WO2020058980A1

WO2020058980A1 - Systems and method for reducing spurious signals in a received signal

Info

Publication number: WO2020058980A1
Application number: PCT/IL2019/051042
Authority: WO
Inventors: Marco Pilas; Eliyahu Eyal BEN TSUR; Ram Yerachmiel GOLDREICH; Nachum LERER
Original assignee: Elbit System Bmd And Land Ew - Elisra Ltd.
Priority date: 2018-09-17
Filing date: 2019-09-19
Publication date: 2020-03-26
Also published as: IL261857B; WO2020058980A4; IL261857A

Abstract

A receiving system for reducing spurious signals in a received signal which includes a first receive, at least a second receiver coupled with processor. The first receiver employs a first mixing-scheme and down-converts a received-signal to a first selected Nyquist-zone, and produces a first down-converted received-signal. The first receiver further samples the first down-converted received-signal and produces a first sampled-signal. The second receiver employs a second mixing-scheme, down-converts the received-signal to a second selected Nyquist-zone, different from the first selected Nyquist-zone, produces a second down-converted received-signal, samples the second down-converted received-signal and produces a second sampled-signal. The processor produces a first frequency-domain-representation of the first sampled-signal and at least a second frequency-domain-representation of the second sampled-signal. The processor further produces a modified frequency-domain-representation from a combination of at least portions the first frequency-domain-representation and at least portions of at least the second frequency-domain-representation.

Description

SYSTEMS AND METHOD FOR REDUCING SPURIOUS SIGNALS IN A

RECEIVED SIGNAL

FIELD OF THE DISCLOSED TECHNIQUE

The disclosed technique relates to reception of Radio

Frequency (RF) signals in general, and to methods and for reducing spurious signals in a received signal, in particular.

BACKGROUND OF THE DISCLOSED TECHNIQUE

Mixing is an operation employed in radio transmission and reception. In essence, the mixing operation includes the multiplication of two signals one with the other. When multiplying two signals, where each exhibits a respective frequency, the resulting signal exhibits frequencies that are the difference between the frequencies of the two signals, and the sum of the frequencies of the two signals, as well as higher order harmonics (i.e., the sum and difference of integer multiples frequencies of the signals being multiplied). Ideally, the result of multiplying two sine waves of different frequencies is as follows:

5ίh(w₁ϋ) · sin(< >₂t) = 1/2{oo5(w₁— w₂)— oo5(w₁ + w₂)} (1 ) Reference is now made to Figure 1 , which is a schematic illustration of a mixer, generally referenced 10, which is known in the art and employed in receiving and transmitting radio signal. The input to mixer 10 is a signal exhibiting a frequency F_SIG> and a Local Oscillator (LO) signal exhibiting a frequency F_L0. The output from mixer 10 is a signal exhibiting a frequency F further explained below. Accordingly, for example, when the frequency, F_SIG. of the signal is 5 Giga Flertz (GHz) and the LO frequency F_Lo is 3GHz, the output signal shall exhibits the frequencies of 2GHz and 8GHz. This characteristic of mixers also results in that there are two frequencies that shall be‘mixed’ to the resulting output signal. The first is at F_LO+F_|F and the second is F_LO-F_|F. Therefore if F_SIG= F_LO+F_|F, any signal exhibiting the frequency of F_LO-F_|F is referred to as an‘image signal and F_LO-F_|F is referred to as the‘image frequency’, F_|M. In the above example, when the IF is selected to be 2GHz, the image frequency, F_|M, is 1 GHz. Similarly, when the IF is selected to be 8GHz, the image frequency, F_!M, is 1 1 GHz.

Radio transmission includes the operation of up-converting the signal to be transmitted. The signal to be transmitted is also referred to as ‘the base-band signal’. The operation of up-converting the signal to be transmitted, also referred to as up-conversion, is achieved by mixing the signal to be transmitted with a carrier signal, which results in an up-converted signal. The frequency of the carrier signal is referred to as the‘carrier frequency’. Up-conversion may be achieved with single stage mixing (i.e., mixing the signal to be transmitted directly with the carrier signal) or with multiple stage mixing (i.e., mixing the signal to be transmitted with intermediate signals). When multiple mixing stages are employed, the frequency of a signal at the output of an intermediate mixing stage is referred to as intermediate frequencies (IF). The output frequency of the last mixing state should equal the carrier frequency.

Conversely, radio reception includes the operation of down-converting the received signal. The operation of down-converting the received signal is also revered to as‘down-conversion’. Similar to up-conversion, down-conversion may be achieved by employing a mixing scheme in which received signal is mixed directly with the carrier signal (i.e., single stage mixing). Alternatively, down-conversion may be achieved by employing multiple mixing stages. When multiple mixing stages are employed, the output frequency of the last mixing stage should equal the base band frequency or a selected intermediate frequency, F . It is noted that the IFs in the down-conversion need not be the same as the IFs in the up-conversion. A receiver which employs only a single mixing stage in which the LO is the carrier signal is referred to as a ‘zero-IF receiver’, and the mixing scheme is referred herein to as‘zero-IF mixing scheme’. A receiver which employs only a single mixing stage in which the LO is different from the carrier signal is referred to as a‘low-IF receiver’, and the mixing scheme is referred herein to as‘low-IF mixing scheme’. A receiver which employs a mixing scheme which includes two or more mixing stages in the down-conversion is referred to as a ‘super-heterodyne receiver’ and the mixing scheme is referred herein to as‘super-heterodyne mixing scheme’. It is noted that in super-heterodyne receiver, one of the mixing stages may up-convert. It is also noted that the resulting output of a super-heterodyne receiver may be zero-if or low-if.

Reference is now made to Figures 2A and 2B, which are schematic illustrations of two spectrum diagrams, generally referenced 20 and 30 respectively, which are known in the art. In Figures 2A and 2B the horizontal axis relates to frequency, denoted ‘/, and the vertical axis relates to power, denoted‘P’. Spectral scheme 20 depicts the spectrum of various signals before mixing while spectral scheme 30 depicts the spectrum of the signal resulting from the mixing operation. With reference to Figure 2A, a signal (e.g., a received signal) 22 located at frequency F _SIG, is to be mixed with a mixing signal 24 exhibiting the frequency F_L0. As explained above, the mixing of the signal 22, with the signal 24 shall result in a signal 32, located at frequency F_|F. However, also as explained above, an image signal 26, located at F_|M, shall also be mixed or‘folded’ into the resulting signal. Image signal 26 causes interference in the resulting signal 32 as depicted in Figure 2B by peak 34.

