Abstract:
The invention provides a system and method of generating a number of inputs to the mixer elements in a direct conversion (homodyne) receiver configuration using Virtual Local Oscillator (VLO) techniques. These generated inputs meet the requirement that they must have a fixed and stable phase-relationship, as well as being correctly related in terms of their power spectra relative to the operating radio (RF), intermediate (IF), and baseband frequencies of the system, and that when applied to the mixer elements they permit the mixer elements to create internally the effect of applying the Local Oscillator signal at a suitable frequency

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to communications, and more specifically to up-conversion and down-conversion, being a frequency generation device providing waveforms for use in a Virtual Local Oscillator-base system.  
           [0002]    Many communication systems up-convert electromagnetic signals from baseband to higher frequencies for transmission, and subsequently down-convert those high frequencies back to their original frequency band when they reach the receiver, processes known as up-conversion and down-conversion (or modulation and demodulation) respectively. The original (or baseband) signal, may be, for example, data, voice or video. These baseband signals may be produced by transducers such as microphones or video cameras, be computer generated, or transferred from an electronic storage device. In general, the high frequencies provide longer range and higher capacity channels than baseband signals, and because high frequency radio frequency (RF) signals can propagate through the air, they can be used for wireless transmissions as well as hard wired or fibre channels.  
           [0003]    All of these signals are generally referred to as radio frequency (RF) signals, which are electromagnetic signals; that is, waveforms with electrical and magnetic properties within the electromagnetic spectrum normally associated with radio wave propagation.  
           [0004]    Wired communication systems which employ such modulation and demodulation techniques include computer communication systems such as local area networks (LANs), point to point signalling, and wide area networks (WANs) such as the Internet. These networks generally communication data signals over electrically conductive or optical fibre channels. Wireless communication systems which may employ modulation and demodulation include those for public broadcasting such as AM and FM radio, and UHF and VHF television. Private communication systems may include cellular telephone networks, personal paging devices, HF (high frequency) radio systems used by taxi services, microwave backbone networks, interconnected appliances under the Bluetooth standard, and satellite communications. Other wired and wireless systems which use RF up-conversion and down-conversion would be known to those skilled in the art.  
           [0005]    For cellular telephones, for example, it is desirable to have transmitters and receivers (which may be referred to in combination as a transceiver) which can be fully integrated onto inexpensive, low power, integrated circuits (ICs).  
           [0006]    As frequencies of interest in the wireless telecommunications industry (especially low-power cellular/micro-cellular voice/data personal communications systems) have risen above those used previously (approximately 900 MHz) into the 1 GHz-5 GHz spectrum, the desire to implement low-cost, power efficient receivers and transmitters has led to intensive research into the use of highly integrated designs, an increasingly important aspect for portable systems, including cellular telephone handsets.  
           [0007]    Several attempts at completely integrated transceiver designs have met with limited success. Other RF receiver topologies exist, such as image rejection architectures, which can be completely integrated on a chip, but lack in overall performance. Although many receivers use the “super-heterodyne” topology, which provides excellent performance, this does not meet the desired level of integration for modern wireless systems.  
           [0008]    Direct conversion architectures demodulate RF signals to baseband in a single step, by mixing the RF signal with a local oscillator signal at the carrier frequency of the RF signal. There is therefore no image frequency, and no image componerts to corrupt the signal. Direct-conversion receivers offer a high level of integratability, but also have several important problems. Hence, direct conversion receivers have thus far proved useful only for signalling formats that do not place appreciable signal energy near DC after conversion to baseband.  
           [0009]    A typical direct conversion or homodyne receiver is shown in FIG. 1. The RF band pass filter (BPF 1 )  102  first filters the signal coming from the antenna  100  (this band pass filter  102  may also be a duplexer). A low noise amplifier  104  is then used to amplify the filtered antenna signal, increasing the strength of the RF signal and reducing the noise figure of the receiver.  
