Abstract:
The invention relates to methods by which radio signals can be transmitted to, and received by, a radio receiver such that the receiver consumes very little power from a battery or energy source. The invention is applicable not only to medical implants, but any application requiring a radio receiver to operate with very low power consumption.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a low power radio frequency receiver, a matching transmitter, a communication method for a low power receiver, and an electromagnetic signal. 
         [0003]    2. Description of the Related Art 
         [0004]    State-of-the-art battery powered medical implants must not only provide the prescribed therapy, but they must also be as small as possible, extremely reliable, and have a long service life. For example, the service life of cardiac pacemakers and implantable cardioverter defibrillators (ICDs) is expected to be seven years or more. Since all electronic devices consume electrical energy, medical implants with batteries that are not readily accessible, or easily replaced, must operate with minimal power consumption. Batteries in medical implants have a small and finite capacity to provide electrical energy, and as such, the operational longevity of devices being powered by them is dependent upon the rate at which the devices consume energy. Since implanted medical devices are required to operate for many years from a limited source of energy, the electronic circuitry in the devices must be designed to operate with minimal power consumption. 
         [0005]    Present day medical implants often utilize radio telemetry to communicate patient medical data and implant status to a physician. This telemetry may occur frequently, as in the case of home monitoring applications, or it may occur less frequently, such as during patient follow-up examinations in a physician&#39;s office. In any event, the radio telemetry circuitry in the implant consumes battery power, and the hence impacts the longevity of the implant. 
         [0006]    In the case of pacemakers and ICDs, the primary purpose of the implant is to regulate a patient&#39;s heartbeat and/or provide a life-saving shock to treat ventricular fibrillation. The implant&#39;s telemetry function is of secondary importance, and because of this, the battery power consumed by the radio telemetry circuitry must be minimal and have negligible impact on the service life of the implant. 
         [0007]    In many battery-powered applications, the architectures of devices may employ special techniques to minimize power consumption. For example, devices may be designed to operate in several different modes, some of which may consume less power than others. Devices may have a “sleep” mode whereby they consume very little power or no power at all, and then one or more operational modes where the devices have more functional capability—at the expense of consuming more power. When operating in this manner to save battery power, the devices may change from one operational mode to another. Changes in operational mode may be determined by a time-sequence under the control of hardware or software. Changes in operational mode may also occur upon reception of a signal, or stimulus from an external device or sensor, or they may result from the reception of radio frequency commands transmitted to the device. 
         [0008]    In a battery-powered device that changes operating modes upon the reception of radio frequency commands, the radio receiver in the device must be powered on, and remain on continuously, or nearly continuously, to ensure the device receives commands, and responds appropriately and in a timely fashion. The time of arrival of a transmitted command is usually not know by the device in advance, and so the receiver must be powered on in anticipation that a command may be transmitted to it at any time. As a consequence, the device&#39;s radio receiver will continuously consume power from the battery even when commands are not being transmitted to it. 
         [0009]    The radio frequency electronics circuitry in a conventional receiver typically consumes a considerable amount of power, and hence the operational longevity of a battery-powered device is significantly reduced as the receiver remains powered on for long periods of time. Because a patient must undergo a surgical procedure to replace an implant when its battery power is nearly depleted, it is highly desirable to minimize the power consumption of radio frequency receivers employed in medical implants. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    This invention describes methods by which radio signals can be transmitted to, and received by, a radio receiver such that the receiver consumes very little power from a battery or energy source. The invention is applicable not only to medical implants, but any application requiring a radio receiver to operate with very low power consumption. 
         [0011]    Description of the Receiver 
         [0012]    According to the invention, the requirements for a low-power receiver are fulfilled by a device comprising: at least one antenna, a resonant circuit, and a nonlinear device. 
         [0013]    Said antenna forms a part of the resonant circuit or it is directly or indirectly coupled to the resonant circuit, according to the invention. 
         [0014]    Said antenna and resonant circuit are tuned to receive a first high frequency carrier of a predetermined first frequency F 1 , and a second high frequency carrier of a predetermined second frequency F 2  simultaneously, which differ from each other by a predetermined intermediate or baseband frequency Fout=|F 2 −F 1 |. 
         [0015]    Furthermore, a nonlinear device is directly or indirectly connected to the resonant circuit in a manner to cause mixing, or heterodyning, of the two carrier frequencies (F 1  and F 2 ), so as to produce an intermediate frequency (IF) or baseband signal Fout. 
         [0016]    The low power receiver is adapted to operate according to the heterodyning method of down converting a high frequency signal to an intermediate frequency (IF) signal, or baseband frequency signal, by mixing the first high frequency carrier and the second high frequency carrier. The intermediate frequency signal, or baseband signal, produced by the mixing or heterodyning behavior of the said nonlinear device, may be used directly as a signal to other electronic circuitry in the device, or the signal may be further demodulated to extract other information that may be contained in the resultant IF signal. 
         [0017]    The low power receiver thus operates as a heterodyning receiver using the second high frequency carrier to replace the otherwise required local oscillator signal in a traditional super-heterodyne receiver architecture. This has the advantage that a very simple receiver design is used for reception of a high frequency carrier. Due to the simple design, which contains very few components, the power consumption of the receiver is very low. 