Image signals are one type of interfering signals known in the art as spurious signals. Image signals result from 2^nd order mixer non-linearity distortion. Additional spurious signals known in the art result from harmonics (e.g., second harmonics, third harmonics etc.) and Direct Current (DC) offset as well intermodulation of second and third order (i.e., IP2 and IP3). Harmonics and intermodulation are generally caused by the non-linearity of the active devices (e.g., amplifiers, mixers) in the receiver. DC offset may be caused by IQ imbalance, which is further elaborated below as well as from second order intermodulation. Spurious signals caused from DC offset are referred to herein as‘DC spurious’. Spurious signals resulting from second order inter-modulations are referred to herein as ΊR2 spurious’. Another spurious signal of interest, results from fourth order mixer non-linearity, and is known as the‘half-IF’ spurious signal. Yet another type of spurious signal is leakage from a switched power supply (i.e., when such a power supply is employed), which exhibits the switching frequency of the power supply.

Another operation employed in radio reception, is sampling. Sampling is the transformation of a continuous signal into discrete quantities, known as samples, from the continuous signal. In time signals, sampling is achieved by measuring the value (e.g., power values, voltage values or current values) of the continuous signal every T_s seconds, referred to as the sampling interval or the sampling period. The sampling frequency or sampling rate, F_s, is the average number of samples obtained in one second (i.e., samples per second). Accordingly, F_s = 1/T_S. To reconstruct the continuous signal from the discrete signal, the sampling frequency should be sufficiently high. Known in the art are the oversampling theorem and the undersampling theorem. According to the oversampling theorem, F_s should be at least twice the highest frequency component of the signal. According to the under sampling theorem, F_s should be at least twice the bandwidth of the signal. However, either when oversampling or undersampling, the spectrum of a sampled signal shall include replicas of the signal at each intervals of F_s/2. These intervals are known as the Nyquist zones.

Reference is now made to Figures 3A and 3B, which are schematic illustrations of two spectral schemes, generally referenced 50 and 60 respectively which are known in the art. In Figures 3A and 3B the horizontal axis relates to frequency, denoted ‘/, and the vertical axis relates to power, denoted‘P\ Spectral scheme 50 depicts the spectrum 52 of a signal to be sampled exhibiting a bandwidth BW. After the signal is sample at a sampling frequency F_s, multiple replicas 62^ 62₂, 62₃,... , 62_n, 62_n+1 are created at frequency intervals of F_s/2. First interval between zero and F_s/2 is referred to as the first Nyquist zone, the second interval between F_s/2 and F_s is referred to as the second Nyquist zone etc. It is noted that the spectrums at the even Nyquist zones are mirror images of the spectrums at the odd Nyquist zones.

U.S. Patent 8,774,334 to Fernando, entitled“Dynamic Receiver Switching” directs to a receiver system which includes a plurality of down-conversion paths and which dynamically selects the number of conversion paths employed. In the example of two down-conversion paths, each path includes respective distinct local oscillators. While one down-conversion path is employed continuously, the second down-conversion path is selectively enabled or disabled based on a detected level of an interferer in the frequency spectrum. When the interferer level is less than a predetermined threshold, a single down-conversion path is employed. When the interferer level is greater than a predetermined threshold, then the two down-conversion paths are employed.

U.S. Patent Application Publication 2007/0183549 to Fifield et al, entitled“Method of, and Receiver For, Canceling Interfering Signals” directs to cancelling an unwanted signal exhibiting a bandwidth which overlies the bandwidth of a wanted second signal. The bandwidth of the unwanted signal is smaller than the bandwidth of the wanted signal. According to the technique disclosed by Fifiled et al, a first signal and a second signals are received and respectively frequency down converted using respective LOs to produce first and second low frequency signals. The frequency of the second LO is selected to mix the center frequency of the narrowband signal to a low or zero IF. Thereafter, the first and second low frequency signals are sampled using synchronized ADCs to produce two digitized signals. The wider bandwidth signal is sampled at a higher sampling rate than the lower bandwidth signal. In other words, the interfering narrow band signal is isolated by employing an ADC with a lower sampling rate. Then the sampling rate of the second digitized signal is adjusted to be the same as the first digitized signal. Thereafter the frequency of the unwanted signal is shifted to be in the same relative position with respect to the wanted signal as it appeared in the received signal. An output signal is derived by obtaining the difference between the wanted and unwanted signals.

SUMMARY OF THE PRESENT DISCLOSED TECHNIQUE

It is an object of the disclosed technique to provide a novel method and system for reducing spurious signals in a received signal. In accordance with the disclosed technique, there is thus provided a receiving system for reducing spurious signals in a received signal. The system includes a first receive, at least a second receiver and a processor. The processor is coupled with the first receiver and with the second receiver. The first receiver employs a first mixing scheme and down-converts a received signal to a first selected Nyquist zone, and produces a first down-converted received signal. The first receiver further samples the first down-converted received signal and produces a first sampled signal. The second receiver employs a second mixing scheme. The second receiver down-converts the received signal to a second selected Nyquist zone, different from the first selected Nyquist zone and produces a second down-converted received signal. The second receiver further samples the second down-converted received signal and produces a second sampled signal. The processor produces a first frequency domain representation of the first sampled signal and at least a second frequency domain representation of the second sampled signal. The processor further produces a modified frequency domain representation from a combination of at least portions the first frequency domain representation and at least portions of at least the second frequency domain representation.

In accordance with another aspect of the disclosed technique, there is thus provided a method for reducing spurious signals in a received signal. The method includes the procedures of down-converting a received signal to a first selected Nyquist zone employing a first mixing scheme, thereby producing a first down-converted signal and down-converting said received signal to at least a second selected Nyquist zone, different from said first selected Nyquist zone, employing at least a second mixing scheme, thereby producing at least a second down-converted signal. The method further includes the procedures of sampling said first down-converted signal, thereby producing a first sampled signal, sampling at least said second down-converted signal, thereby producing at least a second sampled signal, producing a first frequency domain representation of said first sample signal and producing at least a second frequency domain representation of at least said second sample signal. The method also includes the procedure of producing a modified frequency domain representation from a combination of at least portions said first frequency domain representation and at least portions of said second frequency domain representation.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed technique will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:

Figure 1 is a schematic illustration of a mixer which is known in the art;

Figures 2A and 2B, are schematic illustrations of two spectrum diagrams, which are known in the art;

Figures 3A and 3B are schematic illustrations of two spectral schemes, which are known in the art;

Figures 4A-4G, which are schematic illustrations of spectrum diagrams, in accordance with an embodiment of the disclosed technique;

Figure 5 is a schematic illustration of a system for reducing spurious interferences in a received signal, constructed and operative in accordance with another embodiment of the disclosed technique;

Figure 6 is a schematic illustration of a system for reducing spurious interferences in a received signal, constructed and operative in accordance with a further embodiment of the disclosed technique; and

Figure 7 is a schematic illustration of a method for reducing spurious interferences in a received signal, operative in accordance with another embodiment of the disclosed technique.

