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Fully digital radio transmitter: Real or just hype?

17 Mar 2015  | Steve Taranovich

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The DSM output is up-converted to RF by using a bit combiner block and an FPGA's multi-gigabit on-board serialiser. Together these blocks effectively multiply the signal by a square up-conversion to achieve the desired carrier frequency.


Cambridge Consultants all-digital radio transmitter
Cambridge Consultants just demonstrated their all-digital radio transmitter at the Mobile World Congress. The company, founded in 1960 by Cambridge University graduates, has none of the pressures to get to market quickly. These former and present academic-types spun off Cambridge Silicon Radio (CSR) bought by Qualcomm in October 2014. Prior to the acquisition, CSR created the world's first single Bluetooth IC in 2000 and went on to become a leader in the Bluetooth IC industry.

I just had the occasion to speak to Tim Fowler, commercial director, wireless division, Cambridge Consultants and Monty Barlow, director, wireless technology, Cambridge Consultants. Their creation, called "Pizzicato", greatly intrigued me because unlike the previous attempts at the All-Digital radio outlined above and in the five references, Cambridge has taken the design to a new level with their proprietary patented software algorithms and mathematical software prowess.

An interesting note is that they started this design with an old, 3Gbps bitstream from a Xilinx Virtex-5 FPGA serdes port. They use a bandpass sigma-delta converter in the bitstream like the early one bit audio sigma-delta devices (figure 7).


Figure 7: Pizzicato (Image courtesy of Cambridge Consultants)


They still do need a PA and that will be with this present design as Option 1. In the future, Option 2 will use something like a Class S Digital Amplifier. See Figure 8 and Reference 5.


Figure 8: The block diagram of a complete digital transmitter with a GaN Voltage Mode Class S-Power Amplifier (VMCS-PA) (Image courtesy of Reference 5)


Cambridge Consultants are sending me some more detailed information which I will include in a follow-up article. Watch this company because they have some really bright innovator geeks (our brethren) and I expect to see many new enhancements in this technology over the next few years. These types of digital radios can fully take advantage of Moore's Law leading to smaller sizes, lower cost and lower power consumption using next-gen digital IC technology node advancements. An example of the architectures that can benefit from this is the 14 simultaneous cellular base station signals they were able to create with this first prototype.


References
1 All-Digital Synthesizable UWB Transmitter Architectures, Youngmin Park and David D. Wentzloff, PROCEEDINGS OF THE 2008 IEEE INTERNATIONAL CONFERENCE ON ULTRA-WIDEBAND (ICUWB2008), VOL. 2

2 Radio Transmitter Architecture with All-Digital Modulator for Opportunistic Radio and Modern Wireless Terminals, Patrick Wurm, Alexandre A. Shirakawa

3 An FPGA Based All-Digital Transmitter with Radio Frequency Output for Software Defined Radio, Zhuan Ye, John Grosspietsch, Gokhan Memik

4 All-Digital Transmitter with RoF Remote Radio Head, Rui F. Cordeiro, Arnaldo S. R. Oliveira and Jose Vieira.

5 A Watt-class Digital Transmitter with a Voltage-Mode Class-S Power Amplifier and an Envelope Δรค Modulator for 450MHz band, Shinichi Hori, Andreas Wentzel, Makoto Hayakawa, Wolfgang Heinrich, and Kazuaki Kunihiro


About the author
Steve Taranovich is a senior technical editor at EDN with 41 years of experience in the electronics industry. Steve received his MSEE from Polytechnic University, Brooklyn, New York, and his BEEE from New York University, Bronx, New York. He is also chairman of the Educational Activities Committee for IEEE Long Island. His expertise is in analogue, RF and power management with a diverse embedded processing education as it relates to analogue design from his years at Burr-Brown and Texas Instruments. Steve was a circuit design engineer for his first 16 years in electronics. He then served as one of the first field application engineers with Burr-Brown Corp and also became one of their first global account managers, traveling to Europe, India and China.


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