Abstract:
A wireless display tag, adapted to fit within the C-channel of a shelf-edge, or otherwise usable as a hang tag or small identification device, includes, depending on implementation, an active transceiver, a passive transceiver, or both, with analog and digital control portions for managing communications with a host.

Description:
RELATED APPLICATIONS 
     The present invention claims the benefit of priority from the following United States provisional applications: U.S. patent Ser. No. 60/530,819 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Using Amplified Backscatter”; U.S. patent Ser. No. 60/530,818 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Using an Active Transmitter”; U.S. patent Ser. No. 60/530,817 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Using an Active Receiver”; U.S. patent Ser. No. 60/530,816 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Using an Active Transmitter and Diode Receiver”; U.S. patent Ser. No. 60/530,795 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Using Active and Backscatter Transceivers”; U.S. patent Ser. No. 60/530,790 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Unit”; U.S. patent Ser. No. 60/530,783 filed Dec. 18, 2003 entitled “RF Backscatter Transmission with Zero DC-Power Consumption”; U.S. patent Ser. No. 60/530,823 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Initialization; U.S. patent Ser. No. 60/530,784 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) with Environmental Sensors&#39; ”; U.S. patent Ser. No. 60/530,782 filed Dec. 18, 2003 entitled “High Readability Display for a Wireless Display Tag (WDT)” 
     This application is also related to the following U.S. utility applications filed simultaneously herewith: This application is also related to the following U.S. utility applications filed simultaneously herewith: U.S. patent Ser. No. 11/019,660 filed Dec. 20, 2004 entitled “Error Free Method for Wireless Display Tag (WDT) Initialization”; U.S. patent Ser. No. 11/019,494 filed Dec. 20, 2004 entitled “RF Backscatter Transmission with Zero DC Power Consumption”; U.S. patent Ser. No. 11/019,978, filed Dec. 20, 2004 entitled “Wireless Display Tag (WDT) Unit”; U.S. patent Ser. No. 11/019,916, filed Dec. 20, 2004 entitled “Multi User Wireless Display (WDT) Infrastructure and Methods”; and U.S. patent Ser. No. 11/019,705, filed Dec. 20, 2004 entitled “Low Power Wireless Display Tag (WDT) Systems and Methods”. 
    
    
     BACKGROUND OF THE INVENTION 
     Traditional paper pricing labels for products presented on a shelf are being replaced by digital units. Digital units have an LCD display driven by digital logic. They typically are installed on the edge of the retail shelf. In some instances these digital units are capable of radio communication similar to the active radios commonly used by people, for example in car radios and cell phones. 
     Active radio transmission is well known technique of radio transmission where an active power source generates a radio-frequency (RF) wave that is modulated with information and the RF wave excites an antenna. Electro-magnetic radiation propagates from the transmitting antenna to a receiving antenna. A receiver, which may be either active or passive devices, collects the signal, demodulates it, and presents the demodulated information to the user. The advantage of using active radio transmission is that because of the active power source, signal strength is typically good and, hence, there is improved transmission range. However, the use of an active power source results in the need for a larger power supply and generation of heat, both of which are concerns in compact circuitry designs. 
     Other known methods of communication include a backscatter transceiver having a receiver and a transmitter. Backscatter transmission is a technique whereby signals are sent with typically lower power consumption than comparative techniques. The system requires a RF source and the transmitter. The source sends a radio wave over the air. The radio wave propagates from the source to the transmitter&#39;s antenna. What is commonly called a backscatter receiver is actually a diode demodulator for non-constant amplitude carrier reception. A backscatter transmitter does not have an active power source to generate an RF wave(s). An advantage of the backscatter transceiver is low power consumption and, hence, an effective design alternative. However, the problem with the backscatter transceivers is that the signal strength is low and, hence, the range is very limited. Thus, backscatter transceivers are not always effective when longer transmission range is desired. 
     Therefore what is needed is a system and method that communicate using radio communication using minimal power with low heat dissipation to allow for a compact and cost effective design solution, while providing for effective communication range based on the ability to generate strong radio communication signals when required. 
     SUMMARY OF THE INVENTION 
     A system and method are provided communicate using radio communication using minimal power with low heat dissipation in a compact and cost effective design solution. The system includes a method that provides effective communication range based on the ability to generate strong radio communication signals. 