DET AILED DESCRIPTION OF THE EMBODIMENTS

The disclosed technique overcomes the disadvantages of the prior art by providing system and method of reducing spurious signals in a received signal. According to the disclosed technique, a signal received by a first receiver, and is down-converted to a first selected Nyquist zone employing a first mixing scheme, producing a first down-converted signal. The signal is also received by at least a second receiver, in parallel with the first receiver, and is down-converted to a second selected Nyquist zone employing a second corresponding mixing scheme, thereby producing at least a second down-converted signal. The second selected Nyquist zone is different from the first selected Nyquist zone. The first down-converted signal and the second down-converted signal are sampled producing a first sampled signal and a second sampled signal respectively. Thereafter, a first frequency domain representation of the first sample signal is produced as well as a second frequency domain of the second sample signal. A modified frequency domain representation is produced from a combination of at least portions said first frequency domain representation and at least portions of said second frequency domain representation. For example, the modified frequency domain includes values from the first frequency domain representation, of the frequencies where the second mixing scheme introduces interferences. The modified frequency domain further includes values from the second frequency domain representation, of the frequencies where the first mixing scheme introduce interferences. The remaining frequencies are determined from the values of these remaining frequencies in both the first and the second frequency domain representations as further explained below. Thus, the modified frequency domain representation exhibits reduced interference. A time signal may be produced from the modified frequency domain representation. It is noted that the term‘value’ herein above and below relates to either a real or complex value of a characteristic of the signal (e.g., power, voltage, current). These values may be instantaneous values, average (e.g., over a time window) or Root-Mean-Squared (RMS) values.

Reference is now made to Figures 4A-4G, which are schematic illustrations of spectrum diagrams 100, 120, 130, 140, 150, 160 respectively, in accordance with an embodiment of the disclosed technique. In Figures 4A-4G the horizontal axis relates to frequency, denoted f, and the vertical axis relates to power, denoted‘P’. Figures 4A-4G an embodiment of the disclosed technique where the first receiver is a zero-IF receiver and the second receiver is either one of a low-IF receiver or a super-heterodyne receiver. With reference to Figures 4A, spectrum diagram 100 depicts a signal 102, exhibiting as bandwidth of BW, which is to be received by a zero-IF receiver, located at frequency F _SIG . This signal is to be mixed with a mixing signal exhibiting the frequency F_Lo, which is equal to the frequency F_SIG-

With reference to Figure 4B, spectrum diagram 120 depicts signal 102 which is to be received by a low-IF receiver. The signal to be received is to be mixed with an LO signal 122 located at F_L0. This shall result in a signal located at frequency F_|F. Flowever, also as explained above, an image-signal 124 located at F_!M, shall also be mixed or‘folded’ into the resulting signal, and shall interfere with the resulting mixed signal (i.e., the signal at the output of the mixer).

With reference to Figure 4C, spectrum diagram 130 exhibits a signal 132, corresponding to signal 102, which was down-converted by the zero-IF receiver and sample. Since the first receiver is a zero-IF receiver, signal 132 is located at frequency F_B, which is the baseband frequency. Also, signal 132 does not exhibit artifacts relating to image interference. However, signal 132 exhibits DC spurious 134, resulting from IQ imbalance and second order mixer non-linearity distortions. With reference to Figure 4D, spectrum diagram 140 exhibits a signal 142, corresponding to signal 102, which was down-converted by a low-IF receiver and sample. Since the second receiver is a low-IF receiver, signal 142 is located at frequency F . In the example brought forth in Figure 4D, F is located in the third Nyquist zone. Also, since the second receiver is a low-IF receiver, signal 142 exhibits an image-artifact 144, resulting from the mixing of image signal 124 (Figure 4B) into the output of the mixer.

The signals 132 and 142 are transformed to produce a respective frequency domain representation, for example, by performing a Fourier Transform (e.g., a Fast Fourier Transform - FFT) thereon. With reference to Figures 4E and 4F, spectrum diagram 150 depicts the first frequency domain representation signal 152 corresponding to signal 132 and spectrum diagram 160 depicts the second frequency domain representation 162 corresponding to signal 142. As such, first frequency domain representation 152 corresponds to the signal received by the zero-IF receiver and second frequency domain representation 162 corresponds to the signal received by the low-IF receiver. Similar to signal 132 (Figure 4C), frequency domain representation 152 exhibits a DC spurious 154. Similar to Signal 142 (Figure 4D), frequency domain representation 162 exhibits an image-artifact 164. It is noted that the frequency ranges of first frequency domain representations 152 and second frequency domain representation 162 are both between zero and F_s/2 as determined by the sampling frequency.

To reduce the spurious interferences, a modified frequency domain representation is produced, from a combination of at least portions of the values in first frequency domain representation 150 and at least portions of the values of second frequency domain representation 160. Referring to Figure 4G, where spectrum diagram 170 is a schematic illustration of is the modified frequency domain representation 172, and still referring to Figures 4E, 4F. According to one example, the values in the first frequency domain representations 150 are compared with the values in the second frequency domain representation 160. When the values of a frequency in the first frequency domain representations 152 and in second frequency domain representation 162 are similar (e.g., the difference therebetween is below a predetermined threshold), than the value of the corresponding frequency in the modified frequency domain representation is determined from one or both of the two values. For example, the value of a selected one of the first frequency domain representation 152 and the second frequency domain representation 162 is retained. As a further example, the values of the first frequency domain representation 152 and the second frequency domain representation 162 are averaged or the average noise value is employed. When the values of a frequency in the first frequency domain representations 152 and in the second frequency domain representation 162 are different (i.e., either higher or lower), then, the value of the corresponding frequency in the modified frequency domain representation is determined from the frequency domain representation corresponding to the receiver which is known to not exhibit interferences in that frequency. Referring to Figures 4E, 4F and 4G, the value of frequency 0 in first frequency domain representation 152 is different from the values of these frequencies in second frequency domain representation 162. Since, first frequency domain representation 152 corresponds to the zero-IF receiver, which is known to exhibit DC spurious interference, than the value of frequency 0 in second frequency domain representation 162 (i.e., corresponding to the low-IF receiver) are retained as the values of the frequency between 0 in modified frequency domain representation 172.

