Ka-band ADCs and DACs offer the potential to extend software-defined radio to software-defined microwave for satellite communication.
Operators of telecommunication satellites want to be able to offer their customers flexible data and broadcast services anywhere in the world, anytime. Rapidly changing global events like breaking news, the continuous monitoring of aircraft, or the different needs of global time zones, place real-time, daily, or seasonal demands on the coverage, shape, size, and power of the signals transmitted by satellites, as well as the bandwidth and capacity of communication channels contained within these.
The current approach to satellite design requires the specification of the receiver and transmitter to be changed for almost every new application due to mission-specific and individual customer RF needs. This adds unnecessary non-recurring re-design and re-qualification cost and effort to key programs with operators complaining that the cost to develop payloads is prohibitively expensive and delivery takes too long. Today, the global satellite industry is handicapped by the inflexibility, complexity, power consumption, mass, size, and cost of traditional RF frequency conversion. For key suppliers of geostationary Earth orbit (GEO) telecommunication satellites offering up to 50 channels, analogue super-heterodyne convertors add over 40% to the total cost of the payload.
Satellite manufacturers competing for large global tenders want to offer operators flexible communication services, adaptable to real-time user needs and changing link requirements. While there have been significant advances in avionics, with payloads offering customers larger bandwidths and higher throughputs of data, the design of the transponder has in general remained the same for decades. OEMs are limited by current transceiver technology and are motivated by delivering operators increased revenues and efficiency by improving mission flexibility through in-orbit hardware re-configurability. Some primes now include additional hardware that is switched in and out when needed. This approach has resulted in payloads whose mass, power consumption, cost, and inefficiency have all increased with the level of flexibility required by spacecraft owners. Figure 1 illustrates the architecture of a multi-channel telecommunication transponder with on-board digital processing.
Figure 1 The diagram shows the architecture of a traditional digital satellite payload.
Wideband space-grade ADCs were launched 10 years ago, offering the ability to directly digitise L and S-band carriers. For satellites communicating at these frequencies, bandpass under-sampling techniques allowed receivers to directly digitise the RF uplink, removing the need for traditional super-heterodyne down-convertors. This resulted in transponders that were physically smaller, lighter in mass, less power consuming, and lower cost.
Around the same time, the first wideband space-grade DAC was also launched, offering the ability to directly up-convert digital baseband to C-band. The use of return-to-zero analogue outputs reduced the sinc roll-off in the higher Nyquist zones, allowing access to the images at these frequencies. For UHF, L, S, and C-band satellites, the EV12DS130 MUX-DAC enabled transmitters without the need for traditional RF up-convertors, delivering transponders that were physically smaller, lighter in mass, less power consuming, and lower cost (Figure 2).
Figure 2 Wideband space-grade ADCs and DACs enable direct-converting of a digital payload.
Not only did the EV10AS180A and the EV12DS130 eliminate the need for traditional RF frequency conversion, they allowed satellite communication to exploit the advantages of software-defined radio (SDR) offering operators new levels of flexibility, e.g. the ability to change RF frequency plans in-orbit in response to real-time user needs. For transponder manufacturers, SDR allowed them to reduce non-recurring engineering (NRE) and recurring costs by selling a single, generic, multi-mission payload that could be re-used by communication, Earth-observation, navigation, and IoT/M2M applications.
Traditional satellite communication at L and S-band became congested and to avail of larger information bandwidths, operators moved to Ku, K, and Ka-band. To support these higher frequencies, the first wideband, space-grade ADCs and DACs were used to reduce the number of overall RF frequency conversion stages by directly digitising and re-constructing respectively IF carriers (Figure 3).
Figure 3 This is the current architecture of a K-band digital payload.
To support the move to Ka-band, Teledyne e2v started research in 2019 investigating the potential of a novel K-band (18 to 27 GHz) ADC, realised using a 24 GHz front-end, track and hold amplifier and a quad ADC interleaving the four digitiser cores. A prototype was developed and testing revealed that optimising INL calibration for higher frequencies, as opposed to baseband operation, as well as minimising the offset mismatch between individual ADCs, could maximise dynamic K-band performance (Figure 4).
Figure 4 The photo shows the proof-of-concept K-band ADC and the graph shows the measured performance. Source: Teledyne e2v
The ultimate goal of the research is to develop the first Ka-band ADC and DAC for satellite communication to eliminate traditional analogue frequency conversion. This will provide operators increased flexibility in-orbit and real-time RF agility. Further R&D in 2020 discovered there were limits to the performance that could be achieved from the first prototype and to increase signal-to-noise ratio (SNR), spurious free dynamic range (SFDR), and the frequency from K to Ka-band, some fundamental changes would be required.
For the last five decades, Moore’s Law has driven the semiconductor industry, increasing performance and reducing power consumption with each smaller geometry. SDR at L, S, and C-band using direct-converting ADCs and DACs were made possible by exploiting the faster speed and lower power-dissipation benefits of CMOS scaling. However, below 28 nm, Fmax drops from a peak of 360 GHz due to process parasitics and the latest ultra deep-submicron nodes are simply too small to support the development of Ka-band mixed-signal converters. Furthermore, fabrication costs at these geometries is astronomical and not commercially viable for the space industry with its relatively low volumes. Fmax for 90 nm SiGe heterojunction bipolar transistors (HBTs) is currently 600 GHz.
To increase dynamic performance in the higher Nyquist zones and to move from K to Ka-band, a different form factor to that used by the proof-of-concept ADC is required. System-in-package (SiP) allows for significant RF miniaturisation by allowing multiple disparate die to be placed on a single common substrate. Package parasitics at microwave frequencies, particularly for wire-bonded leaded devices, and the choice of materials limit performance at Ka-band. Traditional RF MMICs use LTCC substrates and the research showed that moving to faster organic substrates improves operation at higher frequencies.
