Patent Publication Number: US-2022229171-A1

Title: System and method for microwave imaging

Description:
TECHNOLOGICAL FIELD 
     This disclosure generally relates to microwave imaging, and more particularly relates to microwave imaging using a dual-comb transceiver. 
     BACKGROUND 
     Microwave imaging is a technique used for identifying and evaluating concealed or embedded objects in a structure by using electromagnetic (EM) waves in microwave range. The technique can produce two-dimensional and even three-dimensional microwave images of the objects. The microwave imaging technique is used in different applications such as spectroscopy and Magnetic Resonance Imaging (MM). Traditional microwave imaging systems use waveguide tube to obtain an image. In the traditional microwave imaging systems, transmitting, and receiving antennas are in the form of a waveguide. Therefore, the overall volume of the traditional microwave imaging system is large. 
     Further, the microwave imaging technique can be performed in frequency-domain and in time-domain. In the frequency domain, an excitation signal of a continuous sinewave with frequency swept across a bandwidth of interest is used. Similarly, in the time domain, an excitation signal of an instantaneous short-duration pulse that contains the bandwidth of interest is used. However, designing a high-resolution broadband frequency sweeping circuit in the frequency domain is difficult. As a result, the microwave imaging using time-domain measurements is recognized as a viable alternative to the microwave imaging using frequency-domain measurement. However, microwave imaging systems using time domain measurements (also referred to as “time-domain system”) are bulky and expensive. 
     To lower the overall size of the time domain systems, some available methods use a specialized pulse generation circuit to construct a 3 to 10 GHz time-domain measuring system. However, the time-domain measuring system requires a high-sampling-rate oscilloscope for signal measurement. As a result, a discrete time-domain measurement system is used. However, it is still complicated in terms of jitter control. To that end, there is a need of a system that is self-sustainable (i.e., independent of any external laboratory), and an provide a high resolution in applications of imaging detection. 
     Generally, microwave imaging technology is applicable for disease diagnosis, early cancer detection, food safety and quality control, material characterization, and the like. Further, in many applications of the microwave imaging techniques such as MM, a high signal-to-noise ratio (SNR) is required. In particular, to provide high SNR, maintaining the synchronization and accuracy of repetition rate of signals is challenging. To that end there is a need, of a system that provides high SNR with accurate repetition rate of the signals. 
     BRIEF SUMMARY 
     It is an objective of some of the example embodiments disclosed herein to provide efficient solutions to the problems and challenges discussed above. More specifically, it is an objective of the various embodiments disclosed herein to provide a system having low sampling rate ADC, high SNR, high resolution and which is independent of any external laboratory. 
     According to some embodiments, a system for microwave imaging is provided. The system comprises a dual-comb transceiver module, comprising: at least one transmitter circuit; and a plurality of receiver circuits. The microwave imaging system further comprises a direct digital synthesizer (DDS) circuit configured to generate at least one comb signal, wherein the at least one comb signal is provided to the at least one transmitter circuit, and wherein the at least one transmitter circuit is configured to provide the at least one comb signal to the plurality of receiver circuits. 
     According to an embodiment, the present disclosure provides a method for microwave imaging is provided. The method comprises: transmitting an output signal from a transmitter to at least one receiver module via a channel; transmitting a portion of the output signal from the transmitter to a reference receiver module via an attenuator module; generating a first output signal by the at least one receiver module and a second output signal by the reference receiver module; and determining one or channel parameters associated with the microwave imaging based on the first output signal and the second output signal. 
     According to yet another embodiment, a dual-comb transceiver is provided. The dual-comb transceiver comprises a transmitter module configured to transmit an output signal; at least one receiver module configured to receive the output signal from the transmitter via a channel; and generate a first output signal; and a reference receiver module configured to receive a portion of the output signal transmitted by the transmitter module via an attenuator module; and generate a second output signal, wherein one or more channel parameters associated with the channel are determined based on the first output signal and the second output signal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Having thus described example embodiments of the present disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1A  illustrates a block diagram of a high-level workflow of microwave image generation using a dual-comb transceiver, according to an embodiment of the present disclosure; 
         FIG. 1B  illustrates a block diagram of a dual-comb transceiver used in an imaging device, according to an embodiment of the present disclosure; 
         FIG. 1C  illustrates graphical representation of waveforms of operations of the dual-comb transceiver, according to an embodiment of the present disclosure; 
         FIG. 2  illustrates a block diagram of a transmitter circuitry and receiver circuitry of the dual-comb transceiver, according to an embodiment of the present disclosure; 
         FIG. 3A  illustrates a block diagram of a global distribution architecture of a dual comb transceiver used in a microwave imaging system, according to an embodiment of the present disclosure; 
         FIG. 3B  illustrates another block diagram of the global distribution architecture of dual-comb transceiver shown in  FIG. 3A , according to an embodiment of the present disclosure; 
         FIG. 4A  illustrates a fabricated dual-comb transceiver, according to an embodiment of the present disclosure; 
         FIG. 4B  illustrates a graph showing different waveforms depicting level diagram of transmitter (TX) and receiver (RX) of the dual-comb transceiver shown earlier, according to an embodiment of the present disclosure; 
         FIG. 5A  illustrates a block diagram of the real time image generation implementation in field programmable gate array (FPGA), according to an embodiment of the present disclosure; 
         FIG. 5B  illustrates graphical diagrams showing an impact of averaging on quality of signal and SNR for diverse types of averaging, according to an embodiment of the present disclosure; 
         FIG. 6  illustrates a graph of measured output pulse of transmitter (TX), according to an embodiment of the present disclosure; 
         FIG. 7A  illustrates graphs comprising responses of the dual-comb transceiver to different attenuations in time domain, according to an embodiment of the present disclosure; 