           [0010]    The signal is then split into its quadrature components and down-converted to baseband in a single stage using mixers MI  110  and MQ  120 , and orthogonal signals generated by local oscillator (LO)  132  and 90 degree phase shifter  130 . LO  132  generates a regular, periodic signal which is tuned to the carrier frequency of the incoming wanted signal rather than a frequency offset from the carrier as in the case of the super-heterodyne receiver. The signals coming from the outputs of MI  110  and MQ  120  are now at baseband, that is, having a carrier frequency of 0 Hz. The two signals are next filtered using low pass filters LPFI  112  and LPFQ  122 , are amplified by gain-controlled amplifiers AGCI  114  and AGCQ  124 , and are digitized via analog to digital converters ADI  116  and ADQ  126 .  
           [0011]    Direct conversion RF receivers as illustrated in FIG. 1 have several advantages over super-heterodyne systems in terms of cost, power consumption, and level of integration, however, there are also several serious problems with direct conversion. These problems include:  
           [0012]    noise near baseband (that is, 1/f noise) which corrupts the desired signal. The term “1/f noise” is used to describe a number of types of noise that are greater in magnitude at lower frequencies than at higher frequencies (typically, their magnitude increases roughly with the inverse of the signal frequency);  
           [0013]    local oscillator (LO) leakage in the RF path that creates DC offsets in the down-converted (base-band) output signal. As the LO frequency is the same as the incoming signal being demodulated, any leakage of the LO signal through the mixers  110 ,  120  to their RF port will fall directly into the desired signal&#39;s band and be down-converted to baseband as well;  
           [0014]    local oscillator (LO) leakage into the RF path that causes desensitization. Desensitization is the reduction of desired signal gain as a result of receiver reaction to an undesired signal. The gain reduction is generally due to overload of some portion of the receiver, such as the AGC circuitry  40 ,  42  resulting in suppression of the desired signal because the receiver will no longer respond linearly to incremental changes in input voltage.  
           [0015]    noise inherent to mixed-signal integrated circuits corrupts the desired signal; and  
           [0016]    large on-chip capacitors used as high-pass filters are required to remove unwanted noise and signal energy near DC, which makes integratability expensive. These capacitors are typically placed between the mixers  114 ,  116  and the low pass filters  136 ,  138 .  
           [0017]    What is needed is a simpler and more satisfactory means of generating the signals required for certain Local Oscillator implementations.  
         BRIEF SUMMARY OF THE INVENTION  
         [0018]    The invention provides a simplified and effective system and method for generating a number of inputs to the mixer elements of a direct conversion (homodyne) receiver configuration which uses certain Local Oscillator techniques.  
           [0019]    In this regard, Virtual Local Oscillators are used to provide the equivalent of a local oscillator without using frequency generators having significant spectral components (power) in the input frequency or intermediate frequencies of the receiver circuit, thereby mitigating some of the disadvantages listed above. Our co-pending PCT application (WO0117122: Improved Method and Apparatus for Up- and Down-Conversion of Radio Frequency (RF) Signals, LING, YANG (CA); WONG, LAWRENCE (CA); MANKU, TAJINDER (CA).) describes preferred implementations and relevant sections are included in the detailed description for ease of reference.  
           [0020]    In the implementation of a system using a Virtual Local Oscillator, the circuit that generates the various time-varying signals or waveforms required to operate the VLO invention presents significant design challenges. Designs have been produced which are sufficient to serve the purpose, but they tend to be complex and have higher power consumption.  
           [0021]    The circuit that generates the various time-varying signals or waveforms are required to have a fixed and stable phase-relationship, as well as being correctly related in terms of their power spectra relative to the operating radio (RF), intermediate (IF), and baseband frequencies of the system. Such waveforms, when applied to the mixer, permit the mixer to create internally the effect of applying the Local Oscillator signal at the required frequency. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0022]    Preferred embodiments will be described with reference to the following figures.  
         [0023]    [0023]FIG. 1 represents a typical receiver architecture of a direct conversion or homodyne receiver as known in prior art.  
