Patent Publication Number: US-8542053-B2

Title: High-linearity testing stimulus signal generator

Description:
FIELD OF THE INVENTION 
     The present invention relates to a signal generator, particularly to a high-linearity testing stimulus signal generator. 
     BACKGROUND OF THE INVENTION 
     With the advance of information technology, the audio/video files need higher and higher resolution and demand greater and greater storage capacity. A high-quality terminal device should be equipped with a high-performance data transmission system to transmit an enormous amount of data. Thus, ADC (Analog to Digital Converter), which functions as a conversion interface, demands higher and higher specification, some of which may be far beyond the range that the testing stimulus signal generators can operate. Hence, the high-resolution ADC is usually performed verification by lowering the resolution thereof during testing. Consequently, the test results are usually unpractical. 
     A US Publication No. 20090040199 entitled an “Apparatus for Testing Driving Circuit for Display” discloses an analog-to-digital converter having a ramp generator. The ramp generator generates a linear triangular wave or a ramp wave (the so-called testing stimulus signal) for testing the analog-to-digital converter. The fundamental problems of a ramp generator include whether the linearity of signals can be used for testing the circuit under test having higher and higher resolution, whether it is expensive, whether the test result thereof is as accurate as expected when considering the non-ideality of the fabrication process, whether it can overcome the factors of environmental interference, probe pointing, loads, etc., and whether it is practical to generate testing stimulus signals externally to input to a chip in case of SOC (System-on-a-Chip). A digital-to-analog converter can provide testing stimulus signals. However, a high-resolution digital-to-analog converter built in a chip not only is expensive but also increases the complexity of design and integration of the chip. 
     Another typical method for generating testing stimulus signals is to connect a constant current source to a capacitor. Refer to  FIG. 1 . Via a constant current source  1  and a capacitor  2 , the output current can be converted into the voltage drop of the capacitor  2 , which is a testing stimulus signal desired. The constant current source  1  is provided by incorporating a current mirror with great output impedance. Such a method is instinctive. However, the method can only apply to a chip where a constant current source  1  and a capacitor  2  exist simultaneously. Refer to  FIG. 2 . In practice, the constant current source  1  and the capacitor  2  are non-ideal and have parasitic effects which causes the stray charging curve  3  pretty different from the ideal charging curve  4 . 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to solve the linearity problem of the testing stimulus signals. 
     Another objective of the present invention is to reduce the high cost of high-linearity testing stimulus generators. 
     To achieve the above-mentioned objectives, the present invention proposes a high-linearity testing stimulus generator, which comprises a signal collection unit, a waveform conversion unit, a first voltage-to-current conversion unit, a delay unit, a second voltage-to-current conversion unit, a current comparison unit, an error calculation unit and a compensation unit. 
     The signal collection unit receives an input current signal and outputs a signal. The waveform conversion unit connects with the signal collection unit, converts the signal output by the signal collection unit into a triangular wave voltage signal, and outputs the triangular wave voltage signal via a voltage output terminal. The first voltage-to-current conversion unit and the delay unit connect with the voltage output terminal of the waveform conversion unit. The first voltage-to-current conversion unit converts the triangular wave voltage signal into a first current signal. The delay unit delays propagation time of the triangular wave voltage signal. The second voltage-to-current conversion unit connects with the delay unit and converts the delayed triangular wave voltage signal into a second current signal. The current comparison unit connects respectively with the first voltage-to-current conversion unit and the second voltage-to-current conversion unit to receive the first current signal and the second current signal and then perform comparison thereof to output a current difference signal. The error calculation unit connects with the output terminal of the current comparison unit to receive the current difference signal and perform error calculation to output an error signal. The compensation unit connects with the error calculation unit to receive the error signal and perform signal compensation to output a compensation signal to the signal collection unit. Thus is formed a feedback mechanism. 
     Thereby, when the waveform conversion unit outputs a non-linear triangular wave voltage signal, the feedback mechanism performs compensation adjustment to restore the non-linear triangular wave voltage signal to a linear signal, therefore is able to function as a high-accuracy testing stimulus signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing a constant current source and a capacitor in a conventional technology; 
         FIG. 2  is a diagram schematically showing voltage variation of a charged capacitor in a conventional technology; 
         FIG. 3A  is a block diagram schematically showing the architecture of a high-linearity testing stimulus signal generator according to one embodiment of the present invention; 
         FIG. 3B  is a diagram showing the waveform of signals according to one embodiment of the present invention; 
         FIG. 4  is a circuit diagram showing a voltage-to-current conversion unit according to one embodiment of the present invention; 
         FIG. 5  is a circuit diagram showing a current subtractor according to one embodiment of the present invention; and 
         FIG. 6  is a circuit diagram showing a high-linearity testing stimulus signal generator according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The technical contents of the present invention are described in detail in cooperation with the drawings below. 