         FIG. 7B  illustrates a graph comprising responses of the dual-comb transceiver to different attenuations in frequency domain, according to an embodiment of the present disclosure; 
         FIG. 8  illustrates a flow diagram of microwave imaging using the dual-comb transceiver, according to an embodiment of the present disclosure; and 
         FIG. 9  illustrates another flow diagram of microwave imaging using the dual-comb transceiver, according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be apparent, however, to one skilled in the art that the present disclosure may be practiced without these specific details. In other instances, apparatuses and methods are shown in block diagram form only in order to avoid obscuring the present disclosure. 
     Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. The appearance of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not for other embodiments. 
     Some embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all, embodiments of the present disclosure are shown. Indeed, various embodiments of the present disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like reference numerals refer to like elements throughout. As used herein, the terms “data,” “content,” “information,” and similar terms may be used interchangeably to refer to data capable of being transmitted, received and/or stored in accordance with embodiments of the present disclosure. Thus, use of any such terms should not be taken to limit the spirit and scope of embodiments of the present disclosure. 
     Additionally, as used herein, the term ‘circuitry’ may refer to (a) hardware-only circuit implementations (for example, implementations in analog circuitry and/or digital circuitry); (b) combinations of circuits and computer program product(s) comprising software and/or firmware instructions stored on one or more computer readable memories that work together to cause an apparatus (or a system) to perform one or more functions described herein; and (c) circuits, such as, for example, a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation even if the software or firmware is not physically present. This definition of ‘circuitry’ applies to all uses of this term herein, including in any claims. As a further example, as used herein, the term ‘circuitry’ also includes an implementation comprising one or more processors and/or portion(s) thereof and accompanying software and/or firmware. As another example, the term ‘circuitry’ as used herein also includes, for example, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in a server, a cellular network device, other network device, and/or other computing device. 
     The embodiments are described herein for illustrative purposes and are subject to many variations. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient but are intended to cover the application or implementation without departing from the spirit or the scope of the present disclosure. Further, it is to be understood that the phraseology and terminology employed herein are for the purpose of the description and should not be regarded as limiting. Any heading utilized within this description is for convenience only and has no legal or limiting effect. 
       FIG. 1A  illustrates a block diagram of a high-level workflow  100  of microwave image generation using a dual-comb transceiver, according to an embodiment of present disclosure. 
     A frequency comb is a signal whose spectrum consists of a series of discrete, equally spaced frequency lines. Frequency combs may be generated by using a circuit such as a direct digital synthesizer (DDS). The DDS is a frequency synthesizer technique for generating arbitrary waveforms from a single fixed-frequency reference clock. By using a reference clock frequency, DDS, at step  102 , is configured to generate the signal with accurate repetition rate. In the present disclosure, DDS is used for generating  102  comb 1  signal with the repetition rate of f r +Δf r  and comb 2  signal with the repetition rate of f r . 
     The dual-comb transceiver of the present disclosure associated with the DDS. In the dual-comb transceiver, each of the frequency combs has a different repetition rate. For example, the repetition rate of comb 1  is f r +Δf r  and the repetition rate of comb 2  is (f r ). Therefore, the workflow  100  comprises, at  102 , using the DDS to generate accurate frequency combs (a comb 1  and a comb 2 ). 
     Further, at step  104 , a synchronization operation is performed by the dual-comb transceiver to generate synchronized RF periodic pulses with comb 2  signal. The dual-comb-transceiver uses comb 1  and comb 2  signal as input and synchronizes the comb 1  and comb 2  signal. For the synchronization, the dual-comb-transceiver uses one transmitter (TX) chip to generate globally distributed comb 2  signal and multiple receiver (RX) chips. The comb 1  signal is provided to the TX chip and comb 2  signal is given as input to the multiple RX chips. Therefore, all the RX chips and comb 2  signal are synchronized together producing an analog signal comprising synchronized comb 1  signal and comb 2  signal. 
     Then, at step  106 , an analog to digital convertor (ADC) is used to perform the sampling of the output pulses of the dual-comb transceiver. For microwave image generation, the analog signal at the output of the dual-comb transceiver is required to be sampled and converted to digital signal. To that end, the ADC  106  is required for the sampling of the output signal of the dual-comb transceiver. Then, at step  108 , these samples are provided as input to an FPGA. The output of FPGA is associated with a processing module, including either or a combination of an MCU and a CPU. Further, by using delay, multiply and sum (DMAS) or machine learning (ML) algorithm, microwave images are generated. 
       FIG. 1B  illustrates a block diagram of a dual-comb transceiver  100   b , according to an embodiment of the present disclosure. The dual-comb transceiver  100   b  receives a pair of frequency combs (comb 1 , f r +Δf r    110   b  and comb 2 , f r    110   a ) is used. As used herein, the frequency comb is a source whose spectrum consists of a series of discrete, equally spaced frequency lines. The dual-comb transceiver  100   b  operates in half-duplex mode. (i.e., at one time it will operate either as a transmitter or as a receiver). The dual comb transceiver  100   b  may include a transmitter circuitry  120   a  and a receiver circuitry  120   b . The transmitter circuitry  120   a  may include an input switch  110 , a pulse shaping circuit  112 , a power amplifier (PA)  114 , an RF switch  116  and a coupler  118 . The receiver circuitry  120   b  may include an antenna switch  120 , a low noise amplifier (LNA)  124 , a mixer  126 , a low pass filter (LPF)  128  and a variable gain amplifier (VGA)  130 . 