         [0024]    [0024]FIGS. 2A and 2B illustrate the Virtuat Local Oscillator concept for which the invention is suited.  
         [0025]    [0025]FIG. 3 is a diagram of a preferred embodiment of the invention.  
         [0026]    [0026]FIG. 4 illustrates the various time-varying signals or waveforms produced from the circuit of FIG. 3.  
         [0027]    [0027]FIG. 5 is a diagram of a further preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    Since it is helpful to have some understanding of the concepts of a Virtual Local Oscillator system, we have for completeness, incorporated a brief description of the Virtual Local Oscillator, the subject of a co-pending PCT application (WO0117122: Improved Method and Apparatus for Up- and Down-Conversion of Radio Frequency (RF) Signals, LING, YANG (CA); WONG, LAWRENCE (CA); MANKU, TAJINDER (CA)).  
         [0029]    The Virtual Local Oscillator is concerned with the generation of signals used in the conversion process which have properties that solve the image-rejection problems associated with heterodyne receivers and transmitters and the LO-leakage and 1/f noise problems associated with direct conversion receivers and transmitters.  
         [0030]    A circuit which addresses the problems outlined above, is presented as a block diagram in FIG. 2A. This figure presents a balanced modulator or demodulator  270  in which an input signal x(t) is mixed with two synthesized signals (labelled φ 1  and φ 2 ) which are irregular and vary in the time domain, to effect the desired modulation or demodulation. The two mixers M 1   272  and M 2   274  are standard mixers known in the art, having the typical properties of an associated noise figure, linearity response, and conversion gain. The selection and design of these mixers would follow the standards known in the art, and could be, for example, double balanced mixers. Although this figure implies various elements are implemented in analogue form they can be implemented in digital form.  
         [0031]    The two synthesizers  276  and  278  generate two time-varying functions φ 1  and φ 2  that mixed together within the mixer circuit comprise a virtual local oscillator (VLO) signal. These two functions have the properties that their product emulates a local oscillator (LO) signal that has significant power at the carrier frequency, but neither of the two signals has a significant level of power at the frequency of the LO being emulated. As a result, the desired modulation or demodulation is affected, but there is no LO signal to leak into the RF path.  
         [0032]    The representation in FIG. 2A is exemplary, as any two-stage or multiple stage mixing architecture may be used to implement the invention. As well, the synthesizer for generating the time-varying mixer signals φ 1  and φ 2  may comprise a single device, or multiple devices.  
         [0033]    In current receiver and transmitter technology, frequency translation of an RF signal to and from baseband is performed by multiplying the input signal by regular, periodic, sinusoids. If one multiplication is performed, the architecture is said to be a direct-conversion or homodyne architecture, while if more than one multiplication is performed the architecture is said to be a heterodyne or super-heterodyne architecture. Direct-conversion transceivers suffer from LO leakage and 1/f noise problems which limit their capabilities, while heterodyne transceivers require image-rejection techniques which are difficult to implement on-chip with high levels of performance.  
         [0034]    The problems of image-rejection, LO leakage and 1/f noise in highly integrated transceivers can be overcome by using more complex signals than simple, regular, periodic, sinusoids in the frequency translation process. These signals have tolerable amounts of power at the RF band frequencies both in the signals themselves and in any other signals produced during their generation.  