     In one embodiment, a combination of backscatter and active radio technologies is used to provide both long range and low power. An advantage of this combination is lower power consumption, although other advantages can be determined from the teachings set forth herein by those skilled in the art. 
     In an alternative embodiment, when long range is required, the higher-power consumption active radio is used. 
     In yet another embodiment, when short range is needed, then the lower-power consumption backscatter radio is used. 
     In an embodiment of the system, a digital unit is in communication with an In-Store Server (ISS) computer through either two-way Radio Frequency (RF) backscatter, infra-red (IR), or one-way RF. Two-way RF communication allows for an acknowledgement that the signal from the ISS or an intermediate access point router was received and properly interpreted and that the request successfully carried out is required. The ISS sends the digital unit the price and other information to be displayed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an wireless access point device in communication with a plurality of display units or Wireless Display Terminals (WDTs) proximally located near products on a shelf in accordance with the present invention; 
         FIG. 2   a  shows an active transceiver portion of a WDT in accordance with the present invention; 
         FIG. 2   b  shows a backscatter transceiver portion of a WDT in accordance with the present invention; 
         FIG. 2   c  shows a flow chart for the transceivers of  FIGS. 2   a  and  2   b  in accordance with the present invention; 
         FIG. 3  shows a WDT having an active transceiver and a backscatter transceiver in accordance with the present invention; 
         FIG. 4  shows a ratio detector in accordance with the present invention; 
         FIG. 5  shows a WDT that includes a backscatter transceiver, a super-heterodyne receiver, and a dual-conversion transmitter in accordance with the present invention; 
         FIG. 6  shows a WDT that includes a backscatter transceiver, an image-Reject Receiver, and a dual-conversion transmitter in accordance with the present invention; 
         FIG. 7  shows a WDT that includes a backscatter transceiver, a Weaver receiver, and a open-loop modulated transmitter in accordance with the present invention; 
         FIG. 8  shows a WDT LNA implemented in CMOS of BJT technology in accordance with the present invention; 
         FIG. 9  shows a WDT mixer implemented in CMOS of BJT technology in accordance with the present invention; 
         FIG. 10  shows a WDT in accordance with the teaching of the present invention, wherein the transmitter output is coupled to the receiver input; 
         FIG. 11  shows an active transmitter portion of a WDT in accordance with the present invention; 
         FIG. 12  shows an active quadrature transmitter portion of a WDT in accordance with the present invention; 
         FIG. 13  shows a direct-conversion receiver portion of a WDT in accordance with the present invention; 
         FIG. 14  shows a quadrature receiver portion of a WDT in accordance with the present invention; 
         FIG. 15  shows a Weaver receiver portion of a WDT in accordance with the present invention; 
         FIG. 16  shows a direct conversion receiver and a transmitter portion of a WDT in accordance with the present invention; 
         FIG. 17  show a direct conversion receiver and a dual-up conversion transmitter portion of a WDT in accordance with the present invention; 
         FIG. 18  shows a quadrature receiver and quadrature transmitter portion of a WDT in accordance with the present invention; 
         FIG. 19  shows a Weaver receiver and open-loop PLL transmitter portion of a WDT in accordance with the present invention; 
         FIG. 20  shows a direct conversion transmitter and diode receiver portion of a WDT in accordance with the present invention; 
         FIG. 21  shows a quadrature transmitter and diode receiver portion of a WDT in accordance with the present invention; 
         FIG. 22  shows a direct conversion transmitter and amplified backscatter receiver portion of a WDT in accordance with the present invention; and 
         FIG. 23  shows a quadrature transmitter and amplified backscatter receiver portion of a WDT in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , retailers place products  10  on shelves and indicate the pricing of the products  10  using display units or Wireless Display Terminals (WDTs)  12 . The display units  12  are typically located on shelf edges and display price and other information to aid the consumer as well as the store employees. Although in the present embodiment, the WDT  12  is shown positioned at the shelf edge, the WDT  12  can be located on peg hooks or near products as set forth in U.S. application Ser. No. 11/019,978, titled Wireless Display Tag Unit filed on even date herewith; U.S. application Ser. No. 11/019,660, titled An Error Free Method For Wireless Display Tag Initialization filed on even date herewith; and U.S. application Ser. No. 11/019,494, titled RF Backscatter Transmission With Zero DC Power Consumption filed on even date herewith, all of which are incorporated herein by reference. 