     The dual comb transceiver  100   b  may operate with a plurality of pairs of frequency combs. In the dual comb transceiver  100   b , each pair of frequency combs of the plurality of the pairs of frequency combs has a different repetition rate and each pair of frequency combs of the plurality of the pairs of frequency combs consists of the first comb pulse (comb 1 ) with a repetition rate of f r +Δf r    110   b  and the second comb pulse (comb 2 ) with repetition rate of (f r )  110   a . Each pair of frequency combs of the plurality of the pairs of frequency combs is given as input to the input switch  110  of the dual-comb transceiver  100   b . Further, the input switch  110  selects between the pair of frequencies f r    110   a  and (f r +Δf r )  110   a . The output of the input switch  110  is connected to a pulse shaping circuit  112 . The pulse shaping circuit  112  includes an attenuator, a step recovery diode (SRD), a short-circuited stub, and a Schottky diode. The pulse shaping circuit  112  converts input sine wave into a short duration pulse to generate a pulse of specific input frequency. 
     In the pulse shaping circuit  112 , when the SRD is forward biased, it stores an electric charge and then releases it in a brief period of time when it is reverse biased. The SRD quickly shuts off after releasing the charge. An output signal of the SRD has an instant transition that is used to generate the short-duration pulse. An inverted and delayed version of the SRD output signal is formed by the short-circuited stub connected next to the SRD. The short duration pulse is produced by the SRD output signal and the inverted and delayed signal reflected off the short-circuited stub. A length of the short-circuited stub determines a width of the short duration pulse. Further, the Schottky diode is used to decrease ringing in the synthesized pulse and the attenuator is used to improve input matching. 
     The output of pulse shaping circuit  112  is given as an input to a power amplifier (PA)  114 , where the PA  114  is configured to take weak electrical signal as input and, with the help of an external power source, reproduce a stronger waveform at the output. Thus, the PA  114  amplifies the input signal. The output of the PA  114  is given as input to the radio frequency (RF)  116  switch. The RF switch  116 , also referred to as a microwave switch, allows high-frequency signals to be transferred between various transmission channels or devices. The RF switch  116  forwards the amplified signal to a TX path or to a local oscillator port  116   a  of the mixer  126  in a RX path. The TX path is formed by the coupler  118 , the antenna switch  120 , the RF port  122  and an output port  118   a . The RX path is formed by the mixer  126 , the LPF  128 , and the VGA  130 . The mixer  126 , also referred to as the frequency mixer, is a nonlinear electrical circuit that generates new frequencies by combining two signals (e.g., the amplified periodic pulse and the received periodic pulse). The mixer  126  receives two inputs, and outputs new signals based on the sum and difference of original frequencies of the input signals. There are three ports of the mixer  126 . First port is the input port  124   a , where the input signal that needs to be adjusted in frequency is received. The input signal is usually the incoming signal or equivalent and is usually at a low level in comparison to the other input obtained from a second port. At the second port, input signal is associated with the local oscillator (LO) port  116   a . The third port is the output port  126   a  of the mixer  126 . The output of the mixer  126  is referred to as intermediate frequency (IF). 
     When the RF switch  116  forwards the amplified signal through the TX path, the amplified signal would be passed via the coupler  118 , the antenna switch  120  to the RF port  122 . When the RF switch  116  forwards the PA  114  output pulse to the RX path, the PA  114  pulse is used as the LO signal  116   a  of the mixer  126  for multiplication with the received pulse from the RF port  122 . The antenna switch  120  passes the received pulse to the LNA  124 . The LNA  124  is an electronic amplifier that amplifies a low-power signal without lowering the signal-to-noise ratio appreciably. The LNA  124  boosts the power of both the signal and the noise at its input, but it also introduces some noise. The LNA  124  is configured to reduce the amount of extra noise. The frequency domain output of the mixer  126  is periodic, with a repetition rate of f r . Further, the LPF  128  preserves the lowest period of output signal of the mixer  126  while filtering out the remainder. The LPF  128  decreases the pulse amplitude in the time domain while allowing the system output pulse to be captured using a low sampling rate ADC. The periodic pulse is amplified using the low frequency variable gain amplifier (VGA)  130  after the LPF  128 . The VGA  130  is an electronic device (amplifier) that changes its gain in response to a control voltage (CV) applied to it. The VGA  130  is also used in synthesizers, amplitude modulation, and audio level reduction, among other uses. For the purpose of determining the proper gain and signal, the VGA  130  is utilized. Based on the amplification, the VGA  130  generates the output signals having amplitude value of BB+  130   a  and BB−  130   b.    
       FIG. 1C  illustrates graphical representation of different waveforms associated with operation of the dual comb transceiver  100   b  using short duration RF periodic pulses, according to an embodiment of the present disclosure. A first RF periodic pulse having repetition frequency (PRF) of f r  is multiplied by a second RF periodic pulse with a slightly different PRF of f r +Δf r  to produce a third periodic pulse with the PRF of Δf r  in the dual-comb transceiver  100   b . If one of the RF periodic pulses travels through an RF channel before being multiplied, the outcome is the third comb signal, which has the entire RF channel response (phase and magnitude) mapped in the base band frequency at the same time. In  FIG. 1C , there is shown a plurality of waveforms (e.g., a waveform  132   a , a waveform  132   b , a waveform  134 , a waveform  136   a , a waveform  136   b , and a waveform  138 ) The RF periodic pulse is represented in time domain in waveform  132   a  and in frequency domain in waveform  132   b . The multiplication result of the first RF periodic pulse and the second RF periodic pulses is shown in waveform  134 . The multiplication waveform  134  results in small value of amplitude when the first RF periodic pulse and the second RF periodic pulses have a small overlap, but as the overlap between them rises, their multiplication results in a larger amplitude, and when the first RF periodic pulse and the second RF periodic pulse are aligned, the resultant pulse amplitude reaches its highest value. The analogous spectra of multiplication results of the first RF periodic pulse and the second RF periodic pulses are sampled in frequency domain since they are periodic in time domain. The sampled spectral frequency comb is defined as a set of comb teeth spread at a distance equal to the PRF of the corresponding periodic pulse in time domain in waveform  136   a . In frequency domain, the corresponding representation of the periodic RF pulse and the baseband frequency pulse of the dual-comb operation is illustrated in waveforms  136   b  and  138 , respectively. The distance between the two comb teeth rises with frequency when comparing the two frequency combs in  136   b . This distance should always be less than 
     
