Title of Invention	"AN APPARATUS FOR REDVCING OUTPUT CAPACITANCE OF DIGITAL-TO-TIME DOMAIN CONVERTER FOR VERY HIGH FREQUENCY DIGITAL WAVE FORM SYNTHESIS"
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REDUCING OUTPUT CAPACITANCE OF DIGITAL-TO-TIME DOMAIN CONVERTER FOR VERY HIGH FREQUENCY DIGITAL WAVEFORM SYNTHESIS BACKGROUND 1. Field of the Invention [001] This invention relates to digital waveform synthesis. In particular, the invention relates to digital-to-time-domain conversion. 2. Description of Related Art [002] There are many factors affecting the operating frequency of digital circuits. These factors include environmental conditions (e.g. supply voltage and temperature) and electrical characteristics of the devices implementing the digital circuits. Among these factors, parasitic capacitance is a significant factor that may reduce the switching frequency. [003] In many applications, it is useful to synthesize digital waveforms according to some predefined pattern. It is desirable to be able to synthesize the digital waveforms at frequencies as high as possible. The digital-to-time-domain converter (DTC) is a basic building block used in digital waveform synthesis (DWS). The DTC performance is limited by the large parasitic capacitance that is inherently present on its output node. This parasitic capacitance places a limit on the highest frequency that can be synthesized using DWS. [004] Therefore, there is a need to have an efficient technique to reduce the output capacitance in the DTC. BRIEF DESCRIPTION OF THE DRAWINGS [005] The features and advantages of the present invention will become apparent from the following detailed description of the present invention in which: [006] Figure 1 is a diagram illustrating a system in which one embodiment of the invention can be practiced. [007] Figure 2 is a timing diagram illustrating waveforms of the signals shown in Figure i according to one embodiment of the invention. [008] Figure 3 is a timing diagram illustrating waveforms of the pattern inputs shown in Figure 1 according to one embodiment of the invention. [009] Figure 4 is a diagram illustrating a digital-to-time domain converter shown in Figure 1 according to one embodiment of the invention. [0010] Figure 5A is a diagram illustrating an equivalent circuit at the DTC without the amplifier circuit used in one embodiment of the invention. [0011] Figure 5B is a diagram illustrating an equivalent circuit at the DTC with the amplifier circuit according to one embodiment of the invention. DESCRIPTION [0012] The present invention is a technique to reduce the output capacitance at the digital-to-time-domain converter (DTC) used in digital waveform synthesis (DWS). The present invention includes use of an amplifier circuit at the output of the DTC. The amplifier circuit includes an amplifier having a gain and a feedback resistor. The output capacitance is reduced by an amount approximately equal to the gain of the amplifier. The amplifiei can be implemented by an inverter and the feedback resistor can be implemented by a transmission gate circuit having an n-device and a p-device connected in the ON state. [0013] In the following description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice the present invention. In other instances, well-known electrical structures and circuits are shown in block diagram form in order not to obscure the present invention. [0014] Figure 1 is a diagram illustrating a system 100 in which one embodiment of the invention can be practiced. The system 100 includes a synchronous delay line (SDL) 110. a digital-to-time domain converter (DTC) 120, and a pattern data unit 130. [0015] The SDL 110 receives a clock signal and generates N tap signals, or timing pulses, staggered at Tp/ N intervals, where Tp is the clock period. It should be noted that the use of the SDL 110 is merely one method to generate the tap signals to meet the timing requirements for the DWS. Any other method that provides similar tap signals can be used. Examples of these methods include delayed-lock loop (DLL) and phase-locked loop (PLL) methods. The DTC 120 combines the SDL tap signals with a pattern input to •synthesize an output waveform one bit every Tp/N interval. The pattern data unit 130 provides the input pattern to the DTC 120. The pattern data unit 130 includes a pattern register 132, a pattern shifter 134, and a pattern data generator 136. [0016] The pattern data generator 136 generates new N-bit pattern data for synthesis in each clock cycle. The pattern shifter 134 introduces a phase shift into the synthesized wave by shifting the pattern data by a number of bits that is proportional to the amount of phase shift. The resolution of the introduced phase shift is Tp/N. A left shift corresponds to a negative phase shift and a right shift corresponds to a positive phase shift. Let K be the number of bits to be shifted. Let a be the amount of phase shift. The amount of phase shift is, then, given by: (Equation Removed) [0017] The pattern