Abstract:
A single voltage controlled oscillator (VCO) running at a closed loop controlled fundamental frequency whereby the harmonic products generated by the VCO are used for both transmission and reception. The use of a single VCO and associated supporting circuitry for both transmit and receive portions of the circuit serves to reduce the cost of the transceiver. The radio transceiver comprises a single synthesized oscillator having high harmonic content. The frequency conversions in the receiver make use of the harmonic frequency components of the same oscillator used in performing modulation during transmission.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a radio frequency (RF) communication transceiver, and more particularly relates to a transceiver, such as a Frequency Shift Keying (FSK) transceiver, which employs Time Division Duplexing (TDD) to support full duplex wireless communications. 
     BACKGROUND OF THE INVENTION 
     Currently, high frequency radio frequency (RF) communications are becoming more and more prevalent in the world today. Products touting wireless RF communication links are becoming increasingly popular among consumers. Today, there are an increasing number of new products, including redesigned existing ones, being designed incorporating wireless RF links. 
     Most of the RF communication circuits found in wireless products employ some form of oscillator and synthesizer circuitry in their transceivers. Due to the explosive consumer demand for products sporting wireless communication links, there is a need for low cost transceivers that are suitable for mass manufacture. 
     In addition, the sharing of certain elements between several functions in radio communications transceiver provides a reduction in cost and size, both of which are desirable. 
     Radio communication TDD transceivers that share at least one element are known in the art. U.S. Pat. No. 5,689,819, issued to Nishimura et al., discloses a transceiver wherein a quartz crystal oscillator is used as a common reference signal for the local oscillators in the receive section of the transceiver and for the generation of the carrier signal for use in the transmit portion of the transceiver. The output signal of the quartz crystal oscillator is input to three separate VCOs used to generate the LO signal for the transmitter, the first LO signal for the first IF stage and the second LO signal for the second IF state. A disadvantage of this transceiver is that although the quartz crystal oscillator is shared, an independent VCO is used to generate each LO signal. the quartz crystal oscillator is shared, an independent VCO is used for the generation of each LO signal. 
     U.S Pat. No. 5,123,008, issued to Beesley, discloses a time division duplex transceiver wherein the first receiver local oscillator in a dual conversion superheterodyne receiver doubles as the frequency source for the transmitter. A selected harmonic of a separately generated second receiver local oscillator signal is used to derive the LO for the second receiver. Here too, a disadvantage is that multiple oscillators are required to generate the LOs used in the transceiver. 
     SUMMARY OF THE INVENTION 
     The present invention provides the ability to create a complete communications transceiver utilizing a single oscillator for both transmitting and receiving without comprising performance over other designs that incorporate more than one oscillator. The radio transceiver comprises a single synthesized oscillator having high harmonic content. The transceiver also comprises a power amplifier on the transmit path. On the receive path, the transceiver comprises a low noise amplifier (LNA), a plurality of frequency conversion mixers and a demodulator. 
     A key feature of the present invention includes the implementation of a single voltage controlled oscillator (VCO) running at a closed loop controlled fundamental frequency whereby the harmonic products generated by the VCO are used for both transmission and reception. The use of a single VCO and associated supporting circuitry for both transmit and receive portions of the circuit serves Lo reduce the cost of a transceiver. 
     It is important to note that the principle of utilizing the fundamental and harmonic frequencies of a single oscillator can be applied to a transceiver regardless of the type of modulation scheme employed, e.g., FSK, BPSK, ASK, M-ary modulation, QAM, QPSK, etc. 
     There is provided in accordance with the present invention a time division duplex (TDD) transceiver comprising oscillation means for generating a fundamental frequency and a plurality of harmonic frequencies, wherein the oscillation means employs a single voltage controlled oscillator (VCO), transmitter means for generating a transmit output signal in response to a Tx data input signal, the transmitter means utilizing one or more carrier signals derived from the oscillation means and receiver means for receiving and demodulating a received. signal and in response thereto, generating an Rx output signal, the receiver means utilizing one or more local oscillator signals derived from the oscillation means. 
     The oscillation means comprises a frequency synthesizer coupled to the output of the VCO, the frequency synthesizer having a control input and a reference input, a loop filter connected to the output of the frequency synthesizer, an adder for receiving the Tx data input and for generating a control signal input to the VCO and wherein the VCO is adapted to generate the fundamental frequency and the plurality of harmonic frequencies. 
     The oscillation means comprises a frequency synthesizer coupled to the output of the VCO, the frequency synthesizer having a control input and a reference input, a loop filter connected to the output of the frequency synthesizer, an adder for receiving the Tx data input and generating a control signal input to the VCO and an equalizer coupled to the output of the VCO for providing the desired frequency spectra output for the fundamental frequency and the plurality of harmonic frequencies. 
     The oscillation means comprises a frequency synthesizer coupled to the output of the VCO, the frequency synthesizer having a control input and a reference input, a loop filter connected to the output of the frequency synthesizer, the loop filter operative to generate a control signal input to the VCO and wherein the VCO is adapted to generate the fundamental frequency and the plurality of harmonic frequencies. 
     The oscillation means comprises a frequency synthesizer coupled to the output of the VCO, the frequency synthesizer having a control input and a reference input, a loop filter connected to the output of the frequency synthesizer, the loop filter operative to generate a control signal input to the VCO and an equalizer coupled to the output of the VCO for providing substantially flat frequency response output for the fundamental frequency and the plurality of harmonic frequencies. 
     The transmitter means is adapted to generate either an FSK signal or a BPSK signal. 
     There is also provided in accordance with the present invention a time division duplex (TDD) transceiver comprising oscillation means for generating a fundamental frequency, and its harmonics, wherein the oscillation means employs a single voltage controlled oscillator (VCO), transmitter means for generating a transmit output signal modulated by a Tx data input signal, the transmitter means utilizing one of the harmonic frequency components derived from the oscillation means and receiver means for receiving and demodulating a received signal so as to generate an Rx output signal, the receiver means utilizing the fundamental and the harmonics derived from the oscillation means. 
     The transmitter means is adapted to generate either an FSK signal, a Binary Phase Shift Keying (BPSK) signal, a Quadrature Phase Shift Keying (QPSK) signal or a Quadrature Amplitude Modulation (QAM) signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a block diagram illustrating a digital transceiver utilizing a single oscillator constructed in accordance with the present invention; 
     FIG. 2 is a block diagram illustrating the synthesized VCO/modulator portion of the transceiver adapted for FSK modulation in more detail; 
     FIG. 3 is a schematic diagram illustrating an example implementation of the voltage controlled oscillator in more detail; and 
     FIG. 4 is a block diagram illustrating a BPSK transceiver utilizing a single oscillator constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Notation Used Throughout 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                             Term 
                 Definition 
               