         [0018]    Although the frequencies of the high frequency carriers may be very high, the frequency of the IF or baseband signal may be very low, if the difference between the frequencies of the high frequency carriers is small. In other words the frequencies of the high frequency carriers can be very close in value. Since the IF or baseband frequency is very low, numerous high frequency components and circuitry can be eliminated in the design and implementation of the low power receiver. For example, compared to a traditional super-heterodyne receiver that receives a single high frequency carrier, the following components can be eliminated: RF amplifiers, the local oscillator circuitry (and phase locked loop circuitry if a PLL is used), the local oscillator buffer amplifiers, and interstage impedance matching networks. 
         [0019]    The resultant output signal may be an intermediate frequency (IF) signal, or it may be a baseband signal, and it may be the desired information to be received. In the case where the output signal is an IF signal, the IF signal may be further demodulated by subsequent stages to produce a baseband frequency signal that may contain the desired information to be received. The low power receiver thus is adapted to operate in a heterodyning fashion using the transmitted second high frequency carrier to replace an otherwise required local oscillator signal that is needed in a traditional super-heterodyne receiver. 
         [0020]    In some traditional super-heterodyne receivers more than one intermediate frequency is used. In these multi-conversion receivers, different IF stages with different intermediate frequencies exist to obtain a better signal-to-noise ratio, improved image rejection, and finer tuning resolution. This requires additional local oscillators, mixers, and IF amplifier stages. If the application warrants it, the low power receiver can also employ additional IF stages similar to the methods used for traditional multi-conversion super-heterodyne receivers. 
         [0021]    Another advantage of the invention is that because the low-power receiver&#39;s architecture is considerably simpler than a traditional super-heterodyne receiver, it uses fewer components, and hence the receiver can be made considerably smaller in size. 
         [0022]    According to the invention, the intermediate frequency signal, or baseband signal, is received from a transmitter within a pair of high frequency carriers because the two high frequency carriers already contain the intermediate frequency, or baseband signal, in the superposition of two high frequency carriers. The intermediate frequency, or baseband signal, can also be seen as the beat frequency resulting from the mixing or heterodyning of the two high frequency carriers. 
         [0023]    Since both high frequency carriers are received by the antenna, the high frequency carriers are superimposed within the antenna. The received second high frequency carrier signal replaces the locally generated local oscillator signal in a typical super-heterodyne receiver, and the nonlinear device in the invention replaces the mixer section used in a traditional super-heterodyne receiver. 
         [0024]    In one embodiment of the invention, the low power receiver is adapted to convert the high frequency carriers to an intermediate frequency signal, and then demodulate the intermediate frequency signal with a demodulator so as to convert the IF signal into a baseband signal. The conversion of the high frequency carriers to the intermediate frequency signal is done simply by receiving the two high frequency carriers and mixing, beating, or heterodyning them in the nonlinear device. 
         [0025]    The two high frequency carriers are both received by the antenna. The antenna is tuned to a middle frequency between the first and second high frequency carrier. The superposition and heterodyning of the two high frequency carriers produces the intermediate frequency signal. Thus the conversion from the high frequency carriers to the intermediate frequency signal results from the superposition and mixing, beating, or heterodyning that automatically occurs during reception. 
         [0026]    To obtain an intermediate frequency signal in a traditional super-heterodyne receiver, it is necessary to mix the received high frequency carrier with a signal that is locally generated in the receiver at a different second frequency. The super-heterodyne receiver, as known in the art, therefore contains a local oscillator, which produces the second frequency. The intermediate frequency signal has a frequency equal to the difference between the carrier frequency and the frequency of the local oscillator. A mirror frequency, or image frequency, equal to the sum of the carrier frequency and the local oscillator frequency is also produced in the heterodyning of two signals. The process of producing the IF signal that is equal to the difference of the two high frequency signals is referred to as down-conversion. The process of producing the IF signal equal to the sum of the two high frequency signals is referred to as up-conversion. 
         [0027]    In another embodiment of the invention, the low power receiver is adapted to convert the high frequency carriers directly to a baseband frequency signal. This baseband signal can then be used directly as a control signal by other sections of the device, or it can contain the desired communications signal information, or data, that was transmitted to the device. 
         [0028]    In contrast to a traditional quadrature-amplitude modulation receiver, where two carriers of the same frequency, with a constant phase difference of π/2, are used, the low power receiver according to the invention receives two high frequency carriers at two different frequencies. A receiver for quadrature-amplitude modulated signals can also work with the super-heterodyne method, but has the same disadvantages, because it requires a local oscillator signal to obtain an intermediate frequency signal. 
         [0029]    A high frequency signal, when used for transmission, is referred to as high frequency carrier. A high frequency carrier can be modulated with another signal, or may not be modulated, i.e. it may be just a continuous wave (CW) signal. 
         [0030]    In a first embodiment of the high frequency carrier implementation for the low power receiver, the first and second high frequency carriers are not modulated (i.e they are CW carriers). The low power receiver is adapted to mix the two unmodulated carriers to obtain an intermediate frequency signal, or the baseband frequency signal. The resulting intermediate signal, or baseband signal, contains the desired information (that is, the data representing information) to be received. For example, the presence, or absence, of an intermediate frequency or baseband signal is, in itself, is a binary representation of data representing the desired signal information. 
