Abstract:
A tunable duplexer using voltage-controlled varactors is presented. The center frequency, the pass band, and the stop band are each tunable to meet system requirements. A calibration circuit driving digital to analog converters produces the necessary voltages used in the resonant circuits. The tunable duplexer can be fabricated on a single silicon chip. On-chip transformers can be used to reduce the voltage level of signals in the filters to improve the linearity of the duplexer.

Description:
RELATED APPLICATIONS 
     This application claims priority from U.S. provisional application No. 60/825,387 filed Sep. 12, 2006 entitled “Variable bandwidth tunable silicon duplexer”, incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field of Invention 
     This invention relates to tunable duplexers. 
     2. Prior Art 
     A duplexer is a device that isolates a receiver signal from a transmitter signal while permitting a receiver and transmitter to share a common antenna. The duplexer must be capable of handling the transmitter power and be able to provide sufficient isolation to prevent receiver desensitization due to coupling of the transmitter signal into the receiver. When the transmit and receive frequencies are different, filters may be used to reduce the transmit signal levels to an acceptable low level at the receiver input. 
     Nontunable duplexers used in wide bandwidth systems such as CDMA systems use a well-known method of creating the wide pass band bandpass filters by cascading resonant sections coupled to additional sections. The more sections, the wider the pass band bandwidth. For example, for a CDMA system operating in the AMPSs band, the Tx frequency band is 824 to 849 MHz and the Rx frequency band is 869 to 894 MHz. Fixed nontunable duplexers must be designed to pass all channels in the band. This would require a nontunable filter with a pass band bandwidth of greater than 25 MHz to compensate for process and temperature variations. Many cascaded sections are also needed to achieve this bandwidth and steepness in the transition bands. 
     In the past, duplexers were a fixed design with the frequency bands of the transmit and receive bands predetermined. A tunable duplexer can simplify the design because of the ability to tune to the desired narrowband channel. The need for tunable duplexers existed and the following references reflect the current state of tunable duplexers and tunable filters. 
     U.S. Pat. No. 6,990,327 issued to Zheng et al entitled “Wideband Monolithic Tunable High-Q Notch Filter for Image Rejection in RF Application”, incorporated herein by reference, describes a tunable notch filter which is contained on a single integrated chip. 
     U.S. Pat. No. 6,407,649 issued to Tikka et al entitled “Monolithic FBAR Duplexer and Method of Making the Same”, incorporated herein by reference, describes a monolithic bulk acoustic wave (BAW) duplexer. A patterned piezoelectric material is used as the piexolayer for each of the resonators of the duplexer. 
     U.S. Pat. No. 6,816,714 issued to Toncich, entitled “Antenna Interface Unit”, incorporated herein by reference, describes a ferro-electric tunable duplexer. 
     U.S. Patent Application Publication 2003/0199286, inventor D du Toit, entitled “Smart Radio Incorporating Parascan Varactors Embodied Within an Intelligent Adaptive RF Front End”, incorporated herein by reference, describes a radio with an RF front end containing at least one tunable duplexer. The radio incorporates Parascan® electrically controlled dielectric varactors embedded within the RF front end. Parascan® is a trademarked tunable dielectric material developed by Paratek Microwave, Inc. The RF front end described in the publication contains a tunable duplexer and two tunable filters. 
     An article, entitled “A Novel Electronically Tunable Active Duplexer for Wireless Transceiver Applications”, by B. Sundaram and R. Shastry, published June 2006 in Vol. 54, No. 6 of IEEE Transactions on Microwave Theory and Techniques, describes a 4-port duplexer circuit that uses varactors to enable electronic tuning of the frequency at which isolation is desired. 
     An article, entitled “Adaptive Duplexer Implemented Using Single-Path and Multipath Feedforward Techniques with BST Phase Shifters”, by T. O&#39;Sullivan, R. York, B. Noren, and P. Asbeck, published January 2005 in Vol. 53, No. 1 of IEEE Transactions on Microwave Theory and Techniques, describes a technique to enhance the isolation of a surface acoustic wave duplexer, which reduces the noise levels in the receive band of the system. Feedforward techniques are used to create an adaptive null in the receive band. 
     An article, entitled “A Varactor Tuned RF Filter”, by A. Brown and G. Rebeiz, published Oct. 29, 1999 as a submission for review as a short paper to the IEEE Transactions on MTT, describes an electrically tunable filter at 1 GHz. The resonators used in the tunable filter are stripline interdigital fingers with varactor diodes at the ends. 
     U.S. Patent Application Publication 2005/0148312, inventors Toncich et al, entitled “Bandpass Filter with Tunable Resonator”, incorporated herein by reference, describes a tunable bandpass filter comprising of ferroelectric tunable tank circuits. 
     SUMMARY OF INVENTION 
     The present invention is a duplexer circuit that can be used in applications requiring a single antenna port and communicate in full or half duplex mode. Examples include AMPS, GSM, CDMA and PCS cellular phones. 