     Each of the WDTs  12  communicate via radio frequency with a wireless access point device or Access Point (AP)  14 . The AP  14  can be placed at any convenient location in the store that allows for acceptable radio communication with each of the WDTs  12  that the AP  14  supports. Any number of APs  14  can be used, depending on the number of WDTs  12  that are present in the store and the number of WDTs  12  that each AP  14  is assigned to support. The AP  14  is also in communication, either through a wire medium or wirelessly through the air, with an In-Store Server (ISS) computer, not shown. 
     Referring now to  FIG. 2   a , an active WDT  16  includes a solar cell  1602  and a battery  1604 . The solar cell  1602  may be used to charge the battery  1604  and/or power the circuits. The battery  1602  of the WDT  16  is coupled to an analog circuit unit  1608  that includes an analog-to-digital converter (ADC)  1610 . The analog circuit unit  1608  is coupled to a digital logic unit  1612 . Both the analog circuit unit  1608  and the digital logic unit  1612  are each coupled to an active transceiver  1614  that is coupled to an antenna  1620 . The active transceiver  1614  includes an active transmitter  1616  and an active receiver  1618 , each of which are coupled to a transmitter interface and a receiver interface, respectively, of the digital logic unit  1612 . 
     Referring now to  FIG. 2   b , a backscatter WDT  18  includes a solar cell  1802  and a battery  1804 . The solar cell  1802  may be used to charge the batter  1804  and/or power the circuits. The battery  1802  of the WDT  18  is coupled to an analog circuit unit  1808  that includes an analog-to-digital converter (ADC)  1810 . The analog circuit unit  1808  is coupled to a digital logic unit  1812 . Both the analog circuit unit  1808  and the digital logic unit  1812  are each coupled to and backscatter transceiver  1814  that is coupled to an antenna  1820 . The backscatter transceiver  1814  includes an backscatter transmitter  1816  and an backscatter receiver  1818 , each of which are coupled to a transmitter interface and a receiver interface, respectively, of the digital logic unit  1812 . 
     Referring now to  FIG. 2   c , the process of active and backscatter transmission begins at step  200 , At step  202 , the radio begins communication with the terminal using backscatter transmission mode and the terminal sends the carrier wave to the radio. At step  204 , it is determined if the radio can use digital backscatter transmission. If the radio can use digital backscatter transmission, at step  206 , it is determined that the radio is operating in a protocol environment that uses only two states. Then the process moves to step  212  as discussed below. If at step  204  it is determined that the radio cannot use digital backscatter transmission, then at step  208 , the radio uses analog backscatter transmission. At step  210 , it is determined that the radio is operating in a environment that has greater than two states. At step  212 , the radio modulates the carrier wave signal using changes in load impedance to represent the information that needs to be transmitted. At step  214 , it is determined if the information using backscatter transmission is complete. If so, then the process ends at step  216 ; if the transmission of the information is not complete, then the process returns to step  212  to continue encoding the carrier wave with the information using impedance modulation. 
     Referring now to  FIG. 3 , each WDT  12  acts as a radio transceiver and includes an antenna  20 ; an active transceiver unit  21 , which includes a receiver  22  and a transmitter  24 ; a digital logic component  26 ; and a backscatter transceiver  28 . The radio communication between the WDT  12  and the AP  14 , of  FIG. 1 , is accomplished using the receiver  22  and the transmitter  24 . For information that the WDT  12  is receiving, the receiver  22  takes the incoming radio information from the antenna  20  and processes the information in a manner that the digital logic  26  can use. For information that the WDT  12  is transmitting, the transmitter  24  takes the information from the digital logic  26  and processes the information so that the information can be sent wirelessly, via the antenna  20 , using radio waves. The receiver  22  and transmitter  24  are made primarily with analog circuits. In contrast, the logic  26  is made with digital circuits. 
     The active transceiver  21  of the WDT  12  includes the active transmitter  22  that allows greater range since the signal being transmitted can be larger in power compared to backscatter transmissions by the backscatter transceiver  28 . This is because the signal that is reflected in the backscatter transceiver  28  is limited by the signal that is received. If the signal received is small, then the signal transmitted will be small. 
