OUTPUT SIGNAL PROCESSOR CIRCUIT OF CHARGE TRANSFER DEVICE

Abstract: The preferred embodiments now disclose an output signal processor circuit of a charge transfer device compris­ ing the following: a charge-detect circuit forming output signals of the charge transfer device, where the output signal is provided with the first and second levels; clamp circuit clamping signal output from the charge-detect circuit at a specific level; sampling circuit sampling signals output from the charge-detect circuit; and an integration circuit integrat­ ing signals output from the charge-detect circuit.

Description: The present invention relates to the output signal processor circuit built in a charge transfer device (CTD) such as a charge-coupled device (CCD) or a bucket bridge device (BBD), and more particularly, to the output signal processor circuit of a charge transfer device, which is capable of drastically reducing noise components from output signals. Conventionally, a charge transfer device (CTD) such as a charge-coupled device (CCD) or bucket bridge device (BBD) deals with signals as charges and transfers them synchronous with externally applied clock pulses. However, output signal processors of such conventional charge transfer devices cannot sufficiently reduce noise components from output signals, thus unavoidably causing the signal level to lower. In this regard, output signal processors such as correlated double sampling signal processors comprising a clamp circuit and sample hold circuit, such as those disclosed in IEEE Journal of Solid State Circuits, Volume SC-9, No. 1, February 1974, pages 1-12 and IEEE Journal of Solid State Circuits, Volume SC-15, No. 3, June 1980 pages 373-375, which whilst capable of removing switching transients and eliminating the Nyquist noise, also cannot sufficiently reduce noise components from the low-frequency band signals. In a process of band limiting the thermal noise generated in a correlated double sampling circuit, there is disclosed in IEEE Journal of Solid State Circuits, Volume SC-11, No. 1, February 1976, pages 147-155, an output signal processor circuit incorporating a simple single pole, low pass filter without any provision for controlling the tracing of its operation. Reflecting this, there is a growing demand for the early realisation of a useful output signal processor circuit available for the charge transfer device, which should be capable of effectively and drastically reducing noise components from such bands close to sampling frequencies. In the light of such an urgent problem requiring an early solution, the present invention aims at providing an extremely useful output signal processor circuit available for a charge transfer device, which is capable of drastically reducing noise components from signals output from the charge transfer device without causing the signal level to lower. Another object of the present invention is to provide an output signal processor circuit for the charge transfer device, which is capable of sharply reducing noise components from such bands in the periphery of sampling frequencies fc without causing the signal level to lower, thus drastically reducing noise components that would otherwise reflect onto low-frequency band signals. A further object of the present invention is to provide a useful output signal processor circuit for a charge transfer device, which is capable of sharply reducing noise components from signals output from the charge transfer device by applying the correlative double sampling (CDS) method. Yet another object of the present invention is the provision of a useful output signal processor circuit for a charge transfer device, which, by applying a simplified integral averaging means, is capable of sharply reducing noise components from high-frequency band signals that would otherwise reflect onto low-frequency band signals. Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the present invention will become apparent to those skilled in the art from this detailed description. In accordance with one aspect of the present invention, there is provided an output signal processor circuit of a charge transfer device comprising: a charge detect means for forming both black level and signal level for each charge transfer cycle, a clamping means and sampling means for executing a clamping operation at the black level and sampling operation for each charge transfer cycle; and characterised in that an integral averaging means is provided in the front stage of the said sampling means, said integral averaging means being adapted to have an integral averaging period for each charge transfer cycle and set to be substantially equal to the period of the signal level. Preferably, the charge detect means may comprise a free capacitor provided with a reset switch, where for each transfer cycle, the closed period of the reset switch and the period between the opening of the reset switch and the start of the transmission of signal charge into the free capacitor are both reduced, for allowing the signal level period to be increased such that the averaged value of integration would increase. In accordance with an aspect of the present invention there is also provided an output signal processor circuit for a charge transfer device, in which a charge detect system provides output signals in response to charges provided by the charge transfer device, an integrating means integrates said output signals and a sampling means samples the integrated signals, wherein during each charge transfer cycle the charge detect system is arranged to be reset and then receive said charges and provide said output signals, and characterised in that the integrating means is arranged to integrate said output signals for a period substantially equal to the period from the receipt of the charges to the following reset of the charge detect system, the integrating means not integrating said output signals of the charge detect