A Double Bandpass N-path filter for LTE Carrier Aggregation Receivers in 28nm CMOS Ahmed Elmaghraby, Mohamed Hamouda, Georg Fischer, Robert Weigel, and Thomas Ussmueller Institute for Electronics Engineering University of Erlangen-Nuremberg Cauerstr. 9, 91058 Erlangen, Germany E-mail: ahmed.elmaghraby@fau.de Abstract—A tunable double bandpass integrated filter is introduced. The filter is designed based on the impedance transfer identity of the N-path filtering. The filter’s dual bandpass nature, makes it suitable to be employed as a high Q filter at the input of LTE receivers. It can be used to suppress out-of-band blockers in the non-contiguous carrier aggregation mode. Each of the two passbands can be tuned to be centered at each of the two LTE carriers. The design is simulated for LTE band 2 and the simulation results showed blocker suppression of 10 dB for blockers at the duplex distance and 12 dB of suppression for blockers at double the duplex distance. Keywords—LTE, CA, double BP filters, N-path filters, N-phase generator. I. I NTRODUCTION Recently, SAW-less receivers have replaced the tradi- tional SAW-based ones in most mobile communication radio transceivers [1]. Getting rid of the off-chip SAW bandpass filter is usually accompanied by emplyong an on-chip filtering solution. High Q N-path filters were frequently used as an off- chip solution for SAW-less receivers [2]. Those high Q filters can be placed in front of the LNA, or between the LNA and the Mixer. Placing them in front of the LNA has many advantages as they filter the blockers before they saturate the LNA as well as before they experience third order intermodulation and degrade the dynamic range of the receiver. High Q bandpass N-path filters can be tuned to select the received channel and filters out-of-band frequencies. This channel based identity has become a disadvantage when deal- ing with carrier aggregation (CA) LTE receivers. In CA mode, the LTE mobile receives two different channels at the same time. If the two channels are in different bands, it is called inter-band CA. While if the two channels are in the same band, it is called Intra-band CA. In Contiguous Intra-band CA, the two channels are adjacent to each other. Therefore, they can be treated as one wideband channel. However, in non contiguous CA the two channels whithin the same band are away from each other. This type of CA is a big challenge to any high Q bandpass filter infront of the LNA. As the filter selects one channel and suppresses the other. To avoide this situation the high Q filter can be moved between the LNA and the mixer, provided that the power spliting between the two carriers is done whithin the LNA stage. However, all the advantages of placing the filter infront of the LNA are lost. Z RF Z BB Z BB Z BB Z BB LO 1 LO 2 LO (n-1) LO n Z RF f f LO -f B f LO +f B Z BB f -f B f B Fig. 1. Bandpass to double bandpass impedance transformation using an N-path filter. In this paper, a double bandpass filter is introduced. The dual passbands can simultaneously filter both carriers which allow for the usage of a high Q filter in front of the LNA and support the non contiguous intra-band CA mode. The filter is based on transfering the positive and negative components of a bandpass impedance to the desired passband impedance where the two simultaneous channels are received. Section II, presents the concept of the proposed filter. In section III, the circuit design considerations are discussed. While in section IV, the simulation results of the filter are demonstrated. Finally section V concludes the paper. II. DOUBLE BANDPASS FILTER CONCEPT N-path filters use N switches in parallel. The switches are controlled via a non-overlapping clock signal with a duty cycle equal to 1/N. From the mathematical derivation in [3] it was shown that the impedance seen at the input of the filter is Z in ∼ = R SW + N π 2 sin 2 ( π N ){Z BB (ω - ω LO )+ Z BB (ω + ω LO )} (1) Eqn. (1) implies that N-path filters transfer the baseband impedance Z BB to passband. Therefore, the nature of the baseband impedance will define the type of the realized filter. For example, having a low pass impedance like a capacitor at baseband will result in a bandpass N-path filter. While having a high pass impedance like an inductor at baseband 978-1-4799-460 / $31.00 ©2014 IEEE