9.7 An LTE SAW-less transmitter using 33% duty-cycle LO signals for harmonic suppression

With limited frequency allocation in the radio spectrum, spectral efficiency has always been the core development of communication systems. To accommodate the increase in demand for wireless data services, RF systems have been challenged to provide better in-channel SNR (EVM) and lower out-of-channel emission. As performance requirements become more stringent, second-order RF circuit impairments, that were previously insignificant, have become major design considerations. One example is the Long-Term-Evolution (LTE) [1]. Compared with previous generations, channel bandwidth has been expanded to 9MHz in most of the sub-GHz bands and 18MHz in the GHz bands. For spectral efficiency, the TX OFDM subcarriers are grouped into Resource Blocks (RBs) that can be dynamically allocated within the channel bandwidth. Noise and spurious emission requirements have become more challenging in the sub-GHz bands, so that Counter 3
^d
-order Intermodulation products (CIM3) has been recognized as an important design parameter [2-4] for LTE RF systems. CIM3 is the result of the lower 3
^d
-order intermodulation (IM3) product of signals at around 1×LO and 3×LO by using mixers with 25% or 50% duty-cycle LO. If an f
_BB
tone is being fed to the TX baseband input, after the mixer and the RF amplifier, the TX RF output will produce the desired signal tone at f
_LO
+f
_BB
and an unwanted CIM3 tone at f
_L0
-3f
_BB
[3]. The adverse effects of CIM3 are shown in Fig. 9.7.1, using LTE Band 13 as an illustration. Band 13 has User-Equipment (UE) TX band from 777 to 787MHz, and RX band at -31 MHz away from TX. Extreme cases of full RB and single RB are considered. At full RBs, modulated CIM3 has a bandwidth three times the desired signal, and it folds directly into the TX channel, degrading the TX EVM and the 1st ACLR (E-UTRA). Furthermore, the ACLR of bandwidth-expanded CIM3 falls into the RX band causing desensitization. When single RB is transmitted, the CIM3 may fall into the restricted bands and violate the spectral emission requirement. Consider the Public Safety Band, where the LTE standard dictates that the emission from 769 to 775MHz has to be less than -57dBm/6.25KHz [1]. If the output power at the antenna is +23dBm and only single RB is being transmitted, the power density is 23dBm/180kHz. Normalizing to power density from 180KHz to 6.25KHz, the power density is 8.4dBm/6.25KHz, resulting in a CIM3 requirement of -65.4dB/6.25KHz. This is challenging for linearity, and also for noise requirement in the case of a SAW-less system. CIM3 suppression techniques such as harmonic rejection and power mixing have been proposed [2-5], but these techniques require extra calibrations and/or off-chip filtering components, which will be described in later paragraphs. This work presents a CIM3 suppression technique by removing the undesired 3
^rd
-harmonic component in the LO signal through LO duty-cycle selection. With this direct root-cause elimination method, the TX meets CIM3 and RX band noise requirements for SAW-less LTE RF systems over process and temperature without calibration and off-chip filtering.