system for the remainder of the charge transfer cycle. Brief Description of the Drawings The present invention will be better understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: Figure 1 (a) is the aperture width after applying an integral averaging process; Figure 1 (b) is the frequency response shown after applying the integral averaging process; Figure 1 (c) is the frequency response of noise components when the frequency response relationship shown in Figure 1 (b) is applied to the example shown in Figure 7 (c); Figure 2 is the operation timing chart of the output signal processor circuit of the charge transfer device reflecting the preferred embodiments of the present invention; Figure 3 is a simplified block diagram of an output signal processor circuit of a charge transfer device used for executing operations shown in Figure 2, reflecting the preferred embodiments of the present invention; Figure 4 is a simplified block diagram of the output circuit of a charge transfer device that can be connected to the output signal processor circuit embodied by the present invention; Figure 5 is a simplified circuit diagram used for explaining the correlative double sampling (CDS) method featuring the preferred embodiments of the present invention; Figure 6 is the operation timing chart of the circuit shown in Figure 5; Figure 7 (a) is a chart showing the frequency response of noise components after collecting differences between signals provided with timewise intervals called "τ". Figure 7 (b) is a chart showing the frequency response of noise components when the signal shown in Figure 7 (a) is sampled by applying the sampling frequency fc; and Figure 7 (c) is a chart showing the frequency response of noise components when the signal shown in Figure 7 (b) is sample-held. Detailed Description of the Invention First, before describing one of the preferred embodiments of the present invention, the principle of the "correlative double sampling (CDS) method" embodied by the present invention is described below. The charge transfer device cited in the present invention is typically denoted by either the charge-coupled device (CCD) or the bucket bridge device (BBD). Generally, the output unit of the charge transfer device comprises a detector circuit that converts the signal charge into a specific voltage and an amplifier circuit that draws out the converted voltage. First, the detector circuit containing a free capacitor with reset switch employed by the preferred embodiments of the present invention is described below. The detector circuit first closes the reset switch so that the potential of the free capacitor can be set at the externally supplied reset potential (VRD) and then opens the reset switch to allow the free capacitor to contain a specific voltage on receipt of signal charge. These serial operations are repeatedly executed in every transfer period in order for variable components of the voltage charged in the free capacitor to be extracted from the amplifier circuit. A typical example is shown in Figure 4, in which MOS-FET 1 is made available for the reset switch of the detector circuit, while the positive-negative-coupled reverse-biased diode 2 is made available for the free capacitor. Reset clock (ØR) is transmitted to the gate of MOS-FET 1 which functions as the reset switch. A source-follower circuit comprised of MOS-FETs 3 and 4 makes up the amplifier circuit. Note that a positive-type substrate is used for the circuits shown in Figure 4 and in conjunction with the following description. Such circuits shown in Figure 4 also use signal charge for processing electrons. The same explanation can also be applied to the case where polarity is reversed by inverting the polarity of the drive voltage. The following description refers to the case where the charge-coupled device is made available for the charge transfer device. Note that the same explanation can also be applied to the operation of the bucket bridge device despite its performance characteristics being slightly inferior to those of the charge-coupled device. Referring now to the charge-coupled device, although substantially the low-noise element, it nevertheless causes some noise components in the input unit which generates and feeds the signal charge. Such noise components also exist in the transmission path, detector and amplifier circuits as well, thus eventually causing the signal-to-noise (S/N) ratio of the charge-coupled device to lower. To significantly improve the S/N ratio, the preferred embodiments of the present inventions use the correlative double sampling (CDS) method, the details of which are described below. Figure 5 shows the operation as such circuits using the correlative double sampling method. Signal OS is output from the charge-coupled device provided with the output circuit shown in Figure 4. During this period, the reset clock pulse ØR goes Low before the finally-transferred clock pulse Øj from the charge-coupled device goes Low to allow the black level to appear. Conversely, the signal level appears during the period in which the reset clock pulse ØR goes High after the finally-transferred reset clock pulse Øj is turned Low. Referring now to Figure 5, signal OS is routed via the AC-coupled clamp capacitor (Ccp) into two paths. One is led to clamp switch 5 and the other to buffer amplifier A1. Clamp switch 5 is opened and closed by applying clamp clock pulse Øcp to clamp signal OS which remains in the external potential Vcp during the black level period. The clamped signal OS is then delivered to the sample-hold circuit via buffer A1 that contains the high-impedance input. Signal OS first passes through sample-switch 6 which is opened and closed by the sample-hold clock pulse ØSH, which is then sampled during the signal-level period of signal OS; the sampled signal OS is then delivered to the following sample-hold capacitor (CSH). The potential being sample-held by the sample-hold capacitor (CSH) is eventually output in the form of output signal Vout via buffer A2 containing the high-impedance input. Figure 6 shows the timing relationship between clock pulses and signals shown in Figure 4 and 5. As is clear from those operations described above, the circuit shown in Figure 5 eventually generates an output signal Vout that collects the differences in the potentials between the signal level and the preceding black level of signal OS in every transfer period, and, as a result, the signal value of signal OS from the charge-coupled device (CCD) is eventually output in the sample-held state. At the same time, noise components contained in signal OS is suppressed in the low-frequency bands, thus indicating the response characteristics shown in Figure 7 (a). Symbol "τ" indicates the time difference between the clamp clock pulse Øcp and the sample-held clock pulse ØSH. Nevertheless, in reality, high-frequency-band signals above the level fc/2 (representing Nyquist frequency fN) reflect themselves onto the low-frequency band signals as a result of the sampling operation performed by the sample-hold circuit as shown in Figure 7 (b). The high-frequency-band signal above the level fc/2 eventually generates the response characteristics shown in Figure 7 (c), being affected by the specific frequency characteristics generated by the sample-hold operation. As a result, if signal OS contains such noise components having a relatively high-frequency level in the periphery of the sampling frequency fc, they will easily reflect onto the low-frequency band signals. Consequently, such a circuit as is shown in Figure 5 cannot sufficiently reduce noise components from the low-frequency band signals. We now refer to Figure 1 (a), (b) and (c), and Figures 2 and 3 for operations of the output signal processor circuit of a charge transfer device (CTD) capable of reducing noise components from low-frequency band signals reflected in one of the preferred embodiments of the present invention which is described below. Figure 1 (a), (b) and (c) respectively explain the concept of the operations performed by the output signal processor circuit embodied by the present invention. Of these, Figure 1 (a) shows the aperture width after applying an integral averaging; Figure 1 (b) shows the frequency response after performing the integral averaging operation shown in Figure 1(a), and Figure 1 (c) shows the noise frequency response when the relationship denoted in Figure 1 (b) is applied to the example shown in Figure 7 (c). First, consideration is given to such an aperture circuit containing the aperture width TAP against the time axis "t" shown in Figure 1 (a). The aperture circuit executes the integral averaging operation of the signals inside the aperture width TAP, which are variable according to time factors. Frequency characteristics can easily be sought during this period by converting the signal waveforms shown in Figure 1 (a) by the application of the Fourier series, which causes such frequency characteristics to be generated as shown in Figure 1 (b). In summary, such responsive function is represented by the equation: As a result, the frequency characteristics will be those normally generated by a low-pass filter which causes the response to be reduced to zero by the effect of 1/TAP and by such a frequency multiplied by its integer. In other words, by setting the aperture width TAP at such a period corresponding to the period of the signal level of signal OS or slightly longer than this period, noise components can be removed effectively from high-frequency band signals without causing the signal level to vary at all. As a result, by effectively combining the above method with the correlative double sampling method, noise components can be securely prevented from reflecting themselves onto low-frequency band signals. This is represented in Figure 1 (c). Figure 2 is a timing chart denoting the relationship of the above operations. Of those OS signals generated in Figure 6, a specific signal OS shown in Figure 2 is first clamped by pulse Øcp during the black level period. The clamped signal OS is then integrally averaged by pulse ØAP for a sufficiently long period of time (TAP) including the signal level period, which is then withheld until the next integral averaging starts. Thus, the integrally averaged value in every transfer period is withheld during the period mentioned above, and then sample-held by pulse ØSH. As a result, signal Vout containing the noise response characteristics shown in Figure 1 (c) is eventually output. As is clear from Figure 1 (b), the closer the period TAP to the transfer period 1/fc, the lower the noise response at the level of sampling frequency fc will be. Therefore, the period TAP should be extended as long as possible within the transfer cycle. On the other hand, if the period TAP is extended beyond the signal level period, the integrally averaged signal value will diminish itself. To prevent this, the signal level should also be extended as long as possible within the transfer cycle. To securely realize this, as is clear form Figure 6, the high-level period of pulse ØR, i.e., the closed period of the reset switch, should be as short as possible. Likewise, the black-level period from the fall of pulse ØR to the rise of pulse Øj should also be as short as possible within a range within which clamping can be done by means of pulse Øcp. Figure 3 is a simplified block diagram of the circuit used for executing the operations shown in Figure 2 reflecting one of the preferred embodiments of the present invention. Signal OS from the charge-coupled device having the circuit shown in Figure 4 is first delivered to clamp circuit 11 having the configuration shown in the first-half portion of Figure 5, which is then sent to the integral averaging circuit 12. In other words, signal OS is delivered to the integration circuit 22 comprising resistor RAP and capacitor CAP via switch 