The values of frequency 1 of first frequency domain representation 152 and of second frequency domain representation 162 are similar. As such, the value of frequency 1 in modified frequency domain representation 172 is determined from the values of frequency 1 in first frequency domain representation 152 and in second frequency domain representation 162 (e.g., selecting one of the values or averaging the values). Similarly, the values of the frequencies from frequency 1 up to frequency 6 and from frequency 1 1 to frequency F_s/2 in modified frequency domain representation 172 are determined from the values of the corresponding frequencies of first frequency domain representation 152 and of second frequency domain representation 162.

The frequencies between 6-1 1 are known to include the signal to be received. Also, the value of frequency 8 in first frequency domain representation 152 is different from the value of this frequency in second frequency domain representation 162. Since, second frequency domain representation 162 corresponds to the low-IF receiver, which is known to exhibit image interference, than the values of frequencies 6-1 1 in first frequency domain representation 152 (i.e., corresponding to the zero-IF receiver) are retained as the values of frequencies 6-1 1 in modified frequency domain representation 172. Thus, the artifact due to the interferences (e.g., image or half-IF) in first frequency domain representation 152 is reduced in modified frequency domain representation 172. The above examples for reducing the interferences in a received signal apply also when one of the receivers employs a switch power supply or when both receivers employs switched power supplies of difference frequencies.

In practice, each of the FFTs of the signals corresponding to signals 132 and 142 (i.e., first frequency domain representation 152 and second frequency domain representation 162 respectively), result in a plurality of frequency bins between zero and F_s/2, where each bin is associated with a respective value.

The modified frequency domain representation includes the same number of frequency bins as the FFTs of the signals 132 and 142, between zero and F_s/2, where the value of each bin is determined as described above. In other words, each of first frequency domain representation 152 and second frequency domain representation 162 and modified frequency domain representation 172 is a vector of values, where each entry in the vector corresponds to a value of a frequency bin. The value of each bin modified frequency domain representation 172 is determined by comparing the values of corresponding bins the first frequency domain representation and in the second frequency domain representation as described above.

It is noted that in the example brought forth above in conjunction with Figures 4D, signal 102 was down-converted to the first Nyquist zone (i.e., signal 132 - Figure 4C) and to the third Nyquist zone (i.e., signal 142 - Figure 4D). In general, according to the disclosed technique, signal 102 can be down-converted to any two different Nyquist zones. However, when down-converting to an even Nyquist zone (e.g., second, fourth, six etc.) the spectrum of the resulting signal is a mirror image of the actual spectrum of the signal. On way to address this issue is to re-arrange the FFT vector or to index the FFT vector from end to start. It is further noted that for the sake of the simplicity of the explanation of the disclosed technique, in Figures 4A-4G the value of the signal characteristic of interest (e.g., voltage, current, power) is exemplified as a real valued number. However, as mentioned above, the value may also be a complex number, which includes information pertaining to the amplitude and phase of the signal characteristic on interest. When comparing two complex numbers, the two complex numbers are considered similar when the difference between the magnitudes and the phases are within respective magnitude and phase thresholds.

According to one alternative of the disclosed technique described hereinabove in conjunction with Figures 4A-4G, the receivers employed are, a zero-IF receiver and a low-IF receiver. Reference is now made to Figure 5, which is a schematic illustration of a receiving system, generally referenced 200, for reducing spurious interferences in a received signal, constructed and operative in accordance with another embodiment of the disclosed technique. System 200 includes an antenna 202, an RF interface 204, a first receiver 206_! a second receiver 206₂ and a processor 208. First receiver 206i employs a first mixing scheme (e.g., zero-IF) and includes a first in-phase-channel mixer 210_| and a first quadrature-phase-channel mixer 210_Q, an in-phase-channel Low Pass Filter (referred to as ‘LPF-I in Figure 5) 212_t and a quadrature-phase channel low pass filter (referred to as‘LPF-Q in Figure 5) 212_Q, a first in-phase-channel analog-to-digital converter (referred to as ‘ADC1 -I in Figure 5) 214_| and a first quadrature-phase-channel analog-to-digital converter (referred to as‘ADC1 -Q in Figure 5) 214_Q. First receiver 206_! further includes a fist Local Oscillator (LO) 216 and a first phase shifter 218. For the sake of brevity, the term Ίh-phase’ shall also be abbreviated herein as IP and the term‘quadrature-phase’ shall also be abbreviated herein as QP.

