Tobias D. Werth

Active feedback interference cancellation in RF receiver front-ends

A feedback interference cancellation circuit is presented that uses a control loop to reject blockers in wireless receivers. The concept is based on a translational loop which subtracts a blocker replica in a feedback loop fashion. In contrast to feedforward methods loop selectivity does not depend on exact gain matching of two paths but on the open loop gain which is easier to adjust. The concept is described including the theoretical derivations of the transfer function and stability. Measurements from a prototype chip in 65nm CMOS are presented showing the feasibility of active feedback cancellation. In a desensitization scenario, gain drops by more than 12 dB for a −15dBm blocker at 20MHz offset without feedback interference cancellation while a gain degradation of merely 3 dB is measured with feedback interference cancellation enabled.

A new Q-enhancement architecture for SAW-less communication receiver in 65-nm CMOS

A 2.5V second order RF bandpass filter is presented. A new Q-enhancement structure stacked on a LNA is fabricated in a 65-nm CMOS process with a die area of 1.1 mm × 1.1 mm. The frequency range is adjustable in a 25% range from 1.7 GHz to 2.2 GHz while the Q is adjustable up to 50. Q and the center frequency are adjusted by binary weighted switchable capacitors. The filter draws 42 mA including the buffers from a 2.5 V supply and has an input referred out-of-band 1-dB compression point of 0 dBm. The measured in-band NF is between 5.4 and 6.2 dB.

A 1.3 V, 65nm CMOS, coilless combined feedback LNA with integrated single coil notch filter

An LNA with integrated notch filter is demonstrated. It consists of a coilless two-stage LNA with capacitive feedback and an integrated Q-enhanced notch filter. Thus, this circuit is capable for use in FDD systems like UMTS or WCDMA without additional off-chip interstage filter, especially for highly integrated multi-band multi-standard transceivers as only one area consuming coil is used.

On I/Q-mismatch in active interference cancellation schemes

The performance of active interference cancellation schemes in RF receiver front-ends can be seriously affected by I/Q-mismatch of the active cancellation filter core. Therefore, the influences of I/Q gain and phase mismatch on active feedforward and feedback cancellation schemes are investigated. It is found that I/Q mismatch leads to an image spectrum at the band of interest. The image can be particularly detrimental when blockers that should be filtered by active cancellation appear at the image band thus desensitizing subsequent circuit blocks in the receive chain. A comparison of feedforward and feedback cancellation reveals that feedforward cancellation is much more sensitive to I/Q mismatch effects than feedback cancellation. Simulation results and measurements from a prototype chip are presented to confirm the theoretical results.

Design of frequency agile filters in RF frontend circuits

Frequency agile filter are key components for future cognitive radio applications. Bandpass filters based on the principle of lowpass upconversion allow simple and CMOS-friendly solutions to cognitive radio filter problems. Because the center frequency depends on the LO-frequency only, the band selection is adjusted with the receive mixers LO simultaneously. A frequency agile filter has been incorporated into a 600 MHz to 800 MHz wideband LNA with a gain of 18 dB at 700 MHz. It consumes 8.7 mA from a 1.4 V supply. With a 3 dB-bandwidth of 14 MHz, the filter achieves a selectivity of 9.5 dB at 40 MHz frequency offset. Measurement results of a prototype chip in 90 nm CMOS prove the presented theory.

A frequency agile LNA for cognitive radio applications in the UHF band

A frequency selective LNA for the UHF band is presented that uses an N-Path filtering concept to realize frequency agile filtering between 600 MHz and 800 MHz. A frequency generator like an on-chip PLL can be used to choose the filter band without the need for an additional post-production calibration step. The LNA concept uses the advantages of the source degenerated as well as the shunt feedback LNA. The characteristic of the LNA and the filter function is described in detail. Measurement results from a prototype chip in 90 nm CMOS prove the presented theory. At 700 MHz the LNA offers a gain of 18dB without filtering and 17dB with filtering applied. The noise figure is as low as 3.1dB and increases to 3.9dB with filtering. The linearity in terms of the intermodulation point IP3 is as high as 0 dBm. A single filter gives 9.5 dB selectivity at 40MHz frequency offset. Using only a source inductor, the LNA area occupation is lower than 0.2 mm2.

