Abstract
Due to the use of wide-band analog front-ends in order to cover the many different frequency bands used in Long Term Evolution (LTE), blocker signals received by the antenna may reach the mixer input. As a consequence, square-wave mixer implementations, as, e.g., the 25% duty-cycle current-driven passive mixer, may lead to the down-conversion of these blocker signals by their harmonic response. This contribution describes a harmonic rejection (HR) strategy, which modifies the local oscillator (LO) waveform of a 25% duty-cycle current-driven passive mixer, to suppress specific harmonics. Two HR control signal waveforms are proposed and their performance is evaluated by circuit simulations using a 28nm CMOS technology. By applying the proposed concept, the down-conversion of a blocker by the mixer’s $3^{\text {rd}}$ order harmonic response can be suppressed by more than 30 dB, and the receiver noise figure (NF) is improved by 21.1 dB for a blocker power of −5 dBm at the low-noise amplifier (LNA) input.
Highlights
I N RADIO frequency (RF) transceivers mixers are used to shift the desired RF channel to the baseband (BB) or an intermediate-frequency, where the wanted signal is digitized for further digital signal processing
Due to the use of wide-band analog front-ends in order to cover the many different frequency bands used in Long Term Evolution (LTE), blocker signals received by the antenna may reach the mixer input
The proposed harmonic rejection (HR) strategy is evaluated by circuit simulations using a 28 nm CMOS technology
Summary
I N RADIO frequency (RF) transceivers mixers are used to shift the desired RF channel to the baseband (BB) or an intermediate-frequency, where the wanted signal is digitized for further digital signal processing. Traditional designs using 50% duty-cycle LO signals suffer by an IQ cross-talk problem due to the overlapping ON-states of the switches in the I- and Q-branch This overlapping causes a reduced conversion gain because the RF input current is shared between two branches [2]. Higher order harmonics may be rejected by using more than three square waves [6] or by using an 8-path HR mixer with 12.5% duty-cycle [10] This brief presents an HR strategy which modifies the switch control waveforms of the 25% duty-cycle mixer to suppress specific harmonics. At the same time the beneficial property of nonoverlapping ON states of the switches is maintained which minimizes IQ cross-talk Another advantage of this approach is that no weighting and summation of RF signals as in [6], [11] is needed to realize the rejection of harmonics. Only one transistor is needed in each of the mixer branches I+, I−, Q+ and Q−
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