Abstract:
A phase locked loop (PLL) circuit having a multi-level voltage-current converter and a clock phase locking method using multi-level voltage-current conversion are described. The phase locked loop (PLL) circuit generates an output clock signal that is phase-locked to a reference clock signal. Further, the PLL circuit includes a phase detecting unit, a charge pump unit, a current-voltage converting unit, and a voltage control oscillator. The phase detecting unit detects a phase difference between the reference clock signal and the output clock signal. The charge pump unit generates a pumping voltage in response to an up signal or down signal output from the phase detector. The current-voltage converting unit receives the pumping voltage, converts the pumping voltage into a predetermined first current, and outputs a tuning voltage in response to predetermined selection signals. The voltage control oscillator generates the output clock signal with a frequency that is proportional to the tuning voltage.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to Korean Patent Application No. 2003-49138, filed on Jul. 18, 2003, in the Korean Intellectual Property Office, the disclosure of which is hereby incorporated by reference in its entirety.  
       BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to a semiconductor integrated circuit, and more particularly, to a phase locked loop (PLL) having a multi-level voltage-current converter and a method for locking a clock phase using multi-level voltage-current conversion.  
         [0004]     2. Discussion of the Related Art  
         [0005]     In general, when a clock is needed to operate an integrated circuit in a synchronous system and it receives an external clock signal, a phase locked loop (hereinafter, referred to as a PLL) circuit is used to phase-lock the external clock signal to the internal clock signal.  
         [0006]     In a conventional semiconductor integrated circuit, a PLL circuit is used in a cache memory device for improving the speed of data processing between a central processing unit (CPU) and a dynamic random access memory (DRAM) of a computer or in a high-speed memory device such as synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM). A PLL circuit is also used to detect locking signals during recording processes in optical disks. Optical disks such as audio-CDs, CD-ROMs, or DVDs have a locking signal and data magnetically recorded simultaneously whenever a unit of data begins to be recorded. When reproducing recorded data, optical disk reproducing devices detect a sync signal and perform a locking operation, thereby reading recorded data. As a supply voltage of an integrated circuit decreases, a tuning voltage, which controls a voltage controlled oscillator (hereinafter, referred to as a “VCO”) of the PLL circuit included in the integrated circuit, also decreases. Since an output frequency of the VCO depends on the tuning voltage, when the tuning voltage level decreases, the output frequency of the VCO also decreases. Hence, it is difficult to design a PLL circuit with a wide frequency range while also having a low tuning voltage.  
         [0007]      FIG. 1  illustrates a conventional PLL circuit. Referring to  FIG. 1 , a PLL circuit  100  includes a phase detector  110 , a charge pump unit  120 , and a VCO  130 . The phase detector  110  detects a phase difference between a reference clock signal REF_CLK and an output clock signal FCLK output from the VCO  130  and generates an up signal UP or a down signal DOWN based on the detected phase difference. The charge pump unit  120  generates a predetermined tuning voltage Vc in response to the up signal UP or the down signal DOWN and controls the VCO  130 . The VCO  130  receives the tuning voltage Vc and generates the output clock signal FCLK with a frequency proportional to the tuning voltage Vc. The PLL circuit  100  repeats the aforementioned feedback operation several times until the output clock signal FCLK is phase-locked to the reference clock signal REF_CLK.  
         [0008]     The VCO  130  is a core component of the PLL circuit  100  and must be designed to have a wide range of frequency domains in proportion to the tuning voltage Vc.  FIG. 2  is a graphical illustration of the relationship between the frequency of the output clock signal FCLK and the tuning voltage Vc. Referring to  FIG. 2 , ideally, the frequency of the output clock signal FCLK increases from 640 MHz to 1.1 GHz, as the tuning voltage Vc increases from 600 mV to 1.8V, i.e., the frequency of the output clock signal FCLK has a linear property.  
         [0009]     Since, when the supply voltage of the integrated circuit decreases, the operating voltage of the PLL circuit  100  also decreases, this leads to the decrease of both the level of the tuning voltage Vc and the frequency of the output clock signal FCLK. Thus, when a system including the PLL circuit  100  operates at a high frequency, the system performance decreases because of the decrease in the frequency of the clock signal output from the PLL. One way to cure this phenomenon is to increase the change rate of the frequency of the output clock signal FCLK according to the changes in the tuning voltage Vc of the VCO  130  of the PLL circuit  100 . However, this method increases phase noise of the PLL circuit  100 .  