     The duplexer circuit of an embodiment of this invention is tunable and can be implemented on a single silicon chip. The duplexer circuit contains tunable bandpass filters which are tuned electronically by varying the capacitance of tank circuits using on-chip or off-chip varactors. 
     The duplexer contains two tunable bandpass filters, one for received signals and one for transmitted signals, a combining network, and a calibration circuit for setting the values of the tunable elements of the filters. The calibration circuit can be implemented using a state machine. 
     To achieve the required channel selectivity and the required filter performance, the center frequency, the pass band and the stop band are tunable. The pass band frequency, width, and insertion loss, and the stop band frequency, attenuation, and width, are determined by system requirements. 
     The ability to tune the bandwidth of the pass band of the filter to a desired narrower bandwidth channel reduces the number of resonant sections required. With the frequency tunable duplexer of the present invention, the bandpass filter can be tuned to one narrow band channel, for example 5 MHz, requiring less resonant sections, fewer components and lower cost, while achieving better signal processing performance. 
     The duplexer can be made with low cost lower Q inductors. The tunable filter enables positioning of the transmission zeros, herein referred to as stop band zeros or stop band nulls, in the precise location to meet stop band rejection requirements. This ability in conjunction with being able to use narrow bandwidths in the pass band allow for the use of lower Q inductors to meet the steep pass band to stop band transitions necessary. A high attenuation at a nearby frequency relative to the pass band edge can be achieved. 
     Another advantage of the tunable duplexer is the ability to position stop band zeros such that the nulls created occur at frequencies of known narrowband interfering signals. 
     High linearity is achieved by using on-chip transformers to step the signal voltages down for signal processing, then stepping up the voltage at the interface ports. Distortion is proportional to the voltage across the varactors, therefore, with a lower voltage across the diodes, the distortion is lower and linearity is improved. Linearity is additionally improved by using “back-to-back” or “totem pole” stacked varactor diodes. 
     Both the transmit and receive bandpass filters (BPF) are tuned simultaneously to maintain acceptable input return loss, and acceptable pass band and stop band performance. The tuning is performed by the calibration circuit, which provides tuning voltages to the varactors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the block diagram of the tunable duplexer of the present invention. 
         FIG. 2  shows a filter response of the present invention where the peak frequency and the null frequency are independently tuned. 
         FIG. 3  shows the schematic of the tunable duplexer of the present invention. 
         FIG. 4  shows an example of a schematic of the tunable duplexer of the present invention with varactor capacitance values and inductor values. 
         FIG. 5  shows the response of the filters using the varactor capacitance values and inductor values given in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an embodiment of the tunable duplexer  100  comprising of two varactor-tuned bandpass filters (BPFs), receiver bandpass filter  110  and transmitter bandpass filter  115 , calibration circuit  120 , and a combining network  130 . Duplexed transmitted and received signals couple to TX/RX antenna  140 . The tunable duplexer is fabricated substantially on a single integrated circuit chip. 
     The transmitter signal at the input of the BPF  115  is stepped down in voltage by on-chip transformers. This improves the linear dynamic range of the BPF. Similarily, the receive signal at the input of the BPF  110  can be stepped down prior to filtering and subsequently stepped up after filtering. The combining network  130  can be simply a node that connects the antenna to the output of the transmitter bandpass filter  115  and the input of the receiver bandpass filter  110 . Due to the frequency dependent input and output characteristics of the bandpass filters  110  and  115 , the out-of-band signal energy is reflected and the in-band energy is passed in a low loss or lossless connection at the combining node. The step-up and step-down ratios in the transmit and receive paths can be different ratios. 
     The filter  110  and  115  are comprised of cascaded image-parameter filter sections. Each section can be designed using well-known filter design techniques and circuit simulation tools, for example, Agilent&#39;s Advanced Design System (ADS). The filters  110  and  115  are combined with the combining network to form a duplexer. The center frequency, the pass band and the stop band are each tunable in order to achieve the desired selectivity, pass band and stop band performance. The tuning control is determined by the calibration circuit  120 . 
     Calibration circuit  120  controls the tuning of the varactors used in the bandpass filters  110  and  115 . The calibration circuit  120  tunes each of the filters  110  and  115  by controlling the voltage to the varactors of filters  110  and  115 . The filters  110  and  115  have tunable center frequencies and tunable pass band and stop band bandwidths. Several independent tuning voltages may be required and used to tune the various parameters of the filters. 
       FIG. 2  shows a response of one of the bandpass filters  110  or  115  where f PEAK    202  and fnull 1  to fnull N    204  can be independently tuned. The resonant peak of the filters with a nearby null provides a steep transition from the passband to the stop band. The stop band nulls can be tuned to narrowband interfering, or jammer, frequencies. 