Second receiver 206₂ employs a second mixing scheme (e.g., low-IF) and includes a second IP-channel mixer 220_| and a second quadrature-phase mixer 220_Q, a IP-channel Band Pass Filter (referred to as‘BPF-I in Figure 5) 222_| and a QP-channel BPF (referred to as‘BPF-Q in Figure 5) 222_Q, a second IP-channel ADC converter 224_| and a second QP-channel ADC converter 224_Q. Second receiver 206₂ further includes a second LO 226 and a second phase shifter 228. The oscillation frequency of second LO 226 is different from the oscillation frequency of first LO 216. RF interface 204 at least includes a low noise amplifier. RF interface 204 may further include a pre-selection filter and an image filter arranged in a conventional manner both of which may attenuate the image signal. Processor 208 may be a general purpose processor, a DSP or may be implemented on an FPGA processor or an ASIC. Antenna 202 receives electromagnetic radiation, transforms the received electromagnetic radiation to an electrical received signal and provides the electric received signal to RF interface 204. RF interface 204 at least amplifies the electric received signal and provides the amplified electric received signal to first receiver 206i and to second receiver 206₂. In general, first receiver 206_! down-converts the amplified electric received signal to the first Nyquist zone, samples the down-converted received signal and to produce a first sampled signal. Similarly, second receiver down-converts the amplified electric received signal to a second selected Nyquist zone, different from the first selected Nyquist zone (e.g., the third Nyquist zone), samples the down-converted received signal to produce a second sample signal. First receiver 206_! and second receiver 206₂ provide the first sampled signal and the second sampled signal respectively to processor 208. Processor 208 produces a first frequency domain representation of the first sample signal and a second frequency domain representation from the second sample signal. Processor 208 produces a modified frequency domain representation from a combination of at least portions said the frequency domain representation and at least portions of the second frequency domain representation. For example, processor 208 produces a modified frequency domain representation such that the modified frequency domain includes values from the first frequency domain representation of the frequencies where the second mixing scheme introduces interferences and includes values from the second frequency domain representation, of the frequencies where the first mixing scheme introduce interferences. The remaining frequencies are determined from the values of these remaining frequencies in both the first and the second frequency domain representations. Processor 208 may further produce a time domain signal from the modified frequency domain representation. Specifically, RF interface 204 provides the amplified electric received signal to first in-phase mixer 210_| and a first quadrature-phase mixer 210_Q in first receiver 206_! , and to second in-phase mixer 220_| and a second quadrature-phase mixer 220_Q in second receiver 206₂. In first receiver 206i, first LO 216 produces a first mixing signal and provides the first mixing signal to first quadrature-phase mixer 210_Q and to first phase shifter 218. First phase shifter 218 shifts the phase of the first mixing signal by tt/2 radians and provides the phase shifted first mixing signal to first in-phase mixer 210_|. First in-phase mixer 210_| mixes the amplified electric received signal with the phase shifted first mixing signal to produce a first in-phase down-converted signal. First IP-channel mixer 210_| provides the first in-phase down-converted signal to IP-channel LPF 212_|, which filters the first in-phase down-converted signal. IP-channel LPF 212_| provides the first filtered in-phase down-converted signal to first IP-channel ADC 214_|. First IP-channel ADC 214_| samples the first filtered in-phase down-converted signal and provides the first in-phase sampled signal to processor 208.

Further in first receiver 206i, first quadrature-phase mixer 210_Q mixes the amplified electric received signal with the first mixing signal to produce a first quadrature-phase down-converted signal. First

QP-channel mixer 210_Q provides the first quadrature-phase down-converted signal to QP-channel LPF 212_Q, which filters the first quadrature-phase down-converted signal. QP-channel LPF 212_Q provides the first filtered quadrature-phase down-converted signal to first QP-channel ADC 214_Q. First QP-channel ADC 214_Q samples the first filtered quadrature-phase down-converted signal and provides the first quadrature-phase sampled signal to processor 208.

In second receiver 206₂, second LO 226 produces a second mixing signal. This second mixing signal exhibits a different frequency from the first mixing signal produced by first LO 216. While first LO 216 generates mixing-signal with frequency equal to the signal or carrier frequency, second LO 226 generates a mixing signal such the difference between frequency of the mixing signal and the frequency of the signal being received is equal to an intermediated frequency (e.g., F in Figures 4B and 4D above). Second LO 226 provides the second mixing signal to second QP-channel mixer 220_Q and to second phase shifter 228. Second phase shifter 228 shifts the phase of the second mixing signal by tt/2 radians and provides the second phase shifted mixing signal to second IP-channel mixer 220_|. Second IP-channel mixer 220_| mixes the amplified electric received signal with the phase shifted second mixing signal, to produce a second in-phase down-converted signal. Second IP-channel mixer 220_| provides the second in-phase down-converted signal to IP-channel BPF 222_|, which filters the second in-phase down-converted signal. IP BPF 222_| provides the second filtered in-phase down-converted signal to second IP-channel ADC 224_|. Second IP-channel ADC 224_| samples the second filtered in-phase down-converted signal and provides the second in-phase sampled signal to processor 208.

Further in second receiver 206₂, second QP-channel mixer 220_Q mixes the amplified electric received signal with the second mixing signal to produce a second quadrature-phase down-converted signal. Second QP-channel mixer 220_Q provides the second quadrature-phase down-converted signal to QP-channel BPF 222_Q, which filters the second quadrature-phase down-converted signal. QP-channel BPF 222_Q provides the second filtered quadrature-phase down-converted signal to second QP-channel ADC 224_Q. Second QP-channel ADC 224_Q samples the second filtered quadrature-phase down-converted signal and provides the second quadrature-phase sampled signal to processor 208.

Processor 208 employs the first in-phase sampled signal and the first quadrature-phase sample signal to determine a first frequency domain representation respective of the signal received by first receiver 206_! , for example, by employing the FFT. Similarly, processor 208 employs the second in-phase sampled signal and the second quadrature-phase sample signal to determine a second frequency domain representation respective of the signal received by second receiver 206₂. Processor 208 determines a modified frequency domain representation as described above in conjunction with Figures 4E-4G.

According to one alternative of the disclosed technique described hereinabove in conjunction with Figures 4A-4G, the receivers employed are, a zero-IF receiver and a super-heterodyne receiver. Reference is now made to Figure 6, which is a schematic illustration of a receiving system, generally referenced 250, for reducing spurious interferences in a received signal, constructed and operative in accordance with a further embodiment of the disclosed technique. System 250 includes an antenna 252, an RF interface 254, a first receiver 256i a second receiver 256₂ and a processor 258. First receiver 256i is a zero-IF receiver employing the zero-IF mixing scheme, and includes a first IP-channel mixer 260_| and a first QP-channel mixer 260_Q, a first IP-channel LPF 262_| (referred to as ‘LPF1-I in Figure 6) and a first QP-channel LPF 262_Q (referred to as ‘LPF1-Q in Figure 6), a first IP-channel ADC 264_| (referred to as ‘ADC1-I in Figure 6) and a first QP-channel analog-to-digital converter 264_Q (referred to as‘ADC1 -Q in Figure 6). First receiver 256i further includes a first LO 266 and a first phase shifter 268.

Second receiver 256₂ is a super-heterodyne receiver employing a second mixing scheme (e.g., super-heterodyne mixing ), and includes a second LO 270, an IF mixer 272, a second IP-channel mixer 274_| and a second QP-channel mixer 274_Q, a second IP-channel LPF 276_| (referred to as‘LPF2-I in Figure 6) and a second QP-channel LPF 276_Q (referred to as‘LPF2-Q in Figure 6), a second IP-channel analog-to-digital converter 278_| (referred to as ADC2-I in Figure 6) and a second QP-channel analog-to-digital converter 278_Q (referred to as ADC2-Q in Figure 6). Second receiver 256₂ further includes a third LO 280 and a second a second phase shifter 282. The oscillation frequency of first LO 266, second LO 270 and third LO 280 are all different one with respect to the other. RF interface 254 at least includes a low noise amplifier and may further include a pre-selection filter and an image filter arranged in a conventional manner. Processor 258 may also be a general purpose processor, a Digital Signal Processor (DSP) or may be implemented on a Field Programmable Gate Array (FPGA) processor or an Application Specific Integrated Circuit (ASIC).

