Patent Publication Number: US-10764096-B1

Title: Demodulator and method of demodulating ASK signal

Description:
TECHNICAL FIELD 
     The present application relates to a demodulation of an Amplitude Modulated (AM) signal, particularly to a demodulator, a method of demodulating an Amplitude Shift Keying (ASK) signal, and a Composite Password Card (CPC) including the demodulator. 
     BACKGROUND OF THE APPLICATION 
     ASK modulation is widely used in communication systems. For example, in an Electronic Toll Collection (ETC) system, a Road Side Unit (RSU) typically broadcasts ASK signals (such as a wake-up signal), and a demodulator in a Composite Password Card (CPC) carried by a vehicle typically receives and demodulates the ASK signals. 
     Conventionally, the demodulator may have inconsistent sensibility to ASK signals received from different directions, and may have high power consumption, which may affect life time of a disposable battery in the demodulator. Therefore, a new demodulator with consistent sensibility to ASK signals received from different directions and low power consumption becomes highly desirable. 
     BRIEF DESCRIPTION OF THE APPLICATION 
     According to an embodiment, a demodulator may comprise a first demodulator branch, a second demodulator branch, and a DC circuit. The first demodulator branch may comprise: a first antenna, a first coupling capacitor, a first band pass filter, a first low pass filter, and a first DC blocking capacitor that are electrically connected in series, wherein the first DC blocking capacitor is electrically connected to a load resistor. The second demodulator branch may comprise: a second antenna, a second coupling capacitor, a second band pass filter, a second low pass filter, and a second DC blocking capacitor that are electrically connected in series, wherein the second DC blocking capacitor is electrically connected to the load resistor. The DC circuit may comprise: a DC power supply, a first bias resistor, a first diode, a choke inductor, a second bias resistor, and a second diode that are electrically connected in series, wherein an anode of the first diode is electrically connected to a first spot in the first demodulator branch, wherein an anode of the second diode is electrically connected to a second spot in the second demodulator branch, and wherein the DC circuit comprises a first bypass capacitor and a second bypass capacitor with different capacitances. 
     According to an embodiment, a method for demodulating an ASK signal may comprise receiving the ASK signal with a demodulator, and demodulating the ASK signal with the demodulator. The demodulator may include: a first demodulator branch comprising a first antenna, a first coupling capacitor, a first band pass filter, a first low pass filter, and a first DC blocking capacitor electrically connected in series, wherein the first DC blocking capacitor is electrically connected to a load resistor; a second demodulator branch comprising a second antenna, a second coupling capacitor, a second band pass filter, a second low pass filter, and a second DC blocking capacitor electrically connected in series, wherein the second DC blocking capacitor is electrically connected to the load resistor; and a DC circuit comprising a DC power supply, a first bias resistor, a first diode, a choke inductor, a second bias resistor, and a second diode electrically connected in series. An anode of the first diode is electrically connected to a first spot in the first demodulator branch. An anode of the second diode is electrically connected to a second spot in the second demodulator branch. The DC circuit may comprise a first bypass capacitor and a second bypass capacitor with different capacitances. 
     According to an embodiment, a CPC of an electronic toll collection system may comprise a demodulator configured to demodulate an ASK signal to obtain a base band signal; a wake-up circuit connected to the demodulator and configured to receive the base band signal; and a primary circuit connected to the wake-up circuit. The demodulator may comprise: a first demodulator branch comprising a first antenna, a first band pass filter, and a first low pass filter electrically connected in series and connected to a load resistor; a second demodulator branch comprising a second antenna, a second band pass filter, and a second low pass filter electrically connected in series and connected to the load resistor; and a DC circuit comprising a DC power supply, a first bias resistor, a first diode, a choke inductor, a second bias resistor, and a second diode electrically connected in series. An anode of the first diode is electrically connected to a first spot in the first demodulator branch. An anode of the second diode is electrically connected to a second spot in the second demodulator branch. The DC circuit may comprise a first bypass capacitor with a first capacitance and a second bypass capacitor with a second capacitance different from the first capacitance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present application are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
         FIG. 1  is a block diagram illustrating an ETC system according to an embodiment of the application. 
