1. Software Defined Radio (Enabling Technologies)
Ch5. Superconductor Microelectronics :
A Digital RF Technology for Software Radios
마이크로파공학 연구실
김준철

2. Commercialization of superconductor microelectronics technology promise to
be a key enable of ‘pure’ software radio architectures
This chapter provides a description of the underlying technology and its
potential in both commercial and defense wireless systems

3. 5.1 Introduction
The speed and flexibility enabled by superconductor microelectronics seems
well matched to the goals of proposed software radio architectures
LTS ( Low Temperature Superconductors )
-- Nb ( niobium )
Tc : 9.23K, operate : 4.2K - 5K
Digital RF
-- superconductor logic gates will directly process digital signals at RF or multi-
GHz frequencies.
RSFQ ( Rapid Single Flux Quantum )
-- flexible, high data rate applications (for 3G, 4G wireless services)

4. 5.1.1 Superconductivity and the Josephson Effect
- Phenomenon of Superconductor
1) zero resistance (when cooled below a critical transition temperature (Tc)
2) superconductor can contain magnetic flux only in certain discrete quantities
 called ‘flux quantization’

5. - In order to create digital circuits, an active superconductor component is needed
: the Josephson junction (JJ)
* I < Ic : device exhibits no resistance
* I > Ic : JJ becomes briefly resistive

6. - 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 return to the Zero voltage
- Rapid voltage pulse corresponds to a single flux quantum

7. 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 are
used in cellular base stations
- the use of superconductor material allows the very high Qs to be 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
- an increasingly embraced solution to surmount these obstacles and lies in the concepts
of software radio
- realization of software radio systems presents a host of challenges - chief among them
the unprecedented requirement on analog-to-digital converter (ADC) performance
- this is the area where superconductor microelectronics represents an emerging solution
- with demonstrated ADC, DAC, and DSP components, this technology may well become
a key enabling technology for software radio

8. (table 5.1)  summarizes the performance already achieved with such superconducting
devices to date
- military radio requirements are far more demanding than those for commercial systems

9. 5.2 Rapid Single Flux Quantum (RSFQ) Digital Logic
Rather than relying directly on quantized bundles of magnetic flux as bits, many
effort attempted to use the voltage state of the JJ as a ‘1’ and the superconducting
state as a ‘0’
Power consumption :
Typical latching gate : dissipated about 3 pW
RSFQ technology : dissipated 0.3 pW

10. 5.2.1 Circuit Characteristics
In RSFQ circuits, it in 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 superconducting ring that contains one Josephson junction
plus a resistive shunt outside it

11. 5.2.2 Example RSFQ Logic Gate – RS Flip Flop

12. 5.2.3 RSFQ Data Converter 5.2.3.1 Analog to Digital Converters
- This superconductor ADC design
is especially linear, because the
quantization thresholds are set by a
ratio of fundamental physical
constants ( ) in the SQUID in the
front end
- Common performance netrics for
ADCS are the SINAD and SFDR
SINAD : signal-to-noise and distortion
measurement
 represents the dynamic range of the signal

13. The circuit consists of two major parts
 front end quantizer, digital decimation low pass filter
* front end 1) phase modulator – consist of a signle-junction SQUID
2) phase demodulator
- consist of a time-in terleaved bank of race arbiters(SYNC) followed by a
thermometer to binary encoder (DEC)
ENOB (effective number of bits)

14. 5.2.3.2 Digital-to-Analog Converters

15. 5.2.4 RSFQ scaling Theory
Considerations of the junction fabrication process
: thickness of the tunneling barrier, critical current
density (Jc), thermal fluctuation.
For RSFQ circuits, the high frequency performance
is determined primarily by the width of the SFQ pulse
Although JJs are the central elements of RSFQ
circuits equally important are inductors and resistors

16. 5.3 Cryogenic Aspects
Superconductor RSFQ circuitry must be cooled for operation
(critical temperature : Tc)
The temperature of operation is about one-half Tc
suitable platform for a commercial product
: a closed cycle refrigerator (CCR or cryocooler)
Carnot efficiency
MTBF (mean time between failures)

17. current, developing, and forecasted cryocooler

18. A cost-reliability goal for commercial cryocooler is US $ 1000-10000, 2~10 year lifetime
RSFQ chips heat < heat leaks of radiation and conduction from the surrounding interface
electronics
one of the main concerns is reliability
reality of reliability
* using oil lubricated air-conditioning compressor (increasing life time)
* using noncontacting suspension methods for the piston in the cylinder (increasing MTBF)