       
         
           
             
               f 
               r 
             
             2 
           
         
       
     
     in the entire bandwidth (BW) of the RF pulse to have one-to-one mapping of the RF spectrum to baseband spectrum. This is implied by equation: 
     
       
         
           
             
               
                 
                   
                     
                       B 
                       ⁢ 
                       W 
                     
                     ≤ 
                     
                       
                         m 
                         ⁢ 
                         
                           f 
                           r 
                         
                       
                       2 
                     
                   
                   = 
                   
                     
                       f 
                       r 
                       2 
                     
                     
                       2 
                       ⁢ 
                       Δ 
                       ⁢ 
                       
                         f 
                         r 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where, m stands for compression factor and is equivalent to 
     
       
         
           
             
               
                 f 
                 r 
               
               
                 Δ 
                 ⁢ 
                 
                   f 
                   r 
                 
               
             
             . 
           
         
       
     
     It signifies that the time-domain RF pulse has been increased by m times, or that the RF bandwidth has been reduced by m times. In an example embodiment, the dual-comb transceiver  100   b  shown in  FIG. 1B  is designed to operate in the 1 to 3 GHz frequency range. Some embodiments are based on realization that the frequency range of 1 to 3 GHz is a suitable compromise between spatial resolution and the attenuation of microwave signals by an object subjected to microwave imaging (e.g., a human head). Accordingly, the frequency range 1 to 3 GHz is selected to design the dual-comb transceiver  100   b . To that end, the frequencies f r  and f r +Δf r  are set to 10 MHz and 1 kHz, respectively. As a result, the compression factor m is 10000, implying that RF frequencies of 1 to 3 GHz are mapped to baseband frequencies of 100 to 300 kHz. In some embodiments, when the dual-comb transceiver  100   b  is used in the MRI machine to obtain MM of human head, it is required to generate multiple images of the human head. Accordingly, the present disclosure describes a microwave imaging system which comprises the dual-comb transceiver  100   b  which is configured to provide multiple images of a target object, such as the human head. The detailed description such a microwave imaging system is described further with reference to  FIG. 2 . 
       FIG. 2  illustrates a block diagram of a microwave imaging system  200  including the transmitter circuitry  120   a  and the receiver circuitry  120   b  of the dual-comb transceiver  100   b , according to an embodiment of the present disclosure.  FIG. 2  is described below in conjunction with  FIG. 1B . The dual-comb transceiver  100   b  uses a first direct digital synthesizer (DDS)  220  and a second DDS  212  for generating analog waveforms, usually a sine wave, by generating time-varying signals with PRF of f r +Δf r  and f r , respectively. The advantages of DDS devices are low power, low cost, and single small package, combined with their inherent excellent performance and ability to digitally program and reprogram the output waveform. 
     The TX circuitry  120   a  is connected to the first DDS  220  and a first amplifier  222 . The first DDS  220  generates a sine wave with f r +Δf r  as its frequency. The RX circuitry  120   b  is connected to the second DDS  212  and a second amplifier  214 . The second DDS  212  generates a sine wave with f r  as its frequency. The output frequency f out  of a DDS is given by: 
     
       
         
           
             
               
                 
                   
                     
                       f 
                       
                         o 
                         ⁢ 
                         u 
                         ⁢ 
                         t 
                       
                     
                     ⁡ 
                     
                       ( 
                       M 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           f 
                           
                             c 
                             ⁢ 
                             l 
                             ⁢ 
                             k 
                           
                         
                         