         [0010]     To reduce phase noise, a plurality of VCOs can be used to generate output clock signals in different oscillating frequencies according to the level of the tuning voltage Vc. However, the use of the plurality of VCOs results in high power consumption, and increases the complexity of circuit configuration.  
         [0011]     A need therefore exists for a PLL circuit that generates an output clock signal having a wide frequency band, and also has reduced phase noise and low power consumption.  
       SUMMARY OF THE INVENTION  
       [0012]     The exemplary embodiments of the present invention provide a PLL circuit that controls a wide frequency range while producing a small amount of phase noise, and a method for locking a clock phase using multi-level voltage-current conversion.  
         [0013]     According to one preferred embodiment of the present invention, a phase locked loop (PLL) circuit that generates an output clock signal that is phase-locked to a reference clock signal comprises: a phase detecting unit, a charge pump unit, a current-voltage converting unit, and a voltage control oscillator. The phase detecting unit detects a phase difference between the reference clock signal and the output clock signal and generates an up signal or a down signal based on the detected phase difference. The charge pump unit generates a pumping voltage in response to the up signal or down signal output from the phase detector. The current-voltage converting unit receives the pumping voltage, converts the pumping voltage into a predetermined first current level, and outputs a tuning voltage in response to predetermined selection signals. The voltage control oscillator generates the output clock signal with a frequency that is proportional to the tuning voltage.  
         [0014]     Preferably, the voltage-current converting unit comprises a voltage receiving unit, a current copying unit, and a MUX unit. The voltage receiving unit receives the pumping voltage and converts the pumping voltage into the predetermined first current. The current copying unit copies the predetermined first current output from the voltage receiving unit and generates at least two output voltages. The MUX unit selects one of the at least two output voltages in response to the selection signals and outputs the tuning voltage.  
         [0015]     The voltage receiving unit comprises a first NMOS transistor, a fifth PMOS transistor, a second NMOS transistor, a fourth NMOS transistor, a third NMOS transistor, a sixth PMOS transistor, and a seventh NMOS transistor. A gate of the first NMOS transistor is connected to a node that receives the pumping voltage. The fifth PMOS transistor is connected between a supply voltage and a drain of the first NMOS transistor and whose gate is connected to a node that receives a bias voltage. The second NMOS transistor is connected between a source of the first NMOS transistor and ground. A source of the fourth NMOS transistor is connected to a node that receives the supply voltage and a gate of the fourth NMOS transistor is connected to a node that receives a first voltage between the fifth PMOS transistor and the first NMOS transistor. The third NMOS transistor is connected between the fourth NMOS transistor and the ground. Gate and drain of the third NMOS transistor are connected to a gate of the second NMOS transistor. The third NMOS transistor constitutes a current mirror with the second NMOS transistor. Source and gate of the sixth PMOS transistor are connected to a node that receives the supply voltage. The seventh NMOS transistor is connected between the sixth NMOS transistor and the ground. Gate and drain of the seventh NMOS transistor are connected to each other. The seventh NMOS transistor provides the first current.  
         [0016]     The current copying unit comprises an eighth NMOS transistor, a ninth PMOS transistor, tenth and eleventh PMOS transistors, a first resistor, a second resistor, a twelfth NMOS transistor, and a thirteenth NMOS transistor. Through the eighth NMOS transistor, the first current from the voltage receiving unit flows toward the ground. The ninth PMOS transistor is connected between a node that receives the supply voltage and the eighth NMOS transistor. Drain and gate of the ninth PMOS transistor are connected to each other. Sources of the tenth and eleventh PMOS transistors are connected to a node that receives the supply voltage. Gates of the tenth and eleventh PMOS transistors are connected to the gate of the ninth PMOS transistor. The tenth and eleventh PMOS transistors constitute a current mirror with the ninth PMOS transistor. The first resistor is connected to drains of the tenth and eleventh PMOS transistors. The second resistor is serially connected to the first resistor. The twelfth NMOS transistor is connected between the second resistor and the ground. Through the twelfth NMOS transistor, a sink current that operates the voltage receiving unit flows. Gate and rain of the thirteenth NMOS transistor are connected to the second resistor and a source of the thirteenth NMOS transistor is connected to the ground. The gate of the seventh NMOS transistor of the voltage-current converting unit is connected to a gate of the eighth NMOS transistor, and thus, the seventh NMOS transistor and the eighth NMOS transistor constitute a current mirror. The gate of the third NMOS transistor of the voltage-current converting unit is connected to the gate of the twelfth NMOS transistor of the current copying unit, and thus, the third NMOS transistor and the twelfth NMOS transistor constitute a current mirror.  
