System architecture of an RF-DAC based multistandard transmitter

Abstract

In this paper the system concept of a radiofrequency digital-to-analog converter (RF-DAC) based multistandard transmitter is presented. The RF-DAC combines DA converter and up-conversion mixer in a single building block. Thereby, the analog baseband blocks are eliminated and replaced by easily reconfigurable digital blocks. Requirements on the digital and mixed-signal blocks are discussed. Spurious emissions are a major issue of this 'direct digital modulator'. This is especially true for multiradio devices with several receivers running simultaneously and in close proximity to the transmitter. Therefore, a maximum emission limit is formulated and especially critical frequency bands are identified. Several techniques for reducing the spurious emissions are discussed and performance requirements for an LTE transmitter and its main building block are found. System simulations demonstrate the feasibility of the concept.

Digitally-intensive transceivers for future mobile communications—emerging trends and challenges

“…Among the published architectures and CMOS implementations, which are evolving quickly, digital-polar-, digital-quadrature-and pulse-encoding-based switched-mode transmitter systems are most commonly found and have been proven feasible for wireless communications [8,9]. Pulse encoding approaches generally require additional high-Q filtering after RF signal generation since a considerable amount of quantization noise arises out-of-band.…”

“…Therefore, digital-quadrature and digital-polar architectures utilizing high dynamic range RF-DACs have become popular due to their superior out-of-band performance and output power tuning range. The RF-DAC introduced in [10], combining the functionality of the baseband DAC and the RF mixer, represents a key element of digitally intensive transmitter architectures [8] allowing for an efficient implementation on a single die and leveraging the benefits of CMOS technology scaling.…”

“…As there is a baseband filtering stage before the ADC, the 4th, 5th and 6th terms in (8) are significantly suppressed by this filtering stage and thus can also be ignored. The only signal that drives the strong non-linearity is the Tx leaked signal.…”

A configurable sampling rate converter for all-digital 4G transmitters

“…In the DRFC, the sampling rate of the digital input should be an integer fraction of the LO frequency f LO [4], [5]. When the DRFC is targeted to a specific radio standard, f LO is usually specified to be changeable.…”

A programmable DSP front-end for all-digital 4G transmitters

“…In the DRFC, the sampling rate of the digital input should be a submultiple of the LO frequency f LO [4], [5]. When the DRFC is targeted to a specific radio standard, f LO is usually specified to be changeable.…”