Abstract:
A GPS receiver depends on CMOS technology for both its GPS digital signal processing (DSP) and radio frequency (RF) stages. A resulting increase in RF input noise generated by the CMOS RF input is overcome by placing the antenna on-chip or on a lid of an application specific integrated circuit (ASIC) that includes the DSP and RF stages, and matching it for the best noise figure rather than the best impedance match. The on-chip antenna is matched to the natural high impedance of the CMOS RF input without requiring matching networks that can attenuate already weak signals. Using CMOS technology for both the GPS DSP and RF stages eliminates the need for level shifts between what would otherwise be a 3-volt RF section and a 1.2-volt DSP section.

Description:
FIELD OF THE INVENTION 
   The present invention relates to all-CMOS navigation satellite receivers, and more particularly to methods and systems for eliminating level-shifters in order to conserve operating power, and to methods and systems for matching on-chip antennas for the best noise figure rather than the best impedance match. 
   DESCRIPTION OF THE PRIOR ART 
   The increasing circuit densities and concomitant decreases in semiconductor feature geometries have been driving down the operating voltages needed by the devices. Current CMOS technology implementations are now using one-volt operating voltages. In the past, TTL logic had standardized on 5-volt logic. So when different technologies are intermixed in a single product design, some form of logic level shifting is needed. Such level-shifters consume a relatively large amount of power, and their elimination can increase battery life in portable devices. 
   Gallium arsenide (GaAs) technology has been traditionally used in radio frequency circuitry because of its good high frequency response and low noise operation. But GaAs semiconductor devices operate at higher powers than equivalent CMOS semiconductor devices, and no longer have much of an advantage over CMOS in terms of high frequency operation. But CMOS is inherently more noisy, and sensitive applications like GPS-antenna low noise amplifiers (LNA) are especially challenging. GPS receivers already work with very low receiver signal levels, and just a little bit more noise can seriously degrade performance. 
   Various standardized antenna and transmission line impedances have become standardized and ubiquitous over the years. For example, broadcast television antennas were originally designed with 300-ohm terminal impedances that were coupled to the TV set with 300-ohm twinlead wire. Later designed switch to 75-ohm coax cable for its better immunity to electrical noise, and 300-to-75 ohm baluns were used at the antenna to match the impedance of the antenna to the transmission line impedance. Such matching networks had losses, but these losses were acceptable. Radio communication systems have almost universally adopted 50-ohm impedance antennas and coaxial cables. These too have used matching networks for both the transmitter match and the receiver input match. 
   The prior art generally assumes that the best antenna and receiver performance will be realized when the antenna, transmission line, and input impedances are all matched to be the same. Common practice means each will be 50-ohm impedances. It can be proven that maximum power transfer from the input to the output occurs when the impedances are so matched. But here the present invention depends on some other impedance match being the best for minimum noise figure at the RF input. The traditional 50-ohm impedance is arbitrary, and not controlling in an on-chip antenna design. The best noise figure must be accomplished in an all CMOS design where the input amplifier is a MOS transistor. 
   SUMMARY OF THE INVENTION 
   Briefly, a GPS receiver embodiment of the present invention depends on CMOS technology for both the GPS digital signal processing (DSP) and radio frequency (RF) stages. A resulting increase in RF input noise generated by the CMOS RF input is overcome by placing the antenna on-chip or on the lid of an ASIC package, and matching it for the best noise figure rather than the best impedance match. The on-chip antenna is matched to the natural high impedance of the CMOS RF input without requiring matching networks that can attenuate already weak signals. Using CMOS technology for both the GPS DSP and RF stages eliminates the need for level shifts between what would otherwise be a 3-volt RF section and a 1.2-volt DSP section. 
   An advantage of the present invention is that a GPS receiver and method are provided that requires reduced battery power and can therefore operate for longer periods. 
   Another advantage of the present invention is that a GPS receiver and method are provided that make a single-chip implementation and the concomitant savings possible. 
   These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures. 

