ADC) using ultrahigh-speed and ultra-low-power superconductor technology.  This monolithic superconducting ADC chip will directly convert RF signals from the antenna to digital baseband with an exceptionally high signal-to-noise ratio (SNR) and spurious-free dynamic range (SFDR).

This work is under a $1.5 million, two-year contract from the US Office of Naval Research (ONR).  Hypres is developing a superconducting bandpass ADC able to produce 14 to 16 effective bits with an SFDR of over 100 dB and an SNR of 90 dB at 14 bits.  This bandpass ADC will offer a dynamically programmable bandwidth of 10 to 400 MHz with a 5-GHz center frequency.

Hypres attributes this performance to the ability to produce an accurate high-speed clock with subpicosecond jitter and to the use of a rapid single-flux quantum logic (RSFQ).  Employing Josephson junctions, the RSFQ circuit uses magnetic flux quantization in a phase modulation/demodulation architecture to provide extremely linear and high-speed digitization of analog signals.  Hypres uses a low-temperature niobium superconducting material for this application.

This contract continues an earlier one, a $1.2 million, three-year deal from the ONR for Hypres to develop a digital receiver architecture that complies with the Navy's Joint Tactical Radio System.  The bandpass ADC is a critical component of this digital receiver design.  (Electronic Design)

A report from the military standpoint, from Deborah Van Vechten, program officer, Office of Naval Research, Electronics Department (vanvecd@onr.navy.mil): All Digital Receivers: Breaking Comms 'Stovepipes'

The Office of Naval Research moved the Navy — and all the services — a big step closer to needing only one new radio to talk to all those already in service — the longsought goal of full "interoperability" — by awarding an $8 million one-year contract to Hypres, Inc., for a new all-digital radio receiver.

The company will deliver a demonstration receiver that simultaneously "digitizes" all the signals in the over-the-horizon, lowest military communications bands (HF and VHF) directly from the RF and selects the signals to output by software controlled digital filtering.  Such "software defined functionality" is very different from that of today's heterodyne receivers and is the fundamental innovation required to realize the vision of the DoD-wide joint tactical radio system or JTRS.  With an eventually market of thousands of tactical radios per year, JTRS radios should drastically simplify joint operations and the logistics of manpower reallocation.  Only conceptually minimal changes will later be required for one radio to cover the entire 2 Megahertz to 2 Gigahertz JTRS range, or even the newly announced 2 Megahertz to 55 Gigahertz range.  The JTRS program seeks to develop a generic radio, but is organized into clusters led by the individual services that focus on platform dependent issues.  It is further organized by a joint program office.

The radios now in use, aboard ships, aircraft, and carried by ground units, are largely of a "stovepipe"-type that each handle only one proprietary waveform and require expensive hardware changes to communicate with other radios.  Software defined functionality eliminates such non-interoperablity by allowing the user to change which waveform he wishes to receive and which to transmit.  Radios with this property can be customized for multiple missions, be integrated with legacy hardware, and be upgraded by the easy and cheap insertion of software modules instead of new hardware.

The new JTRS radios will also replace older analog communications components with digital technology, a change made possible by the high clock speeds now available in digital processors.  Doing so reduces the complexity and cost of the radios.

The virtues of the technology underlying the Hypres digital receiver are its inherent accuracy and very high processing speed.  Both enable the receiver to handle multiple simultaneous signals spread over considerably wider communications bandwidths.  The Hypres receiver will be able to listen to signals of varying data rates that have been "layered" on top of each other, improving data transmission rates.  These signals can include those "spread" over wide bands by the commercially important communications technique called code division multiple access, or CDMA.

The move to digital reception, allows the user to "make copies" of data, and process it with different digital filters to reveal the different signals, a technique that largely eliminates the need for parallel analog hardware and adaptive analog pre-processing.  Hypres is already teamed with Boeing, winner of the JTRS "Cluster 1" contract for the Army.  The company now is hoping to get selected for "Cluster 3", the maritime JTRS, which is managed by the Space and Naval Warfare Systems Command.

The Hypres software defined digital receiver technology provides a simultaneous "stare and scan" capability that may meet the EW needs of one of ONR's flagship efforts, the advanced multi-function radio frequency system concept, now called AMRF-C.  The AMRF-C initiative, set for a major demonstration next year, aims at developing a highly reconfigurable set of antenna apertures to handle all shipboard radio communications, radar and electronic warfare systems.

More from the military viewpoint on the latest developments from Anna Leese de Escobar, SPAWAR RF (anna.leese@navy.mil): R F Systems Applications of Superconducting Electronics

Superconducting Electronics (SCE) technology has the potential for achieving transformational performance for Navy and Joint Applications.  Recent major advances in miniaturizing the size, efficiency, and most importantly, reliability of cryogenic cooling systems have resulted in sharp increases in the insertability of Superconducting Electronics (SCE) into existing RF systems to achieve impressive performance improvements.  Entirely new capabilities are enabled by this performance that promise solutions to long-standing problems in the Communications and Signals Intelligence domains.  These new SCE Transformational Technology components can be divided into two major classes: HTS or high temperature superconductors and LTS or low temperature Superconductors.