                           2 
                           N 
                         
                       
                       × 
                       M 
                     
                     = 
                     
                       
                         f 
                         res 
                       
                       * 
                       M 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Where, M stands for binary tuning word, f clk  stands for an internal reference clock frequency (system clock) and N stands for a length of the phase accumulator in bits, which may be 32 bits in some examples. For example, if f clk =100 MHz and N=32 then f res =0.023283-Hz, and f out =0.023283×M, where M=1, 2, 3, . . . , 2 32 . For each of the two DDS, the DDS  220  and the DDS  212 , input is a signal with frequency f clk , output is a signal with frequency f out . Therefore, for the DDS  220 , f out =f r +Δf r  and for the DDS  212 , f out =f r . The binary tuning word M is a difference between the frequencies of the DDS  220  and the DDS  212 . Thus, by varying M and using equation (1), the frequencies of the DDS  220  and the DDS  212  can be set appropriately, as the corresponding output frequency f out . Furthermore, by using variable resolution frequency (f res ) associated with each of the DDS, frequency combs (comb 1  with PRF of f r +Δf r  and comb 2  with PRF of f r  are generated. By using DDS, the accuracy between frequency repetition rates f r  and f r +Δf r  is maintained based on equation (1). To that end, f res  is a configurable value and may be varied in varied embodiments, as desired by a user, or based on the application in which a dual-comb transceiver based on the DDS provided above is used. 
     Through a driver  208 , the output of the RX circuitry  120   b  is connected to an ADC  210 . The first DDS  220 , the second DDS  212  and the ADC  210  in the microwave imaging system  200  receive same clock  224 , which is shared by a clock distribution network  218  which is configured to distribute the clock  224  generated by the clock generator  216  to the different components of the microwave imaging system  200  (like the first DDS  220 , the second DDS  212  and the ADC  210 ). By the use of the single clock  224  for the multiple components of the microwave imaging system  200 , better synchronization between the different components may be achieved. 
     Apart from better synchronization between different components within the system, the microwave imaging system  200  also provides synchronization of comb 2  signal, which is of frequency f r , among different receiver circuitries. This is achieved by usage of a global distribution architecture illustrated in  FIG. 3A . 
       FIG. 3A  illustrates a block diagram of a global distribution architecture  300   a  of the comb 2  signal  312 , according to an embodiment of the present disclosure. The global distribution architecture  300   a  is proposed to maintain the signal synchronization of comb 2  signal  312  among a plurality of receiver circuitries. In this architecture, the plurality of receiver circuitries include: a receiver circuit RX 1   306 , a receiver circuit RX 2   308  and a receiver circuit RXN  310 . Further, the comb 2  signal  312  is provided to a second transmitter circuit (TX 2 )  304  and to each of the plurality of receiver circuits  306 ,  308  and  310 . To that end, the comb 2  signal  312  is generated by using a DDS circuit, such as the DDS circuit  220  and the DDS circuit  212  illustrated in  FIG. 2 , to generate accurate frequency which is used in global distribution architecture  300   a . The comb 2  signal  312  is globally distributed to the plurality of receivers like RX 1   306 , RX 2   308  and RXN  310 . Therefore all the receivers are synchronized together. Additionally, comb 1  signal  302   a  is given as input to a first transmitter (TX 1 ) circuit  302  and output is taken from a first output port RFOUT  302   b.    
     Further, the comb 2  signal  312  is obtained from a second output port RFOUT port  304   a  of the second transmitter (TX 2 ) circuit  304  and is provided to the plurality of receivers RX 1   306 , RX 2   308 , and RXN  310 . Each of the plurality of receivers RX 1   306 , RX 2   308  and RXN  310  has six input ports. In each of the plurality of receivers RX 1   306  to RXN  310 , first port is comb 2  signal  312  port, which is configured to receive comb 2  signal  312  as input. The other ports is used as ports RFIN 0   306   a , RFIN 1   306   b , RFIN 2   306   c , RFIN 3   306   d , and RFIN 4   306   e  in the receiver circuit RX 1   306 ; RFIN 0   308   a , RFIN 1   308   b , RFIN 2   308   c , RFIN 3   308   d , and RFIN 4   308   e  for the receiver circuit RX 2   308 , and RFIN 0   310   a , RFIN 1   310   b , RFIN 2   310   c , RFIN 3   310   d , and RFIN 4   310   e  for the receiver circuit RXN  310  are configured to receive RF pulses. Another possible architecture for global distribution of comb 2  signal is by using a transceiver circuit, which is illustrated in  FIG. 3B . 
       FIG. 3B  illustrates a block diagram showing another global distribution architecture using a dual-comb transceiver, according to an embodiments of the present disclosure. 
       FIG. 3B  is described below in conjunction with  FIG. 3A . The global distribution architecture shown in  FIG. 3B  is based on a dual-comb transceiver system  300   b . The dual-comb transceiver system  300   b  comprises a single chip or an integrated circuit package which includes all the components for implementing the global distribution architecture analogous to the global distribution architecture  300   a  shown in  FIG. 3A  on a single chip or package. 
     The dual-comb transceiver system  300   b  includes a first path  322 , a second path  332  and a third path  334 . The first path  322  is for generating the RFOUT pulse  330  for comb 1   336   a . The second path  332  is used to generate the RFOUT pulse  330  for comb 2  signal  336 . The third path  334  is for global distribution of comb 2  signal  336  to the plurality of receivers. The global distribution of comb 2  signal  336  to the plurality of receivers helps to maintain the synchronization between each of them. The first path is travelled by the comb 1  signal  336   a , and the second path  332  is travelled by comb 2  signal  336 . The comb 2  signal  336  is generated by the DDS  354  which can generate exactly accurate repetition frequency. 
     In the third path  334 , the comb 2  signal  336  is given to a plurality of receivers at their corresponding input ports, such as: a first input port RFIN 0   338   a , a second input port RFIN 1   338   b , a third input port RFIN 2   338   c , a fourth input port RFIN 3   338   d , and a fifth input port RFIN 4   338   e.    
     From the corresponding input ports, the global distribution comb 2  signal  336  is forwarded as the RF pulse to such as: a first LNA  350   a , a second LNA  350   b , a third LNA  350   c , a fourth LNA  350   d  and a fifth LNA  350   e , respectively. Further, the output of each LNA is given as input to VGA or corresponding RX Baseband module. For example, a first RX Baseband  314   a , a second RX Baseband  314   b , a third RX Baseband  314   c , a fourth RX Baseband  314   d , and a fifth RX Baseband  314   e  through their corresponding mixer modules, which include: a first mixer  340   a , a second mixer  340   b , a third mixer  340   c , a fourth mixer  340   d , and a fifth mixer  340   e , respectively. The output signal of each RX Baseband is generated by their corresponding ADCs, viz: a first ADC  352   a , a second ADC  352   b , a third ADC  352   c , a fourth ADC  352   d , and a fifth ADC  352   e , respectively. The output signals from the any of the RX baseband circuits is then used for further processing in applications of microwave imaging. For example, a delay-multiply-and-sum (DMAS) algorithm or machine learning algorithm may be used to generate images for microwave imaging, based on the output of the dual comb transceiver  100   b  or  300   b  described above. 
