Direct-digitization architectures in modern RF systems
Radiofrequency (RF) systems are usually based on transmission and reception paths, where the analog RF signal is conditioned for further processing on a digital state. Analog mixers have been traditionally key parts for this signal conditioning.
Mixing steps translate the signal from its original frequency to the desired band. Classical architectures on RF systems use at least one intermediate frequency (IF) between baseband and the final carrier frequency. This scheme applies to both transmission and reception paths, and the process will be carried out as an upconversion or a downconversion respectively [1].
Reasons for using IF are variate, but it is mainly considered that some frequency constraints will be eased off for the RF design and fabrication when using lower frequencies. This is especially important in the design of band-pass filters because the lowered center frequency results in a softer requirement for the Q-factor [2]. IF-based receivers are known as superheterodyne receivers, and core amplifiers and filters of most architectures work over the intermediate frequency.
However, some of the most decisive restrictions when using higher frequencies affect conversions between digital and analog states. Despite that, direct-digitization or homodyne digital receivers are a certain technological trend from an architectural view of RF systems. The microwave signal is straightly sampled on RF and digitally converted down to baseband. Industry-leading companies are investing on direct-digitization solutions for their new RF systems, as we can see on the last chapter of this article.
This makes engineers wonder — what are the benefits of direct-digitization?
RF-sampling: challenges and benefits
RF-sampling is a challenging project when working with high frequencies. High-speed Analog-to-Digital-Converters (ADC) are strictly required on these designs, even when undersampling solutions are taking the market [3][4]. Input bandwidth and sampling rate should be high enough to support several GHz inputs.
On a digital receiver, sampling RF signals require paying special attention to the noise figure (F) of the ADC. The noise will be degraded so that the analog input requires gain compensation: low noise amplification (LNA) approximately equal to the gain of the mixer, amplification and filtering steps of an equivalent superheterodyne receiver [5].
Benefits of using digital RF architectures are clear. The design and manufacturing of analog circuitry get strongly reduced to be replaced by commercial off-the-shelf (COTS) digital devices. At this point, the increasing processing capacity of FPGAs will do the job in the digital world, performing digital down-conversion (DDC), digital demodulation and other digital signal processing (DSP) tasks. Reduction of components usually affects positively the mean-time-between-failures (MTBF) and the average lifetime of the system and lowers the costs of production.
Digital solutions do not only work on the reception path. On transmitters, RF signal can be directly generated in RF by using high-speed Digital-to-Analog-Converters (DAC). The process usually requires previous combinations of digital upconversion (DUC), digital modulation and direct digital synthesis (DDS) techniques.
Industry and market solutions
DAC and ADC integrated circuits with low noise-floor, high resolution, wide input bandwidth and fast sampling rates are recently appearing on the market developed by leading manufacturers like Analog Devices, Texas Instruments or Xilinx[6][7][8]. Nonetheless, National Semiconductor already started the race back in 2011 releasing the ADC12Dxx00RF family, when they were acquired by Texas Instruments.
Analog Devices, the top seller of converting technology nowadays, is releasing various high speed and resolution -up to 16 bits- ADC compatible with the JESD204B standard. Evaluation kits ease the reduction of time-to-market for new developments.
RFSOC UltraScale+ series from Xilinx, the leader in FPGA and CPLD technologies, integrates RF reception with digital processing, revealing one of the most promising paradigms for future RF systems.
Moreover, sample and reference designs already demonstrated successful direct digitization of L, S or C-bands[9] using ADC12DJ3200 from Texas Instruments.
That is the reason for what manufacturers of digital boards have already started to design and promote commercial solutions for RF reception and generation, based on fast DAC, ADC and FPGA technologies. Abaco-4DSP released different products based on Xilinx and Texas technologies, like VP430 board or FMC digitizer boards for FPGA carriers. Vadatech offers also FMC developments with several GHz input bandwiths[11].