High Temperature Superconductor (HTS) analog filter technology has been widely accepted by the cellular telephone industry because of the need for narrow bandwidth and sharp-skirted filters.  The very impressive interference rejection and greatly improved receiver noise figures (i.e. higher sensitivity) due to cryogenic cooling of the first LNA in the system have allowed fewer cellular telephone towers to provide service to an ever increasing user base.  HTS SCE is practical for military purposes because cryogenic cooling systems have demonstrated an astonishing 850,000 hours mean time between failures (MTBF) in commercial environments not unlike those found in shipboard applications and can be as small as a Coke can.  The Navy has the same or worse EMI problems as the commercial world.

But now cryogenic cooling allows HTS filters to be used in this high EMI environment, precisely excising own-ship interference.  Additionally, cryogenic cooling of low noise figure receiver pre-amplifiers (LNA's) increases sensitivity in the bands of interest, resulting in the possibility of detecting mid-VHF and UHF signals 20 - 200% farther away (depending on environmental conditions).

Similarly, improvements in link margin can be expected by using HTS filters and cryogenically cooled LNA's in digital SATCOM or LOS communications terminals.  Larger link margins can be used to increase data rates, reduce transmit power or reduce radar cross-sections by reducing required antenna gain and hence the resulting dish antenna sizes.  Further, optimization of a communications system with such tight channels and managed co-site interference should result in more utilization of allocated bandwidth.  The minute loss of the HTS components enables an additional transformational capability: the possibility of utilizing HTS subsystems to perform analog signal processing such as signal excision, or even transforms (e.g. Fourier), before the noise figure of the system is "set" at the first LNA.

Low temperature superconductor (LTS) digital technology can be used to improve two main RF areas — small antennas and high-speed digital circuits in RF front-end systems.  Commercial LTS coolers exist and could be further improved in reliability and size by the same process that improved the HTS ones.  LTS Superconducting QUantum Interference Devices (SQUIDs) offer an opportunity for extremely small and extremely wideband (1MHz-1GHz), antennas that would allow for consolidation and size reduction of listening systems while achieving environmentally noise limited performance.  In addition to the logistic benefits, this type of antenna performance will allow increased range against conventional RF signals and will allow shipboard efforts to detect a whole new class of signals previously not detectable.

The physics of LTS suggests complex logic speeds of up to 160GHz are possible, in fact, simple digital circuits at over 700GHz have been demonstrated.  Extremely high-speed digitizers can extend the concept of software defined radios by directly digitizing signals at full-bandwidth RF rather than at a typically narrowband, intermediate frequency(IF) as used in the present non-LTS state of the art.  By doing so, these digitizers eliminate the noise and distortion and analog part complexity inherent in the heterodyne frequency conversion step from RF to IF and increase the bandwidth available for processing.  Fast digitization techniques will allow the sorting of agile signals based on their bearing, for example.  By exploiting the RF environment on a totally digital basis, even the most complex signals could be observed with the flexibility and adaptability that comes with complete software re-programmability.  LTS Digital-RF™ is a technology which offers simultaneously: high speed and sensitivity with quantum accuracy, ultra low noise and power dissipation, ideal interconnects, and simple fabrication.

Satellite Communications.  Large, costly satellite reflectors represent a significant cost factor in a communications system.  Many of these systems are being challenged to maintain data quality due to increased electromagnetic (EM) noise from terrestrial sources as well as a higher population of geosynchronous satellites on the celestial equator.  The sharp frequency passband of a superconducting filter may eliminate these sources of interference, improve the gain-to-noise temperature ratio (G/T) of the antenna, and possibly improve the link budget by several decibels.  Smaller satellite reflectors on ships and vehicles are also excellent candidates for upgrading with Superconducting Electronics front ends.  Satellite applications could also benefit from the high speed of LTS digital circuits, allowing more channels or bandwidth efficiency.

Communications nodes.  Aircraft, ships, vehicles and unmanned aerial vehicles (UAVs) equipped with multisystem communications suites are being developed to link large areas of the battle space.  A key issue for successful node operations is ensuring EM compatibility and eliminating co-site interference.  Superconducting filters represent one of the most effective ways of eliminating self-induced IM.  In addition, as mentioned above, all the benefits of software defined radio systems are enabled by a high-speed analog to digital converter at the front end of the system with no down conversion necessary.  Also the sensitivity increase afforded by cryogenic operation of the first LNA in the system would increase link margins and affect the entire communication system, allowing for use of the additional margin as the system designers wish, as described above.

UAV Antennas and Radio Frequency Distribution Systems (RFDS).  For UAVs, size, weight and power are severely limited. High-Temperature Superconducting (HTS) Antennas allow use of practical "Electrically Small" antennas that provide acceptable gain.  Small sizes allow attractive size reductions of two to one for spiral antennas and ten to one for loop antennas.  Superconductivity can allow small antennas in tightly packed arrays with broadband, low frequency, heretofore impossible, performance.  Adaptive arrays can be used at any frequency in principle, even at more than one frequency at once.  An increase in the number of elements allows an increase in the number of jammers nulled.