         [0017]     When the PLL circuit is included in a synchronous memory device such as synchronous dynamic random access memory (SDRAM), it further includes a mode register (MRS) for providing the first through third selection signals.  
         [0018]     The PLL circuit further comprises a selection signal generating circuit that comprises a frequency-voltage converting unit, a reference voltage generating unit, a first amp unit, a second amp unit, a first switch, a first switch, a second switch, and a decoding unit. The frequency-voltage converting unit converts a frequency of the reference clock signal into a first voltage. The reference voltage generating unit generates a predetermined first reference voltage and a predetermined second reference voltage. The first amp unit compares the first voltage to the predetermined first reference voltage. The second amp unit compares the first voltage to the predetermined second reference voltage. The first switch forwards an output of the first amp unit to a first latch in response to the reference clock signal. The second switch forwards an output of the second amp unit to a second latch in response to the reference clock signal. The decoding unit inverts an output of the first latch, outputs a first selection signal as the inverted output of the first latch, receives the inverted output of the first latch and the inverted output of the second latch, outputs a second selection signal, and outputs a third selection signal as the output of the second latch. The decoding unit comprises an inverter and a NOR gate. The inverter inverts the output of the first latch and outputs the first selection signal. The NOR gate receives an output of the inverter and an output of the second latch and outputs the second selection signal.  
         [0019]     According to another preferred embodiment of the present invention, a clock phase locking method for phase-locking an output clock signal to a reference clock signal comprises detecting a phase difference between the reference clock signal and the output clock signal, generating a pumping voltage in response to the detected phase difference, receiving the pumping voltage and converting the pumping voltage into a predetermined first current, copying the predetermined first current and generating a first through third output voltages, selectively outputting one of the first through third output voltages as a tuning voltage in response to one of a first through third selection signals, and generating the output clock signal with a frequency that is proportional to the tuning voltage.  
         [0020]     Preferably, the first through third selection signals are provided by a register such that they are selectively activated according to a frequency range of the reference clock signal.  
         [0021]     According to still anther preferred embodiment of the present invention, a clock phase locking method for phase-locking an output clock signal to a reference clock signal comprises detecting a phase difference between the reference clock signal and the output clock signal, generating a pumping voltage in response to the detected phase difference, receiving the pumping voltage and converting the pumping voltage into a predetermined first current, copying the predetermined first current and generating first through third output voltages, converting a frequency of the reference clock signal into a predetermined voltage, generating a first reference voltage and a second reference voltage, comparing the predetermined voltage to the first reference voltage and generating a first selection signal, comparing the predetermined voltage to the second reference voltage and generating a second selection signal, performing a NOR operation on the first selection signal and the second selection signal and generating a third selection signal, selectively outputting the first through third output voltages as a tuning voltage in response to the first through third selection signals, and generating the output clock signal with a frequency that is proportional to the tuning voltage.  