         [0031]    In a second embodiment of the high frequency carrier implementation for the low power receiver, the first or the second high frequency carrier is a reference carrier, which is not modulated (i.e. a CW carrier), and the other high frequency carrier is modulated with a data signal. The low power receiver is adapted to mix the unmodulated CW reference carrier with the modulated high frequency carrier to obtain a modulated intermediate frequency signal, or baseband frequency signal, that contains the data signal. 
         [0032]    In a yet another embodiment of the high frequency carrier implementation for the low power receiver, the first and the second high frequency carriers are modulated with a data signal. The resultant intermediate frequency signal, or baseband signal, will contain the data signals that were modulated onto the high frequency carriers. The signal information is regained by demodulating the received intermediate frequency or baseband signal. This has the advantage that one of the high frequency carriers replaces an otherwise needed local oscillator signal, from which the intermediate frequency is produced. 
         [0033]    In a first embodiment of the antenna configuration, the receiver comprises two separate antennas each tuned to receive only one of the high frequency carriers. The receiver then combines and mixes, or heterodynes, the high frequency carriers to produce the intermediate frequency or baseband signal. 
         [0034]    In a second embodiment of the antenna configuration, the receiver comprises only one antenna with which it receives the two high frequency carriers. In this embodiment the two high frequency carriers are combined and superimposed in the antenna. The receiver mixes, or heterodynes, the already combined high frequency carriers to produce the intermediate frequency or baseband signal. 
         [0035]    It should be noted that the amplitudes of the transmitted high frequency carriers may be equal or different. If the antenna or the resonant circuit is tuned to the first high frequency carrier, the received signal may be higher than for the second high frequency carrier. In this case it may be useful to transit a greater amplitude or stronger signal for the second high frequency carrier to obtain adequate signal amplitudes for both high frequency carriers within the receiver. 
         [0036]    In one embodiment of the invention the low power receiver comprises a first band-pass filter, which is connected directly, or indirectly, to the output of the antenna. In this embodiment the first band-pass filter has a pass band that is corresponds to the carrier frequencies. The band-pass filter will attenuate unwanted signals at frequencies outside the frequency band of the desired transmitted signals. Attenuating undesired signals, by filtering, will reduce the potential for interference and improve the quality and reliability of the desired communications link. 
         [0037]    It should be noted that the first band-pass filter may also have a pass-band around a frequency between the first and second high frequency carrier frequencies. In this case both high frequency carriers can pass through the band-pass filter with similar attenuation. It is desirable that the filter&#39;s attenuation to the desired high frequency carriers be as low as possible. 
         [0038]    It is also possible that the first band-pass filter may be tuned to one high frequency carrier frequency, and the antenna may be tuned to the frequency of the same high frequency carrier. The attenuation for this high frequency carrier is lower then for the other high frequency carrier. 
         [0039]    It should be noted that more than one first band-pass filter can be employed to obtain steeper roll-off and greater attenuation for signals outside the filter&#39;s pass band. Each high frequency carrier can have a respective band-pass filter. 
         [0040]    In a further embodiment of the invention, the low power receiver incorporates a second band-pass filter, or a low-pass filter, that is connected to the output of the nonlinear device. The second band-pass filter, or low-pass filter, has a pass band corresponding to the intermediate frequency or the baseband frequency. The second band-pass filter, or low-pass filter, is used to eliminate or attenuate the high frequency carrier signals and unwanted signals resulting from the mixing, or heterodyning, process. 
         [0041]    In still another preferred embodiment, the second band-pass filter, or low-pass filter, is connected to an intermediate frequency (IF) or baseband amplifier. In this embodiment the low power receiver is adapted to filter and to amplify the desired IF or baseband signal. In this embodiment, the IF or baseband amplifier is used to obtain a higher signal level for the further processing of the received signal. The filter and amplifier may also serve to provide impedance matching between sections of the receiver for maximum signal power transfer. The amplifier may also have a specific frequency response and operate in a fashion similar to a bandpass filter, or low-pass filter, to further attenuate unwanted signals. 
         [0042]    In a further preferred embodiment of the invention the nonlinear device or the baseband amplifier is connected to a level detector circuit. In this embodiment the low power receiver is adapted:
       1. to analyse the level of the IF or baseband signal, or the filtered and amplified IF or baseband signal, with the level detector circuit, and   2. to produce a wakeup signal depending on the level of the IF or baseband signal, or the filtered and amplified IF or baseband signal, with the level detector circuit.       
 
         [0045]    This embodiment of the invention facilitates implementing a low power receiver that can be operated continuously, nearly continuously, or be turned on for long intervals of time, while consuming minimal power from a battery. When the low power receiver detects a control signal from an external transmitter, it can wake up other circuitry in the device, or enable other programmed operating routines, that may have been powered-off to conserve battery power. The circuitry and/or operating routines can encompass device functions that require greater power consumption from the battery, such as data communications circuitry, processor circuitry, or the like. 
         [0046]    In one embodiment of the invention, the signal from the output of the nonlinear device, the IF output, or baseband amplifier output, is connected to a data detection circuit. The low power receiver is adapted to convert the IF or baseband signal, or the filtered and amplified IF or baseband signal, with the data detection circuit, into a digital data signal, and to output the digital signal to other circuitry for subsequent processing. This data signal can be used to convey information to the device such as identification information, operating instructions, or software or firmware updates. 
