Abstract:
The present invention relates to the field of fingerprint identification technologies, and provides a method and a system for processing fingerprint sensing signals, and a fingerprint identification terminal. The method includes a frequency mixing step by mixing a collected high frequency fingerprint sensing signal with a first high frequency signal to obtain a low frequency signal; and an amplification step by amplifying the low frequency signal. By using the character that capacitance impedance is inversely proportional to signal frequency, the present invention shifts a fingerprint sensing signal with a high frequency into a signal with a low frequency through frequency spectrum shifting and performs signal amplification on the signal with a low frequency, which can overcome the difficulty in amplifying the high frequency fingerprint sensing signal and thus improves the signal-to-noise ratio SNR.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the field of fingerprint identification technologies and, more particularly, to a method and a system for processing fingerprint sensing signals and a fingerprint identification terminal. 
       BACKGROUND 
       [0002]    Fingerprint has almost become a pronoun for biometric features identification due to the lifelong invariance, uniqueness and accessibility. While a fingerprint identification technology has also been widely applied to various terminal devices, such as a mobile terminal, a banking system, an attendance system, or the like, and is frequently used for providing security access to sensitive electronic devices and/or data. 
         [0003]    A capacitive fingerprint detection circuit is a front analog circuit that is operable to read fingerprint information.  FIG. 1  shows a circuit structure of a known design, and its principle is to conduct a driving signal (for example, a driving signal with a frequency of 400 KHz) onto a touch plane; when a finger touches the plane, a touch operation can transmit a fingerprint sensing signal into a system through a capacitor on the plane. 
         [0004]    Chinese invention patent application CN103376970 A discloses another capacitive fingerprint sensor, where the capacitive fingerprint sensor can be formed by an array of sensing elements. Each capacitive sensing element of the array records a voltage that changes with the capacity coupling capacitance, and the capacitive sensing element is located between the finger and a sensing chip, and the finger can be capacitively coupled to each capacitive sensing element of the sensor, so that the sensor can sense the capacitance between each capacitive sensing element and a fingerprint muscle. The sensing chip detects a capacitance signal through sensing the voltage changes on the capacitive sensing element. Also, the sensing chip can detect a capacitance signal by sensing the change of charges that are received by the capacitive sensing element. 
         [0005]    There is a problem in the two fingerprint identification technologies described above and in other existing designs that because the capacitance C in the feedback loop of an amplifier is very small, it is very difficult to amply the fingerprint sensing signal and the SNR (signal-to-noise ratio) is low. 
       SUMMARY 
       [0006]    An embodiment of the present invention is to solve a first technical problem by a method for processing fingerprint sensing signals, which aims at improving the signal-to-noise ratio SNR of the fingerprint sensing signal. 
         [0007]    An embodiment of the present invention is defined as a method for processing fingerprint sensing signals. The method includes a frequency mixing step by performing frequency mixing of a collected high frequency fingerprint sensing signal with a first high frequency signal to obtain a low frequency signal; and an amplifying processing step by performing amplification on the low frequency signal. 
         [0008]    Before the frequency mixing step, the method further includes a signal conversion step by converting the collected high frequency fingerprint sensing signal in a voltage form into a high frequency fingerprint sensing signal in a current form and performing the frequency mixing step based on the high frequency fingerprint sensing signal in a current form. 
         [0009]    An embodiment of the present invention is to solve a second technical problem by a system for processing fingerprint sensing signals. The system includes a frequency mixer configured to perform frequency mixing of a collected high frequency fingerprint sensing signal with a first high frequency signal to obtain a low frequency signal; and a capacitive feedback amplifier configured to perform amplification on the low frequency signal, where an inverting input end of the capacitive feedback amplifier is connected to the output end of the frequency mixer, a feedback capacitor is connected between the inverting input end and an output end of the capacitive feedback amplifier, and an non-inverting input end of the capacitive feedback amplifier is connected to a bias voltage. 