         [0022]     Therefore, according to the preferred embodiments of the present invention, the frequency range of the output clock signal of the VCO is divided according to the tuning voltage that is selectively generated in response to the first through third selection signals, thereby reducing the power consumed and generating an output clock signal with a wide frequency range that is phase-locked to the reference clock signal while producing a small amount of jitter and phase noise.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The features of the present invention will become more apparent by describing in detail an exemplary embodiment thereof with reference to the attached drawings in which:  
         [0024]      FIG. 1  is a block diagram of a conventional PLL circuit;  
         [0025]      FIG. 2  is a graphical illustration of an operating characteristic of a voltage controlled oscillator (VCO) of  FIG. 1 ;  
         [0026]      FIG. 3  is a block diagram of a PLL circuit according to a preferred embodiment of the present invention;  
         [0027]      FIG. 4  is a schematic diagram of a voltage-current converting unit of the PLL circuit of  FIG. 3 ;  
         [0028]      FIG. 5  is a schematic diagram of a selection signal generating circuit;  
         [0029]      FIG. 6  is a graphical illustration of an operating characteristic of the PLL circuit of  FIG. 3 ; and  
         [0030]      FIG. 7  illustrates noise simulation results obtained using the PLL circuit of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0031]     The embodiments of the present invention will now be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. In the drawings, like reference numerals are used to refer to like elements throughout.  
         [0032]      FIG. 3  is a schematic block diagram illustrating a PLL circuit according to a preferred embodiment of the present invention. A PLL circuit  300  includes a phase detector  310 , a charge pump unit  320 , a voltage-current converting unit  330 , and a VCO  340 . The phase detector  310  detects a phase difference between a reference clock signal REF_CLK and an output signal FCLK output from the VCO  340  and outputs an up signal UP or a down signal DOWN based on the detected phase difference. The charge pump unit  320  outputs a pumping voltage Vp in response to the up signal UP or the down signal DOWN. The voltage-current converting unit  330  receives the pumping voltage Vp and outputs different tuning voltages Vc in response to predetermined first through third selection signals A, B, and C. The voltage-current converting unit  330  includes a voltage receiving unit  332 , a current copying unit  334 , and a MUX unit  336 . The VCO  340  generates the output clock signal FCLK at a frequency proportional to the received tuning voltage Vc. The output clock signal FCLK is also called a recovered clock signal in the sense that the output clock signal FCLK is phase-locked to the reference clock signal REF_CLK.  
         [0033]     The voltage-current converting unit  330  is illustrated in detail in  FIG. 4 . Referring to  FIG. 4 , the voltage receiving unit  332  receives the pumping voltage Vp and converts the pumping voltage Vp into a predetermined current I 6 , the current copying unit  334  copies the current I 6  and outputs a first through third voltages Va, Vb and Vc, and the MUX unit  336  selects one of the first through third voltages Va, Vb, and Vc in response to the predetermined first through third selection signals A, B, and C.  
         [0034]     The voltage receiving unit  332  includes a first transistor M 1  through a seventh transistor M 7 . The first transistor M 1  has a gate connected to a node that receives the pumping voltage Vp. The fifth transistor M 5  is connected between a node that receives a supply voltage VDD and a drain of the first transistor M 1  and has a gate connected to a bias voltage Vbias. The second transistor M 2  is connected between a source of the first transistor M 1  and a voltage VSS or ground. The fourth transistor M 4  has a source connected to a node that receives the supply voltage VDD and has a gate connected to a node that receives a first voltage V 1  and is connected between the fifth transistor M 5  and the first transistor M 1 . The third transistor M 3  is connected between the fourth transistor M 4  and the voltage VSS or ground and has a gate and a drain connected to a gate of the second transistor M 2 . The third transistor M 3  and the second transistor M 2  constitute a current mirror. The sixth transistor M 6  has a source and a gate connected to a node that receives the supply voltage VDD. The seventh transistor M 7  is connected between the sixth transistor M 6  and the voltage VSS or ground and has a drain and a gate connected to each other.  
         [0035]     In the voltage receiving unit  332 , sink current flows through the third transistor M 3 . The fifth transistor M 5  and the second transistor M 2  operate in a triode region. The first node voltage V 1  changes in response to a change in the pumping voltage Vp, however, the amount of change of the first node voltage V 1  is much smaller than that of the pumping voltage Vp, and the change of the first node voltage V 1  is proportional to the change of the pumping voltage. The current I 6  flowing through the sixth transistor M 6  is based on a current I 4  flowing through the fourth transistor M 4  having a gate connected to the first node voltage V 1 .  
