8-Path tunable RF notch filters for blocker suppression

Abstract

Abstract:Periodically time-variant passive 8-path notch filters are demonstrated in 65nm CMOS technology, with a notch frequency tunable from 100MHz to 1.2GHz with a clock signal. In a 50Ω environment, filter insertion loss in the pass band is 1.4-2.8dB, while the rejection at the notch frequency is >20dB.Given their P 1dB > +2dBm and IIP3> +17dBm, the filters can protect radio receivers from blocking over a wide tuning range. TextThe huge growth of the number of wireless devices makes wireless coexistence an increasingly relevant issue. If radios operate in close proximity, blockers as strong as 0dBm may occur, driving almost any receiver in compression (note that 0dBm in 50Ω corresponds to a pk-pk voltage of half a 1.2V supply). Thus RF blocker filtering is highly wanted. However, fixed filters are undesired when aiming for multi-band, software defined or cognitive radio transceivers. Passive LC filters show limited Q and tunability. Recently frequency translated filtering has been proposed as a potential solution direction for high Q filtering [1][2][3][4][5]. In [1,2] we showed that by applying the "N-path concept" 2[6], more than a decade of center frequency range with good linearity, compression point (P 1dB >0dBm, IIP3 >14dBm) and low noise is feasible for a bandpass (BP) filter. In [3] a notch filter with a combination of active and passive mixers is applied in a feedforward path realizing a BP filter.Moreover in [5] the low input impedance of a transimpedance amplifier with feedback is upconverted to create a notch filter at low frequencies (80MHz) suppressing TX leakage in an FDD system. In this work we explore the possibility to realize a notch filter applying the N-path concept at RF frequencies and in a completely passive way. A single-ended (SE) and a differential 8-path notch filter with passive frequency mixing are presented. The filters are power-matched in the input and output in the passband and provide a low insertion loss, high compression point and also low noise property, thus they can be utilized in front of a receiver to provide rejection of high power blockers with a large frequency tuning range. A prototype including a SE and a differential notch filter is implemented in 65nm CMOS technology (see Fig. 4.2.2 and 4.2.7). The circuit operates directly in a 50Ω system. In the differential architecture the notch is suppressed at the even harmonics of f s resulting in a wider passband. The second mixer in switches is set to 300mV to avoid reliability issues at high input swings). Larger switch would increase the insertion loss at high frequencies due to the parasitic capacitance and would require higher digital power drive. In the SE filter C 1 =7pF is chosen in each path targeting to suppress a 6MHz width blockers (e.g. TV channels in the cognitive radio or TV tuner applications). In the differential architecture two capacitors are in series and in order to get the same RC product as the SE version we have doubled the capacitor value (C 2 =14pF).For each filter, a divide-by-8 ring counter i...

Effects of LO harmonics and overlap shunting on N-phase passive mixer based receivers

“…With process scaling allowing for fast, low resistance switches, some designs have removed the LNAs at the front end of the receiver and connected a passive mixer directly to the antenna [1]- [4]. These mixer-first architectures have now achieved noise figures competitive with LNA-first designs [3], [4], as well as providing RF impedance tunability (both for matching and filtering) to the front end by exploiting the impedance translation property of these mixers [5]- [7].…”

A 0.2–3.6-GHz 10-dBm B1dB 29-dBm IIP3 Tunable Filter for Transmit Leakage Suppression in SAW-Less 3G/4G FDD Receivers

“…In Section II, the IIP3 requirements are summarized for cross-modulation distortion resulting from TX leakage. In Section III, the use of inductorless CMOS-based -path filters are introduced based on passive mixing with wide tuning range and high linearity [12]- [16] and the advantages and disadvantages of these schemes are discussed. These filters provide high by frequency shifting the impedance seen at baseband through the mixers.…”

Active N-path Filters: Theory and Design

“…4.4(c) [112] due to the fact that its low-frequency counterpart is not band-limited. Of course in this case, if the voltage around the switched capacitor part is taken as output, the methodology can be exploited to find the transfer function there, T(s), and finally the actual output transfer function can be found by (1-T(s)).…”

“…Secondly, the baseband current −V bi /R s is only present for 1/N th of the time, so its effect on the baseband voltage V bi can be modeled by a shunt resistance of NR s . Therefore, V bi (∆ω) as a function of input voltage,V in (ω lo + ∆ω), will be: [112] where the methodology should be utilized indirectly.…”