     The active transmitter  24  transmits the signal using single conversion, dual-conversion, direct modulation of a VCO or PLL. However, the scope of the present invention is not limited by the techniques can be used for transmission The signal transmitted is not limited by the signal that was received. The power is limited by governmental regulations. 
     In one embodiment, the WDT  12  includes active circuits and devices, such as a super-heterodyne or direct conversion radio for communications. The active radio allows greater range since the receivers can be made electrically quieter. There are fundamental limits on how small of a radio signal can be recovered include the effects of thermal noise levels relative to the signal levels. The active transceiver  21  of the WDT  12  adds only a small amount of noise to the fundamental minimum level of noise, as set forth and computed in equation (1) below. The added noise can be less than 1 dB, which is in contrast to the minimum backscatter excess noise of about 114 dB.
 
 P   noise   =kTB=− 174 dBm in 1 Hz bandwidth
 
     k=Boltzmann&#39;s constant=1.38*10 −23  J/K 
     T=temperature, ° K 
     B=bandwidth, Hz 
     Backscatter Transceiver, Direct Conversion Receiver, and Direct-Conversion Transmitter 
     Referring now to  FIG. 3 , the WDT  12  includes a backscatter transceiver  28  and an active transceiver  21 ; the WDT  12 , in this embodiment, involves a direct-conversion receiver  22  and direct-conversion transmitter  24 . The direct conversion receiver  22  and the direct conversion transmitter  24  result in a reduced parts count; therefore, lower cost, and potentially lower power consumption. 
     The backscatter transceiver  28  has a receiver  30  that includes a diode  32 . The receiver  30  is known as a “crystal radio” and the diode  32  is a low-turn-on voltage device, such as a Schottky diode. The receiver  30  is used when the WDT  12  is deployed in an amplitude modulated (AM) communication environment. The incoming RF signal is rectified with the diode  32 . The rectified signal is sent to an audio amplifier from the Schottky diode and the incoming signal from the antenna  20  is AM using the Schottky diode. A filter, often a simple resistor-capacitor filter, is used to filter out the remaining RF, carrier, portion of the signal, leaving the lower-frequency modulating information. 
     Referring now to  FIG. 4 , in an alternative embodiment, wherein the WDT  12  is deployed in a frequency modulated (FM) communication environment, an FM discriminator receiver  40  replaces the receiver  30  of  FIG. 3 . The receiver  40  includes diodes  42  and a transformer  44 . The receiver  40  uses passive circuitry for the backscatter communications. Note that both circuits are passive. The receiver  40  changes or modulates the frequency of the high-frequency signal to the lower-frequency modulating information. The receiver  40  operates within the linear range of operating point of the diodes  42 . When the instantaneous frequency of the carrier signal from the AP 14  increases slightly, the output amplitude linearly increases. The inverse occurs when the frequency decreases slightly. Therefore, the FM signal is detected with a passive circuit. 
     An alternative embodiment includes a circuit called the ratio detector, wherein the instantaneous frequency operation causes an equivalent voltage in equal amplitude but opposite polarity across either side of the secondary. A tap is used to set the level. Therefore, the output is based on a ratio. Because it is a ratio, the undesired AM signal is rejected and only the desired FM signal is detected. 
     The impedance terminating the transmitter&#39;antenna can be in one of three general states: open, short, or the same impedance as the antenna&#39;s characteristic impedance as disclosed in the Related Applications filed filed on even date herewith entitled RF Backscatter Transmission with Zero DC Power Consumption. If the terminating impedance is an open, then the signal propagates without change. If the impedance terminating the antenna is equal to the antenna&#39;s characteristic impedance, then the power reflected from the antenna is as much as the antenna absorbs. The characteristic impedance is created electronically by allowing a controlled current from a controlled current source to flow through the diode. If the impedance terminating the antenna is a short (i.e. low impedance), then the power reflected from the antenna is approximately four times the value when connected to the antenna&#39;s characteristic impedance. 