         [0036]     The current copying unit  334  includes the eighth transistor M 8  through the thirteenth transistor M 13 , a first resistor R 1 , and a second resistor R 2 . The eighth transistor M 8  has a gate connected to the gate of the seventh transistor M 7  of the voltage receiving unit  332  and constitutes a current mirror along with the seventh transistor M 7 . The ninth transistor M 9  is connected between a node that receives the supply voltage VDD and the eighth transistor M 8  and has a drain and a gate connected to each other. The tenth transistor M 10  and the eleventh transistor M 11  have their sources connected to a node that receives the supply voltage VDD, their gates connected to the gate of the ninth transistor M 9 , and both constitute a current mirror along with the ninth transistor M 9 . The first resistor R 1  is connected to the drains of the tenth transistor M 10  and the eleventh transistor M 11 . The second resistor R 2  is serially connected to the first resistor R 1 . The twelfth transistor M 12  has a gate connected to the gate of the third transistor M 3  of the voltage receiving unit  332  between the second resistor R 2  and the voltage VSS or ground. The gate and drain of the thirteenth transistor M 13  are connected to the second resistor R 2  and the source of the thirteenth transistor M 13  is connected to the voltage VSS or ground.  
         [0037]     The current I 6  flows through the seventh transistor M 7  of the voltage receiving unit  332  and flows into the eighth transistor M 8 , the ninth transistor M 9 , the tenth transistor M 10 , and the eleventh transistor M 11 . The current I 4  flows through the third transistor M 3  of the voltage receiving unit  332  and flows into the twelfth transistor M 12 . A predetermined tail current I tail flows into the thirteenth transistor M 13  when the twelfth transistor M 12  is turned off. Current flowing through the tenth transistor M 10  and the eleventh transistor M 11  generates the first through third voltages Va, Vb, and Vc at both terminals of the first resistor R 1  and the second resistor R 2 .  
         [0038]     The MUX unit  336  switches the first voltage Va into a tuning voltage Vcon in response to the first selection signal A, the second voltage Vb into the tuning voltage Vcon in response to the second selection signal B, or the third voltage Vc into the tuning voltage Vcon in response to the third selection voltage C.  
         [0039]     When the PLL circuit  300  of  FIG. 3  is applied to a synchronous memory device such as synchronous dynamic random access memory (SDRAM), the first through third selection signals A, B, and C are stored in a mode register (MRS) that is activated according to a frequency range of the reference clock signal REF_CLK. The selection signals are provided to the PLL circuit  300  of  FIG. 3 . Alternatively, the first through third selection signals A, B, and C may be provided by a selection signal generating circuit.  
         [0040]     According to one preferred embodiment of the present invention, the PLL circuit  300  further comprises a selection signal generating circuit.  FIG. 5  shows a schematic diagram of the selection signal generating circuit  500 . Referring to  FIG. 5 , the selection signal generating circuit includes a frequency-voltage converting unit  510 , a reference voltage generating unit  520 , a first amp unit  532 , and a second amp unit  534 , a first buffer  542 , a second buffer  544 , a first switch  546 , a second switch  548 , a first latch  552 , a second latch  554 , and a decoding unit  560 . The selection signal generating circuit  500  generates the first through third selection signals A, B, and C according to the frequency range of the reference clock signal REF_CLK.  
         [0041]     The frequency-voltage converting unit  510  converts the frequency of the reference clock signal REF_CLK into a predetermined voltage Vf. The reference voltage generating unit  520  generates a first reference voltage Vref 1  and a second reference voltage Vref 2 . The first amp unit  532  compares the voltage Vf to the first reference voltage Vref 1  and the second amp unit  534  compares the voltage Vf to the second reference voltage Vref 2 . The first buffer unit  542  and the second buffer unit  544  buffer the reference clock signal REF_CLK and turn on the first switch  546  and the second switch  548 . The first switch  546  forwards an output of the first amp unit  532  to the first latch  552  and the second switch  548  forwards an output of the second amp unit  534  to the second latch  554 . The decoding unit  560  includes an inverter  562  and a NOR gate  564 . The inverter  562  inverts an output of the first latch  552  and outputs the first selection signal A. The NOR gate  564  receives an output of the second latch  554  and outputs the second selection signal B. The output of the second latch  554  is the third selection signal C.  