         [0035]    The preferred criteria for selecting such functions φ 1  and φ 2  are:  
         [0036]    (i) for the signal x(t) to be translated to baseband, φ 1 (t)*φ 2 (t) must have a frequency component at the carrier frequency of x(t);  
         [0037]    (ii) in order to minimize spurious response problems, φ 1 (t)*φ 2 (t) must have less than a tolerable amount energy at frequencies other than the carrier frequency of x(t) or at least far enough away that these image frequencies can be significantly filtered on-chip prior to down-conversion;  
         [0038]    (iii) in order to minimize LO leakage problems, the signals φ 1  and φ 2  must not have significant amounts of power in the RF output signal bandwidth. That is, the amount of power generated at the output frequency should not effect the overall system performance of the transmitter or receiver in a significant manner;  
         [0039]    (iv) also to avoid LO leakage found in conventional direct conversion and directly modulated topologies, the signals required to generate φ 1  and φ 2 , or the intermediate signals which occur, should not have a significant amount of power at the output frequency;  
         [0040]    (v) φ 2 *φ 2  (sometimes written simply φ 2 φ 2 ) should not have a significant amount of power within the bandwidth of the up-converted RF (output) signal. This ensures that if φ 1  leaks into the input port, it does not produce a signal within the RF signal at the output. It also ensures that if φ 2  leaks into node between the two mixers, it does not produce a signal within the RF signal at the output; and  
         [0041]    (vi) if x(t) is an RF signal, φ 1 *φ 1 *φ 2  should not have a significant amount of power within the bandwidth of the RF signal at baseband. This ensures that if φ 1  leaks into the input port, it does not produce a signal within the baseband signal at the output.  
         [0042]    These signals can, in general, be random, pseudo-random, or periodic functions of time, and may be either analogue, or digital time-varying signals or waveforms.  
         [0043]    It would be clear to one skilled in the art that virtual LO signals may be generated which provide the benefits of the invention to greater or lesser degrees. While it is possible in certain circumstances to have almost no LO leakage, it may be acceptable in other circumstances to incorporate virtual LO signals which still allow a degree of LO leakage.  
         [0044]    An exemplary set of acceptable waveforms is presented in FIG. 2B, plotted in amplitude versus time. Five cycles of the VLO signal are presented, labelled φ 1 φ 2 . It is important to note that at no point in the operation of the circuit is an actual φ 1 φ 2  signal ever generated; the mixers receive separate φ 1  and φ 2  signals, and mix them with the input signal using different physical components. Hence, there is no LO signal which may leak into the circuit. The states of these φ 1  and φ 2  signals with respect to the hypothetical φ 1 φ 2  output are as follows:  
                                       φ1φ2   φ1   φ2                   Cycle 1-LO   HI   LO       Cycle 1-HI   LO   LO       Cycle 2-LO   HI   LO       Cycle 2-HI   LO   LO       Cycle 3-LO   LO   HI       Cycle 3-HI   LO   LO       Cycle 4-LO   HI   LO       Cycle 4-HI   LO   LO       Cycle 5-LO   LO   HI       Cycle 5-HI   HI   HI                  
 
         [0045]    While these signals may be described as “aperiodic”, groups of cycles may be repeated successively. For example, the pattern of the φ 1  and φ 2  input signals presented in FIG. 2B which generate the φ 1 φ 2  signal, repeat with every five cycles. Longer cycles could certainly be used.  
         [0046]    It would be clear to one skilled in the art that many additional pairings of signals may also be generated. The more thoroughly the above criteria (i)-(vi) for selection of the of the φ 1  and φ 2  signals are complied with, the more effective the invention will be in overcoming the problems in the art.  
         [0047]    The topology of the virtual local oscillator is similar to that of other two stage or multistage modulators and demodulators, but the use of irregular, time-varying mixer signal provides fundamental advantages over known transmitters and receivers, including:  
         [0048]    minimal 1/f noise;  
         [0049]    minimal imaging problems;  
         [0050]    minimal leakage of a local oscillator (LO) signal into the RF output band;  
         [0051]    removes the necessity of having a second LO and various (often external) filters; and  
         [0052]    has a higher level of integration as the components it does require are easily placed on an integrated circuit. For example, no large capacitors or sophisticated filters are required.  
         [0053]    Since the mixers in most transceivers act as solid state switches being turning on and off, it is preferable to drive the mixers using square time-varying signals or waveforms rather than sinusoids. Square time-varying signals or waveforms with steep leading and trailing edges will switch the state of the mixers more quickly, and at a more precise moment in time than sinusoid waveforms.  