         [0010]    An embodiment of the present invention is to solve a third technical problem by a fingerprint identification terminal. The fingerprint identification terminal includes a plurality of fingerprint sensing pixels distributed in an array manner, where each fingerprint sensing pixel is connected to a system for processing fingerprint sensing signals. The system includes: a frequency mixer configured to conduct frequency mixing processing on a high frequency fingerprint sensing signal collected with a first high frequency signal to obtain a low frequency signal; and a capacitive feedback amplifier configured to conduct amplifying processing on the low frequency signal, wherein the inverting input end of the capacitive feedback amplifier is connected to the output end of the frequency mixer, a feedback capacitor is connected between the inverting input end and the output end of the capacitive feedback amplifier, and the non-inverting input end of the capacitive feedback amplifier is connected to a bias voltage. 
         [0011]    The system may further include a voltage-current converter configured to convert the high frequency fingerprint sensing signal in a voltage form collected into a high frequency fingerprint sensing signal in a current form, and output the high frequency fingerprint sensing signal in a current form to the frequency mixer, so that the frequency mixer conducts frequency mixing processing based on the high frequency fingerprint sensing signal in a current form. 
         [0012]    The voltage-current converter may include a first switch tube, where the first end of the first switch tube is connected to a power supply, and the control end of the first switch tube is connected to a bias voltage; a second switch tube, where the first end of the second switch tube is connected to the second end of the first switch tube, the second end of the second switch tube is served as an output end and is connected to one input end of the frequency mixer, and the control end of the second switch tube is configured to input the high frequency fingerprint sensing signal in a voltage form; and a third switch tube, where the first end of the third switch tube is connected to the second end of the first switch tube, the second end of the third switch tube is connected to the ground, and the control end of the third switch tube is configured to input a reference voltage. 
         [0013]    The capacitive feedback amplifier may include a fourth switch tube, where the first end of the fourth switch tube is connected to a power supply, and the control end of the fourth switch tube is connected to a bias voltage; a fifth switch tube, where the first end of the fifth switch tube is connected to the second end of the fourth switch, and the control end of the fifth switch tube is served as a non-inverting input end of the capacitive feedback amplifier and is connected to a bias voltage; a sixth switch tube, where both the first end and the control end of the sixth switch tube are connected to the second end of the fifth switch tube, and the second end of the sixth switch tube is connected to the ground; a seventh switch tube, where the first end of the seventh switch tube is connected to the second end of the fourth switch tube, the control end of the seventh switch tube is served as the inverting input end of the capacitive feedback amplifier and is connected to the output end of the frequency mixer, and the second end of the seventh switch tube is served as the output end of the capacitive feedback amplifier and is connected to the control end of the seventh switch tube through a capacitor; and an eighth switch tube, where the first end of the eighth switch tube is connected to the second end of the seventh switch tube, the control end of the eighth switch tube is connected to a bias voltage, and the second end of the eighth switch tube is connected to the ground. 
         [0014]    By utilizing the feature that the capacitance impedance is inversely proportional to the signal frequency, the present invention performs frequency mixing of a high frequency fingerprint sensing signal with a first high frequency signal by way of frequency spectrum shifting and performs signal amplification at a low frequency after the frequency spectrum shifting, which can thus overcome the difficulty in signal amplification of a high frequency fingerprint sensing signal and improve the signal-to-noise ratio SNR thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a circuit diagram of a capacitive fingerprint detection circuit provided by the prior art; 
           [0016]      FIG. 2  is a flow chart of implementing a method for processing fingerprint sensing signals according to an embodiment of the present invention; 
           [0017]      FIG. 3  is a block diagram of frequency spectrum shifting after frequency mixing processing on 400 KHz and 390 KHz according to an embodiment of the present invention; 
           [0018]      FIG. 4  is a logic structure chart of a fingerprint identification terminal according to an embodiment of the present invention; 
           [0019]      FIG. 5  is an exemplary circuit diagram of a voltage-current converter in  FIG. 4 ; and 
           [0020]      FIG. 6  is an exemplary circuit diagram of a capacitive feedback amplifier in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In order to make the objects, technical solutions and advantages of the present invention more clear, the present invention will be further described in details hereinafter with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only for explanation of the present invention only, but are not intended to limit the present invention. 