         FIG. 2  is a circuit diagram illustrating a demodulator according to an embodiment of the application. 
         FIGS. 3A-3E  are drawings illustrating routes of various signals according to an embodiment of the application. 
         FIGS. 4A-4B  are drawings illustrating wave forms of signals according to an embodiment of the application. 
         FIG. 5  is a flow chart illustrating a method of demodulating an ASK signal according to an embodiment of the application. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Various aspects and examples of the application will now be described. The following description provides specific details for a thorough understanding and enabling description of these examples. Those skilled in the art will understand, however, that the application may be practiced without many of these details. 
     Additionally, some well-known structures or functions may not be shown or described in detail, so as concise purpose and to avoid unnecessarily obscuring the relevant description. 
     The terminology used in the description presented below is intended to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific examples of the application. Certain terms may even be emphasized below, however, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description section. 
     Without loss of generality, reference will be made to illustrative embodiments by taking a demodulator and a method of demodulating an ASK signal in an ETC system as example. Those of ordinary skills in the art understand that this is only to describe the application clearly and adequately, rather than limit the scope of the application, which is defined by the appended claims. 
       FIG. 1  shows a block diagram illustrating an ETC system  1  according to an embodiment. The ETC system  1  may comprise a Road Side Unit (RSU)  2  and a Composite Password Card (CPC)  3 . 
     In practice, the RSU  2  may use a base band signal (e.g., with a frequency at 14 KHz) to change the amplitude of a carrier signal (e.g., with a frequency at 5.8 GHz) to obtain a resultant ASK signal, and may emit the resultant ASK signal to the CPC  3 . 
     The CPC  3  may typically be installed inside a vehicle (not shown) on a windshield or at other suitable place of the vehicle for example. The CPC  3  may include a demodulator  200 , a wake-up circuit  220  and a primary circuit  240  for example. 
     The demodulator  200  may receive and demodulate the resultant ASK signal to obtain the base band signal. The wake-up circuit  220  may detect a frequency of the base band signal for example. If the frequency meets at least one preset condition (e.g., the frequency falls within a defined frequency range), the wake-up circuit  220  may recognize that a wake-up signal has been received, and then may wake up the primary circuit  240  for example. The primary circuit  240  then may establish a connection with the RSU  2  to fulfill a payment for example. 
       FIG. 2  shows a circuit diagram illustrating the demodulator  200  according to an embodiment of the application. The demodulator  200  may include a first demodulator branch  200 A and a second demodulator branch  200 B that are electrically connected in parallel. The first demodulator branch  200 A is electrically connected between a first antenna ANT 1  configured to receive an ASK signal and a load resistor RL. Similarly, the second demodulator branch  200 B is electrically connected between a second antenna ANT 2  configured to receive an ASK signal and the load resistor RL. The load resistor RL is grounded. 
     In some embodiments, the first demodulator branch  200 A may include: a first antenna ANT 1  to receive an ASK signal, a first coupling capacitor Cb 1 , a first Band Pass Filter BPF 1  to pass the ASK signal, a first Low Pass Filter L 1  to pass a demodulated signal, a first DC blocking capacitor C 1 , and a first current limiting resistor R 1 , which are all electrically connected in series. The first current limiting resistor R 1  is electrically connected to a first end of the load resistor RL. The second end of the load resistor RL is grounded. 
     Similarly, second demodulator branch  200 A may include: a second antenna ANT 2  to receive an ASK signal, a second coupling capacitor Cb 2 , a second Band Pass Filter BPF 2  to pass the ASK signal, a second Low Pass Filter L 2  to pass a demodulated signal, a second DC blocking capacitor C 2 , and a second current limiting resistor R 2 , which are all electrically connected in series. The second current limiting resistor R 2  is electrically connected to the first end of the load resistor RL. 