         [0042]     The selection signal generating circuit  500  operates as follows. For purpose of illustration, it is assumed that the first reference voltage Vref 1  generated in the reference voltage generating unit  520  is about 0.48V and the second reference voltage Vref 2  is about 0.57V. Further, the frequency-voltage converting unit  510  has such a characteristic that the voltage Vf decreases when the frequency of the input reference clock signal REF_CLK is higher than a predetermined frequency. For instance, if a reference clock signal REF_CLK with a frequency of 14.89 MHz or higher is input to the frequency-voltage converting unit  510 , the level of the voltage Vf is lower than 0.48V.  
         [0043]     If the level of the voltage Vf of the frequency-voltage converting unit  510  is lower than 0.48V, the output of the first amp unit  532  and the output of the second amp unit  534  are generated at logic low levels. Thus, the first selection signal A is generated at a logic high level, and the second selection signal B and the third selection signal C are generated at logic low levels. If the voltage Vf is between about 0.48V and about 0.57V, the output of the first amp unit  532  is generated at a logic low level and the output of the second amp unit  534  is generated at a logic high level. Thus, the first selection signal A and the third selection signal C are generated at logic low levels and the second selection signal B is generated at a logic high level. If the voltage Vf is higher than 0.57V, the output of the first amp unit  532  and the output of the second amp unit  534  are generated at logic low levels. Thus, the first selection signal A and the second selection signal B are generated at logic low levels and the third selection signal C is generated at a logic high level.  
         [0044]     Referring to  FIG. 4 , the tuning voltage Vcon is switched to the first voltage Va in response to the first selection signal A, the tuning voltage Vcon is switched to the second voltage Vb in response to the second selection signal B, and the tuning voltage Vcon is switched to the third voltage Vc in response to the third selection signal C.  
         [0045]      FIG. 6  is a graphical illustration of the frequency of the output clock signal FCLK generated by the PLL circuit  300  with respect to the tuning voltage Vcon compared to the operating characteristics of the conventional PLL circuit. Referring to  FIG. 6 , the frequency of the output clock signal FCLK output by the conventional PLL circuit increases linearly from 640 MHz to 1.1 GHz as the tuning voltage Vc increases from 0.6V to 1.8V. In contrast, the output clock signal FCLK output by the PLL circuit  300  has a frequency that ranges from 952.96 MHz to 1.1 GHz in response to activation of the first selection signal A, from 804.48 MHz to 952.95 MHz in response to activation of the second_selection signal B, and from 640 MHz to 804.47 MHz in response to activation of the third selection signal C. That is, the PLL circuit  300  outputs clock signals that have the same frequency range as the output clock signal FCLK output by the conventional PLL circuit, however, the frequency range output by PLL  300  is divided into plural parts. According to this embodiment, the frequency range is divided into 3 parts.  
         [0046]     Although the overall frequency range of gain of the VCO  340  of  FIG. 3  that generates the output clock signal FCLK according to the tuning voltage Vcon generated in response to the first through third selection signals A, B, and C is similar to the frequency range of gain of the conventional VCO  130  of  FIG. 1 , the gain of the VCO  340  of  FIG. 3  is reduced to ⅓ of the gain of the conventional VCO  130  of  FIG. 1 .  FIG. 7  shows the simulated noise signal characteristics for the conventional VCO  130  of  FIG. 1  at 300 MHz/V, and that of the VCO  340  of  FIG. 3  at 100 MHz/V. Once gain of the VCO  340  of  FIG. 3  is reduced to ⅓ of the gain of the conventional VCO  130  of  FIG. 1 , jitter and phase noise can be reduced to about 9.5 dB. Therefore, while jitter and phase noise is reduced, the frequency range of the PLL is maintained at a desired level.  
         [0047]     Therefore, with the above described preferred embodiments, the PLL circuit is capable of generating an output clock signal FCLK that has three frequency ranges corresponding to three levels of tuning voltages Va, Vb and Vc, which are selectively generated in response to three selection signals A, B, and C. The PLL&#39;s power consumption is reduced and the output clock signal FCLK has reduced jitter and phase noise while having a wide frequency range.  
         [0048]     While the above exemplary embodiments are described with the frequency range of the output clock signal being divided into three parts in response to three selection signals, it is readily appreciated by one of ordinary skill in the art that the frequency range of the output clock signal may also be divided into two or more parts in response to two or more selection signals. Also, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.