     Referring again to  FIG. 3 , the direct conversion receiver  22  operates by amplifying, then mixing the frequency down to baseband. An optional low-noise amplifier (LNA)  220  increases the modulated RF signal to decrease the signal&#39;s susceptibility to noise. The LNA  220  is followed by a mixer  224 . The signal frequency of the local oscillator (LO)  222  is the same as the incoming RF signal. Therefore, the output frequency of the mixer  224  is the desired modulated signal. Optionally, there are filters to reject undesired signals (not shown). The filters may be before the LNA  220 , in between the LNA  220  and the mixer  224 , and finally after the mixer  224 . 
     The direct-conversion transmitter  24  includes a power amplifier  240  and is driven by the modulating signal which is fed to a mixer  244 . A LO&#39;s  242  frequency of the mixer  244  is the desired RF output frequency. The output of the mixer  244  is at the desired RF output frequency and the signal modulated. The output power is increased by the PA  240 . Then the signal that is generated by the PA  240  is fed to the antenna  20 . 
     Backscatter Transceiver, Super-Heterodyne Receiver, and Dual-Conversion Transmitter 
     Referring now to  FIG. 5 , in an alternative embodiment, a device  50  includes a backscatter transceiver  52 , a single-conversion active radio receiver  54 , and dual-conversion transmitter  56 . A single-conversion receiver with IF output is also called a super-heterodyne receiver. The backscatter transceiver  52  is similar in operation to the backscatter transceiver  28  of  FIG. 3 . 
     The single-conversion receiver  54  functions by converting the incoming signal to an intermediate frequency (IF). Since the IF is typically at a lower frequency, the filters are easier to implement. Furthermore, the gain of the device  542  is higher, so the overall system gain is larger. Moreover, the filters can be designed to provide rejection of the undesired image frequency. 
     The receiver  54  includes a LNA  540  at the input to decrease the desired signals&#39; susceptibility to noise. The LNA  540  output is fed to a mixer  542  which typically reduces the frequency. A local-oscillator (LO)  544  stimulates the other input port of the mixer  542 . The LO  544  is specifically chosen so that the output frequency of the mixer  542  is higher than the baseband signal. The active receiver  54  differs from the active receiver  22  of  FIG. 3  in that the difference in output frequency significantly changes the behavior of the active receiver  54 . The IF higher than the baseband prevents DC offsets, reduces IM 2  requirements, greatly reduces LO antenna radiation, to mention a few features. 
     The active transmitter  56  increases the output frequency in two steps. First, mixers  58  and  59  with its associated LO are used for each step. In alternative embodiments, order to reduce the circuitry requirements of the LO, the two LOs can be integer multiples of each other. That way, a divider can be driven by the larger frequency LO, and generate the smaller-frequency LO. A PA is used to increase the output power to drive the antenna. 
     Backscatter Transceiver, Image-Reject Receiver, and Dual-Conversion Transmitter 
     Referring now to  FIG. 6 , in another embodiment, the device  60  includes a backscatter transceiver  62 , an image-rejection receiver  64 , and a dual-conversion transmitter  66 . The backscatter transceiver  62  is similar in operation to the backscatter transceiver  28  of  FIG. 3 . 
     The receiver  64  is an image-rejection configuration, wherein the undesired image frequency is mathematically cancelled. The antenna  68  is connected to a LNA  642  to reduce noise. The output of the LNA  642  is connected to two mixers  644  and  646 . The LO of the mixers  644  and  646  are separated by 90°. In addition to the 90° separation caused by the LO, and additional 90° shift is introduced by shifting the output of mixer  646  by 90° using a delay block  648 . Thus, there is a 180° separation between the output of mixer  644  and the mixer  646 . Then the output from the mixer  644  and the mixer  646  are summed by connecting the outputs together. In this manner, the undesired image frequency is reduced or nearly eliminated. 
     The baseband from a digital logic unit  61  drives the input of the dual-conversion transmitter  66 . The baseband is separated into real and quadrature components. Each component drives each of the mixers  664 ,  666 , and  668 . The mixers  666  and  668  have the same LO frequency, but are separated by 90° whereas the LO frequency for the mixer  664  operates at a different frequency than the LO frequency of the mixers  666  and  668 . Thus, the phase of the LO for the mixers  666  and  668  is separated by 90°. Thereby the undesired sideband is reduced. The output frequency is lower then the RF output frequency. This is done because the circuitry to reduce the undesired sideband has better performance at lower frequencies. After the signal is generated, it is increased in frequency again, this time to the RF output frequency. A PA  662  is used to increase the output power as required to drive the antenna. 