21 which is closed by pulse ØAP during the period TAP in each transfer cycle. The value of the integrated time constant τ AP = RAP.CAP is determined by its relationship with the TAP period. The integration circuit substantially makes up the sample-hold circuit itself, and therefore, the integrally averaged signal during the TAP period is stored in capacitor CAP until the next integral averaging operation starts. This signal is then delivered to the final-stage sample-hold circuit 13 via buffer amplifier A3 which is provided with a high-impedance input. The final-stage sample-hold circuit 13 comprises a configuration similar to the latter-half portion of the circuit shown in Figure 5. The difference is that in Figure 3, the Sample-hold circuit 13 samples the processed signals using pulse ØSH during the period in which the integrally averaged value is still held in the preceding circuit. With reference to Figures 2 and 3, the foregoing description referred to the case in which an integral averaging circuit 12 is inserted between the clamp circuit 11 and the sample-hold circuit 13. This configuration may be substituted by other means. For example, if the integral averaging circuit 12 is of such a configuration capable of executing the integral averaging operation within the signal level of signal OS to allow both the integrally averaged signal and the unprocessed black-level signal to exist in every transfer period, then it is apparent that frequency characteristics identical to those shown in Figures 2 and 3 can be realized by arranging the entire system in order, namely, the integral averaging circuit, clamp circuit, and the sample-hold circuit. In summary, the essence of the present invention is to effectively combine the execution of the signal clamping operation at the black level with the sample-hold operation after averaging the integrated values at the signal level. As is clear from the foregoing detailed description, the preferred embodiments of the present invention make it possible for the output signal processor circuit to drastically reduce noise components from high-frequency-band signals as well as such noise components that will otherwise reflect onto low-frequency band signals merely by adding a simplified integral averaging means to the output signal processor circuit of the charge transfer device. The present invention provides another preferred embodiment in which the correlation double sampling circuit is provided with an integration circuit that constantly integrates both the signal level and the black level for a specific period of time. The invention being thus described, it will be obvious that it may be varied in many ways. Such variations are not to be regarded a a departure from the scope of the invention, which is intended to be defined by the following claims.

Claims: An output signal processor circuit for a charge transfer device comprising: a charge detect means (1,2,3,4) for forming both black level and signal level for each charge transfer cycle, a clamping means (11) and sampling means (13) for executing a clamping operation at the black level and sampling operation for each charge transfer cycle; and characterised in that an integral averaging means (12) is provided in the front stage of the said sampling means (13), said integral averaging means (12) being adapted to have an integral averaging period for each charge transfer cycle and set to be substantially equal to the period of the signal level. An output signal processor circuit of a charge transfer device according to claim 1, wherein: the integral averaging means (12) is provided in the front stage of said clamping means (11), said clamping means being provided in the front stage of the said sampling means (13). An output signal processor circuit of a charge transfer device according to claim 2, wherein said sampling means (13) samples results of an integral-averaging operation executed by said integral averaging means (12). An output signal processor circuit of a charge transfer device according to claim 1, wherein: the said integral averaging means (12) is provided between the sampling means (13) and clamping means (11), said clamping means (11) being provided in the front stage of the sampling means (13). An output signal processor circuit of a charge transfer device according to claim 4, wherein said sampling means (13) samples results of an integral-averaging operation executed by said integral averaging means (12). An output signal processor circuit of a charge transfer device according to claim 1, wherein: the charge detect means comprises a free capacitor (2) provided with a reset switch (1), where for each transfer cycle, the closed period of the reset switch and the period between the opening of the reset switch and the start of the transmission of signal charge into the free capacitor are both reduced, for allowing the signal level period to be increased such that the averaged value of integration would increase. An output signal processor circuit for a charge transfer device, in which a charge detect system (1,2,3,4) provides output signals in response to charges provided by the charge transfer device, an integrating means (12) integrates said output signals and a sampling means (13) samples the integrated signals, wherein during each charge transfer cycle the charge detect system is arranged to be reset and then receive said charges and provide said output signals, and characterised in that the integrating means (12) is arranged to integrate said output signals for a period substantially equal to the period from the receipt of the charges to the following reset of the charge detect system, the integrating means (12) not integrating said output signals of the charge detect system for the remainder of the charge transfer cycle.

Inventors: WATANABE, TAKASHI
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1985
Main classification: G11C19
IPC codes: G11C19, G11C27, H04N5, H03H11