         [0047]    Another embodiment of the low power receiver, according to the invention, comprises a high frequency amplifier, which is connected to the antenna or the resonant circuit. The high frequency amplifier is adapted to amplify the first and second high frequency carriers. The high frequency amplifier may also provide filtering of the desired high frequency carrier signals to reduce interference from unwanted signals in the adjacent radio spectrum. 
         [0048]    Another possibility is that the high frequency amplifier is connected between the first band-pass filter and the nonlinear device. In this case the amplifier feeds the bandpass filtered and amplified signal to the nonlinear device. 
         [0049]    In the case where the low power receiver contains two antennas, or two resonant circuits, there may be two or more high frequency amplifiers employed, each connected to one antenna or resonant circuit. 
         [0050]    In another embodiment of the invention, the low power receiver is adapted to receive more than two high frequency carriers at different frequencies, and to obtain an output signal from intermodulation products produced by nonlinear mixing of the high frequency carriers. In this case the output signal is generated by nonlinear mixing of high frequency carriers, i.e. Fout=(nF 1 ±mF 2 ±pF 3 ± . . . ) where n, m, p, . . . are integers, and F 1 , F 2 , F 3  are high frequency carriers. 
         [0051]    It should be clear that if more than one intermediate frequency is to be received, there may be several band-pass filters applied in parallel. 
         [0052]    In a preferred embodiment, the nonlinear device is a single diode, an array of diodes, a switching device, or a device with a strong 2 nd  order transfer function. 
         [0053]    There are several methods that can be used to obtain a device with the needed 2 nd  order transfer function for mixing, beating, or heterodyning. It should be noted that a device exhibiting a strong second-order transfer function provides an efficient mechanism for converting two transmitted RF signals to an intermediate frequency signal or baseband signal. Alternatively, any ON-OFF switching device can also be used for mixing, heterodyning, or detection, since such devices also exhibit second-order transfer function characteristics. If more than two high frequency carrier signals are transmitted, other nonlinear transfer functions, such as 3 rd  order and higher, are efficient for the detection process. 
         [0054]    When an ON-OFF switching device is used as the nonlinear device, it can also provide signal detection. In this case the output of the nonlinear device is the desired output signal. 
         [0055]    In another embodiment of the invention the low power receiver is adapted to receive spread spectrum carriers. Spread spectrum techniques may utilize frequency hopping, or direct sequence spreading, or any other spread spectrum techniques known in the art. 
         [0056]    Spread spectrum techniques result in spreading the transmitted energy over a wider frequency band. This can be important when government regulatory agencies impose limits on the maximum allowable radiated power limits for the high frequency carriers. In some countries, the regulatory agencies allow spread spectrum signals to be transmitted at power levels higher than signals that do not employ spread spectrum techniques. It may be beneficial to use spread spectrum techniques to transmit the signals at higher power levels so as to overcome radio frequency propagation losses. Transmitting at higher power levels to overcome radio frequency propagation losses improves the performance and reliability of the communications link. Another benefit is that the signals are more resistant to transmission impairments such as multipath fading and interference. 
         [0057]    There are two fundamental methods for producing spread spectrum signals: (1) direct-sequence spread spectrum, and (2) frequency hopping spread spectrum. 
         [0058]    Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a “pseudorandom noise” signal. This pseudorandom noise signal (PN) is a sequence of 1 and −1 values, called “chips”, operating at a rate much higher than that of the original data signal. Multiplying the data to be transmitted by a high-rate PN sequence causes the energy of the original signal to be spread over a much wider frequency band. Since the sequence of the chips produced by the transmitter is known by the spread spectrum receiver, the receiver can use the same PN sequence, to counteract the effect of the pseudorandom noise sequence on the received signal, in order to reconstruct the original information. 
         [0059]    In one preferred embodiment of the invention, direct-sequence spread spectrum techniques may be employed if the two transmitted high frequency carriers use the same PN chip sequence at the same period in time. When spreading in this manner, the instantaneous intermediate frequency, or baseband frequency, recovered by the low-power receiver will be the same as if spreading had not been employed. The low-power receiver will recover the transmitted signal information. 
         [0060]    In frequency hopping spread spectrum systems, the transmitter&#39;s carrier frequency hops over a band of frequencies using a specific hop sequence. The associated spread spectrum receiver also tunes over the same frequency band using an identical frequency hop sequence at the same period in time. In this way, the transmitter and receiver are always operating on the same RF frequency, or channel, and hence the receiver is able to recover the transmitted information. 
         [0061]    In one preferred embodiment of the invention, spread spectrum frequency hopping techniques may be employed if the two transmitted high frequency carriers frequency hop using the same hop sequence and hop at the same, or nearly same, instant in time. It is desirable that the two high frequency carriers maintain their desired frequency difference relationship while hopping during the spectrum spreading sequence. When frequency hopping in this manner, the intermediate frequency of the two high frequency carriers will maintain the desired relationship and it will be possible to recover the desired control signal or transmitted data with the low-power receiver. 