   
     IN THE DRAWINGS 
       FIG. 1A  is a functional block diagram of a conventional GPS receiver that mixes GaAs and CMOS semiconductor technologies; 
       FIG. 1B  is a functional block diagram of a GPS receiver embodiment of the present invention that uses only CMOS technology; 
       FIG. 2  is a functional block diagram of an implementation of the GPS-RF part of the receiver of  FIG. 1B ; 
       FIG. 3  is a schematic diagram of a GPS receiver embodiment of the present invention and represents the physical placement of a meander antenna on-chip and peripheral to a CMOS RF-baseband core disposed in a semiconductor chip; and 
       FIG. 4  is a schematic diagram of a GPS receiver embodiment of the present invention and represents the physical placement of a meander antenna placed on the lid of an ASIC package and peripheral to a CMOS RF-baseband core disposed in a semiconductor chip. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A  illustrates a prior art GPS receiver, and is referred to herein by the general reference  100 . The GPS receiver includes a 50-ohm microwave patch antenna  102  connected to a gallium-arsenide (GaAs) technology radio frequency (RF) stage  104 . Digital samples of the downconverted GPS satellite transmissions are provided to a GPS digital signal processor (DSP)  106 . The 50-ohm antenna  102  is typically connected with a 50-ohm transmission cable to an impedance matching network  108 . Such provides the best impedance matching so maximum signal power is transferred. But the best impedance match may not represent the best signal injection noise figure. 
   The RF stage  104  includes a conventional GPS-RF receiver  110  that uses GaAs technology and therefore requires a three-volt power supply. It outputs three-volt logic that must be level-shifted down to one-volt by level shifters  112 . The level shifting is implemented in the CMOS device  106 . A GPS navigation computer  114  produces position solutions from the samples shifted by shifters  112 . But such level shifting consumes a significant amount of power relative to the GPS navigation computer  114 . So their elimination can represent a substantial power savings and offer extended battery life during portable operation. 
   Now compare the prior art of  FIG. 1A  to the present invention represented in  FIG. 1B .  FIG. 1B  illustrates a GPS receiver embodiment of the present invention, and is referred to herein by the general reference numeral  120 . The GPS receiver  120  comprises a meander-line on-chip antenna  122  to receive the microwave transmissions of orbiting GPS satellites. The antenna is a high-impedance, on-chip design that is matched for the best noise-figure. This is represented by a lossless matching network  124  which provides a modest amount of signal gain. A GPS radio frequency (RF)  126  and baseband digital signal processor (DSP)  128  tune and demodulate GPS satellite signals. Such baseband digital signal processor (DSP)  128  has been marketed separately by e-Ride (San Francisco, Calif.) under its trademark OPUS. The input to RF  126  will typically be a high-impedance input of a low-noise amplifier (LNA) stage. 
   Maintaining a traditional 50-ohm impedance scheme when the antenna and connections are fully integrated on the semiconductor device is arbitrary and can lead to increased noise levels in this application. The higher input impedances may be achieved by using parallel-resonance points in the antenna rather than the more traditional series-resonant points. The feed-point at which an antenna is tapped will also effect the impedance. 
     FIG. 2  represents a GPS RF chip embodiment of the present invention, and is referred to herein by the general reference numeral  200 . The GPS RF-DSP chip  200  comprises an on-chip antenna  202  with a high output impedance tap and lossless matching network  203  that matches to the high input impedance of a CMOS technology low noise amplifier (LNA)  204 . An arbitrary 50-ohm impedance is not used because a matching network that generates noise would be needed between the antenna  202  and LNA  204 . In fact, the match between antenna  202  and LNA  204  is not adjusted for best impedance match, and therefore maximum power transfer, but rather best noise-figure. An RF bandpass filter (GPS-SAW)  206  precedes an RF amplifier  208 . An intermediate frequency (IF) is generated by a mixer  210  that receives a synthesized local oscillator (LO) frequency. A tunable IF filter (BPF)  212  removes the mixer images. The downconverted baseband is amplified by an automatic gain control (AGC) stage  214 . A 2-bit quantizer  216  produces magnitude and sign digital samples that are output by buffers  218  and  220 . 