         [0054]    Turning to FIGS. 3, 4 and  5  we will now describe various preferred embodiments of the invention.  
         [0055]    Note that throughout the figures and descriptions, reference is made to amplifier stages which are not balanced. Those skilled in the art would recognise that this is a simplification to assist in the explanation of the invention, and that the use of balanced amplifiers would be typical.  
         [0056]    Preferred embodiments of the invention comprise a ring oscillator operatively connected to a number of logical gates arranged to produced the required time-varying signals. As shown in the FIG. 3, a first preferred embodiment of the invention comprises a series of five inverting amplifiers  300 ,  302 ,  304 ,  306 ,  308 , followed by a non-inverting amplifier  310  connected as a ring, the output of each of the first four inverting amplifiers  300 ,  302 ,  304 ,  306  being connected to the input of the next inverting amplifier, the output of the last inverting amplifier  308  being connected to the input of the non-inverting amplifier  310  and the output of the non-inverting amplifier  310  being connected to the input of the first inverting amplifier  300 . The output of the fifth inverting amplifier  308  is also connect to a buffer amplifier  330  to produce the time-varying signal φ 1     I   (t). The output of the non-inverting amplifier  310  is also connect to another buffer amplifier  335  to produce the time-varying signal φ 2     I   (t). The outputs of the first inverting amplifier  300  and the third inverting amplifier  304  are connected to the two inputs of a first two-input exclusive-OR gate  320  to produce a time-varying signal φ 1     Q   (t), and the outputs of the second inverting amplifier  302  and the fourth inverting amplifier  306  are connected to the two inputs of a second two-input exclusive-OR gate  325  to produce a time-varying signal φ 2     Q   (t). In this case, all of the time-varying signals φ 1     I   (t), φ 1     Q   (t), φ 2     I   (t) and φ 2     Q   (t) are square-waves and are used as inputs to various balanced mixers in the associated receiver circuit.  
         [0057]    Each of the five inverting amplifiers  300 ,  302 ,  304 ,  306 ,  308 , and the non-inverting amplifier  310  have a delay control input, all of which are connected together and driven by the output of a low-pass filter  360 . The input of the low-pass filter (LPF)  360  is driven by the output of a Phase Discriminator (PD)  350  (or phase comparison circuit) whose inputs are the output of a reference Local Oscillator  355  and the output of a ‘divide-by-N’ (N) circuit  345  driven by the output of the non-inverting amplifier  310 , thereby forming a phase locked loop. This Phase Locked Loop (PLL) circuitry provides frequency stability for the ring oscillator by comparing the phase of the signal generated by the ring oscillator with that provided by the local oscillator, in a manner well-understood by those skilled in the art.  
         [0058]    By appropriate selection of the outputs of the stages, and the application of simple ‘exclusive-OR’ (XOR) logic gates, a number of time-varying signals are generated which have the required stable relationships in frequency and phase. FIG. 4 shows the time-varying signals as generated by the circuit of FIG. 3 at various points in the circuit. Referring to both figures, the outputs of the buffer amplifiers  300 ,  302 ,  304 ,  306 ,  308 ,  310 , are shown as A φ1Q (t)  400 , B  402 , C  404 , D  406 , E  408  and F φ1I (t)  410 , and those of the XOR gates  320 ,  325 , are shown as B⊕D φ2I (t)  420  and C⊕E φ2Q (t)  430 . The time-varying signals labelled A φ1Q (t)  400 , F φ1I (t)  410 , B⊕D  φ2I (t)  420  and C⊕E φ 2Q (t)  430  bear the necessary relationships to one another to be useful in a modulator or demodulator taking advantage of the principles of a Virtual Local Oscillator.  