From Abhijit Karandikar, Product Manager, Superconductor Technologies, Inc., (abhijit@suptech.com), comes information about his company's HTS contributions to the advancement of cryogenics in telecommunications:

High-temperature superconductors — a short history

The phenomenon of superconductivity was first observed in 1911, when Dutch physicist H. Kamerlingh Onnes used liquid helium to cool mercury to 4.2 Kelvin.  Yet it took another 75 years before viable, commercial applications of the science began to appear.  A key first step occurred in 1986, when physicists Karl Muller and J. Georg Bednorz of IBM's Zurich Research Laboratory discovered that ceramics from a class of materials called perovskites become superconductors at approximately 35 Kelvin.  One year later, Paul Chu of the University of Houston announced the discovery of Yttrium Barium Cooper Oxide (YBCO), a compound that becomes superconducting at 90 Kelvin.  Even higher temperatures were reached soon thereafter, including the production of bismuth compounds (BSCCO) that are superconductive up to 110 Kelvin and thallium compounds (TBCCO) that are superconductive up to 127 Kelvin.  The discovery of these "high-temperature superconductors" — which could use cost-effective liquid nitrogen (rather than liquid helium) as the refrigerant — at last opened up the commercial market for superconducting solutions.

STI — leader in HTS for wireless networks

In 1987, Nobel Prize winner Dr. J. Robert Schrieffer teamed up with three venture capitalists to form Superconductor Technologies Inc. (STI), a Santa Barbara CA company that would develop and exploit the new technology of high temperature superconductivity (HTS).  The STI team examined the many potential applications for HTS, which includes magnetic resonance imaging (MRI), levitating trains and ultra-efficient power lines, and chose to focus on the radio frequency segment of the wireless communications market.

One of the biggest challenges STI faced in order to penetrate the demanding telecommunications market was to find a cryogenic cooler (or "cryocooler") that was small, lightweight, power-efficient and extremely reliable.  After an unsuccessful search for a suitable cryocooler among existing manufacturers, STI made the strategic decision to invest the considerable resources required to develop and manufacture a cooler on its own.  Developing a cryocooler in-house has allowed the company to exert greater control over this critical component and has led to significant technological improvements not otherwise possible. (See cover.)

STI's advanced dewar and cryogenic cooling technology, in conjunction with superior system integration and exceptional customer service, enables SuperLink Rx to exceed the wireless industry's strict standards for performance and reliability.  STI customers have now deployed over 3,800 SuperLink Rx units worldwide, with a cumulative run-time of over 42 million hours.  Each unit is maintenance-free, enjoys an "uptime" of 99.9%, and has a field-proven MTBF of over 500,000 hours.  The reliability of the latest generation of cryogenic coolers is astounding: with 2,000 coolers logging a total of 12 million hours, only two have been known to fail.  This translates to a demonstrated MTBF that is measured in millions of hours.

Since 1987, Superconductor Technologies has become the global leader in superconducting products for wireless networks. STI's SuperLink Solutions increase capacity utilization, lower dropped and blocked calls, extend coverage an enable higher wireless transmission data rates.  The company's flagship product-SuperLink Rx-incorporates HTS technology to create a cryogenic receiver front-end (CRFE) used by wireless operators to enhance network performance while reducing capital and operating costs.  With its high-Q, brick-wall filtering, remarkably low noise figure, and additional "cold" grain (via the cryogenically-cooled low-noise amplifier), SuperLink Rx is the perfect receiver, providing the strongest possible link between mobile customers and their wireless networks.

From Thom Davis, Chief, Space Cryogenic Cooling Technology Group, Space Vehicles Directorate, Kirtland AFB, NM (thom.davis@kirtland.af.mil) these comments on Air Force Research Laboratory Development (from a paper presented at the recent CEC Conference):

Currently under development are a range of Stirling, pulse tube, reverse Brayton and Joule-Thomson cycle cryocoolers to meet current and future Air Force and Department of Defense requirements-at 10K, 35K, 60K, 95K, and multistage requirements at 35/85K.  Working with industry partners, the AF Research Lab is developing advanced cryogenic integrating technologies to reduce current cryo system integration penalties and design time, including continued development of gimbaled transport systems, 35K and 10K thermal storage units, heat pipes, cryogenic straps and thermal switches.

Near term goal is completing the HCC 35/85K cryocooler and electronics, an enabling cooling technology and risk reduction for space surveillance concepts.  More far term objectives involve advanced concepts to achieve cooling requirements.  AFRL is working with Technical Applications Inc., Boulder CO, to develop a cryocooler using Micro-Electrical-Mechanical Systems (MEMS) for space cryogenic cooling.  The cryocooler development program is providing critical path cryocoolers for Missile Defense Agency, Air Force and DOD programs.