               
                   
                   
               
             
             
               
                   
                 ASK 
                 Amplitude Shift Keying 
               
               
                   
                 BPF 
                 Band Pass Filter 
               
               
                   
                 BPSK 
                 Binary Phase Shift Keying 
               
               
                   
                 FSK 
                 Frequency Shift Keying 
               
               
                   
                 IF 
                 Intermediate Frequency 
               
               
                   
                 LNA 
                 Low Noise Amplifier 
               
               
                   
                 LO 
                 Local Oscillator 
               
               
                   
                 PLL 
                 Phase Locked Loop 
               
               
                   
                 PSK 
                 Phase Shift Keying 
               
               
                   
                 QAM 
                 Quadrature Amplitude Modulation 
               
               
                   
                 QPSK 
                 Quadrature Phase Shift Keying 
               
               
                   
                 RF 
                 Radio Frequency 
               
               
                   
                 TDD 
                 Time Division Duplex 
               
               
                   
                 VCO 
                 Voltage Controlled Oscillator 
               
               
                   
                   
               
             
          
         
       
     
     A block diagram illustrating an FSK transceiver utilizing a single oscillator constructed in accordance with the present invention is shown in FIG.  1 . The transceiver, generally referenced comprises a transmit portion and a receive portion. The multiple harmonic synthesized VCO/modulator component is shared by both transmit and receive portions. In addition, the transmit portion comprises a power amplifier  16 . 
     A common design goal for communication transceivers is to reduce the cost of the transceiver. This can be accomplished by reducing the component count. In a time division duplex (TDD) transceiver, the transmit path and receive path are not active at the same time. Thus, the receiver does not receive while the transmitter is transmitting and vice versa. 
     A key feature of the present invention is that it is possible to use the same oscillator for receiving and transmitting when implementing a TDD based transceiver. The TDD based transceiver may be adapted to transmit and receive any type of digital modulation scheme. Within the TDD transceiver, the same oscillator can be shared since reception and transmission do not occur simultaneously. 
     Note that the transceiver shown in FIG. 1 is adapted to transmit and receive FSK signals and is presented as an example to illustrate the principles of the present invention. One skilled in the RF communication arts can apply the principle of utilizing one or more harmonics generated by a single oscillator to other transmitter, receiver or transceiver configurations employing various types of modulation. 
     The data to be transmitted, i.e., Tx data, is input to the synthesized VCO/modulator that functions to generate a modulated transmit signal. The output signal is generated utilizing the 3 rd  harmonic of the output of the single oscillator as the carrier signal for the transmitter. The output of the power amplifier is input to the single pole double throw switch  13  which functions to switch the coupling of the antenna  12  between the receive portion and the transmit portion of the transceiver  10 . In FIG. 1, the switch  13  is shown in the transmit position whereby the output of the power amplifier is input to the band pass filter (BPF)  14 . The signal output of the BPF  14  is input to the antenna  12 . 
     The receive signal path of the transceiver  10  comprises the antenna  12 , BPF  14 , switch  13 , low noise amplifier (LNA)  20 , BPF  22 , down converter (mixer)  24 , first intermediate frequency (IF) BPF  26 , second mixer  28 , second IF BPF  30  and demodulator  34 . The second harmonic frequency of the output of the single oscillator is used as the local oscillator signal for the down converter mixer  24 . The fundamental frequency of the output of the single oscillator is used as the local oscillator signal for the second mixer  28 . 