     Backscatter Transceiver, Weaver Receiver, and Open-Loop Modulated Transmitter 
     Referring now to  FIG. 7 , in an alternative embodiment a device  70  is shown with a backscatter transceiver  72 , a Weaver Receiver  74 , and an Open-Loop Modulated Transmitter  76 . The backscatter transceiver  72  is similar in operation to the backscatter transceiver  28  of  FIG. 3 . The Weaver Receiver  74  is a variation of the image-rejection mixer, which is set forth above. The Weaver architecture uses quadrature LOs because the wide-band phase shifters are difficult to practically implement. 
     The Weaver receiver  74  takes the signal from an antenna  78  and uses a low noise amplifier (LNA)  740  optionally to decrease the signal&#39;s susceptibility to noise. Then that amplified signal is sent to a pair of mixers  742  whose LO is driven in quadrature. The output of the mixers  742  is sent to another pair of mixer  744  with their LO driven in quadrature. The outputs of the second pair of mixers  744  are then summed. As long as the quadrature phase is correct, and the gains matched, then the image is cancelled. Note that optional filters may be inserted, one between the top pair of mixers  742  and  744  and another between the bottom pair of mixers  742  and  744 . The filter is needed if the gain of the undesired sideband is large enough to cause circuit degradation. 
     The transmitter  76  uses the baseband signal and modulates a filter capacitor  764  of a Phase Lock-Loop  762 . By this means, the LO frequency is varied by the modulating signal. This is used to drive a PA. The PA increases the output power to drive the antenna. 
     Note that although only 4 combinations of active transceivers were described above, all can be mixed to form any of 16 combinations. 
     Circuits 
     Referring now to  FIG. 8 , an LNA  80  and an LNA  82  are shown with CMOS or BJT technologies, respectively. The source and emitter respectively is degenerated. This shifts the input impedance in addition to lowering the gain. An LC tank circuit is connected to the drain/collector respectively. 
     Referring now to  FIG. 9 , a mixer  90  and  92  can be also implemented in CMOS or BJT technologies, respectively. The popular “Gilbert Cell” technology is shown. However, other circuits can be used. 
     Hookup at the Antenna 
     Referring now to  FIG. 10 , a WDT  100  includes a transmitter  102  having an output connected to a receiver input  104 . Furthermore, an active radio  110  and backscatter receiver  106  share an antenna  108 . This is the preferred embodiment for a half-duplex radio where either the receiver or transmitter is on individually. Likewise, the backscatter and active radio are not on at the same time. Therefore, the power control on the not-currently used radio sections can be disabled. The not-currently used radio sections will not affect the reception or transmission of information. This is accomplished by removing power from the unused circuitry. The unused circuitry will then be in a high-impedance state since g m  is related to current through the devices. The high-impedance state has little effect on the desired circuitry that is in an “on” condition. Device M 2   110  is used as a switch to isolate the transmitter during receive mode, and to connect the tank to the power supply during transmit mode. 
     In an alternative embodiment, a full duplex radio can be implemented with the addition of filters and/or combiners. The additional elements are place in series with the inputs of the receivers and/or the output of the transmitters. In this way, the transmitter&#39;s signal does not interfere with the receiver, and the receiver picks up the desired signal, not its own undesired transmitters&#39; signals. In addition to interference, the undesired transmit signal can overload the receiver. This has the effect of distorting the desired received signal, or worse yet overwhelming it completely so it can not be received. Another less-often-seen effect is the increase of the effective receiver noise figure. The undesired transmitter is in effect noise. The amount of undesired transmit signal that is picked up by its own corresponding receiver increases the effective noise floor. Therefore, the desired signals are increasingly difficult to receive. 
