Wesley N. Allen

Bandwidth-optimal single-tunable-element matching network for antenna tuning in mobile handsets

This paper presents a single-tunable-element matching network that is optimized for the GSM850 and GSM900 bands. It is designed to counteract the detuning of mobile handset antennas by optimizing the transducer gain of the network. A simple model that emulates the antenna detuning effects based on extensive measurements from the literature is first presented. Based on this model it is shown that a matching network with a single tunable element suffices to cover the two aforementioned bands. Implementations for a planar inverted-F antenna (PIFA) as well as a proof-of-concept for the iPhone 4™ antenna are presented. The measured results, in good agreement with the modeled data, indicate that transducer gain improvements of up to 3.5 dB (53%) and 5.6 dB (57%) can be obtained for the PIFA and iPhone 4™ implementations, respectively.

Three-Bit and Six-Bit Tunable Matching Networks with Tapered Lines

Two tunable matching networks, one with three bits and one with six bits, are designed, fabricated and measured. The tuning elements of the networks are the Radant RMSW- 100 SPST MEMS switches. These switches connect fixed line- loading capacitances to the networks. Tapered transmission lines are used instead of uniform lines in the networks. The 6-bit network exhibits two bands with bandwidths of 46% and 29% and Smith Chart coverage of at least 68%. The 3-bit network exhibits a single band with a bandwidth of 44% and a Smith Chart coverage of at least 23%. A particular load is included in the bandwidth and coverage calculations if a) it can be matched with a reflection of less than −10 dB and b) the S-parameters of the tunable matching network when configured to match this load satisfy |S11| 2 + |S21| 2 ≥ 0.5, i.e., its measured efficiency is higher than 50%.

Bandwidth-optimal single shunt-capacitor matching networks for parallel RC loads of Q ≫ 1

In this paper we demonstrate an optimized matching network that maximizes the matching bandwidth for parallel RC loads with quality factors greater than unity. The matching network includes a transmission line and a single shunt capacitor. We systematically investigate the dependance of the networks bandwidth on the impedance and electrical length of the transmission line. Results show that high impedance lines always yield maximum bandwidth for parallel RC loads with Q ≫ 1. A 4.3∶1 MEMS varactor-based tunable matching network is measured as a vehicle to demonstrate these results. In particular, we find that up to a two-fold increase in the 10-dB reflection bandwidth is possible by utilizing high impedance lines. Wih a single 4.3∶1 MEMS varactor a measured bandwidth of over 136% is obtained.

Open-loop temperature-compensated tuning of a 2-pole absorptive bandstop filter

For the first time, an open-loop temperature-compensated control scheme is presented for a tunable 2-pole absorptive bandstop filter. The scheme includes a post-fabrication calibration that extracts the filter poles and zeros and generates a model of the zeros with respect to bias condition across −35 to 100° C. The bias voltages needed to achieve a target notch at a sensed temperature are determined based on this model. Measured filter tuning shows 20–48 dB improvement in isolation from 1.7–2.1 GHz compared to tuning without temperature compensation and frequency variation of less than ±0.03% from −35 to 100° C.

Tuning limits of shunt varactor diodes for maintaining high OIP 3 in arbitrary circuits

For the first time, an analytical formula is presented for calculating the OIP3 of any arbitrary circuit with a single shunt varactor diode. Additionally, numerical calculation of the limits on varactor tuning range when a specific OIP3 is desired is discussed, and a commercial varactor is found to maintain 44% of its tuning range with a OIP3 > 37 dBm.

A continuously tunable 95–138 MHz bandpass resonator with 40 dBm IIP3

This paper reports on the design of a frequency-tunable lumped-element bandpass resonator with wide tuning range (1.45:1) and high third-order input intercept point (IIP3 > 40 dBm). The filter tuning is realized by means of a continuously variable capacitance that is formed by a hybrid combination of a back-to-back varactor pair-operated in the high DC bias region-and a switched network of static capacitors. An experimental prototype was designed, built and measured for proof-of-concept demonstration purposes. It exhibited a tunable frequency between 95 and 138 MHz (1.45:1) and insertion loss between 1.83 and 2.25 dB. Within this frequency range, the in-band IIP3 was measured above 40 dBm.

Wideband diode-based reconfigurable matching network operating at 36 dBm input power

This paper presents wideband reconfigurable matching networks operating at high power in the GSM850 and GSM900 bands to counteract the detuning of mobile handset antennas caused by user interaction. High breakdown voltage varactor diodes are used as tuning elements. Single-diode, shunt-series diode, and two-diode topologies are investigated for power handling, and the effect of input power on matching network transducer gain, GT, is studied. Results show that a shunt-series diode topology can handle input powers of up to 36 dBm while maintaining GT degradation of less than 0.1 dB.

Transient response enhancement of RF MEMS tuners using digital signal processing

The transient response time and overshoot of an RF MEMS filter tuning membrane are enhanced in this work. For the first time, a dead-beat control is applied to the bias voltage of an underdamped RF MEMS membrane to achieve faster and smoother tuning. The presented method runs in real-time using a Digital Signal Processor (DSP). The parameters used by the DSP are extracted from the step response of the membrane during post-fabrication calibration. As a result, the presented methodology is systematic and immune to fabrication process variations. The presented methodology is applied on an MEMS membrane, used in tunable silicon RF filters, fabricated on an SOI wafer. Measured results show that the shaped bias voltage shortens the settling time (by 1.2–2.6x), and a significantly lowers the overshoot (by 8–60x) when compared to the voltage step response. The presented work does not require a feedback system, making it a feasible solution to embed in communication devices.

Fully autonomous multiple-jammer suppression

For the first time, a fully autonomous jammer suppressor is presented that can suppress two jammers. A system consisting of a 4-pole absorptive bandstop filter (ABSF), a novel multi-jammer detector, and an open-loop filter tuner is described and demonstrated. The ABSF enables high isolation at two frequencies with planar resonators. The jammer detector can identify the location of two jammers in 100 ms-order speeds and can be designed to trade off bandwidth and accuracy while maintaining detection speed. The open-loop filter tuner enables accurate tuning of the ABSF without costly filter feedback through the use of a post-fabrication-calibrated surrogate model. With a single jammer, the system provides fully autonomous suppression of 3055 dB from 1600–2100 MHz. In the presence of two jammers, up to 56 dB suppression is measured with an average of 30–48 dB suppression in the ABSF absorptive region of 1910–2100 MHz.

Tunable bandpass-bandstop filter cascade for VHF applications

A lumped-element bandpass-bandstop filter cascade with the capability of independently allocating its poles and zeros is demonstrated with commercially available components. The filter cascade consists of 14 tunable resonators to form advanced filtering functions. Eight resonators implement a reconfigurable fourth-order band-pass filter with tunable resonant-couplings, while the remaining six are used to form three second-order absorptive band-stop filters to create deep isolation levels. The filter cascade can be continuously tuned from 100 to 255MHz with high isolation (70dB at 5% away from the pass-band edge). Resonant coupling structures act as intrinsic switches to achieve reconfigurable signal routing. Therefore, switchless multi-band filter bank operation is also achieved.