   A voltage controlled oscillator  222  is used in a phase locked loop (PLL) to synthesize a variety of operating frequencies. Different frequency plans are implemented by programmable dividers  224 ,  226 , and  228 . A phase detector  230  compares the final division to a crystal generated reference frequency. A feedback difference control signal is returned through a loop filter  232  to keep VCO  222  on frequency. A divider  234  provides a final LO frequency to mixer  210 . Master (MCK) and other (PBK) clocks are derived from the VCO  222  by dividers  236  and  238 , and output buffer  240 . A first crystal oscillator (Xo)  242  and a second crystal oscillator (Xo)  244  are hardware temperature-compensated by a network  246 . Such corresponds to temperature curve flattening circuit  110  in  FIG. 1 . Selectable dividers  248  and  250  provide a choice in crystal reference frequency. Such is used by tuner  252  to operate BPF  212 . A voltage regulator  254  provides for the correct rail voltage for a CMOS chip with sub-0.20 micrometer geometry, e.g., about one volt. The all-CMOS design allows both the RF and DSP sections to be implemented on the same semiconductor chip. 
   Embodiments of the present invention use an on-chip antenna tapped at a high-impedance point that produces optimum noise figure at the RF input. The input impedance of a MOSFET is inherently capacitive, so providing a good impedance match to a 50-ohm resistance without degrading noise performance would appear to be difficult. See, EECS 522 ANALOG INTEGRATED CIRCUITS PROJECT, WINTER 2002; “A 1.9 GHz Low Noise Amplifier”, authored by Jerome Le Ny, Bhavana Thudi, Jonathan McKenna. Simply putting a 50-ohm resistor across the input terminals of a common source amplifier injects thermal noise and attenuates the signal ahead of the transistor. Such results in unacceptably high noise figures. A common-gate configuration can be used where the input impedance looking into the source terminal is 1/gm. A critical balance of device size and bias current can provide the desired match. But, the noise figure for high frequency operation would be excessive due to transistor gate current noise. A third configuration uses a resistor shunt and series feedback to fix the input and output impedances of the LNA. But this has relatively higher power dissipation compared to others with similar noise performance. Shunt-series amplifiers are naturally broadband, and LC tuning techniques to reduce power consumption cannot be used. It also needs quality on-chip resistors which are generally not available in CMOS technologies. An inductive source degeneration method may be the best. With a series input inductance (Ls), a real term in the input impedance can be generated without using resistors that can degrade noise performance. Tuning the amplifier input becomes necessary, which can be a favorable narrow band approach for a GPS application. The input impedance is that of a series RLC network, with a resistive term that is directly proportional to the inductance value. At series resonance, the input circuit impedance is purely real and proportional to Ls. By choosing Ls appropriately, the best noise match can be obtained. A gate inductance Lg is set by the resonance frequency once the Ls is chosen to satisfy the necessary input impedance. 
   See, Thesis: “AN INVESTIGATION OF ON-CHIP ANTENNA CHARACTERISTICS RELATED TO ENERGY HARVESTING APPLICATIONS”, by Dmitry Gorodetsky, B.Sc. in Electrical Engineering, University of Pittsburgh, 1999. The antenna input impedance, Z A , usually refers to the impedance seen looking into the terminals of the antenna. This is different from Z IN , because Z IN =Z A +Z FEED . The antenna impedance has a real and an imaginary component and is given by: Z A (f)=R A (f)+jX A (f). The input resistance, which is the real part of the antenna input impedance, will usually consist of two components, R L , the loss resistance of the antenna and R r , the radiation resistance of the antenna. These two components appear in series. 