     To aid in understanding the principles of the present invention, examples of actual frequencies will be given. For example, let the oscillator fundamental frequency within the synthesized VCO equal 800 MHz. The second and third harmonic frequencies are, therefore, 1.6 and 2.4 GHz, respectively. During transmission, the third harmonic, i.e., 2.4 GHz, is used as the carrier signal for modulating the input Tx data so as to generate a FSK output signal. 
     During reception of a signal at 800 MHz, the single oscillator within the synthesized VCO is adapted to generate a signal whose fundamental frequency is shifted by approximately 1 MHz, i.e., to 799 MHz. Thus, the second harmonic frequency is 1,598 MHz. This signal is applied to the down converter  24 . The fundamental frequency of 799 MHz is applied to the second mixer  28 . The result is that a 2.4 GHz received input signal is mixed down to 3 MHz. 
     This signal is then input to the demodulator  34  which functions to reproduce, i.e., recover, the transmitted baseband signal. The demodulator may comprise any suitable circuit appropriately adapted to demodulate the received signal in accordance with the modulation technique employed by the transceiver. For example, in a transceiver designed to receive a FSK modulated signal, the demodulator may comprise a discriminator and a slicer or, alternatively, a PLL and a slicer, both of which may be used to generate the transmitted baseband signal. The discriminator may comprise a quadrature detector or equivalent circuit. 
     A block diagram illustrating the synthesized VCO/modulator portion of an FSK transceiver in more detail is shown in FIG.  2 . The synthesized VCO, generally referenced  18 , is capable of generating a plurality of harmonic frequencies. The synthesized VCO comprises a VCO  42 , frequency synthesizer or phase locked loop (PLL)  46  and loop filter  48 . Modulation of the input Tx data is performed via summer  44  placed in the loop between the loop filter and the VCO. The synthesized VCO is PLL controlled in order to generate an accurate fundamental frequency. The fundamental frequency can be selected via the digital control input to the PLL  46 . A digital reference frequency is also input to the PLL  46 . 
     The output of the frequency synthesizer  46  is input to the loop filter  48 . The output of the loop filter is input to summer  44  which functions to modify the output of the loop filter in accordance with the input Tx data thus performing FSK modulation. To frequency modulate the transmitter, a baseband signal is summed with the control voltage intended for the VCO and output by the loop filter, hence forming a modulator. The output of the summer  44  is input to the VCO  42 . 
     Note that synthesized VCO loops comprising the PLL  46 , loop filter  48 , summer  44  and VCO  42  are well known in the art. One skilled in the art could modify or substitute the circuit shown in FIG. 2 with other equivalent configurations without departing from the scope of the present invention. 
     The output of the VCO  42  is optionally input to an equalizer  40  that functions to obtain the desired power levels for the various harmonics generated by the VCO  42 . The output of the VCO comprises the fundamental frequency and N harmonics. The equalizer can be constructed to equalize any or all of the harmonic frequencies output by the VCO. The equalizer is added to the VCO output in order to generate higher harmonic products which are utilized as the carrier frequency for transmission and as one or more local oscillator signals for receiving. 
     The Tx data signal comprises logic levels properly adjusted so as to achieve the desired frequency deviations in the modulated signal output by the transmitter. In the case of FSK, the Tx data signal is added onto the control signal at the tuning port of the oscillator that is described in more detail hereinbelow. 
     The resultant modulation bandwidth at the output of the transmitter is multiplied by the transmitter path multiplication ratio. In the example presented herein, the transmitter path multiplication ratio is equal to 3. In other words, the frequency modulation bandwidth on the third harmonic output by the VCO is three times wider than that produced by the data signal using the fundamental frequency of the VCO. 
     During transmission, the VCO  42  is modulated by the Tx data signal. The transmitted frequency is the center frequency of the VCO multiplied by an integer factor K. The transmitted carrier frequency may be expressed by the multiplication shown below in Equation 1. 
     