         [0022]    Because the capacitance impedance is inversely proportional to the signal frequency: Z=1/(j*2*3.14*f*C), wherein Z represents the impedance, f represents the signal frequency, C represents the capacitance value of the feedback capacitor, and j represents an imaginary unit. That is, the capacitance impedance at a low frequency is higher than the capacitance impedance at a high frequency. According to the formula above, the frequency f reduces from 400 KHz to 10 KHz, the capacitance value C is unchanged, the impedance Z is increased to 40 times of the original; that is, the embodiment of the present invention implements a gain that is dozens of times of that of the circuit in the past in the case that the same circuit element values are used, so as to improve the signal noise ratio SNR. 
         [0023]    Based on the foregoing principle, the flow of the processing method for fingerprint sensing signals according to an embodiment of the present invention is as shown in  FIG. 2 , wherein the steps thereof are elaborated as follows. 
         [0024]    Step S 101  is a frequency mixing step: conducting frequency mixing processing on a collected high frequency fingerprint sensing signal with a first high frequency signal to obtain a low frequency signal. 
         [0025]    A high frequency driving signal (for example, the signal frequency is 400 KHz) is conducted to a touch plane, when the signal is touched by fingers and conducted to a capacitor, a high frequency fingerprint sensing signal is obtained. The high frequency fingerprint sensing signal and a first high frequency signal (for example, the signal frequency is 390 KHz) are mixed to produce a low frequency signal (the signal frequency is 10 KHz), wherein the frequency spectrum shifting is as shown in  FIG. 3 , and it can be seen that a 40-times gain is produced. 
         [0026]    Step S 102  is an amplifying step: conducting amplifying processing on the low frequency signal. 
         [0027]    To facilitate subsequent integral processing on the high frequency fingerprint sensing signal, the high frequency fingerprint sensing signal needs to be converted from a voltage form to a current form before amplifying, and then the high frequency fingerprint sensing signal in a current form obtained after the converting is mixed. 
         [0028]      FIG. 4  shows a logic structure of a fingerprint identification terminal provided by the embodiment of the present invention. To facilitate description, parts related to the embodiment of the present invention are illustrated only. 
         [0029]    Referring to  FIG. 4 , the fingerprint identification terminal includes a plurality of fingerprint sensing pixels distributed in an array manner, wherein each fingerprint sensing pixel is connected to a processing system for fingerprint sensing signals, wherein A represents an output buffer amplifier. The processing system includes a frequency mixer  1  and a capacitive feedback amplifier  2 , wherein the frequency mixer  1  is configured to conduct frequency mixing processing on the collected high frequency fingerprint sensing signal with a first high frequency signal (produced by a first high frequency signal generator) to obtain a low frequency signal; then the capacitive feedback amplifier  2  conducts amplifying processing on the low frequency signal, wherein the inverting input end of the capacitive feedback amplifier  2  is connected to the output end of the frequency mixer  1 ; moreover, a feedback capacitor C is also connected between the inverting input end and the output end of the capacitive feedback amplifier, while the non-inverting input end of the capacitive feedback amplifier is connected to a bias voltage BIAS. 
         [0030]    To facilitate subsequent integral similarly, the processing system further includes a voltage-current converter  3 , wherein the voltage-current converter  3  is configured to convert the high frequency fingerprint sensing signal in a voltage form collected into a high frequency fingerprint sensing signal in a current form, and outputs the high frequency fingerprint sensing signal in a current form to the frequency mixer  1 , so that the frequency mixer  1  conducts frequency mixing processing based on the high frequency fingerprint sensing signal in a current form. 
         [0031]      FIG. 5  shows a specific structure of the voltage-current converter  3 , which includes a first switch tube Q 1 , a second switch tube Q 2 , and a third switch tube Q 3 , wherein the first end of Q 1  is connected to a power supply VDD, and the control end of Q 1  is connected to a bias voltage BIAS; the first end of Q 2  is connected to the second end of Q 1 , the second end of Q 2  is served as an output end and is connected to one input end of the frequency mixer  1 , and the control terminal of Q 2  is configured to input the high frequency fingerprint sensing signal in a voltage form; the first end of Q 3  is connected to the second end of Q 1 , the second end of Q 3  is connected to the ground, and the control end of Q 3  is configured to input a reference voltage. All the first switch tube Q 1 , the second switch tube Q 2  and the third switch tube Q 3  above can be implemented using a PMOS tube. 