Antenna 252 receives electromagnetic radiation, transforms the received electromagnetic radiation to an electrical received signal and provides the electric received signal to RF interface 254. RF interface 254 at least amplifies the electric received signal and provides the amplified electric received signal to first receiver 256i and to second receiver 256₂. Similar to system 200 (Figure 5), first receiver 256^ down-converts the amplified electric received signal to the first Nyquist zone, samples the down-converted received signal and to produce a first sampled signal. Second receiver down-converts the amplified electric received signal to a second selected Nyquist zone, different from the first selected Nyquist zone (e.g., the third Nyquist zone), samples the down-converted received signal to produce a second sample signal. First receiver 256i and second receiver 256₂ provide the first sampled signal and the second sampled signal respectively to processor 248. As described above in conjunction with Figures 4E-4G), processor 258 produces a first frequency domain representation of the first sample signal and a second frequency domain representation from the second sample signal. Processor 258 produces a modified frequency domain representation a combination of at least portions said first frequency domain representation and at least portions of said second frequency domain representation. For example, processor 258 produces a modified frequency domain representation such that the modified frequency domain includes values from the first frequency domain representation, of the frequencies where the second mixing scheme introduces interferences. The modified frequency domain further includes values from the second frequency domain representation, of the frequencies where the first mixing scheme introduce interferences. The remaining frequencies are determined from the values of these remaining frequencies in both the first and the second frequency domain representations. Processor 258 may further produce a time domain signal from the modified frequency domain representation

Specifically, RF interface 254 provides the amplified electric received signal to first IP-channel mixer 260_|, to first quadrature-phase mixer 260_Q in first receiver 256i, and to IF mixer 272. In first receiver 256_! , first LO 266 produces a first mixing signal and provides the first mixing signal to first QP-channel mixer 260_Q and to first phase shifter 268. First phase shifter 268 shifts the phase of the first mixing signal by tt/2 radians and provides the phase shifted first mixing signal to first in-phase mixer 260_|. First IP-channel mixer 260_| mixes the amplified electric received signal with the phase shifted first mixing signal to produce a first in-phase down-converted signal. First IP-channel mixer 260_| provides the first in-phase down-converted signal to first IP-channel LPF 262_h which filters the first in-phase down-converted signal. First IP-channel LPF 262_| provides the first filtered in-phase down-converted signal to first IP-channel ADC 264_|. First IP-channel ADC 264_| samples the first filtered in-phase down-converted signal and provides the first in-phase sampled signal to processor 258.

Further in first receiver 256^ first QP-channel mixer 260_Q mixes the amplified electric received signal with the first mixing signal to produce a first quadrature-phase down-converted signal. First QP-channel mixer 260_Q provides the first quadrature-phase down-converted signal to first QP-channel LPF 262_Q, which filters the first quadrature-phase down-converted signal. First QP-channel LPF 262_Q provides the first filtered quadrature-phase down-converted signal to first QP-channel ADC 264_q. First QP-channel ADC 264_Q samples the first filtered quadrature-phase down-converted signal and provides the first quadrature-phase sampled signal to processor 258.

In second receiver 256₂, second LO 270 produces a second mixing signal and provides this second mixing signal to IF mixer 272. IF mixer 272 mixes the amplified electric received signal with the second mixing signal to produce an intermediate received signal. IF mixer 272 provides the intermediate signal to second IP-channel mixer 274_| and to second QP-channel mixer 274_Q. Third LO 280 produces a third mixing signal. Third LO 280 provides the third mixing signal to second

QP-channel mixer 274_Q and to second phase shifter 282. Second phase shifter 282 shifts the phase of the third mixing signal by tt/2 radians and provides the third phase shifted mixing signal to second IP-channel mixer 274_|. Second IP-channel mixer 274_| mixes the intermediate received signal with the phase shifted third mixing signal to produce a second in-phase down-converted signal. Second IP-channel mixer 274_| provides the second in-phase down-converted signal to second IP-channel LPF 276_|, which filters the second in-phase down-converted signal. Second IP-channel LPF 276_| provides the second filtered in-phase down-converted signal to second IP-channel ADC 278. Second IP-channel ADC 278_| samples the second filtered in-phase down-converted signal and provides the second in-phase sampled signal to processor 258.

Further in second receiver 256₂, second QP-channel mixer 274_Q mixes the intermediate received signal with the third mixing signal to produce a second quadrature-phase down-converted signal. Second QP-channel mixer 274_Q provides the second quadrature-phase down-converted signal to second QP-channel LPF 276_Q, which filters the second quadrature-phase down-converted signal. Second QP-channel LPF 276_Q provides the second filtered quadrature-phase down-converted signal to second quadrature-phase ADC 278_Q. Second QP-channel ADC 278_Q samples the second filtered quadrature-phase down-converted signal and provides the second quadrature-phase sampled signal to processor 258.

Processor 258 employs the first in-phase sampled signal and the first quadrature-phase sample signal to determine a first frequency domain representation respective of the signal received by first receiver 256_! , for example, by employing the FFT. Similarly, processor 258 employs the second in-phase sampled signal and the second quadrature-phase sample signal to determine a second frequency domain representation respective of the signal received by second receiver 256₂. Processor 258 determines a modified frequency domain representation as described above in conjunction with Figures 4E-4G.

The exemplary system described above in conjunction with Figure 5 employed a zero-IF receiver and a low-IF receiver and the exemplary system described in conjunction with Figure 6 employed a zero-IF receiver and a super-heterodyne receiver. It is, however, noted that the disclosed technique is not limited to these combinations. Any combination of two different receivers with mixing schemes which down convert the received signal to two different Nyquist zones may be employed.