         [0062]    In still another embodiment of the invention, the transmitter is adapted to transmit at least two pairs of high frequency carriers, and the first pair of high frequency carriers has a difference in frequency that is equal to the difference in frequency of the second pair of high frequency carriers. Both high frequency carrier pairs will produce the same intermediate frequency, or beat frequency, in the receiver, i.e. Fout=|F 2 −F 1 |=|F 4 −F 3 |. The two pairs of transmitted high frequency carriers convey the same signal information, and hence they are redundant. Thus the desired signal information is radiated twice and received twice since the superposition of the first pair of high frequency carriers is redundant with the superposition of the second pair of high frequency carriers. Transmitting a second pair of signals, that contain redundant information, is advantageous in communications links that experience multipath fading. If one of the pair of carriers experiences destructive interference due to multipath fading, it is unlikely the second pair will be affected since their wavelengths will be different; and multipath fading is dependent upon the carrier wavelength. As a consequence, the receiver will be able to recover the transmitted information and maintain a reliable communications link even in the presence of severe multipath fading. Therefore the communication link established with the proposed low power receiver is more resistant to link failure due to multipath fading. This can be attributed to the redundancy of the desired signal in two pairs of high frequency carriers. 
         [0063]    Description of the Transmitter 
         [0064]    According the invention, the signals required for the aforementioned low power receiver are fulfilled by a transmitter comprising at least one antenna, and at least one high frequency amplifier, and at least one signal source providing at least two high frequency carrier signals. The transmitter is adapted to transmit a first high frequency carrier and a second high frequency carrier signal simultaneously. The high frequency carrier signals are adapted to be down-converted to an intermediate frequency signal or baseband frequency signal in the low power receiver. 
         [0065]    According to the invention, the transmitter produces a pair of radiated high frequency carrier signals such that the signals contain an intermediate frequency signal, or a baseband signal, when superimposed. The intermediate frequency, or baseband frequency, Fout, is the difference frequency of the high frequency carriers F 1  and F 2 , i.e. Fout=|F 2 −F 1 |. This has the advantage that when the two high frequency carriers are received by the low power receiver, the intermediate frequency signal is already contained in the superposition of the two high frequency carriers, wherein the otherwise needed local oscillator signal required in a super-heterodyne receiver for obtaining the intermediate frequency signal is replaced by one of the high frequency carriers. Thus a simplified receiver design can be utilized, which contains less high frequency electronic circuitry, and thus can be designed to consume very low power. The transmitter disclosed in the invention provides a signal to replace the local oscillator signal that is required in receivers based upon the super-heterodyne and homodyne principles of operation. 
         [0066]    It should be noted that the amplitude of the two high frequency carriers may be equal or may be different, depending on the transmitter and/or receiver design. 
         [0067]    The transmitter can comprise one radio frequency amplifier and antenna for all high frequency carriers, or one radio frequency amplifier and antenna for each radiated high frequency carrier. Alternatively, the transmitter may employ one radio frequency amplifier for each separate high frequency carrier, and combine the outputs of the amplifiers into one or more antennas. 
         [0068]    It may be desirable to use a separate amplifier for each high frequency carrier to simplify the design of the transmitter. By using a separate amplifier for each high frequency carrier, the amplifiers&#39; linearity requirements are less critical, and the amplifier may be less costly to manufacture. High frequency amplifiers that simultaneously amplify multiple carrier signals require a high degree of linearity to prevent intermodulation and distortion of the signals. In addition, high frequency amplifiers that only amplify one high frequency carrier at a time can be designed to be more power-efficient than amplifiers designed to simultaneously amplify more than one carrier signal. Amplifiers operating with high power efficiency are smaller, lighter in weight, easier to cool, and cost less to manufacture than high-linearity amplifiers. 
         [0069]    In one embodiment of the transmitter according to the invention, the transmitter does not include a modulation unit. The transmitter is adapted to only transmit two high frequency continuous wave (CW) carrier signals. Either one or both CW carrier signals may be enabled or disabled as desired. The presence, or absence, of one, or both, CW carrier signals can be used as a command signal to the low power receiver 
         [0070]    In another embodiment of the transmitter according to the invention, the transmitter can transmit a signal containing information to the receiver. The transmitter comprises at least one modulation unit that is adapted to modulate at least one high frequency carrier signal with a data signal. The data signal may be either a digital or an analog signal. The data signal can either be a data stream, or repeated information like an identification code. The carriers may be amplitude modulated (AM), ON-Off Key modulated (OOK), frequency modulated (FM), phase modulated (PM), quadrature-amplitude modulated (QAM), or any combination thereof. 
         [0071]    In another embodiment of the transmitter according to the invention, both high frequency carriers may be modulated. The carriers may be amplitude modulated (AM), ON-Off Key modulated (OOK), frequency modulated (FM), phase modulated (PM), quadrature-amplitude modulated (QAM), or any combination thereof. The data signal may be either a digital or an analog signal. The data signal can either be a data stream, or repeated information like an identification code. The modulation technique can add up to a combined modulation in the intermediate frequency signal as further described below. 
         [0072]    In one embodiment of the invention the transmitter is adapted to quadrature-amplitude modulate the data signal on at least one of the high frequency carriers. 
         [0073]    If the data signal is quadrature-amplitude modulated on one high frequency carrier, each transmitted symbol can convey more than one data bit. There are 8 QAM, 16 QAM, up to 256 QAM methods known in the art. 
         [0074]    In another embodiment of the invention, one high frequency carrier may be amplitude modulated, and the second high frequency carrier may be phase modulated. The resulting intermediate frequency signal, which is the superposition of the two high frequency carriers, is then a QAM signal or pseudo-QAM signal. 