     Isolated Sub Blocks 
     In an alternative embodiment, a WDT can include an active transceiver without the backscatter transceiver. Referring now to  FIG. 11 , an active transmitter is shown coupled to an antenna. The active transmitter allows greater range since the signal being transmitted can be larger in power compared to backscatter transmission. This is because the signal that is reflected in a backscatter transmitter is limited by the signal that is received. If the signal received is small, then the signal transmitted will be small. There are no significant exceptions to this rule. An active transmitter does not use backscatter transmission techniques. The signal is received, and then transmitted using single conversion, dual-conversion, direct modulation of a VCO or PLL. However, other techniques can be used. All the techniques are well understood by one skilled in the art. The signal transmitted is not limited by the signal that was received. The power is limited by governmental regulations. 
     Referring again to  FIG. 11 , a direct-up conversion active transmitter is shown. The digital logic outputs a modulated signal. The mixer up-converts the signal to the desired radio frequency. The power amplifier increases the power to the desired level. 
     Referring now to  FIG. 12 , an alternative embodiment is a quadrature modulator. The digital logic outputs two signals in quadrature. Two mixers up-convert the signals. The local oscillators for each mixer are 90° out of phase. The outputs of the two mixers are then combined in order to mathematically cancel the image frequency. 
     The active receiver is made from active circuits and devices. The active radio allows greater range since the receivers can be made electrically quieter. The active radio circuits used in the WDTs can be super heterodyne or direct conversion. However, other techniques can be used, such as regenerative and super-regenerative receivers. 
     Referring now to  FIG. 13 , a direct-conversion receiver includes an antenna, a LNA, a mixer, and digital logic. The signal is collected at the antenna. The LNA amplifies the signal to increase the signal strength with minor loss in signal to noise ratio. The mixer down-converts the signal from radio frequencies to baseband frequencies by mixing it with the LO. The digital logic then processes the baseband signal and extracts the useful information. 
     Referring now to  FIG. 14 , an alternative embodiment for an active receiver is a quadrature receiver. The LNA amplifies the signal to increase the signal strength with minor loss in signal to noise ratio. The mixers down-converts the signal from radio frequencies to baseband frequencies by mixing it with the LO. The two LOs are separated in phase by 90°. The delay section further delays one mixer output signal by 90°. Therefore, if the image signal is presented at the antenna, it is mathematically rejected. 
     Referring now to  FIG. 15 , an alternative embodiment for a receiver is a Weaver Receiver. A Weaver Receiver minimizes the problems associated with a direct-conversion receiver, including: dc-voltage offset, LO re-radiation, and high-IIP2 (second-order input-intercept point) requirements. The Weaver Receiver is a variation of the image-rejection mixer. Wide-band phase shifters are difficult to practically implement. Therefore, the Weaver architecture uses quadrature LOs, 
     Referring now to  FIG. 16 , an alternative embodiment for an active transceiver includes the combination of a direct-conversion receiver and a direct-conversion transmitter. 
     Referring now to  FIG. 17 , an alternative embodiment for an active transceiver includes the combination a direct conversion-receiver and dual-up-conversion transmitter. 
     Referring now to  FIG. 18 , an alternative embodiment for an active transceiver includes the combination of a quadrature receiver and a quadrature. 
     Referring now to  FIG. 19 , an alternative embodiment for an active transceiver includes the combination of a Weaver Receiver and an open-loop modulated PLL transmitter. 
     Referring now to  FIG. 20 , an alternative embodiment for an active transceiver includes the combination of an active transmitters with a diode receiver. 
     Referring now to  FIG. 21 , an alternative embodiment for an active transceiver includes the combination of a quadrature transmitter and a diode receiver. 
     In an alternative embodiment, the diode receiver&#39;s performance can be improved by adding an amplifier between the antenna and the diode to amplify the signal. 
     Referring now to  FIG. 22 , an alternative embodiment includes the combination of a direct-conversion up-converter transmitter and an amplified backscatter receiver. Note that there is potentially a feedback loop through the receive section of the radio to the transmit section. This will have to be broken electrically (analog or digitally) or mechanically during the WDT transmit time to eliminate this feedback. This can be implemented with a CMOS switch in the receive path. The switch is opened up during the receive time to isolate the receiver from the transmitter. 
     Referring now to  FIG. 23 , an alternative embodiment includes the combination of a quadrature transmitter and an amplified backscatter receiver. 
     Having fully described various embodiment and various alternatives, those skilled in the art will recognize, given the teachings herein that numerous alternatives and variations exist that do not depart from the invention and it is therefore intended that the invention not be limited by the forgoing description.