New RF Strategies for Software Radio
Software defined radio system engineers can now exploit new technology to perform digital signal processing much closer to the antenna than ever before. Various strategies include the latest wideband data converters, monolithic receiver chips, compact RF tuners, new FPGA families, and remote data acquisition modules using gigabit serial interfaces. Each approach presents benefits and tradeoffs that must be considered in choosing the optimal solution for a given application.
Wideband A/D Converters
A new class of monolithic A/D converters capable of sampling rates of 5 GHz and higher allows engineers to directly digitize analog RF signals covering a frequency span of more than 2 GHz. Now wideband communications and radar signals can be captured in a single data stream, eliminating the complexity of splitting a given band into parallel, adjacent sub-bands and the inevitability of input signals straddling them. While these new converters appear to simplify software radio architectures, they also impose many limitations and tradeoffs.
Antenna RF signals must first be amplified, filtered, and possibly downconverted in frequency to match the input voltage range and usable input bandwidth of the A/D converter. Normally, an amplifier boosts the strongest signal to the full scale input range of the A/D. Any further amplification to boost weaker in-band signals will cause overloading the A/D, destroying the signal integrity for all signals. Thus, even one strong interferer will reduce the achievable dynamic range for weaker signals. This significant tradeoff occurs whenever a single A/D is used to handle a large number of signal types across a wide frequency span.
Filtering is imperative to eliminate all energy outside the frequency span of interest. Otherwise, aliasing will fold out-of-band noise and adjacent signals into the digitized signal stream, degrading signal-to-noise performance and adding spurious signals.
As sampling rates increase, A/D converters deliver lower ENOB (effective number of bits) ratings. For example, a 5-GSample/sec, 10-bit A/D converter may only deliver an ENOB of 7.6 bits. This tradeoff is often a critical factor for component selection and system architecture.
Finally, A/D data arriving at several GSamples/sec will overload most digital signal processors. High-speed A/Ds often include data de-interleaving hardware to simplify the electrical interface, but even so, every data sample must somehow be processed, stored, or transferred. The latest families of FPGAs are especially well suited, not only in dealing with these extremely high data interface rates, but also in processing signals in real time.
As a product example, the Pentek 71741 3.6 GHz A/D and DDC XMC module is shown in Figure 1. It features a 12-bit, 3.6 GSample/sec A/D converter coupled to a Virtex-7 FPGA. The A/D de-interleaves samples into eight parallel 12-bit streams, delivering samples to the LVDS ports of the FPGA at 450 MSamples/sec each. Inside, eight parallel engines implement a DDC (digital down converter) that tunes across the 1.8-GHz input band. It performs frequency translation to baseband and provides digital filtering of the complex baseband output samples. Selectable output bandwidths of 90, 180, or 360 MHz, representing tunable slices of the input spectrum, are delivered to the system through a native PCIe Gen 3 x8 interface.
A growing class of new monolithic silicon receivers offers an impressive integration of diverse RF analog circuitry required to implement a complete software radio tuner front end. These low-cost devices accept input signals directly from the antenna, and deliver amplified, translated, and filtered analog baseband outputs suitable for lower-speed, high-resolution A/D converters or demodulator chips.
As an example, the Maxim MAX2112 targets satellite settop and VSAT applications, including 8PSK modulation and Digital Video Broadcast (DVB-S2) applications. It uses an LNA to boost antenna input signals falling between 925 and 2175 MHz, as well as a programmable gain RF amplifier for 80 dB of overall gain control.
An integrated VCO and programmable fractional-N frequency synthesizer drive a quadrature mixer to tune across the entire input frequency range, downconverting any input signal to I+Q baseband. These baseband signals are band limited with a pair of low-pass filters, programmable from 4 to 40 kHz.
This single chip offers an extremely high level of integration, dramatically reducing the size and cost of the receiver, and is ideal for applications restricted in space, power, weight and cost, or requiring a large number of channels.
Not surprisingly, these benefits come with a performance tradeoff. While these devices work well for applications requiring only modest signal-to-noise ratios like satellite signal reception, they are not suitable for some of the more demanding government and military systems for communications, signals intelligence, and radar.
Higher dynamic range requirements like these require better RF analog signal processing, including multi-conversion designs, amplifiers with lower noise figures, local oscillators with better phase noise and wider tuning ranges, mixers that minimize unwanted spurs, and filters with better pass band flatness, roll off, and stop band performance. Other critical factors include packaging, shielding, isolation, voltage regulation, vibration tolerance, and thermal performance. Overall performance levels of the system are achieved by progressively improving the weakest signal chain elements in iterative cycles until the desired result is reached.
Each incremental improvement boosts system level performance such as lower bit error rates for digital communication systems, improved target detection range and classification accuracy for radar systems, higher intelligibility of voice interceptors, and the enhanced precision of target location and trajectory for weapons control systems.