         [0075]    The digital data may be pre-processed, or filtered, before it is modulated onto the high frequency carriers. Known pre-processing steps include analog filtering, and/or digital filtering, to reduce the spectral occupancy of the signal information. Filter types may include Gaussian, Butterworth, Root-raised Cosine, etc. Examples for digital modulation techniques that may be used include, but are not limited to amplitude modulation (AM), On-Off Keying (OOK), frequency-shift keying (FSK), minimum-shift keying (MSK), Gaussian-frequency-shift keying (GFSK), Gaussian-minimum-shift keying (GMSK), phase-shift keying (PSK), orthogonal frequency division multiplexing (OFDM), which is also a QAM technique, quadrature phase-shift keying (QPSK), etc. 
         [0076]    In still another embodiment of the invention, the transmitter is adapted to transmit at least two pairs of high frequency carriers, and the first pair of high frequency carriers has a difference in frequency that is equal to the difference in frequency of the second pair of high frequency carriers. Both high frequency carrier pairs will produce the same intermediate or beat frequency, in the receiver, i.e. Fout=|F 2 −F 1 |=|F 4 −F 3 |. The two pairs of transmitted high frequency carriers may be modulated using the methods described earlier, and convey the same signal information, and hence they are redundant. Thus the desired signal information is radiated twice and received twice since the superposition of the first pair of high frequency carriers is redundant with the superposition of the second pair of high frequency carriers. 
         [0077]    Transmitting a second pair of signals, that contain redundant information, is advantageous in communications links that experience multipath fading. If one of the pairs of carriers were to experience destructive interference due to multipath fading, it is unlikely the second pair will be affected since their wavelengths will be different; and multipath fading is dependent upon the carrier wavelength. As a consequence, the receiver will be able to recover the transmitted information and maintain a reliable communications link even in the presence of severe multipath fading. Therefore the communication link established with the low power receiver is more resistant to communication link failure caused by multipath fading. This can be attributed to the redundancy of the desired signal in two pairs of high frequency carriers. 
         [0078]    In one embodiment of the invention, the transmitter is adapted to transmit the data using spread spectrum techniques. This has the advantage that the radiated electromagnetic energy is spread over a broader frequency band, and therefore the average power spectral density of the radiated energy is reduced. This is especially important where state regulations on radiated power limit the maximum legal transmitter power level. Transmitters employing spread spectrum techniques are often allowed to transmit at higher power levels than those that do not use spread spectrum. Transmitting at higher power levels is advantageous for increasing the operating range of the communications link, or increasing the link reliability, or both. 
         [0079]    Another benefit of using spread spectrum techniques is that the signal is harder to detect, and therefore more secure for sensitive data. In some cases the spread spectrum signal is not detectible unless the receiver knows the spread spectrum technique a priori. 
         [0080]    Description of the Modulation and Signal 
         [0081]    The following modulation techniques apply to the communication method according to the invention.
       1. None of the simultaneously transmitted high frequency carriers may be modulated. The carriers, (F 1  and F 2 ), may be high frequency continuous wave (CW) signals. In this case, the desired signal is the frequency difference signal, i.e. Fout=|F 2 −F 1 |.   2. It should be noted that either one high frequency carrier, or more high frequency carriers, may be modulated. The carrier, or carriers, may utilize amplitude modulation (AM), On-Off Key modulation (OOK), frequency modulation (FM), phase modulation (PM), quadrature-amplitude modulation (QAM), or any combination of AM, OOK, FM, PM, or QAM modulation techniques.   3. This data signal may be a data stream or a single repeated sequence such as an identification code.   4. The data signal can either be an analog signal or a digital signal.   5. The high frequency carrier, or carriers, may be modulated with a cyclic signal, which can be frequency coded to address one specific receiver. This would be one bit of information—either the frequency of the cyclic signal matches a predetermined frequency or not.   6. In one embodiment of the communication method according to the invention, one high frequency carrier is amplitude modulated with a first part of the signal information, and a second high frequency carrier is phase modulated with a second part of the signal information, wherein the resulting intermediate frequency signal, which is the superposition of the two high frequency carriers, is quadrature-amplitude modulated and contains the full signal information.   7. The high frequency carrier signals may also employ spread spectrum techniques such as direct sequence spread spectrum or frequency hopping techniques.       
 
         [0089]    The invention has been described in different embodiments. It should be noted that the features of the different embodiments can be combined, if they do not exclude each other, or if it is otherwise specified. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0090]    The foregoing, and other objects, advantages, and novel features of the present invention can be understood and appreciated by reference to the following detailed description of the invention, taken in conjunction the accompanying drawings, in which: 
           [0091]      FIG. 1  shows schematically a frequency spectrum of a transmitter with two carriers. 
           [0092]      FIG. 2  depicts schematically a possible simplified design of a transmitter  200  according to one embodiment of the invention. 
           [0093]      FIG. 3  shows an embodiment of a simplified schematic design of a receiver  300  according to the invention. 
           [0094]      FIG. 4  shows schematically a transmitter frequency spectrum of another embodiment of the invention. 