         [0059]    The delay introduced by each of the buffer amplifiers  300 ,  302 ,  304 ,  306 ,  308 , and  310  which comprise the Ring Oscillator is shown as ‘d’. Variation of this delay affects the actual oscillation frequency of the Ring Oscillator and may be used as previously described in the provision of a phase locking arrangement, but their relative differences will affect how closely the signals  φ1Q (t)  400 , F φ1I (t)  410 ,  φ2I (t)  420  and  φ2Q (t)  430  emulate the LO of a direct conversion receiver when used in the virtual local oscillator concept. These differences can be minimized through the use of differential amplifier, so that the same amplifier can be used for all sections of the ring oscillator and proper integrated circuit layout techniques to match the loading of each amplifier stage. Inverters  330  and  335  are also used to match the delay of the XORs  320  and  325 .  
         [0060]    Although the use of the phase locking loop arrangement is included here because the inherent frequency stability of the ring oscillator may not be sufficient for the VLO application, it is not a necessary element of the invention. Other mechanisms may be used to provide the frequency stability required by a particular application of the invention.  
         [0061]    Other embodiments of the invention use different combinations of logic to derive time-varying signals which have phase and frequency relationships useful in the implementation of Virtual Local Oscillators for use in modulation and demodulation and like circuits or systems. Embodiments with more stages within the ring of the ring oscillator may be used to derive a lesser or greater number of related time-varying signals using different logic elements arranged to combine various outputs of the stages of the ring oscillator, these logic elements may include, but are not limited to, buffers, ‘exclusive-OR’ (XOR), ‘AND’, and, ‘OR’ gates.  
         [0062]    In a second preferred embodiment illustrated in FIG. 5, seven inverting amplifiers  500 ,  502 ,  504 ,  506 ,  508 ,  510 ,  512  and a non-inverting amplifier  514  form the ring oscillator, the outputs of the first  500 , third  504  and fifth  508  amplifiers are combined through an XOR gate  520  to generate  φ2I (t), and the outputs of the second  502 , fourth  506  and sixth  510  amplifiers are combined through a second XOR gate  525  to generate  φ2Q (t). The outputs of the seventh  512  and eighth  514  stages are buffered  530 ,  535  to produce  φ   1Q (t) and  φ1I (t) respectively. The remaining elements, namely the low-pass filter  560 , the Phase Discriminator  550 , the reference Local Oscillator  555  and the ‘divide-by-N’ circuit  545  form the Phase Locked Loop (PLL) circuitry providing frequency stability for the ring oscillator as before.  
         [0063]    In further embodiments, I inverting amplifier stages (where I is an odd integer, value five or more) and a single non-inverting amplifier stage arranged as a ring oscillator may be used; the outputs of the odd-numbered stages from 1 to (I−2) are combined using an XOR gate to generate  φ2I (t), the outputs of the even-numbered stages from 2 to (I−1) are combined using a second XOR gate to generate  φ2Q (t), and the output of the Ith inverting amplifier stage and the output of the non-inverting amplifier stage are buffered to generate φ 1Q (t) and φ 1I (t) respectively.  
         [0064]    In cases where balanced amplifiers are used more stages can be added to the ring oscillator as long as there is an even number of stages in the oscillator. Outputs-of the-odd stages must be combined to create the inphase φ signals and outputs of the even stages must combined to create to the quadrature φ signals. More than two φ signals may be generated for each of the inphase and quadrature arms if all the φ signals for each arm are added modulo-2 to give a 50% duty cycle square-wave at the RF frequency. Any logic elements can be used to generate the φ signals as long as the delay from all the ring oscillator outputs to the φ outputs is matched well enough that spectrum of all the φ signals added together modulo-2 has a large tone at the RF frequency and does not contain significant power at frequencies other than the RF frequency. In this context, “significant” means “large enough to cause spurious response problems which degrade the overall receiver performance to unacceptable levels”.  
         [0065]    A person skilled in that art will realise that the invention has application elsewhere, and it is the intention of the inventor that this description covers those situations and applications insofar as they are not already known and in use in the field. A person skilled in the art will realise that the embodiments described may be varied in detail without losing or detracting from the inventive concept described herein, and it is our intention to encompass such variations in design within the description and claims.