       
           F   TX   =K·F   FUND   (1) 
       
     
     where 
     F TX  is the frequency of the transmitter output 
     F FUND  is the fundamental frequency of the VCO 
     K is the ratio between the transmit frequency and the fundamental frequency of the VCO. 
     The modulated and multiplied product of the VCO is then amplified via power amplifier (FIG. 1) and applied to the antenna through a switch, which enables the receiver and transmitter to share a single antenna and band pass filter. 
     During reception, the VCO is not modulated, and the first and second local oscillator signals are generated using the harmonic products output of the VCO to provide total frequency conversion that converts the received frequency to the center frequency of the demodulator  34  (FIG.  1 ). 
     Note that the present invention is not limited to transceivers having a particular number of conversions. The use of more than one frequency conversion in the receiver, as utilized in the transceiver example presented herein, however, provides improved suppression of interference and improved image frequency rejection. 
     The frequency to which the receiver is tuned can be expressed by the sum shown below in Equation 2. 
     
       
           F   REC   =M·   FUND   +N·F=F   IF   (2) 
       
     
     where 
     M and N are integers and 
     F REC  is the received frequency 
     F FUND  is the fundamental frequency of the VCO 
     M·F FUND  is the first local oscillator frequency 
     N·F FUND  is the second local oscillator frequency 
     F IF  is the intermediate frequency (IF), i.e., demodulator frequency 
     Note that F REC  represents the received frequency when the transceiver performs double conversion with lower side injection in both conversions. One skilled in the art, however, can apply the present invention to a transceiver with any number of conversions, and employ upper side injections as well. 
     During the time the transceiver is receiving, the modulation port is inactive. With reference to FIG. 1, the received signal is delivered from the antenna, through the switch to the LNA, then filtered and converted to the first Intermediate Frequency (IF) via mixer  24 . The signal output of the mixer is then filtered again and converted by the second mixer  28  to the second IF which is also the frequency of the demodulator. 
     The demodulator functions to demodulate the converted signal to yield the original baseband signal. Since reception and transmission do not occur simultaneously, the same synthesized VCO may be used for both. During the time the transceiver is transmitting, the synthesized VCO is programmed (via the digital control input) to generate a frequency as expressed in Equation 3:                F   SYNTH     =       F   TX     K             (   3   )                                
     In the transceiver example presented herein, the value of K is equal to 3. During the time the transceiver is receiving, the synthesized VCO is programmed to generate a frequency as expressed in Equation 4 below.                F   SYNTH     =         F   REC     -     F   IF         M   +   N               (   4   )                                
     A schematic diagram illustrating an example implementation of the voltage controlled oscillator in more detail is shown in FIG.  3 . The VCO circuit, generally referenced  50 , comprises two resonators adapted to produce oscillations. The output of the summer  44  (FIG. 2) is input to the V in  terminal. The signal is coupled via resistor  72  to a resonator comprising inductor  54  and varactor  52 . The signal is input through capacitor  56  to the base of NPN transistor  64  and to the combination of capacitors  66 ,  68 . The emitter of transistor  64  is connected to ground via resister  70 . The collector is connected to a resonator comprised of inductor  60  and capacitor  62 , while capacitor  58  connected to ground provides filtering. The two resonators in the VCO  50  are constructed so that their respective resonant frequencies are skewed and do not overlap. 
     A block diagram illustrating an FSK transceiver utilizing a single oscillator constructed in accordance with the present invention is shown in FIG.  4 . The transceiver, generally referenced  110 , comprises a transmit portion and a receive portion. The multiple harmonic synthesized VCO/modulator component  88  is shared by both transmit and receive portions. The transceiver  110  is similar in construction to the transceiver of FIG. 1, the difference being the addition of a modulator  86  adapted to perform some type of amplitude shift keying (ASK) or phase shift keying (PSK), e.g., BPSK, QPSK, QAM, etc. In addition, the transmit portion comprises a power amplifier  84 . 
     The data to be transmitted, i.e., Tx data, is input to the modulator  86 . The output of the modulator is generated utilizing the 3 rd  harmonic of the output of the single oscillator as the carrier signal for the transmitter. The modulated signal is input to power amplifier  84 . The output of the power amplifier is input to the single pole double throw switch  106  which functions to switch the coupling of the antenna  80  between the receive portion and the transmit portion of the transceiver  110 . The antenna is coupled to the switch  106  via BPF  82 . 
     The receive signal path of the transceiver  110  comprises the antenna  80 , BPF  82 , switch  106 , low noise amplifier (LNA)  90 , BPF  92 , down converter (mixer)  94 , first intermediate frequency (IF) BPF  96 , second mixer  98 , second IF BPF  100  and demodulator  104 . Note that the demodulator  104  is adapted to be appropriate for the type of modulation employed by the transceiver. The second harmonic frequency of the output of the single oscillator is used as the local oscillator signal for the down converter mixer  94 . The fundamental frequency of the output of the single oscillator is used as the local oscillator signal for the second mixer  98 . 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.