     In some embodiments, the RL can be an input resistance of an amplifier in the following circuit (such as the wake-up circuit  220  as shown in  FIG. 1 ) for example. 
     In some embodiments, the first Low Pass Filter L 1  and the second Low Pass Filter L 2  may include inductors. 
     As shown in  FIG. 2 , the demodulator  200  further includes a DC circuit  200 C. In some embodiments, the DC circuit  200 C may include a DC power supply Vcc, a first bias (and blocking) resistor Rb 1 , a first diode D 1 , a choke inductor L 11 , a second bias (and blocking) resistor Rb 2 , and a second diode D 2 , which are all electrically connected in series. The first diode D 1  and the second diode D 2  can be Schottky Barrier Diodes for example. 
     During operation, the Vcc supplies DC power at an end of the DC circuit  200 C. At the other end of the DC circuit  200 C, the cathode of the second diode D 2  is grounded. In the DC circuit  200 C, the first diode D 1  and the second diode D 2  are arranged in an electrical direction from the Vcc to the ground. 
     The anode of the first diode D 1  is electrically connected to a first spot S 1  that is electrically connected to both the first Band Pass Filter BPF 1  and the first Low Pass Filter L 1  in the first demodulator branch  200 A. Similarly, the anode of the second diode D 2  is electrically connected to a second spot S 2  that is electrically connected to both the second Band Pass Filter BPF 2  and the second Low Pass Filter L 2  in the second demodulator branch  200 B. 
     Thereby, the first diode D 1 , the choke inductor L 11 , and the second bias resistor Rb 2  of the DC circuit  200 C are electrically connected in series between the first spot S 1  of the first demodulator branch  200 A and the second spot S 2  of the second demodulator branch  200 B. 
     The DC circuit  200 C further includes a first bypass capacitor C 11  with a small capacitance (such as 10 pF), and a second bypass capacitor C 21  with a large capacitance (such as 100 nF). The cathode of the first diode D 1  is grounded via the first bypass capacitor C 11  to bypass an ASK modulated signal (e.g., at a frequency of 5.8 GHz). The cathode of the first diode D 1  is also grounded via the second bypass capacitor C 21  to bypass a demodulated signal (e.g., at a frequency of 14 KHz). 
       FIGS. 3A-3E  are drawings illustrating routes of various signals according to an embodiment of the application. 
       FIG. 3A  shows a modulated signal route in the first demodulator branch  200 A. For example, a modulated signal can be an ASK modulated signal at a Radio Frequency (RF) of 5.8 GHz that is received by the first antenna ANT 1  of the first demodulator branch  200 A.  FIG. 3B  shows a demodulated signal route in the first demodulator branch  200 A. For example, the demodulated signal can be a base band signal at a video frequency of 14 KHz. 
       FIG. 3C  shows a modulated signal route in the second demodulator branch  200 B. For example, the modulated signal can be an ASK modulated signal at a Radio Frequency (RF) of 5.8 GHz that is received by the second antenna ANT 2  of the second demodulator branch  200 B.  FIG. 3D  shows a demodulated signal route in the second demodulator branch  200 B. For example, the demodulated signal can be a base band signal at a video frequency of 14 KHz. 
       FIG. 3E  shows a DC current route in the DC circuit  200 C. During operation, the DC current supplied from the Vcc flows through the first bias resistor Rb 1 , the first diode D 1 , the choke inductor L 11 , the second bias resistor Rb 2 , and a second diode D 2 , and finally into the ground. 
     An example is provided below to illustrate how the demodulator  200  works. For example, those elements as shown in  FIG. 2  may have exemplary values, Cb 1 =Cb 2 =10 pF, Rb 1 =Rb 2 =300 Kohm; L 1 =L 2 =100 nH, C 1 =C 2 =100 nF, R 1 =R 2 =3.3 Kohm, RL=10 Kohm, C 11 =10 pF, C 21 =100 nF; L 11 =100 nH, etc. 
