Communication signals are not usually generated on the assigned spectrum band, but rather they are generally synthesized in baseband and then moved to the proper center frequency. This process is carried out by the so-called frequency modulation stage of transmitters, which are implemented, practically with no exception, in the analog domain.
Analog frequency modulation stages typically show low flexibility and scalability. Indeed, the frequency shift applied by them is usually fixed, though there exist some techniques to achieve a variable shift along certain band, with a range which is normally proportional to the complexity of the corresponding stage. Moreover, these stages are only able to process adjacent channels, as they cannot place two independent signals over two far frequencies. A good example of these characteristics can be found in cable modulators: in order to move a block of 6 or 8MHz channels all along the band between 50MHz and 1GHz, these devices apply a fixed spectral shift to a band near 2.5GHz, and then they downshift the entire block to the desired frequency.
The development of very high speed Digital-to-Analog converters in the 2000s, with sample rates up to several GHz, was a remarkable milestone in the timeline of communication transmitters design. These devices, usually called RFDACs by manufacturers, are able to synthesize digitally generated signals directly over radiofrequency bands, so they remove the need for any kind of analog frequency modulation stage. Hence, not only the drawbacks of these elements are overcome through the use of RFDACs, but also additional advantages and capabilities for communications transmitters are presented:
1. Flexibility: the frequency modulation stage is implemented in the digital domain, so any signal can be shifted to an arbitrary frequency within the RFDAC operation band.
2. Scalability: a single RFDAC can synthesize multiple independent channels placed over arbitrary frequencies even in a dynamic way, so a transmitter could turn on or mute any channel depending on its needs.
3. Frequency agility: digital signal processing stages have the ability of switching the frequency of any active channel almost instantaneously along very wide bands, with ranges up to several GHz.
Currently RFDACs are mostly used in multimedia broadcasting over cable networks, taking the advantage that a single converter is able to synthesize the full spectrum used in these applications, i.e., 160 QAM carriers with a bandwidth of 6MHz each –North American channelization– or 120 8MHz-QAM carriers –European channelization– along the band from 50MHz to 1004MHz.
Gradiant has achieved a deep know-how of the direct-digital synthesis techniques field, and currently many researchers are involved in several projects in which RFDACs play a main role. As a result, several intellectual property blocks have been developed, along with a fully functional multichannel direct-digital synthesis RF upconverter compatible with any commercial RFDAC.