Radar, satellite communications, software radio or wireless communications are examples of technological markets [12][13] directly affected by this advancements. Direct-digitization architectures bring opportunities to reduce the size of RF systems, increase the reliability and lifetime of products, lower manufacturing costs and decrease the complexity of RF transmitters and receivers.
Until now, IF was certainly needed in GHz-systems. Today, radiofrequency technology has evolved into a new stage in which digital solutions are getting more and more relevance every day.
Innovation is always spinning forward. Just like a Drill.
References
[1] F. Marki and C. Marki, “Mixer Basics Primer, A Tutorial for RF & Microwave Mixers”, Marki Microwave, Inc, 2010. [Online]. Available: https://www.markimicrowave.com/assets/appnotes/mixer_basics_primer.pdf [Accessed May 6, 2018].
[2] T.R. Kuphaldt, “Practical Guide to Radio-Frequency Analysis and Design”, All About Circuits. [Online]. Available: https://www.allaboutcircuits.com/textbook/radio-frequency-analysis-design/selected-topics/the-benefits-of-an-intermediate-frequency-in-rf-systems/ [Accessed May 10, 2018]
[3] L. Frenzel, “Fast ADC Facilitates Direct RF Sampling at Higher Frequencies”, Electronic Design, 2017. [Online]. Available: http://www.electronicdesign.com/analog/fast-adc-facilitates-direct-rf-sampling-higher-frequencies [Accessed May 10, 2018]
[4] W. Kester, “ What the Nyquist Criterion Means to Your Sampled Data System Design”, Analog Devices, 2009. [Online]. Available: http://www.analog.com/media/en/training-seminars/tutorials/MT-002.pdf [Accessed May 10, 2018]
[5] T. Neu, “ Direct RF conversion: From vision to reality”, Texas Instruments, 2015. [Online]. Available: http://www.ti.com/lit/wp/slyy068/slyy068.pdf [Accessed May 10, 2018]
[6] “ ADC32RF45 Dual-Channel, 14-Bit, 3-GSPS RF-Sampling Analog-to-Digital Converter (ADC)”, Texas Instruments, 2016. [Online]. Available: http://www.ti.com/product/ADC32RF45 [Accessed May 10, 2018]
[7] “ AD9689 14-Bit, 2.0 GSPS/2.6 GSPS, JESD204B, Dual Analog-to-Digital Converter”, Analog Devices, 2018. [Online]. Available: http://www.analog.com/en/products/analog-to-digital-converters/standard-adc/high-speed-ad-10msps/ad9689.html [Accessed May 10, 2018]
[8] “Integrated RF-Class Analog. RF Data Converters in a Programmable MPSoC”, Xilinx, 2018. [Online]. Available: https://www.xilinx.com/products/technology/rfsampling.html [Accessed May 10, 2018]
[9] “ Direct RF-Sampling Radar Receiver for L-, S-, C-, and X-Band Using ADC12DJ3200 Reference Design”, Texas Instruments, 2017. [Online]. Available: http://www.ti.com/tool/TIDA-01442 [Accessed May 10, 2018]
[10] “ The VP430: direct RF processing system”, Abaco Systems, 2018. [Online]. Available: https://www.abaco.com/products/vp430-rfsoc-board [Accessed May 10, 2018]
[11] “FMC211 — FMC ADC 10-bit @ 2.6 GSPS Module”, Vadatech, 2015. [Online]. Available: https://www.vadatech.com/media/FMC211_FMC211_Datasheet.pdf [Accessed May 10, 2018]
[12] “PSR-NG: Design of a digital receiver-processor for the Indra PSR 2D S-Band radar”, Indra, 2017. [Online]. Available: https://www.indracompany.com/en/indra/psr-ng-design-digital-receiver-processor-indra-psr-2d-s-band-radar [Accessed May 11, 2018]
[13] D. Smetana, “High-speed ADC/DAC and FPGAs drive the design of next-generation SATCOM systems”, Military Embedded Systems, 2016. [Online]. Available: http://mil-embedded.com/articles/high-speed-drive-design-next-generation-satcom-systems/ [Accessed May 11, 2018]