   Ignoring the consequences to achieving an optimum noise figure, the maximum power transfer theorem states that in order to obtain maximum power from an antenna, its input impedance (Z IN ) must be a conjugate of the load&#39;s impedance. When this is achieved, half of the power will be absorbed in the antenna and half will be utilized in the load. After the maximum power transfer theorem has been satisfied, the next step is to limit the power losses within the antenna. This can be done by minimizing R L  and maximizing R r . In other words, this is an adjustment of the efficiency of the antenna. If the characteristic impedance and the antenna input impedance are equal, the input impedance is real and simply seen is Z 0 . In this case, the length of the feed becomes irrelevant to the impedance seen at its  13  terminals. This is the condition of perfect matching of the load to the line. When this occurs, all power is absorbed by the load and there is no reflection. Usually in RF work the transmission lines are designed so that their characteristic impedance is real at the specified frequency. The value of the characteristic impedance is typically 50-ohms or 75-ohms. Such enables the connection of multiple units using standard coaxial cables. 
   Optotek Ltd. (Ottawa, Ontario, Canada) Application Note 34, “Amplifier Load Mismatch Design Using MMICAD”, by D. Fitzpatrick (http://www.optotek.com/pdfs/note34.pdf) describes a method for matching circuits to achieve an optimum noise figure. According to a an article published by Focus Microwaves (Dollard-des-Ormeaux, Quebec, Canada), the noise behavior of a transistor is fully described by the minimum noise figure (NF min ), equivalent noise resistance (R n ), real and imaginary part of optimum noise source impedance (Y opt =G opt +j Bopt ). The noise figure only depends on the source impedance (Z s =1/Y s ) seen by the device. Between the four noise parameters exist a simple relation,
 
Noise Figure  NF ( Y   s )= NF   min +( R   n   /G   s )*| Y   s   −Y   opt | 2 ,
 
where Y s =G s +jB s . This is the equation of a set of circles on a Smith Chart (noise circles) for which the value of the noise figure is the level on each circle. (See, http://www.focus-microwaves.com/Products/ProductLinks/noise_sys.html.)
 
   Different types of antennas can be incorporated on-chip. For example, a slot antenna can made by cutting a slot in a discarded section of waveguide and downsized for an on-chip version. In a CommsDesign article, “Wireless transceiver-on-chip now possible”, by R. Colin Johnson, Aug. 3, 2004, http://www.commsdesign.com/showArticle.jhtml?articleID=26805541, a spiraling slot was cut into a 1-cm2 metal layer on the top of the chip. By tailoring the antenna&#39;s shape, the researchers were able to match the antenna&#39;s impedance to the rest of the transceiver without requiring a lossy passive matching network. Such technique could also be used to match for the optimum noise figure. 
     FIG. 3  represents a GPS receiver embodiment of the present invention, and is referred to herein by the general reference numeral  300 . The GPS receiver  300  includes a silicon semiconductor chip  302  on which are integrated a CMOS technology GPS-RF and baseband core  304  and a meander antenna  306 . The RF input of the GPS-RF baseband core  304  and meander antenna  306  have impedances in excess of a megohm to reduce the noise that would be generated by the matching network if they were fixed for traditional 50-75 ohm impedances. Only two external components complete the receiver, e.g., a bandpass SAW-filter  308  and a reference oscillator  310 . Such GPS receiver  300  is a battery operated portable, and therefore very sensitive to the operating power demands of its active circuits. 
     FIG. 4  represents a GPS receiver embodiment of the present invention, and is referred to herein by the general reference numeral  400 . The GPS receiver  400  includes an application specific integrated circuit (ASIC) package lid  402  which encloses a CMOS technology GPS-RF and baseband core (RF/BB)  404  and a meander antenna  406 . Such antenna can be fabricated using thick film and thin film techniques on ceramic or silicon. The RF input of the GPS-RF baseband core  404  and meander antenna  406  have impedances in excess of a megohm to reduce the noise that would be generated by the matching network if they were otherwise fixed for traditional 50-75 ohm impedances. The GPS receiver is completed with a bandpass SAW-filter  408  and a reference oscillator  410 . 
   Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the scope of the invention.