As a result, there is a continuum of required software radio performance levels matching the operational objectives and constraints of a wide range of systems. At the low end, the monolithic receiver described above may suffice, while a very sensitive SIGINT receiver might require a large, highly sophisticated RF subsystem.
Compact Slot Receivers
Some applications need to cover only a limited range of input signal frequencies, such as an upper-band GSM receiver handling signals between 1700 and 2000 MHz. For these band-limited systems, simpler and less expensive single- conversion RF tuner architectures can still deliver good performance. In these systems, a single local oscillator and mixer downconvert the RF signal to a lower-frequency IF signal compatible with a high-resolution A/D converter. Of course, judicious selection of amplifiers and filters, and careful analysis and suppression of mixer products are essential design tasks.
These types of RF tuners are often called “slot receivers,” a name inspired by the narrow tuning range. They can be ideal for dedicated applications where limited frequency coverage, cost, size, and weight allow placement of the tuner at or near the antenna.
In the slot receiver shown in Figure 2, an input band pass filter rejects signals outside of the defined RF tuning “slot,” helping to eliminate both out-of-band noise and discrete signal interferers. The mixer and tunable local oscillator translate the RF input down to an IF frequency of 225 MHz. An IF bandpass filter excludes all signals outside an 80-MHz band centered at 225 MHz, delivering an analog output suitable for 14- or 16-bit A/D converters.
Low-noise amplifiers and programmable attenuators in the signal chain boost antenna signal levels to match the full-scale input voltage of the A/D. These slot receivers cover the 400-MHz slot between 800 MHz and 3 GHz. An overlap of 100 MHz between adjacent slots ensures that any 80-MHz signal band can be accommodated.
Remote Software Radio Receivers
Delivering RF signals from the antenna to the receiver system presents many challenges, especially when using long coaxial cables. The higher the frequency, the more signal loss in the cable. To mitigate this, LNBs (low noise blocks) located on the antenna are commonly used to downconvert signals above 4 GHz (C-band and higher) to a lower frequency typically often in the L-band (1-2 GHz).
Nevertheless, cables carrying these analog signals still suffer degradation and present EMI radiation and susceptibility issues. Not only do coaxial cables impose a tangible weight impact in aircraft and UAVs, they also become maintenance burdens for the extremely long runs and the salt environment aboard ships. Digitizing signals right at the antenna by using some of the techniques discussed above offer receiver system engineers new ways to overcome problems with analog signal transmission over cables.
Fortunately, the transmission and distribution of these digitized antenna signals can now be handled by new industry-standard gigabit serial digital links and protocols. For example, GbE and 10-GbE links are now so widely deployed in computer networks, data processing centers, and WAN/LAN servers, that commercial competition has driven down costs of components, switches, bridges, cables, and other infrastructure.
To address these markets, FPGA vendors not only offer built-in PCIe ports, they also offer native lightweight gigabit serial protocols such as Xilinx’s Aurora and Altera’s SerialLite. These, along with SerialFPDP, are ideal for delivering raw A/D or baseband I+Q samples from an FPGA-based front end located at the antenna. At the receiving end, host bus adapters are available for all of these protocols, and many embedded systems processors have native interfaces for SerialRapidIO and PCIe.
Each of these gigabit serial links supports both copper and optical interfaces. Rapidly advancing technology for optical transceivers and cables delivers increasingly higher performance, while lowering both cost and power consumption. Single-mode fiber cables can connect data from remote receivers up to 10 km away. This benefits large antenna array installations that must collect signals from a grid of widely spaced antennas.
Optical cables are free from EMI radiation, eliminating interference to other electronics in tightly packed manned and unmanned aircraft, as well as offering security against eavesdropping. They are also immune to EMI pickup from powerful transmitters, motors, and generators found in ship borne installations. Lastly, optical cables are much lighter than copper cables and are highly resistant to moisture, salt, and chemicals.
Software Radio Revisited
The added benefits of antenna site software radio receivers are numerous. However, there is no substitute for appropriate analog RF signal conditioning prior to A/D conversion, and each technique presents its own application-specific tradeoffs.
Antenna-site FPGAs can implement essential DDC functions, and native interfaces can deliver digital baseband samples across industry-standard digital gigabit serial links. Because these links are full-duplex, the same cable provides a reverse path for control and status functions from the host.
Digital receiver data can be easily distributed to multiple destinations using low-cost switches and readily archived on storage servers, as shown in Figure 3.
For sensitive signals and classified information, data encryption can be easily included at the antenna before digital transmission. Additional pre-processing algorithms such as radar pulse-compression, FFT energy calculations, scanning, and threshold detection can be incorporated within the FPGA to reduce transmission data rates and offload these processing tasks from the host system.
It is apparent that many applications can benefit from pushing front-end software radio functions up the mast to the antenna as a viable alternative to traditional analog input rack-mounted receiver systems.
This article was written by Rodger Hosking, Vice President and Co-Founder of Pentek, Inc., Upper Saddle River, NJ. For more information, Click Here .
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