19. 5.4 Superconductor SDR for commercial Application
5.4.1 Superconductor in wireless communications
Growth of commercial wireless communication technology evloving forward a ‘software radio’
implementation
 Because of spectrum regulation
The only way to fit in more callers or higher rate data transfer or both is to increase their
‘spectral efficiency’
due to the competition of many providers need the cost/performance ratio
so, CCR is needed, in practice, cryocooled high temperature superconductor (HTS) filter are
now being installed in cell base station receiver with good success
cryocooled LTS (low temperature superconductor) mixed signal electronics is probably the
only technology with the speed, linearity, and sensitivity to allow direct RF digitization and
subsequent fast digital signal processing
Beside, there are now reliable 5K cryocoolers with an appropriate market, further
improvements in efficiency, size, and costs are expected

20. 5.4.2 Advantages of superconductor Receivers
Using LTS -- possibility of digital RF
 replacing analog components with digital components
Advantages
* far less distortion
* digitizing a wider band of the spectrum
* the high sensitivity
Receiver noise Temperature calculation
Using superconducting receiver
 reducing noise temperature ( from 1000 ~ 2000K to 250K )

21. For conventional Receiver
Digital RF superconductor receiver is used,
directly digitizing the signal carrier
We conclude that by employing a digital RF approach with a superconductor ADC, we move
from a system limited by the noise figure of its components to one limited by its environment.

22. 5.4.3 Trends in Spread Spectrum Communications
Interference Limited System - the Myth and the Reality
The Myth : low noise receivers are not useful in interference limited systems, because
thermal noise compared to interference from other users
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
Benefits form Lower Receiver Noise
Receiver noise power density N0=KBTS
From Viterbi
benefit
1) the lower Pr must be, the lower our transmitted power, leading to less drain on our
battery
2) callers can use a higher required Eb/No to meet a lower bit error rate requirement or
support higher symbol constellation
3) the number of simultaneous users (M+1) increase
4) using a higher data rate (Rb)

24. 5.4.4 High Power Amplifier Linearization
The high power amplifiers (HPAs) of any base station transmitter system consume a
significant part of the cost and power budget
overcome inherent nonlinearities in the HPA transfer function
1) extends the bandwidth of operation
2) fewer total HPAs
 HPAs of lower quality (lower cost) , to enable the system to still function correctly
superconductor ICs may have the ability to allow real time digital adaptive linearization at RF

25. 5.4.5 Digital RF Transceiver
How almost all the functions required by
a digital RF transceiver could be
integrated monolithically into a ‘radio on a
chip’
analog designer of a bandpass ADC
circuits faces two large problem
1) linearity of the down- conversion over
a wide frequency and dynamic range
2) matching of the anti-alias low pass filter
and digitizers
 digital RF bandpass receiver does not
suffer from these problems

26. 5.5 Superconductor SDR for Military Application
5.5.1 Co-site interference (CSI)
CSI stems from the simultaneous use of multiple radio protocols/frequencies/systems in
close proximity
CSI ‘s three main manifestations
1) trying to receive a weak spread spectrum signal in large interfering signals
2) multipath confusion
3) impulsive interference from hopping signals
Superconductor electronics and the high dynamic range, wideband ADCs and DACs can
overcome these problems by employing brute force fidelity and speed to resolve the issues.

27. 5.5.2 Digitally dehopping spread spectrum signals
need complexity of the dedicated analog systems to access spread spectrum wave forms
however it is possible by using a high SFDR receiver
ex) digitally dehopping, sharp skirt HTS analog filter using superconductor
5.5.3 Satellite Communications
Narrowband (5Khz) DAVA system’s poor link margin
Why? Transponder’s nonlinearities, SOQPSK demodulator’s legacy implementation
However, by using a low noise cryogenic SQUID- based front end, reduce loss
A digital RF approach is possible that it is processing increasing throughput at once, sorting
out the relevant channels, and synchronization
Because of the possible sensitivity of RSFQ circuits, one can actually reduce the noise floor
of its fundamental limit

28. 5.5.4 Accommodating New Wave forms
Hallmarks of the SDR approach
* compatibility for future waveforms
* accessing hop rate
* complete denial of access to the wave form
* RSFQ circuits computational speed
* fundamental accuracy of RSFQ DACs
* allowing the flexibility to add new algorithms
5.5.5 Massive Time Multiplexing
Speed of RSFQ circuits can produce independent lulti – TeraOPS
reducing both capitalization and operational costs
offering better utilization of resources

29. 5.5.6 Conclusions
Using the voltage state of a Josephson junction to suing quanta of magnetic flux to represent
information bits, has radically transformed the capability limits of superconducting technology.
The advances in reliability of cryocoolers has further meant that superconducting devices
(HTS filters) have begun to be deployed in commercial wireless networks
evolution of digital RF technology, coming as it does at the same time as the emerging
commercial acceptance of the concepts software 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