     Some embodiments are based on the recognition that a selection of an appropriate path for transmission is done based on a selection signal input LO_sel  346  as select line of a first multiplexer (mux)  342  module, which selects either of: the comb 2  signal as input (comb 2 _in)  336 , or the signal which is produced from an oscillator OSC  320  as input. The OSC  320  signal is passed through the phase locked loop PLL  316  and a pulse generator PG  326  to the input of the first mux  342 . When the selection signal input LO_sel  346  of the first mux  342  is high, then comb 2 _in  336  is passed as LO signal  344  to each of mixer modules  340   a - 340   e , in the plurality of receiver modules, else OSC signal  320  is passed as LO signal  344  to the plurality of mixer modules  340   a - 340   e . Each mixer uses the same LO signal  344  as comb 2 _in  336  or OSC signal  322 , which is passed through PLL  316  and PG  326 . 
     The first path  322  and the second path  332  is selected using second mux  348 . When the select line Comb_sel  324  is low, then first path  322  is selected otherwise second path  332  is selected. If select line Comb_sel  324  of the second mux  348  is high, then the output of the comb 2  signal  336  is received through the PLL  316 _ 2 , PG  326 _ 2  and PA  328  from an output port RFOUT  330  else comb 1  signal&#39;s output is received through the PLL 316 , PG  326  and PA  328  from the output port RFOUT  330 . The second mux  348  receives an input from the OSC  320 , which is producing an input signal that has frequency of 40 MHz. The output of the DDS  354  is provided as input  318  to the second mux  348 . By using DDS  354 , the accuracy in repetition rate between multiple receivers (RX 1   306 , RX 2   308 , and RXN  310 ) and comb 2  signal  336  may be achieved. 
     Thus, the first path  322  incudes signal transmission in the sequence OSC  320 -&gt;mux  348 -&gt;PLL  316 -&gt;PG  326 -&gt;PA  328  a transmit the signal out from the output port RFOUT  330 . Similarly, the second path  332  includes signal transmission in the sequence OSC  320 -&gt;PLL  316 _ 2 -&gt;PG  326 _ 2 -&gt;mux  342 -&gt;go to all receiving paths. Alternately, the mux  342  may also select the signal from outside by using comb 2 _in  336 +LNA  350   f.    
       FIG. 4A  illustrates an integrated circuit diagram of fabricated dual-comb transceiver  400 , according to an embodiments of the present disclosure. The dual comb transceiver  400   a  is analogous to the dual-comb transceiver  100   b  and accordingly, all the components of dual-comb transceiver  100   b  are fabricated on a printed circuit board (PCB). From the left side of the PCB, a peripheral component interconnect Express (PCIe) connector is utilized to apply input sine waves with frequencies of f r  and f r +Δf r  and receive the output differential periodic pulse in baseband (BB) with the amplitude of BB+  130   a , and BB−  130   b . The operations of the dual-comb transceiver  400  ( 100   b ) is already explained above with reference to  FIG. 1B . 
     All the components like DDS, PA  114 , VGA  130 , SRD, RF switch  116 , clock generator  216 , clock distributor  218 , mixer  126 , ADC  210  driver  208 , LPF  128  and like are utilized in the fabrication of the dual-comb transceiver  400  which is shown in  FIG. 4A . For example, an AD9913 DDS may create sine wave signals with a maximum frequency of 104 MHz and a frequency resolution of 0.058 Hz or greater at speeds up to 250 MSPS. A current parameter of the digital-to-analog converter (DAC) can be used to alter the DDS output voltage level. The AD7760 ADC is a 2.5 MSPS, 24-bit sigma-delta ADC that passes the Nyquist criterion for high-resolution sampling of the 104-300 kHz baseband pulse. Because the output periodic pulse is repeated with PRF f r  of 1 kHz, the output pulse has a period of 2500 ADC samples. The remaining components were chosen to fulfill the bandwidth and link budget constraints. 
       FIG. 4B  illustrates a graph  402  of a transmitter (TX) pulse  402   a  and a receiver (RX) pulse  402   b  of the dual-comb transceiver  100   b  shown in  FIG. 1B , according to an embodiment of the present disclosure. In  FIG. 4B , X-axis represents the components of the dual-comb transceiver  100   b  and their corresponding voltage (mVpp) is represented on the Y-axis. All the components of the dual-comb transceiver  110   b  are used and the channel  204  between TX and RX is attenuated by 40 dB. The graph of TX pulse  402   a  begins after the pulse-shaping circuit  112 , which generates the wideband pulse of frequency 1 to 3 GHz. The graph of RX pulse  402   b  is dramatically reduced following the LPF, as illustrated by point  402   b   1 . 
       FIG. 5A  illustrates a block diagram  500  of real time microwave image generation implementation in a field-programmable gate array (FPGA)  516 , according to an embodiment of the present disclosure. The FPGA  516  includes a MUX  504  and a block memory  510 . A first block  526  is used as a select line of the MUX  504 . The averaging function is implemented in the FPGA  516  for real time averaging to improve the signal-to-noise ratio (SNR) and shorten the averaging time. One block of data is one period of the output signal, which is 1 millisecond (ms) and equals 2500 ADC samples. The FPGA  516  is set up to add the current block&#39;s samples  508  to the incoming block&#39;s samples  512 . To produce the sum of all incoming ADC data blocks  502 , the Read-Modify-Write procedure is used. The first Block select signal  526  is set to 1 when the first data block (ADC data of first one period) arrives, and the ADC data  514  is written to a block memory  510 . The first Block is 0 for all future data blocks. The previous ADC data  512  is read from the block memory  510 , combined with the incoming data  502 , and written to the same position. After all of the blocks have been collected, the averaged data is generated by reading all of the data  514  from the FPGA  516  memory and dividing it by a number of data blocks  518  using a division block  520  of a microcontroller (MCU)  522 . The FPGA  516  can also insert or delete one ADC data block to align the data block timing after receiving a predetermined amount of data blocks to counteract data block timing inaccuracy. This function is customizable by MCU  522  to provide for flexibility. An output  524  of the MCU  522  is given to a processing module, such as a CPU for further processing and image generation. For example, a delay-multiply-and-sum (DMAS) algorithm or machine learning algorithm may be used to generate images for microwave imaging, based on the output of the dual comb transceiver  100   b  or  300   b  described above. 
       FIG. 5B  illustrates graphical representations  528  of an impact of averaging on quality of a signal  528   a ,  528   b  and SNR  528   c  for diverse types of averaging, according to an embodiment of the present disclosure.  FIG. 5B  depicts an example of a dual-comb system, averaged 1024 times and windowed at 0.05 ms. In the graph  528   a , Y-axis represents the voltage (V), and X-axis represents the time (ms)  528   a   1 . Assess the impact of averaging number on the quality of a dual-comb system output signal in time domain is shown in the graph  528   a , the system output is captured with different averaging numbers such as 16, 64, 128, 256, 512, 1024, 2048, and 4096, while each measurement is repeated 10 times and the channel attenuation is set to 40 dB. The FFT of the time-domain signal in graph  528   b  is shown from 104 KHz to 300 KHz. In the graph  528   b , y-axis represents power (dBm) and x-axis represents frequency (KHz)  528   b   1 . In the graph  528   c , SNR or signal-to-noise ratio, is defined on y-axis and number of averaging on x-axis  528   c   1  as a measure of magnitude variations caused by noise in the output signal spectrum in frequency domain. as a result, the SNR for a time-domain signal, such as y, is defined as 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       N 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         R 
                         ⁡ 
                         
