Abstract:
An image pickup device for generating three pulses 120 degrees out of phase with one another and having the same period, to clamp, sample and hold, and reset an image signal output from an image pickup element. The pulse width of the sample and hold pulse is less than one third of the period. The image signal is read from the image pickup element using three transfer pulses 120 degrees out of phase with one another and having the same period. Each of the transfer pulses is less in width than one third of the period, and also serves as a timing signal for sampling and holding the image signal. In one embodiment, the image pickup device may have three read-out devices for reading out three horizontal lines at one time (not, however, in phase), with a separate train of transfer pulses being used for each of the three lines.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image pickup device and more particularly to a drive apparatus for the image pickup device. 
     2. Related Background Art 
     When a solid state image pickup element, for example, a CCD, is used in a television camera, the use of so-called double-correlation sampling of the output circuit of the camera can reduce the reset noise to improve the S/N ratio. In this instance, in the double-correlation sampling circuit, the three (clamp, sample and hold, and reset) pulses are required to have predetermined phase differences among them. 
     In such a case, if the clamp, sample and hold (hereinafter referred to as the S/H), and reset pulses are used, each being 120 degrees out of phase with the others and each having a duty cycle of 1/3, one pulse will rise while one of the other two pulses is falling. Thus, noise is likely to be mixed into the desired signal during clamping or sampling and holding and a fixed pattern noise, etc., will increase. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to eliminate such prior art drawbacks. 
     In order to solve these problems, a first embodiment of the present invention is an image pickup device for generating three pulses, 120 degrees out of phase with one another and having the same period, to clamp, sample and hold, and reset an image signal output from an image pickup element, in which the pulse width of the sample and hold pulse is less than one third of the period. 
     A second embodiment of the present invention is a image pickup device for reading an image signal using three transfer pulses 120 degrees out of phase with one another, in which each of the transfer pulses is less in width than one third of the period and serves as a timing signal for sampling and holding the image signal. 
     According to such embodiments of the present invention, the mixing of noise due to other pulses in the signal during sampling and holding is prevented by rendering the pulse width of the sample and hold pulse less than one third of the period. 
     Other objects and features of the present invention will be apparent from the following detailed description of the preferred embodiments and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the structure of a frame transfer-type CCD to which the present invention can be applied; 
     FIG. 2 is a block diagram showing the structure of an output amplifier 4 shown in FIG. 1; 
     FIG. 3A is a timing chart showing the conventional waveforms of pulses φ1, φ2 and φ3, shown in FIG. 2, for clamping, sampling and holding, and resetting purposes; 
     FIG. 3B is a timing chart showing the waveforms of pulses φ1, φ2 and φ3 according to one embodiment of the present invention for clamping, sampling and holding, and resetting purposes. 
     FIG. 4 is a block diagram showing the structure of a frame transfer-type CCD to which the present invention can also be applied; and 
     FIG. 5 is a block diagram showing the relationship between horizontal shift registers 3-1 to 3-3 and output amplifiers 4-1 to 4-3, as shown in FIG. 4, and pulses φ1, φ2 and φ3 for clampling, sampling and holding, and resetting purposes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the present invention will now be described with respect to the drawings. 
     While one embodiment of the present invention will be described by taking frame transfer type CCD as an image pickup element as an example, it should be noted that the present invention is not limited to such examples. The present invention is also applicable to interline transfer type CCDs and other image pickup devices. 
     FIG. 1 shows the structure of a frame transfer type CCD. In FIG. 1, an image pickup (photoreceptor) unit 1 produces electric charges is accordance with the incident light and stores them therein. A memory (storage) 2 receives the electric charges produced by unit 1 and stores them temporarily. A horizontal register unit 3 sequentially reads the stored charges in storage 2 in units of a line. An output amplifier 4 is provided on the side of horizontal register unit 3 for converting the charges to a voltage and performing double-correlation sampling. 
     FIG. 2 is a circuit diagram showing the details of output amplifier 4. In FIG. 2, a floating diffusion (hereinafter referred to as the FD) 5 converts the charges transferred by horizontal register 3 to a voltage. A FET 6 constitutes a reset circuit which erases a voltage across FD5. A FET 7 connected to a voltage source +V DD  and a current source (which is connected to a circuit substrate) constitutes a source follower which transmits the potential level of FD 5 to the next stage. A low-pass filter (hereinafter referred to as the LPF) 8 delays a source signal from source follower 7 and eliminates white noise occurring int he substrate and high frequency components of a transfer clock. A capacitor Cc or 9 AC-couples the signal delayed by the LPF to the next stage. A clamp circuit 10 clamps the potential of capacitor 9 to a reference potential V REF . A FET 11 connected to voltage source +V DD  and to a current source, constitutes a source follower which transmits the signal AC-coupled by capacitor 9 to the next stage. A FET 12 charges therethrough a hold capacitor CH 13 with the source potential of source follower 11. A FET 14 connected to voltage source +V DD  and current source constitutes a source follower which transmits the potential stored across capacitor CH 13 to the next stage. FETs 12 and 14 and capacitor CH 13 constitute a sample and hold circuit. 