A comparison of bandwidth setting concepts for Q-enhanced LC-tanks in deep-sub micron CMOS processes

In this paper LC-resonator bandwidth setting techniques in deep-sub micron CMOS processes are examined and compared with the main focus on Q-enhancement, linearity, noise, power consumption and controllability at low supply voltages. Simulations have been performed using an ideal amplifier model to achieve results independent of the purpose of the circuit. The Q-enhancement is achieved by positive feedback into the gates of a cross coupled MOSFET transistor pair. The different Q-tuning concepts include current control techniques as well as capacitive subdivision in the feedback path. As result of this work, a combination of tail current controlled and capacitive subdivision has been found to be the best mean to achieve highest linearity at good controllability. Simulation results show that a quality factor around 100 is possible at a current consumption of maximum 3 mA. The equivalent input noise does not exceed 1.5 nV/radic(Hz) with total harmonic distortion around 0.005% which makes the circuit reasonable for application in LNAs as well as for transmitter structures.

System design considerations for SAW-less front-ends at the example of GSM DCS1800

In this paper, system design implications for SAW-less front-ends are derived from a comparison of a conventional and a SAW-less direct conversion receiver which uses an active interference cancellation technique for front-end filtering. Specifications for the GSM DCS1800 band are cited and a level plan is established with block level specs obtained from circuit level simulations. The level plan is used to judge on trade-offs that have to be made when the SAW band select filter is omitted. It is found that omitting the filter pays a “SAW-less dividend” in the form of improved reference sensitivity as SAW-filter insertion loss does not occurr. Moreover, a trade-off in the proposed SAW-less front-end is identified between selectivity and phase noise due to reciprocal mixing and an optimum sensitivity is determined which maximizes sensitivity for a given blocking condition.

A fully integrated Q-enhanced notch filter LNA for TX blocker suppression in FDD systems

An LNA with integrated Q-enhanced notch filter for use in FDD transceiver systems has been demonstrated. The amplifier core is coilless, made possible by the use of capacitive shunt-series feedback to simplify matching. Additionally, a Q-enhanced notch filter is integrated which attenuates TX blockers thus making an external interstage SAW filter redundant. A prototype was produced on a 65nm CMOS process. Without filter operation it draws 16.6mA from a 1.3V supply, with filter switched on the Q-enhancement adds 2.3–4.1 mA, depending on the demanded selectivity. The filter can be disconnected from the LNA. At 100 Ω source impedance the input reflection factor S11 does not exceed −15 dB for the chosen frequency. The tuning range of the filter is 1500–1780 MHz for RX. The LNA supplies 24.5dB gain with a noise figure of NF = 2.3 dB. For 9 dB selectivity, the gain at RX drops to 20dB and the NF increases to 4.1 dB. The point where the RX gain drops 1dB due to a TX blocker is increased from −23.3 dBm to −19.5 dBm thus showing a proof-of-concept.

A novel notch filter LNA for use in multi-band, multi-standard cellular receivers

A novel LNA with integrated notch filter for multi-band, multi-standard cellular communication is presented in this paper. The LNA is capable to serve multiple FDD bands of the LTE standard and its cobanded UMTS counterparts. Due to its architecture also TDD bands including GSM bands DCS1900 and PCS1800 and LTE TDD bands can be covered. Due to the integrated notch filter which is reused for all covered bands external interstage filters can be omitted thus saving PCB area and component costs. Exemplary for LTE band II, simulations reveal a noise figure of 1.96 dB at 1930 MHz and 10.3 dB selectivity at 80 MHz duplex distance. The inband-IIP3 reaches -10.27 dBm, while IIP3DPX = -10.4 dBm and IIP3HDPX = -2.6 dBm. The circuit works from a 1.4 V supply, drawing 15.3 mA on a commercial 90 nm process.