According to another embodiment of the disclosed technique, spectral pre-mapping may be employed to identify the frequencies in which interference occurs in each of the receivers. According one pre-mapping example, the above described examples for reducing the interferences in a received signal may be employed in communication systems that do not continuously transmit (e.g., Time Division Multiple Access - TDMA systems or Time Division Duplexing - TDD systems). In such systems, the time periods in which the receivers do not receive a signal (i.e., the receiver should receive noise) are employed to identify the frequencies in which interference occurs in each receiver. In such time periods, any value corresponding to a frequency in either the first frequency domain representation or the second frequency domain representation, which is greater than the expected noise, is determined as a spurious signal. Thus, in time-periods in which the receiver does receive a signal, the value of a frequency in the frequency domain representation corresponding to the receiver which does not exhibit interferences in that frequency, is selected as the value of the corresponding frequency in the modified frequency domain representation. The remaining frequencies in the modified frequency domain representation are determined from the values of corresponding remaining frequencies in both the first and the second frequency domain representations as described above.

According to yet another example of spectral pre-mapping, when the receiving system according to the disclosed technique and the transmitter are synchronized, the frequencies which exhibit spurious signals may be pre-mapped by employing a synchronization signal with a known spectrum. Consequently, an expected received spectrum of this synchronizing signal may also be determined. Each of the first frequency domain representation and the second frequency domain representation is compared with this expected spectrum. When the value of a frequency in one of the frequency domain representation is different from the expected received spectrum, than that frequency is determined as including a spurious signal and the value from the other frequency domain representation shall be employed (i.e., during reception after pre-mapping) as the value of the corresponding frequency in the modified frequency domain representation. The remaining frequencies in the modified frequency domain representation are determined from the values of corresponding remaining frequencies in both the first and the second frequency domain representations as described above.

The expected received spectrum may be determined by employing a propagation gain model (e.g., Friis’s equation combined with the system gains). Alternatively, the values of the expected received spectrum may be in an initial arbitrary scale. These values are then scaled according to the ratio between the average of the initial values of the expected received spectrum and the average of the values of first frequency domain representation and/or of the second frequency domain representation. Thus, the values of the first frequency domain representation and the second frequency domain representation can be compared with the scaled values of expected received spectrum. In general, the scales of the expected received spectrum and the first and second received spectrums may be normalized. When the transmitter and the receiving system according to the disclosed technique are not synchronized an auxiliary dedicated transmitter may be employed to transmit the signal of known spectrum.

Reference is now made to Figure 7, which is a schematic illustration of a method for reducing spurious interferences in a received signal, operative in accordance with another embodiment of the disclosed technique. In procedure 300, a received signal is down-converted to a first selected Nyquist zone employing a first mixing scheme, thereby producing a first down-converted signal. With reference to Figures 4A, 5, and 6, first receiver 206i (Figure 5) or first receiver 256i (Figure 6) down convert a signal, such as signal 102 to the first Nyquist zone. After procedure 300 the method proceeds to procedure 304.

In procedure 302, the received signal is down-converted to a second selected Nyquist zone, different from the first selected Nyquist zone, employing at least a second corresponding mixing scheme, thereby producing at least a second down-converted signal. The second mixing scheme may be a low-IF receiver or a super-heterodyne receiver. With reference to Figures 4A, 5, and 6, first receiver 206₂ (Figure 5) or first receiver 256₂ (Figure 6) down convert a signal, such as signal 102 (Figure 4A) to the second first Nyquist zone. After procedure 302 the method proceeds to procedure 306.

In procedure 304, the first down-converted signal is sampled, thereby producing a first sampled signal. With reference to Figures 4C, 5 and 6, first in-phase ADC 214_| and first quadrature-phase ADC 214_Q (Figure 5), or first in-phase ADC 264_| and first quadrature-phase ADC 264_Q (Figure 6) produce a first sampled down-converted signal such as signal 132 (Figure 4C). After procedure 304, the method proceeds to procedure 308.

In procedure 306, at least the second down-converted signal is sampled, thereby producing at least a second sampled signal. With reference to Figures 4D, 5 and 6, second in-phase ADC converter 224_| and second quadrature-phase ADC converter 224_Q (Figure 5), or second in-phase ADC 278_| and second quadrature-phase ADC 278_Q (Figure 6) produce a second sampled down -converted signal such as signal 142 (Figure 4D). After procedure 304, the method proceeds to procedure 310.

In procedure 308, a first frequency domain representation from the first sample signal. A frequency domain representation may be produced by applying the Fourier Transform to the first sampled signal. With reference to Figures 4E, 5 and 6, processor 208 or processor 258 produce a frequency domain representation of the first sampled signal. After procedure 308, the method proceeds to procedure 312.

In procedure 310, at least a second frequency domain representation from at least the second sample signal is produced. With reference to Figures 4E, 5 and 6, processor 208 or processor 258 produce a second frequency domain representation of at least the second sample signal. After procedure 310, the method proceeds to procedure 312.

In procedure 312 a modified frequency domain representation is produced, a combination of at least portions said first frequency domain representation and at least portions of said second frequency domain representation. According to one alternative, the modified frequency domain representation is produced by comparing the values of the frequencies in the first frequency domain representation with the values of the corresponding frequencies in the second frequency domain representation. The modified frequency domain representation includes values from the first frequency domain representation of the frequencies where the second mixing scheme introduces interferences, and includes values from the second frequency domain representation, of the frequencies where the first mixing scheme introduce interferences. The remaining frequencies are determined from the values of these remaining frequencies in both the first and the second frequency domain representations. For example, the values of the remaining frequencies are determined from the value of a selected one of the first frequency domain representation or the second frequency domain representation. As a further example, the values of the remaining frequencies are determined by averaging values of these frequencies in the first frequency domain representation and in the second frequency domain representation are averaged. Alternatively, the average noise value is employed. With reference to 4E-4G, Figures 5 and 6, processor 208 (Figure 5) or processor 258 (Figure 6) produce a modified frequency domain representation, such as modified frequency domain representation 172 (Figure 4G) from the first frequency representation (e.g., first frequency representation 152 - Figure 4E) and the second frequency representation (e.g., first frequency representation 172 - Figure 4F). In procedure 314, a time domain signal is produced from the modified frequency domain representation. The time is produced, for example, by employing the Inverse Fourier Transform. With reference to Figures 5 and 6, processor 208 (Figure 5) or processor 258 (Figure 6), produce a time domain signal from.

