The fully digital radio transmitter: Is it real or more hype?
Cambridge Consultants are claiming the world’s first fully digital radio transmitter built only from computing power. There are no analog components like a high-speed D to A converter with amplifier, although I would think they would need a Power Amplifier (PA) to broadcast a great distance. This is a Digital Radio transmitter and not part of a Software Defined Radio (SDR) architecture which requires analog components. They are demonstrating the transmitter at the Mobile World Congress (MWC).
The Pizzicato radio (my Italian wife tells me that Pizzicato means pinched or to pluck as in a stringed instrument). More about this new architecture later; now let’s see what came before this effort leading up to the Cambridge solution.
Many so-called All-Digital Radios have been tried in the past. Here are some that stand out, but in my skeptical analog brain I find it hard to conceive a truly All-Digital Radio, only possibly some main architectural sections as is evidenced in the following examples from some EEs in IEEE tech papers.
The pulsed UWB transmitter
In Reference 1 we see a 2008 design for pulsed-ultra-wideband transmitter architectures. The developers have two designs: delay line-based and ring oscillator-based. To illustrate the advantage of a digital format architecture, the author shows that the two architectures are synthesized and place-and-routed (PAR) using existing design tools---no custom routing or care as is typical in analog layout and routing. Another advantage of a digital system is that when the desired center frequency and bandwidth are slightly off due to variations induced by PAR, those frequency-related parameters can be easily tuned digitally. See Figure 1 for the architecture block diagram.
Figure 1 Reference 1 architecture for the “all-digital” pulsed UWB transmitter. (Image courtesy of Reference 1)
Not bad, but not my idea of a fully-flexible all-digital radio design.
Radio transmitter architecture with all-digital modulator
Reference 2 is a mid-2000 design with somewhat limited application to an “Opportunistic Radio” which shares spectrum in licensed and unlicensed bands for secondary users. The architecture used for this need of a flexible and reconfigurable radio type is a direct modulation type architecture based on an all-digital I&Q modulator followed by a linear power amplifier. With a digital modulator, the D to A conversion is able to take place at the speed of RF, so no less than 2× the channel frequency of the signal in order to meet the Nyquist criteria (Figure 2).
Figure 2 The “all-digital” radio transmitter architecture based on sigma-delta modulation. (Image courtesy of Reference 2)
The “all-digital” modulator is based upon the sigma-delta filtering plus a set of analog band-pass BAW-CRF (bulk acoustic wave-coupled resonator filter) filter where only one path to the filter is active at any one time. The D to A converter is clocked by and “all-Digital” RF synthesizer. There is no Power Amplifier (PA) used so this solution is very linear and good for a Zigbee protocol with low power and shorter range. Noise shaping is done at higher frequencies similar to the original sigma-delta designs that handles lower audio-type frequencies.
In most of these architectures, we look five or ten years in the future to the maturity of such architectures. The maximum speed of the sigma-delta modulators needs to be increased, higher power levels and the PA issue needs to be addressed, and power system efficiency needs to be improved.