     In FIG. 2, clamp pulse φ2 is also applied as a horizontal transfer pulse to horizontal register 3. Clamp, sample and hold, and reset pulses φ1, φ2 and φ3 are applied 120 degrees out of phase with one another. FIG. 3A shows the waveforms of signals at several points of the circuit when the duty cycle of pulses φ1, φ2 and φ3 is 1/3. In FIG. 2, when clamp pulse φ1 becomes low (t=t1), a signal charge is injected into FD 5 from horizontal register 3 and stored in FD 5 when reset pulse φ3 becomes high (t=t2). When reset pulse φ3 becomes high (t=t2), the charges injected into FD 5 are displaced and the potential at point A is clamped to reference potential V REF . When reset pulse φ3 becomes low (t=t3), reset occurs at point A, clamp pulse φ1 goes from high to low (t=t4), and a signal component with such reset noise superimposed thereon appears at point A. The signal at point A is delayed via source follower 7 and LPF 8 to appear at points B and C. So long as clamp pulse φ1 is high (t3 to t4), the potential at point C is clamped to reference potential V REF  and the reset noise level is so clamped. When S/H pulse φ2 becomes high (t=t4), signal components from which the reset noise is removed appear at the output and the signal level present when the pulse φ2 falls is held. 
     One drawback is that as shown in FIG. 3A, when the duty cycle of each pulse signals φ1, φ2 and φ3 is 1/3, noise which occurs due to rises and falls of other pulses and which is delayed by the LPF if likely to be superimposed on the he signal present during the S/H pulse, so that fixed pattern noise occurs. Another drawback is that as shown at  ○7   in FIG. 3A, the time T&#34; during which a signal component occurs at the output V OUT  is shorter than T&#39;. 
     The method of driving the particular embodiment for eliminating such drawbacks will be described with respect to FIG. 3B. The particular embodiment is characterized in that the pulse width of the clamp, S/H and reset pulses φ1, φ2 and φ3 to drive the several elements of the output amplifier is equal to, or less than, 1/3 of the period of these pulses, as shown at  ○1  ,  ○2   and  ○3   in FIG. 3B. 
     Thus, for example, while S/H pulse φ2 is low and sampling is not conducted, clamp pulse φ1 or reset pulse φ3 may fall or rise. Thus noise due to such fall or rise may be mixed in the output at points B and C in FIG. 2. Consequently, while the S/H pulse is high and sampling is being conducted, no noise due tot he rise and fall of clamp and reset pulses φ1 and φ2 will be mixed in the outputs at B and C points of FIG. 2 and fixed pattern noise is prevented from occurring. 
     In addition, according to the particular embodiment, the time T&#39; during when a signal component is occurring at the output V OUT , as shown at ○7   in FIG. 3B, is more than that shown at  ○7   in FIG. 3A. 
     A second embodiment of the present invention will be described with respect to FIG. 4 which is a block diagram of a frame transfer type CCD to which the present invention is applied. 
     Like reference numerals are used to denote like parts in FIGS. 1 and 4 and more detailed description of corresponding elements will be omitted. In FIG. 4, reference numerals 3-1, 3-2 3-3 denote horizontal registers which read signals in memory (storage) 2. The reason why the number of horizontal registers is three in the particular embodiment will be described next. An image pickup device such as that shown in FIG. 4 is constructed so as to provide a read out of a relatively large number (shown as three) horizontal lines at one, to improve the resolution. If the number of the horizontal lines read out at once increases, however, the number of horizontal registers to read the image signal will increase. On the other hand, the time required to read signals representative of images in a horizontal line is determined by a standard television signal. Thus the frequency of a clock signal to drive such horizontal registers will necessarily increase and thus the transfer efficiency will decrease. 
     Thus in the particular embodiment, three horizontal registers as shown by 3-1, 3-2 and 3-3, are provided so that each of the registers receives a corresponding 1/3 of the image signals in a horizontal line. 
     Reference numerals 4-1 to 4-3 denote the output amplifiers shown in FIG. 1, each of which is constructed as shown in FIG. 2. 
     Next, the relationship among these output amplifiers 4-1 to 4-3, horizontal registers 3-1 to 3-3, and pulses φ1, φ2 and φ3 applied to these elements will be described with respect to FIG. 5, in which a pulse generator 20 generates pulses φ1, φ2 and φ3 120 degrees out of phase with one another and each having a pulse width less than 1/3 of its period. 
     Pulses φ3, φ2 and φ1 are applied to a reset signal terminal RESET, a S/H signal terminal S/H and a clamp signal terminal CLAMP, respectively, or output amplifier 4-1, and the pulse φ1 is also applied to horizontal shift register 3-1. 
     Pulses φ1, φ3 and φ2 are applied to a reset signal terminal RESET, a S/H signal terminal S/H and a clamp signal terminal CLAMP, respectively, of output amplifier 4-2, and the pulse φ2 is also applied to horizontal shift register 3-2. 
     Pulses φ2, φ1 and φ3 are applied to a reset signal terminal RESET, a S/H signal terminal S/H and a clamp signal terminal CLAMP of output amplifier 4-3, and the pulse φ3 is also applied to horizontal shift register 3-3. 
     That is, in the particular embodiment shown in FIG. 5, the clock signals input to drive clock terminals CLOCK of the three horizontal shift registers 3-1 to 3-3 are the same in period and 120 degrees out of phase with one another, so that image signals are sequentially output from horizontal shift registers 3-1 to 3-3 to the corresponding output amplifiers 4-1 to 4-3. 
     Signals having the same period and 120 degrees out of phase with one another are also input to the reset signal terminals RESET, S/H signal terminals S/H and clamp signal terminals CLAMP of output amplifiers 4-1 to 4-3, as shown in FIG. 5. 
     Thus in the particular embodiment, only three different pulses are required to drive output amplifiers 4-1 to 4-3 and horizontal registers 3-1 to 3-3 and not other kinds of pulses are needed. 
     The present invention is also applicable to the image pick up device shown in FIGS. 4 and 5. Namely, pulses φ1, φ2 and φ3 generated by pulse generator 20 having the same period, are 120 degrees out of phase with one another and are each shorter in pulse width than 1/3 of the period. invention, a sample and hold pulse is shorter in width than 1/3 of its period, so that noise due to other pulses is prevented from being mixed in the signals.