As mentioned above, the disclosed technique reduces interferences such as image signal (i.e., which is a 2^nd order mixer non-linearity distortion) as well as IQ imbalance and second mixer and fourth order mixer non-linearity distortion (e.g., half-IF). Image interferences and harmonics were explained above. Following is an explanation regarding IQ imbalance. IQ imbalance is caused since the gain of the two in-phase and quadrature phase channels, employed to produce the in-phase signal and quadrature-phase signal are not identical, and the phase shifter (e.g., phase shifters 218 or phase shifter 228 - Figure 5, phase shifter 266 or phase shifter 280 - Figure 6) do not shift the phase of the LO by exactly TT/2. This may cause amplitude and phase errors of the resulting complex signal. Since these errors do no change in time or change slowly, these errors are considered as DC spurious errors.

The disclosed technique enables to reduce spurious signals and thus improve the spectral“cleaness” of received signal and consequently improve the Signal to Noise Ratio (SNR) of the system. The disclosed technique enables to upgrade existing receiving systems by adding another receiver in parallel with the existing receiver. When designing new systems, the complexity of the new system may be reduced, for example, since the requirements on image filtering may be less strict.

It will be appreciated by persons skilled in the art that the disclosed technique is not limited to what has been particularly shown and described hereinabove. Rather the scope of the disclosed technique is defined only by the claims, which follow.

Claims

1. A receiving system for reducing spurious signals in a received signal comprising:

a first receiver, employing a first mixing scheme, down-converting a received signal to a first selected Nyquist zone, and for producing a first down-converted received signal, said first receiver further being for sampling said first down-converted received signal and for producing a first sampled signal;

at least a second receiver, employing a second mixing scheme, said second receiver being for down-converting said received signal to a second selected Nyquist zone, different from said first selected Nyquist zone, and for producing a second down-converted received signal, said second receiver further being for sampling said second down-converted received signal and for producing a second sampled signal;

a processor, coupled with said first receiver and with said second receiver, said processor being for producing a first frequency domain representation from said first sampled signal and a second frequency domain representation from said second sampled signal, said processor further being for producing a modified frequency domain representation from a combination of at least portions of said first frequency domain representation and at least portions of at least said second frequency domain representation, by comparing said first frequency domain representation with said second frequency domain representation,

said processor determines the value of a frequency in said modified frequency domain representation from at least one of the said values of the corresponding frequency in said first frequency domain representation and said second frequency domain representation, when the values of a frequency in said first frequency domain representation and in said second frequency domain representation are similar, and

said processor selects the value of the corresponding frequency in the modified frequency domain representation from a frequency domain representation corresponding to one of said first receiver and said second receiver which is known to not exhibit interferences in that frequency, when the values of a frequency in said first frequency domain representation and in said second frequency domain representation are different.

2. The system according to claim 1 , wherein frequencies which do not exhibit interferences are determined by spectral pre-mapping identify the frequencies in which interference occurs in each of said first receiver and said second receiver.

3. The system according to claim 2, wherein when said receiving system is employed in a communication systems that do not continuously transmit, time periods in which the receivers do not receive a signal are employed for said spectral pre-mapping,

wherein in said time periods, any value corresponding to a frequency in either said first frequency domain representation or said second frequency domain representation, which is greater than the expected noise, is determined as a spurious signal.

4. The system according to claim 2, when said receiving system is employed in synchronized communication systems, a synchronizing signal is employed for spectral pre-mapping,

wherein an expected received spectrum of a synchronizing signal is determined, wherein each of the first frequency domain representation and the second frequency domain representation is compared with this expected spectrum,

wherein When the value of a frequency in one of the frequency domain representation is different from the expected received spectrum, than that frequency is determined as including a spurious signal.

5. The system according to claim 1 , wherein each of said first mixing scheme and said second mixing scheme is one of zero-IF, low-IF and super-heterodyne.

6. The system according to claim 5, wherein said first mixing scheme is zero-IF and said second mixing scheme is low-IF.

7. The system according to claim 1 , wherein said first mixing scheme is zero-IF and said second mixing scheme is super-heterodyne.

8. The system according to claim 1 , wherein said processor further produces a time-domain signal from said modified frequency domain representation.

9. A method for reducing spurious signals in a received signal comprising the procedures of:

down-converting a received signal to a first selected Nyquist zone employing a first mixing scheme, thereby producing a first down-converted signal;

down-converting said received signal to at least a second selected Nyquist zone, different from said first selected Nyquist zone, employing at least a second mixing scheme, thereby producing at least a second down-converted signal;

sampling said first down-converted signal, thereby producing a first sampled signal;

sampling at least said second down-converted signal, thereby producing at least a second sampled signal;

producing a first frequency domain representation from said first sample signal;

producing at least a second frequency domain representation from said second sample signal; and

producing a modified frequency domain representation from a combination of at least portions said first frequency domain representation and at least portions of said second frequency domain representation, by comparing said first frequency domain representation with said second frequency domain representation, wherein when the values of a frequency in said first frequency domain representation and in said second frequency domain representation are similar, than the value of the corresponding frequency in said modified frequency domain representation is determined from at least one of the said values, and

wherein when the values of a frequency in said first frequency domain representation and in said second frequency domain representation are different, then, said processor selects the value of the corresponding frequency in the modified frequency domain representation from a frequency domain representation corresponding to one of said first receiver and said second receiver which is known to not exhibit interferences in that frequency.

10. The method according to claim 9, wherein frequencies which do not exhibit interferences are determined by spectral pre-mapping identify the frequencies in which interference occurs.

1 1. The method according to claim 10, wherein in communication systems that do not continuously transmit, time periods in which a signal is not received are employed for said spectral pre-mapping, wherein in said time periods, any value corresponding to a frequency in either said first frequency domain representation or said second frequency domain representation, which is greater than the expected noise, is determined as a spurious signal.

12. The method according to claim 10, wherein in synchronized communication systems, a synchronizing signal is employed for spectral pre-mapping,

wherein an expected received spectrum of a synchronizing signal is determined,

wherein each of the first frequency domain representation and the second frequency domain representation is compared with this expected spectrum,

13. The method according to claim 9, further including the procedure of producing a time domain signal from the modified frequency domain representation.

14. The system according to claim 9, wherein each of said first mixing scheme and said second mixing scheme is one of zero-IF, low-IF and super-heterodyne.

15. The system according to claim 14, wherein said first mixing scheme is zero-IF and said second mixing scheme is low-IF.

16. The system according to claim 9, wherein said first mixing scheme is zero-IF and said second mixing scheme is super-heterodyne.