                           ( 
                           f 
                           ) 
                         
                       
                     
                     = 
                     
                       2 
                       ⁢ 
                       0 
                       × 
                       
                         log 
                         10 
                       
                       ⁢ 
                       
                         
                            
                           
                             
                               Y 
                               ⁡ 
                               
                                 ( 
                                 f 
                                 ) 
                               
                             
                             _ 
                           
                            
                         
                         
                           σ 
                           ⁡ 
                           
                             ( 
                             f 
                             ) 
                           
                         
                       
                     
                   
                   , 
                   dB 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     where Y(f) is the FFT of y, j is the absolute value, (Y(f)) is the averaging operation, and σ(f) is the standard deviation of Y(f). The graph  528   c  depicts the SNR versus the number of averaging points for five different frequencies, 200 KHz  528   c   2 , 150 KHz  528   c   3 , 104 KHz  528   c   4 , 250 KHz  528   c   5 , and 300 kHz  528   c   6 . As shown in  FIG. 5B , after  2000  averaging, the SNR remains flat, and increasing the number of averaging does not improve the SNR. and an ADC quantization noise dominates the noise. It should be noted that the highest SNR is obtained at 200 kHz  528   c   2 . The ADC Quantization noise is a quantization error model which is introduced by the quantization of the ADC. 
       FIG. 6  is a graph that illustrates the measured output pulse of TX  402   a  according to an embodiment of the present disclosure. In the graph  602 , x-axis represents the time in nano second (ns) and y axis represents the voltage (V). In graph  604 , x-axis represents the frequency in GHz and y-axis represents the power in dBm. The constructed TX/RX module is used to implement the dual-comb transceiver  100   b  with one transmitter and one receiver. The response of the dual-comb transceiver  300   b  in time domain and frequency is depicted in  602  and  604 , respectively. The peak-to-peak voltage of the output pulse of the TX  402   a  is 1.85 Vpp, as shown in graph  602 . The FFT of the output pulse of the TX  402   a  is shown in graph  604 . The output pulse of the TX  402   a  has a maximum power of 33 dBm at 2.40 GHz, as shown in graph  604 . The pulse&#39;s 10-dB bandwidth is 2.66 GHz, with in the range of 0.36 to 3.02 GHz. 
       FIG. 7A  illustrates graphs  700  comprising responses of the dual-comb transceiver  100   b  to different attenuations in time domain, according to an embodiment of the present disclosure. In the graphs  700 , voltage is representing y-axis and time (ms) is representing x-axis. These graphs show the baseband (BB) output of a dual-comb transceiver in response to various channel attenuations of 20 dB  702   a,  30 dB  702   b,  40 dB  702   c,  50 dB  702   d,  60 dB  702   e , and 70 dB  702   f . The graph of output voltage amplitude is indicating non-linear behavior, according to the time-domain measurement and the output voltage amplitude is equivalent to the noise voltage. The output voltage changes linearly for the 30 dB  702   b,  40 dB  702   c,  50 dB  702   d , and 60 dB  702   e  channel attenuations. 
       FIG. 7B  illustrates a graph  704  showing the dual-comb transceiver  100   b  responses to different attenuations in frequency domain, according to an embodiment of the present disclosure. The frequency response of dual-comb transceiver from 104 to 300 kHz is shown on the x-axis. The Power (dBm/10 KHz) of the signal represents on the y-axis. This frequency range is translated to 1 to 3 GHz RF response. The frequency response is calculated by taking FFT from the time-domain measured results of  FIG. 7A . The length of the time-domain signal used for taking FFT is 0.1 ms, therefore the frequency resolution is 10 kHz. By limiting the length of the time-domain signal further, the noise is reduced. The signal peak is also aligned to be located in the middle of the period. In this  FIG. 704 a    represents attenuations of 20 dB,  704   b  represents attenuations of 30 dB,  704   c  represents attenuations of 40 dB,  704   d  represents attenuations of 50 dB,  704   e  represents attenuations of 60 dB, and  704   f  represents attenuations of 70 dB. When the channel attenuation is 20 dB  704   a , the distance between the 20 dB  704   a  and 30 dB  704   b  curves is less than 10 dB, which is due to RX saturation and non-linear response. When the channel attenuation is 70 dB  704   f , noise affects the signal level above 230 kHz. 
       FIG. 8  illustrates a flow diagram of microwave imaging using dual-comb transceiver  100   b ,  300   a  or  300   b , according to an embodiment of the present disclosure. At step  802 , the pair of frequency combs (comb 1  and comb 2 ) which are different in repetition rate is generated by using DDS. Comb 1  signal have repetition rate of f r +Δf r  and comb 2  signal have repetition rate of f r . DDS is able to maintain the accuracy of repetition rates between the multiple signals. At step  804 , the pair of frequency comb is given as input to the input switch  110  of the dual-comb transceiver  104  and each pair of frequency comb consists of a first frequency comb pulse (comb 1 ) and second comb pulse (comb 2 ). For example, first frequency comb pulse (comb 1 ) has repetition rate of f r +Δf r    110   a  and a second frequency comb pulse (comb 2 ) has repetition rate of f r    110   b . Further, at step  806 , the output of the input switch  110  is provided to the pulse shaping circuit  112  to generate the short duration pulse. At step  808 , the short duration pulse is amplified by using the power amplifier  114  and passed to the RF switch  116  at step  810 . The output of RF pulse follows either TX path or the RX path. At step  812 , if the RF pulse is passing via RX path, then step  818  is executed otherwise, step  814  is executed. At step  814 , the output of the RF switch  116  is transmitting through the coupler  118  to the antenna switch  120  further, at step  816 , the antenna switch  120  is passing the RF pulse to the RF port  122 . At step  818 , the transmitted signal (which is transmitted to the RF port  122 , when the RF pulse follows the TX path) is received by the antenna switch  120  from the RF port  122 . Further, at step  820 , the antenna switch  120  sends the receiving pulse to the LNA  124  which amplifies the RF pulse and sends to the mixer  126 . At step  822 , the output of the PA  114  is again used as LO signal  116   a  to the mixer via RF switch  116 . Further at step  824 , the output of the mixer  126  is passing through the LPF  128  to get only low frequency signal. At step  826 , the output of the LPF  128  is provided to the VGA  130  and receives the RF pulse with amplitude of BB+  130   a  and BB−  130   b . Further at step  828 , the output of VGA  130  is sampled. For example, the output of the VGA  130  is passing to the ADC  210  through the drive  208  to get data samples. Further at step  830 , output of the ADC is stored in the block memory of the FPGA. For example, the output of the ADC is 210 is stored in the block memory  510  of the FPGA  516  At step  832 , data from the block memory of the FPGA is provided to the processing module. For example, data from the block memory  510  of the FPGA  516  is provided to microcontroller  522  and CPU  524 . At step  834 , by using delay-multiply-and-sum (DMAS) algorithm or with machine learning algorithm, microwave images are generated. 
       FIG. 9  illustrates another flow diagram of a method  900  for microwave imaging, according to an embodiment of the present disclosure. The method  900  starts at step  901  and proceeds to step  903 . 
     At step  903 , at least one comb signal is generated by a DDS circuit. The at least one comb signal has an output frequency of f out . For example, referring to  FIG. 2 , the DDS  220  circuit generates an output signal (corresponding to a frequency comb comb 1  or comb) of frequency f r +Δf r . (or f r ) Thus, it may be considered that f out =f r +Δf r  (or f r ) 
     Then, at step  905 , the generated at least one comb signal is provided as an input to a transmitter circuit. For example, referring to  FIG. 3A , the transmitter circuit TX 2   304  receives the input signal as comb 2  signal. This signal may be generated by a DDS circuit, (such as the DDS circuit  212  or the DDS circuit  220 ). 
     Further, at step  907 , the at least one comb signal may be transmitted from the at least one transmitter circuit to a plurality of receiver circuits for microwave imaging. For example, referring again to  FIG. 3A , the comb 2  signal output by the transmitter circuit TX 2   304  is provided as input to the plurality of receiver circuits, RX 1   306 , RX 2   308  and RXN  310 . 
     Also, the DDS circuit is configured to generate the output signal based on a clock signal which is provided as an input to the DDS circuit. The output frequency f out  of a DDS is given by: 
     