       FIG. 3  shows the schematic of an embodiment of the present invention, duplexer  300 . Distortion is proportional to the voltage across the varactors; therefore, if the voltage is lower across the varactors, distortion is lower and linearity is improved. In this embodiment of the present invention, a lower voltage is achieved across the varactors by using step-down transformers. Transformer  310  steps down the transmit signal. Transformer  302  steps down the received input signal and steps up the transmitted output signal, preferably with a ratio in the range of 1 to 10, or higher or lower ratios. The ratio is dependent on the center frequency and insertion loss requirements. The combining network may be a node as shown as node  314  that connects the antenna transformer to the transmit and receive filters. Filter block  306  shows the circuitry of one section of the Tx bandpass filter. The duplexer  300  may contain one or more instances of filter block  306 . Filter block  308  shows the circuitry of one section of the Rx bandpass filter. The number of filter blocks  306  and  308  needed is dependent on the required bandwidth and loss for the system. The inductors  315  and  316  in filter blocks  306  and  308  may be on-chip or off-chip. Transformers  310  and  311  may be on-chip transformers that step down the transmitter power amplifier signal and step up the signal coupled to the receiver. 
     Transformers  302 ,  310  and  311  also function to transform impedance where the impedance of the filters can be higher or lower than the impedance of the antenna, transmitter circuit, and receiver circuit. For the ratios shown, the impedance of the filters would be lower by a factor of the square of N. The ratio of each transformer does not need to be the same. 
     The calibration state machine  315  supplies the voltages to the voltage controlled varactors of filter blocks  306  and  308 . The calibration state machine  315  produces the varactor control voltages for determining the center frequencies of the Tx and Rx bandpass filters and the voltages for determining the bandwidth and stop band frequencies for each filter. In one embodiment, the varactor control voltages are produced by digital-to-analog (D/A) converters driven by the calibration state machine  315 . The voltages may also be produced by D/A converters driven by a microprocessor. 
     To improve linearity, the varactors of filter sections  306  and  308  can be high voltage varactors. The high voltage varactors can be well-to-substrate junctions or can be fabricated with either existing process steps present on a standard low-cost IC process or with the addition of one or more process steps. An example fabrication technique to form a varactor is using the collector-base junction of a bipolar transistor by appropriately adjusting the implant doses to create a large capacitance tuning range across a high voltage range. 
     An alternative to achieving improved linearity without the use of high voltage varactors is to switch in multiple fixed value capacitors in parallel with the varactors in the filter. Since a larger proportion of the total capacitance now consists of linear capacitance, the linearity improves and the varactor may no longer need to be of a high-voltage type. 
     Tuning can be open loop with the voltages driven from a table of predetermined or premeasured values. The predetermined voltage values can be determined during filter synthesis. A set of varactor control voltages are generated for each desired tuning frequency of the duplexer. Additionally, temperature coefficients may be predetermined and added to a table to account for temperature variations. Utilizing temperature monitoring, the predetermined or premeasured voltage values can be adjusted by the temperature coefficients stored in the table. 
     Alternatively, tuning can be performed using an injected test tone to measure critical filter frequencies, such as the center frequency, bandwidth, and stop band nulls. The injected signal can be swept across the desired operating frequency range to verify the position of critical operating frequencies and provide information for making adjustments to tuning voltages. Open loop tuning can be aided by a calibration measurement made when the system is powered up, periodically, or each time a channel is changed, or each time a call is initiated. The calibration measurement can update the table of values or correction factors. 
     By implementing the varactors on-chip, techniques for compensating for environmental changes may be improved. Specifically, the temperature coefficients of the on-chip varactors will match very well and this allows the use of a reference varactor whose temperature and temperature coefficient will match very closely with the varactors used in the filters. The capacitance of the reference varactor can be monitored and its tuning voltage can be automatically adjusted to ensure constant capacitance. Alternatively, a full tuning curve can be measured. The reference varactor tuning information can be used to update the tuning voltage of the filter varactors, thereby ensuring very accurate compensation of capacitance drift due to temperature changes, power supply voltage changes, ageing, and other sources of drift. 
     The Tx and Rx filter topologies are influenced by the relationship of the transmit and receive frequencies. The Tx stop band frequency generally corresponds to the Rx pass band and the Rx stop band should fall into the Tx pass band. For the example shown in  FIG. 3 , the Rx pass band is higher in frequency than the Tx pass band. 
     The varactor capacitance values are interrelated and are determined when the duplexer is synthesized according to system requirements. The values of the voltages that correspond to the capacitance values reside in the calibration state machine or microprocessor. Each filter section produces one null. Therefore, as the number of sections increases, the number of stop band null frequencies increase. 
       FIG. 4  shows an example of a schematic of the tunable duplexer of the present invention with actual varactor capacitance values and inductor values. The Tx pass band for this example is 824 to 829 MHz and the Tx stop band is 869 to 894 MHz. The Rx pass band is 869 to 874 MHz and the Rx stop band is 824 to 829 MHz. 
       FIG. 5  shows the response of the filters using the varactor capacitance values and inductor values given in  FIG. 4 . 
     The duplexer of the present invention can be used with transceivers that have selectable frequencies. When the transceiver is tuned to the desired transmit and receive frequencies, the tunable duplexer will also tune to match the frequencies of the transceiver.