         [0032]      FIG. 5  shows a circuit which is a differential pair, wherein the working principle thereof is as follows: the first end of Q 1  is connected to a power supply VDD; when the voltage of BIAS and VDD satisfy a certain condition (for example, BIAS is less than VDD), Q 1  is switched on to provide a constant current for Q 2  and Q 3 , Q 2  is switched on through a high frequency voltage signal (i.e., the high frequency fingerprint sensing signal), and Q 3  is switched on through a reference voltage. If the reference voltage keeps unchanged, the high frequency voltage signal is increased (i.e., the high frequency voltage signal is greater than the reference voltage), then the current of the second end of Q 2  is decreased according to the features of the PMOS tube; when the high frequency voltage signal is decreased (i.e., the high frequency voltage is less than the reference voltage), then the current of the second end of Q 2  is increased, and the current of the second end of Q 2  changes inversely with the change of the high frequency voltage signal. 
         [0033]    Moreover,  FIG. 6  shows a specific structure of the capacitive feedback amplifier  2 , which includes a fourth switch tube Q 4 , a fifth switch tube Q 5 , a sixth switch tube Q 6 , a seventh switch tube Q 7 , and an eighth switch tube Q 8 , wherein the first end of Q 4  is connected to a power supply VDD, and the control end of Q 4  is connected to a bias voltage BIAS; the first end of Q 5  is connected to the second end of the fourth switch, and the control end INP of Q 5  is served as a non-inverting input end of the capacitive feedback amplifier  2  and is connected to a bias voltage BIAS; both the first end and the control end of Q 6  are connected to the second end of Q 5 , and the second end of Q 6  is connected to the ground GND; the first end of Q 7  is connected with the second end of the fourth switch tube, the control end INN of Q 7  is served as the inverting input end of the capacitive feedback amplifier  2  and is connected to the output end of the frequency mixer  1 , the second end of Q 7  is served as the output end OUTPUT of the capacitive feedback amplifier  2  and is connected to the control end INN of Q 7  through a capacitor; the first end of Q 8  is connected to the second end of the seventh switch tube, the control end of Q 8  is connected to a bias voltage BIAS, and the second end of Q 8  is connected to the ground GND. The fourth switch tube Q 4 , the fifth switch tube Q 5  and the seventh switch tube Q 7  above can be implemented using a PMOS tube, while the sixth switch tube Q 6  and the eighth switch tube Q 8  can be implemented using an NMOS tube. 
         [0034]      FIG. 6  shows a circuit which is an operational amplifier, wherein the working principle thereof is as follows: the effect of Q 4  is same as that of Q 1  in  FIG. 5 ; if INP keeps unchanged, and INN is increased (i.e., INN is greater than INP), then the current of Q 7  is decreased according to the features of the PMOS tube, then the drain voltage of Q 7  (i.e., OUTPUT) will be greatly reduced. Because signals between INN and OUTPUT are inverting and the gain there between is very high, virtual ground is formed at the INN end, and a feedback capacitor is connected between INN and OUTPUT, then the current flowing in INN will be completely integrated into the capacitor, so as to achieve the effect of amplifying and integrating the low frequency signal. 
         [0035]    The foregoing is merely preferred embodiments of the invention, but is not intended to limit the invention; and any modification, equivalent replacement, improvement and the like made within the spirits and principles of the invention shall all fall within the protection scope of the invention. 
       INDUSTRIAL APPLICABILITY 
       [0036]    In conclusion, the present invention utilizes the character that capacitance impedance is inversely proportional to signal frequency, mixes the high frequency fingerprint sensing signal with the first high frequency signal through frequency spectrum shifting, and then amplifies the signal after shifting the signal to a low frequency, so as to implement a gain that is dozens of times of that of the circuit in the past in the case that the same circuit element values are used, so as to improve the signal noise ratio SNR of the fingerprint sensing signal. On the other hand, before the fingerprint sensing signal enters the amplifier, the fingerprint sensing signal is converted from a voltage form to a current form, which facilitates integrating.