       
         
           
             
               
                 
                   
                     
                       f 
                       
                         o 
                         ⁢ 
                         u 
                         ⁢ 
                         t 
                       
                     
                     ⁡ 
                     
                       ( 
                       M 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           f 
                           
                             c 
                             ⁢ 
                             l 
                             ⁢ 
                             k 
                           
                         
                         
                           2 
                           N 
                         
                       
                       × 
                       M 
                     
                     = 
                     
                       
                         f 
                         res 
                       
                       * 
                       M 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Where, M stands for binary tuning word, f clk  stands for an internal reference clock frequency (system clock generated by a clock generator) and N stands for a length of the phase accumulator in bits. For example, if f clk =100 MHz and N=32 then f res =0.023283-Hz, and f out =0.023283× M, where M=1, 2, 3, . . . , 2 32 . Furthermore, by using variable resolution frequency (f res ) associated with the DDS, frequency combs (comb 1  with PRF of f r +Δf and comb 2  with PRF of f r ) are generated. By using DDS, the accuracy between frequency repetition rates f r  and f r +Δf is maintained based on equation (1). To that end, f res  is a configurable value and may be varied in varied embodiments, as desired by a user, or based on the application in which a dual-comb transceiver based on the DDS provided above is used. 
     The generation of the frequency comb signal based on relation given above by the DDS circuit and providing the same comb signal as input to the plurality of receivers in the manner described in various embodiments above, provides the advantages of accurate frequency generation, synchronization between the plurality of receivers, maintenance of high SNR and accurate image generation in microwave imaging applications. 
     Many modifications and other embodiments of the present disclosure set forth herein will come to mind to one skilled in the art to which the present disclosure pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the present disclosure is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Moreover, although the foregoing descriptions and the associated drawings describe example embodiments in the context of certain example combinations of elements and/or functions, it should be appreciated that different combinations of elements and/or functions may be provided by alternative embodiments without departing from the scope of the appended claims. In this regard, for example, different combinations of elements and/or functions than those explicitly described above are also contemplated as may be set forth in some of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.