     As shown in  FIG. 2 , since the first demodulator branch  200 A has the first coupling capacitor Cb 1  and the first DC blocking capacitor C 1  to block DC current, the DC current supplied from the Vcc in the DC circuit  200 C may not flow into the first demodulator branch  200 A at the first spot S 1 . 
     Similarly, since the second demodulator branch  200 B has the second coupling capacitor Cb 2  and the second DC blocking capacitor C 2  to block DC current, the DC current that is supplied from the Vcc in the DC circuit  200 C and passes through the first diode D 1 , the choke inductor L 11 , and the second bias resistor Rb 2  may not flow into the second demodulator branch  200 B at the second spot S 2 . 
     The cathode of the first diode D 1  of the first demodulator branch  200 A is connected the anode of the second diode D 2  of the second demodulator branch  200 B via the choke inductor L 11  and the second bias resistor Rb 2 . The cathode of the second diode D 2  is grounded. 
     The choke inductor L 11  (e.g., with inductance 100 nH) in the DC circuit  200 C may block RF signals (e.g., modulated signals at frequency of 5.8 GHz) from the second demodulator branch  200 B into the DC circuit  200 C for example. The second bias resistor Rb 2  of the DC circuit  200 C may block video frequency signals (e.g., demodulated signals at a frequency of 14 KHz) from the second demodulator branch  200 B into the DC circuit  200 C. 
     The first current limiting resistor R 1  and the second current limiting resistor R 2  with suitable resistances can prevent a demodulated signal cross-entering the first demodulation branch  200 A from the second demodulation branch  200 B, and vice versa. 
     In order to solve a problem of RF signal grounding and video signal grounding of the first demodulator branch  200 A, the first bypass capacitor C 11  with a small capacitance (e.g., about 10 pF) is used to bypass the modulated signals (such as 5.8 GHz RF signals), and the second bypass capacitor C 21  with a large capacitance (e.g., about 100 nF) is used to bypass the demodulated signals (such as 14 KHz video signal). The first bias resistor Rb 1  and the second bias resistor Rb 2  in the DC circuit  200 C together function as a bias resistor. 
     With the dual antenna structure of the demodulator as described above, the demodulator has a more consistent sensibility for signals received from different directions. In addition, since the second demodulator branch can multiplex or reuse a bias current supplied from the DC power supply Vcc for the first demodulator branch, the DC power consumption of the demodulator can be greatly reduced (even nearly by 50%). 
       FIGS. 4A-4B  illustrate wave forms of signals according to an embodiment of the application.  FIG. 4A  shows a wave form of an ASK modulated signal (at a radio frequency of e.g., 5.8 GHz) received by the first antenna of the first demodulator branch, for example.  FIG. 4B  shows a wave form of a demodulated signal (at a frequency of e.g., 14 KHz) in the first demodulator branch  200 A, for example. 
       FIG. 5  is a flow chart illustrating a method  500  of demodulating an ASK signal according to an embodiment of the application. In an embodiment of the application, in step  502 , receiving the ASK modulated signal with a demodulator  200  as shown in  FIG. 2 , in step  504 , demodulating the ASK modulated signal with the demodulator  200  to obtain a demodulated signal or a base band signal as discussed above. 
     Those of ordinary skill in the art understand that the wave form of signal  332  is shown as linear to illustrate the change of wave forms more clearly, rather than to limit the scope of the present application. 
     Features and aspects of various embodiments may be integrated into other embodiments, and embodiments illustrated in this document may be implemented without all of the features or aspects illustrated or described. 
     One skilled in the art will appreciate that although specific examples and embodiments of the system and methods have been described for purposes of illustration, various modifications can be made without deviating from the spirit and scope of the present application. Moreover, features of one embodiment may be incorporated into other embodiments, even where those features are not described together in a single embodiment within the present document. Accordingly, the application is described by the appended claims.