     Refer to  FIG. 3A  and  FIG. 3B .  FIG. 3A  is a block diagram schematically showing the architecture of a high-linearity testing stimulus signal generator according to one embodiment of the present invention.  FIG. 3B  is a diagram showing the waveform of signals according to one embodiment of the present invention. The present invention proposes a high-linearity testing stimulus signal generator, which comprises a signal collection unit  10 , a waveform conversion unit  20 , a first voltage-to-current conversion unit  30 , a delay unit  40 , a second voltage-to-current conversion unit  50 , a current comparison unit  60 , an error calculation unit  70 , and a compensation unit  80 . 
     The signal collection unit  10  receives an input current signal  11  and outputs a signal. The waveform conversion unit  20  connects with the signal collection unit  10 , converts the signal output by the signal collection unit  10  into a triangular wave voltage signal  21 , and outputs the triangular wave voltage signal  21  via a voltage output terminal  22 . It should be particularly mentioned herein that the triangular wave voltage signal  21  is unstable unless it is linearly modified. The details thereof will be described later. The first voltage-to-current conversion unit  30  and the delay unit  40  connect with the voltage output terminal  22  of the waveform conversion unit  20 . The first voltage-to-current conversion unit  30  converts the triangular wave voltage signal  21  into a first current signal  31 . The delay unit  40  delays propagation time of the triangular wave voltage signal  21 . The second voltage-to-current conversion unit  50  connects with the delay unit  40  and converts the delayed triangular wave voltage signal  21  into a second current signal  51 . Refer to  FIG. 4  a circuit diagram showing a voltage-to-current conversion unit according to one embodiment of the present invention. Both the first and second voltage-to-current conversion units  30  and  50  use the same circuit to perform voltage-to-current conversion. 
     The current comparison unit  60  connects respectively with the first voltage-to-current conversion unit  30  and the second voltage-to-current conversion unit  50  to receive the first current signal  31  and the second current signal  51  and then perform comparison thereof to output a current difference signal  61 . In one embodiment, the current comparison unit  60  is a current subtractor. Refer to  FIG. 5  for a circuit diagram showing a current subtractor according to one embodiment of the present invention. The current comparison unit  60  has two current input terminals  62  to receive the first and second current signals  31  and  51 . The current comparison unit  60  has an output terminal  63  to output the current difference signal  61 . The first current signal  31  is basically similar to the second current signal  51  except there is a time difference existing therebetween. In current comparison, the subtraction of the second current signal  51  and the first current signal  31  is performed to obtain the current difference signal  61 , which is similar to a square wave signal. 
     The present invention may further have a reference current output unit  90  connecting with a current input terminal  71  of the error calculation unit  70  and providing a reference signal  91  for the error calculation unit  70  to perform error calculation. The error calculation unit  70  connects with the output terminal  63  of the current comparison unit  60  to receive the current difference signal  61 . In one embodiment, the error calculation unit  70  is a current subtractor, which respectively receives the reference signal  91  and the current difference signal  61  to perform error calculation and then output an error signal  72 . If the triangular wave voltage signal  21  is a non-linear signal, the current difference signal  61  is not an accurate square wave signal. However, the reference signal  91  is a standard square wave signal. Therefore, the error calculation unit  70  calculates the difference between the current difference signal  61  and the reference signal  91  to obtain the error signal  72 . In one embodiment, the error signal  72  is a current signal. 
     The compensation unit  80  connects with the error calculation unit  70  to receive the error signal  72  and then perform signal compensation to output a compensation signal  81  to the signal collection unit  10 . Thus is formed a feedback mechanism. In one embodiment, the compensation unit  80  performs multiple amplification to the error signal  72  to obtain the compensation signal  81 . In signal compensation, the compensation signal  81  is used to promote the linearity of the triangular wave voltage signal  21 . 
     Refer to  FIG. 6  a circuit diagram schematically showing a high-linearity testing stimulus signal generator according to one embodiment of the present invention. The signal collection unit  10  uses a p-type MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and an n-type MOSFET to perform voltage-to-current conversion. The waveform conversion unit  20  is a circuit containing capacitors and resistors, thus the capacitors are charged and discharged to convert the triangular wave voltage signal  21 . In one embodiment, there are two delay units  40  connecting with the waveform conversion unit  20  and respectively connecting with the first voltage-to-current conversion unit  30  and the second voltage-to-current conversion unit  50 . The delay units  40  respectively delay the signals to the first voltage-to-current conversion unit  30  and the second voltage-to-current conversion unit  50  through different propagation time, whereby the signal received by the second voltage-to-current conversion unit  50  is slower than the signal received by the first voltage-to-current conversion unit  30  to achieve signal delaying effect. The compensation unit  80  performs multiple amplification to the error signal  72  by using the transistors, which is a skill known in the art and will not be repeated herein. The compensation signal  81 , which has been amplified, is a voltage signal. The voltage signal is converted into a current signal by the transistors of the signal collection unit  10 . 
     In conclusion, the present invention uses the feedback mechanism of the compensation unit  80  to perform linearity modification and promote the linearity of the triangular wave voltage signal  21 . The present invention performs the feedback modification via a current mechanism. As the current mode provides high response speed, the present invention is exempted from the interference caused by device drift. Therefore, the present invention can effectively promote the linearity of the testing stimulus signals.