In 2020, a second prototype was developed combining two CMOS, interleaved, quad ADCs and a SiGe 30 GHz track and hold amplifier. Flip-chip die with lower parasitics at higher frequencies were mounted onto a low-dielectric constant organic substrate and placed in a compact 33×19 mm SiP, as shown in Figure 5. Improved performance was measured at K-band.
Figure 5 The second prototype of the K-band ADC showed improved performance. Source: Teledyne e2v
Following the research carried out in 2019 and 2020, Teledyne e2v plans to release samples of the first Ka-band ADC for space applications in the second half on 2021. The SiP product will include a 40 GHz, front-end, track and hold amplifier to allow direct sampling of Ka-band carriers.
To complement the development of the Ka-band ADC, a 12-bit, 12 GSPS, 25 GHz DAC will also be offered to enable software-defined microwave (SDM) satellite communication. The EV12DD700 quadruples the sampling frequency, the re-constructed bandwidth, and the range of frequencies that a baseband digital input can be directly up-converted to compared to the original, space-grade SDR DAC, the EV12DS130. The new EV12DD700 contains a novel 2RF mode allowing access to the images in the higher Nyquist zones at K-band.
This dual device also offers ×4, ×8, and ×16 interpolation ratios to reduce the input data rate as well as programmable, digital anti-sinc filters to flatten the output response from both channels in the frequency domain. Real and complex I/Q data can be re-constructed and each DAC has independent adjustment of gain, interpolation factor, and digital up conversion (DUC) local-oscillator frequency. An integrated DDS can generate a ramp, a CW tone, or a chirp signal, and fast frequency hopping is also supported to secure and protect the downlink. Separate from the DACs’ return-to-zero up-converting modes, the use of DUC can translate a baseband input with reduced instantaneous bandwidth to the higher Nyquist zones using fewer serial links.
Figure 6 The photo shows the EV12DD700 DAC and the graph is its direct up-converting modes. Source: Teledyne e2v
To support satellite communication, particularly beamforming applications, both the ADC and DAC contain features that synchronise gain and phase delay across multiple channels to guarantee deterministic latency and processing. After power-up, a SYNC input pulse resets all the dividers within the clocks paths of both devices to ensure the circuits restart deterministically. A SYNCO output connects to another device for multi-device locking.
The digital interfaces of the ADC and the DAC are realised using 12 Gbps high-speed serial links and the ESIstream protocol. This is based on 14b/16b encoding with each frame containing scrambled data to ensure timing transitions as well as two bits of overhead: one for disparity to control dc balancing and the other as a toggling synchronisation monitor. When combined with the above ADC/DAC SYNC and SYNCO signals, the links support multi-device synchronisation and deterministic latency. Free ESIstream IP is available for space-grade FPGAs!
The following YouTube videos demonstrate the features of the prototype Ka-band ADC and DAC.
For the first time, the prospect of Ka-band ADCs and DACs offer the potential to extend SDR to SDM for satellite communication. This will allow operators to change RF frequency plans and transponder operation in-orbit, in response to real-time user needs and link requirements. Technology-demonstrator satellites will be able to offer telecommunication, Earth-observation, IoT, and navigation services, as well as de-risking new multi-mission concepts, by re-configuring the specification and functionality of a single payload.
RF agility and resilience will allow operators to maximise the return from their expensive spacecraft assets in response to changing communication and market needs. The ability to re-configure and re-use the same transponder hardware is highly disruptive, will reduce NRE and recurring costs, will prolong the mission lifetime of hardware, and will lower the overall price to access satellite communication. The use of Ka-band ADCs and DACs will deliver major SWaP benefits to RF payloads!
The ability to change a payload’s RF uplink/downlink carrier frequencies, instantaneous processed bandwidth, waveform and modulation types, and the fundamental service offered by re-configuring an FPGA in-orbit, represents a game-changing advance for satellite communication. ‘SoftSats’ will enable many new mission types and transponder architectures, and I’d like to understand how you would exploit this unique technology for future applications. For example, will you still locate your transceivers within the main payload? Would you consider positioning Ka-band ADCs and DACs at the receive and transmitting antennae, directly processing the uplink and downlink carriers respectively, and then connect to on-board digital processors using high-speed electrical or optical links as illustrated in Figure 7?
Figure 7 This diagram shows a distributed satellite receiver architecture. Source: Teledyne e2v
First samples of the Ka-band ADC and DAC will become available this year with procurement and qualification options, as well as radiation-hardness data, to be released shortly after.
To offer the space industry further integration and on-board processing benefits, SiPs will also be offered combining microwave ADCs and DACs with qualified FPGAs in a compact form factor (Figure 8). The first product will baseline Xilinx’s XQRKU060 device as illustrated below, with additional space-grade FPGAs planned as part of the overall roadmap.
Figure 8 The planned product concept combines RF ADCs and DACs with Xilinx’s XQRKU060 FPGA.
Until next month, the first person to tell me the difference between the DAC’s RF and 2RF modes will win a Courses for Rocket Scientists World Tour t-shirt. Congratulations to Lorenzo from Italy, the first to answer the riddle from my previous post.
This article was originally published on EDN.
Dr. Rajan Bedi is the CEO and founder of Spacechips, which designs and builds a range of advanced, L to Ku-band, ultra-high-throughput on-board processors and transponders for telecommunication, Earth-observation, navigation, Internet, and M2M/IoT satellites. Spacechips’ Design Consultancy Services develop bespoke satellite and spacecraft sub-systems, as well as advising customers how to use and select the right components, and how to design, test, assemble, and manufacture space electronics.