A Multiband LTE SAW-Less CMOS Transmitter with Source-Follower-Driven Passive Mixers, Envelope-Tracked RF-PGAs, and Marchand Baluns

Kihara, Takao; Sano, T.; Mizokami, Masakazu; Furuta, Yoshikazu; Hokazono, Mitsuhiko; Maruyama, Toru; Heima, T.; Sato, H.

doi:10.1587/transele.e96.c.774

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“…Baseband predistortion that compensates the nonlinearity is not possible, due to the required wider filter bandwidths and the associated penalty in out-ofband noise. In [2], an LC tuned load is used to filter out 3 rd -order harmonics of the modulator and reduce the 3LO-generated C-IM3. This comes with a considerable area penalty and limits the tuning range.…”

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A multiband 40nm CMOS LTE SAW-less modulator with −60dBc C-IM3

Ingels

Furuta

Zhang

et al. 2013

2013 IEEE International Solid-State Circuits Conference Digest of Technical Papers

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Due to the increasing demand for communication bandwidth combined with the scarceness of free spectrum, the complexity and versatility of 4 th -generation modulation schemes is greater than ever. In particular, the LTE standard defines multiple RF bands and groups OFDM modulated subcarriers into Resource Blocks (RB) that can be flexibly used within the allocated channel bandwidth. When the transmitted power is concentrated in one or a few RBs, counter-intermodulation products (C-IM) may fall directly or through cross-modulation into the RX band and degrade FDD performance. They may also fall into protected bands and violate spectral emission requirements.The main contributor to C-IM3 comes from the baseband 3 rd -order nonlinearity. In the mixer, the baseband's HD3 is upconverted to LO-3BB (Fig. 19.6.1.A). In a typical TX chain, signal swings are kept rather high to maintain a good SNR and in combination with low power supply voltages in nm CMOS this turns out to be a critical issue. A less important contribution originates in the mixer, where upconversion with the 3 rd harmonic of the LO creates a component at 3LO-BB that can generate C-IM3 due to intermodulation with the wanted signal in the non-linear PA ( Fig. 19.6.1.B).Note that C-IM3 from the RFIC may further generate components at LO+5BB and LO-7BB through intermodulation with the wanted signal in the HPA (Fig. 19.6.1.C).C-IM3 has only recently been recognized as important and few publications deal with the problem. C-IM3 can be improved by increasing the modulator's intrinsic linearity [1]. However, this increases the design effort and power consumption. Baseband predistortion that compensates the nonlinearity is not possible, due to the required wider filter bandwidths and the associated penalty in out-ofband noise. In [2], an LC tuned load is used to filter out 3 rd -order harmonics of the modulator and reduce the 3LO-generated C-IM3. This comes with a considerable area penalty and limits the tuning range. Furthermore, baseband linearity still needs to be sufficient to avoid direct C-IM3 generation, which is achieved by supplying the baseband from the 2.8V supply.The solution proposed here exploits harmonic cancellation [3,4] at baseband to reject the 3 rd -order baseband harmonics and improve the transmitter's C-IM3 performance. By combining 3 baseband phases rotated over 0, 45° and 90° and scaled with the factors 1, √2 and 1, 3 rd -order intermodulation products can ideally be completely suppressed (Fig. 19.6.2.A). The LO does not need any modification and still uses the traditional 4 phases at 90°, which limits the complexity of the LO generation.Apart from C-IM3, more traditional requirements such as RX-band noise and ACLR remain stringent for 3G/4G transmitters, so this work builds further on previous results and consists of an active-passive baseband filter followed by a voltage-sampling passive mixer and a programmable pre-power amplifier [5]. When directly applied to this architecture, harmonic rejection would result in a tripling of the baseban...

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