           [0095]      FIG. 5  shows one possible embodiment of a nonlinear electronic circuit that can be used in the low-power receiver to generate an intermediate frequency signal, or a baseband signal, from the superposition and mixing of two or more high frequency carrier signals. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0096]    The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
         [0097]      FIG. 1  shows, schematically, the frequency spectrum of a transmitter with two high frequency carriers at different frequencies F 1  and F 2 . The high frequency carriers may not have modulation sidebands (i.e. they may be only CW carriers), or one, or both carriers may have modulation sidebands  106 . None, one, or all high frequency carriers may be modulated. In the following the high frequency carriers may be referred to simply by carriers. The carriers of two frequencies mix with each other and produce new signals at the frequency F 1 +F 2  and at delta F=|F 2 −F 1 |. Delta F can be used for a low power receiver design, because although F 1  and F 2  may be very high, delta F can be very low. The signal at frequency delta F is called intermediate frequency signal. If delta F is very small, it may also be referred to as a baseband signal. Because of this relationship between two carriers, or more than two carriers, the design of the receiver can be simplified as described further below. 
         [0098]      FIG. 2  schematically depicts a possible simplified design of a transmitter  200  according to one embodiment of the invention. The transmitter is adapted to simultaneously transmit two different carriers at a first high frequency F 1 , and a second high frequency F 2 , which is different from the first frequency. In this embodiment the transmitter  200  comprises a control and data unit  202  to provide reference frequencies, and is optionally adapted to perform analog and/or digital baseband modulation. The transmitter further comprises two high frequency sources  204  and  206 , which are connected to the control and data unit  202  at their inputs. These high frequency sources generate the two carriers at different frequencies. The first radio frequency source  204  generates the first carrier at the frequency F 1  and the second high frequency source  206  generates the second carrier at the frequency F 2 . The high frequency sources  204 ,  206  are connected at their respective output to a combiner  208 , which linearly combines the two high frequency signals. The combiner  208  is connected to an antenna  210 , which radiates the high frequency signals over the air. The high frequency sources  204  and  206  may utilize separate high frequency amplifiers to increase the signal power of the carriers prior to being combined. Optionally, the combiner  208  may also include high frequency amplifiers to further increase the power level of the signals to be transmitted. 
         [0099]    In another embodiment of the antenna configuration, the combiner stage  208  is not used, and the high frequency sources  204 , and  206 , are individually connected to separate antennas. 
         [0100]    In one embodiment, one high frequency source  204  is adapted to modulate a baseband signal to the high frequency carrier and the other high frequency source  206  is adapted to produce a continuous wave (CW) high frequency signal, which acts as a reference signal for a receiver. In another embodiment both high frequency sources  204  and  206  are both adapted to modulate baseband signals onto the two carriers of different high frequencies. 
         [0101]    In one embodiment of the invention one high frequency carrier is amplitude modulated and the other high frequency carrier is phase modulated. The resulting intermediate frequency produced in the low power receiver, which is the superposition of the two carriers, is a quadrature-amplitude modulated (QAM) signal. 
         [0102]    In another embodiment of the invention, more than one pair of high frequency sources are provided. The sources may be combined and transmitted using one antenna, or the sources may not be combined, and each source will transmit its signal from a separate antenna, i.e. one antenna for each source. Transmitting more than one pair of signals can be used to improve the reliability of the communications link by reducing the susceptibility to signal fading caused by multipath interference. 
         [0103]    In one embodiment of the invention the control and data unit is a programmable controller, an application specific integrated circuit (ASIC), or a digital signal processor (DSP), or a combination thereof. In another embodiment the high frequency sources are numerically controlled oscillators (NCO), crystal oscillators, or phase-locked frequency synthesisers. 
         [0104]      FIG. 3  shows an embodiment of a simplified schematic design of a receiver  300  according to the invention. In this embodiment the receiver comprises an antenna  302 , which is connected to a first band-pass filter  304  being connected to a nonlinear or switching device  306 . The nonlinear or switching device is connected to an intermediate frequency bandpass filter, or a baseband filter, and an amplification unit  308 . This unit is connected to a level detector unit  310 , and to a data detection unit  314 . 
         [0105]    The antenna is tuned to receive two high frequency carriers at frequencies F 1  and F 2 , which are of similar frequencies. The two high frequency carriers are fed to the first band-pass filter  304 . This bandpass filter has a narrow pass band around a middle frequency between F 1  and F 2 . The filter is designed to allow both high frequency carriers to pass through the filter with little attenuation, while attenuating unwanted signals outside the filter&#39;s pass band. 
         [0106]    The filtered high frequency signals are fed to the nonlinear or switching device  306 . The superposition and mixing, beating, or heterodyning of the two high frequency carriers in the nonlinear, or switching device,  306  results in two additional frequencies, Fsum=F 1 +F 2 , and Fout=|F 2 −F 1 |. The desired intermediate frequency signal, or baseband signal, is the signal at frequency Fout=|F 2 −F 1 |. The IF signal, or baseband signal, is fed to the IF filter, or baseband filter, and amplification unit  308 . The IF or baseband filter passes the desired signal with very little attenuation, and suppresses the residual high frequency components at F 1  and F 2 , as well as at Fsum. 
         [0107]    The filtered and amplified IF or baseband signal is fed to the level detector unit  310 , which detects if the signal exceeds a predetermined level. If the predetermined level is exceeded the level detector unit generates a wake up signal  312  to activate other parts of the device, e.g. a more power consuming communication device and/or a corresponding processor can be activated. The signal is possibly also fed to a data detection unit  314 . In one embodiment of the invention the data detection unit may detect and output a data signal  316 . Alternatively, the output of the data detection unit may also be used as a wakeup signal. 
         [0108]    In one embodiment the data signal is an identification code for the device in which the receiver is installed. Thus only the receiver with the matching identification code may wake up when a transmitter transmits the corresponding identification code. In another embodiment of the invention, the transmitter may transmit a broadcast message to all receivers within reception range, and all devices will respond to the transmitted message. In yet another embodiment of the invention, the data detection unit can feed the data from the received baseband signal to other parts of the device to allow the device to receive data from a transmitter. 
         [0109]      FIG. 4  shows a schematic of the transmitter frequency spectrum of another embodiment of the invention. With this implementation of the invention, it is possible to reduce communication link failures due to multipath fading. Multipath fading means that radio signals can be severely attenuated at certain locations due to destructive interference caused by signal reflections from nearby structures. 
         [0110]    In this embodiment two pairs of high frequency carriers, (four signals),  402  and  404 , are used to transmit data. The two pairs of high frequency carriers  402  and  404  have the same frequency difference delta F. 
         [0000]      ( F 4− F 3)=( F 2− F 1)=delta  F.    
         [0111]    In the receiver, the intermediate frequency, delta F, is received due to the above described superposition and heterodyning of each pair of high frequency carriers. Thus on the receiver side the two high frequency carrier pairs have the same intermediate or baseband frequency. Although the receiver receives only one intermediate frequency signal, or baseband frequency signal, it consists of two pairs of independent high frequency signals due to superposition. 
         [0112]    In this embodiment of the invention the high frequency carrier pairs F 1 , F 2  and F 3 , F 4  carry the same signal information. That means they are redundant and thus the information in the resulting intermediate frequency signals is redundant. When the signals are received at the receiver without reflections, or multipath interference from nearby structures, the resultant received intermediate frequency signal is a superposition and heterodyning of the two pairs of high frequency carriers. When multipath fading occurs on one high frequency carrier pair, the receiver will still receive the other high frequency carrier pair, and can receive the signal information on its respective intermediate frequency signal. It is very unlikely that multipath fading occurs on the two high frequency pairs at the same receiver location since the high frequency carrier pairs are in a different frequency bands and therefore their wavelengths are different. Although the communication link may fail with the F 1  and F 2  pair, the link remains established by F 3  and F 4  pair. The same situation is true if the F 3  and F 4  pair experience multipath interference. The receiver will likely receive the desired signal information transmitted by the F 1  and F 2  pair of high frequency carrier signals. 
         [0113]      FIG. 5  shows one possible embodiment of a nonlinear electronic circuit  500  that can be used in the low-power receiver to generate an IF frequency signal, or a baseband frequency signal, by the superposition and mixing of two or more high frequency carrier signals. The input to the nonlinear stage is labeled RF Input, and it receives the high frequency carriers from the antenna and band-pass filter (not shown in this diagram). The balun transformer  501  converts a single-ended signal to a differential signal, and provides an impedance transformation to increase the voltage level of the received signal. The differential signal comprised of the high frequency carriers is coupled to diodes  505 ,  506 ,  508  and  509 . These diodes are configured to operate as a full-wave rectifier, and develop an output signal across load resistor  507 . Capacitors  502  and  504  serve to block direct current flow, and also present a high impedance to block the generated IF or baseband output signal, but allow the high frequency carrier signals to couple from the balun transformer to the diodes. The nonlinear behavior of the full wave rectifier results in the desired intermediate frequency (IF) signal, or baseband signal, being developed across load resistor  507 . Inductors  503  and  510  present a high impedance to the high frequency carriers, but allow the desired IF output signal, or baseband output signal, to pass with minimal attenuation. Inductor  503  also provides a ground return path for the IF or baseband output signal. The desired IF output signal, or baseband signal, is available at the terminals labeled IF or Baseband Out. Capacitor  511 , in conjunction with inductor  503  and inductor  510 , operates as a low-pass filter to attenuate the high frequency carriers, and the unwanted mixing product resulting from the sum of the two carriers (Fsum). The cutoff frequency of this output filter section can be adjusted to accommodate the desired output frequency of the IF, or baseband output signal, by adjusting the component values accordingly. 
         [0114]    It should be noted that this is only one of many possible methods for implementing a nonlinear circuit to develop the desired IF signal or baseband signal from the high frequency carrier input signals. For example, the input balun transformer could be replaced with an inductor-capacitor network to provide impedance transformation and/or convert a single-ended signal to a differential signal. Also, if the nonlinear circuit was coupled to the output terminals of a dipole antenna, the balun stage would not be needed since a dipole antenna produces a signal output that is already in a differential form. It is also possible to implement the nonlinear circuit using a single diode such that single-ended to differential conversion is not required and the need for a balun can be eliminated. Furthermore, there are techniques know in the present art for producing nonlinear operation using other diode configurations, such as “half bridge” configuration, and the “ring quads” configuration as found in conventional double-balanced mixers. Zero-bias Schottky diodes, with very low barrier potential, result in good signal sensitivity and conversion efficiency, however, other diode types can be used and will result in varying degrees of circuit performance. Tunnel diodes and Back diodes and also be used, but they are significantly more expensive than Schottky diodes. 
         [0115]    Although an exemplary embodiment of the present invention has been shown and described, it should be apparent to those of ordinary skill that a number of changes and modifications to the invention may be made without departing from the spirit and scope of the invention. This invention can readily be adapted to such devices by following the present teachings. All such changes, modifications and alterations should therefore be recognized as falling within the scope of the present invention.