register 132 stores the shifted pattern data from the pattern shifter 134. The pattern register 132 updates the pattern data presented to the DTC 120 with proper set-up and hold times for sampling by the tap signals from the SDL 110. The pattern to be synthesized, in general, changes in each clock, either by being replaced by a new pattern data from the pattern data generator 136, or by being shifted by the pattern shifter 134, or by both. When the pattern changes, it is invalid during the first portion of the reference clock period and may be sampled incorrectly by the tap signals from the SDL 110. To overcome this difficulty, a delay scheme is used by the pattern register 132. This delay scheme is illustrated in Figure 3 and will be described later. (0018] Figure 2 is a timing diagram illustrating waveforms of the signals shown in Figure ] according to one embodiment of the invention. The signals include the reference clock signal CLK, the tap signals TAP[0:N-1] from the SDL 110, the synchronized pattern input PI[0:N-1] from the pattern register 132, the shifted pattern data from the pattern shifter SPD[0:N-1] 134, and the original pattern data PD[0:N-1] from the pattern data generator 136. J0019] In this illustrative example, the number of bits N for the pattern data is 8. There are 8 corresponding tap signals from the SDL 110, TAP[0] to TAP[7]. It is noted that the tap signals are divided into three groups. The first group, group A in Figure 3, corresponds to the first N/4 tap signals, i.e., TAP[0.T], The second group, group B in Figure 3, corresponds to the middle N/2 tap signals, i.e., TAP[2:5], The third group, group C in Figure 3, corresponds to the last N/4 tap signals, i.e., TAP[6:7]. The tap signals in the first and the third groups are de-asserted in the middle of the clock period. The tap signals in the middle group are de-asserted at the start of the clock period. By de-asserting the tap signals of the first and third group in the middle of the clock period, solid low and high times can be achieved, as will be shown later. It should be noted that the boundaries between the three groups are a design choice. In other embodiments, the number of groups and the groupings may be changed. As seen in Figure 3, the tap signals TAP[0:7| are asserted synchronously with the corresponding pattern input PI[0:7] so that reliable data clocking can be achieved. [0020] The pattern data PD[0:7] changes every clock. In the example illustrated in Figure 2, the pattern shifter 134 introduces a 3-bit left shift corresponding to a phase shift of-] 35°. The pattern register 132 delays the shifted pattern data SPD[0:7] by one clock to provide the pattern input PI[0:7]. The pattern input bits are then sampled by the tap signals TAP[0] to TAP[7] to generate the output signal as a synthesized waveform. [0021] Figure 3 is a timing diagram illustrating waveforms of the pattern inputs shown in Figure 1 according to one embodiment of the invention. The pattern input bits PI[0:7J are provided by the pattern data generator 136, shifted (if necessary) by the pattern shifter 134, and synchronized by the pattern register 132. [0022] The pattern input bits are delayed by the pattern register 132 in three groups: A, B, and C Group A includes the lower N/4 bits, i.e., PI[0:1]. Group B includes the middle N/2 bits, i.e., PI[2:5]. Group C includes the upper N/4 bits, i.e., PI[6:7]. The inputs in group A are delayed by one half a clock period so that they have nominally one half a clock period to stabilize, i.e., they have one half a clock period of setup time, before being sampled by the lower N/4 tap signals TAP[0:1] as illustrated in Figure 3. Also, as illustrated in Figure 3, the inputs in group A nominally have a hold time of one fourth a clock period from tap signals TAP[0:1]. Similarly, the inputs in group B and C are delayed by a full clock period and one and a half clock periods, respectively, so that they nominally have one fourth a clock period of setup time to the middle N/2 tap signals TAP|2:5] and the upper tap signals TAP[6:7]. Also, the inputs in group B nominally have a hold time of one fourth a clock period from tap signals TAP[2:5], while the inputs in group C nominally have a hold time of one half a clock period from tap signals TAP[6:7). [0023] The shaded areas shown in the waveforms in Figure 3 designate the approximate sampling points of the tap signals from the SDL 110. As discussed above, in each case, healthy set-up and hold times for the tap signals surround the shaded areas. [0024] Figure 4 is a diagram illustrating the digital-to-time domain converter (DTC) 120 shown in Figure 1 according to one embodiment of the invention. The DTC 120 includes a transmission gate array 410, an amplifier circuit 430, and a selector 440. [0025] The transmission gate array 410 receives the pattern input PI[0:N-1] from the pattern data unit 130 (Figure 1) to transfer one of the N pattern inputs to the circuit output based on an active signal from N select signals SEL[0:N-1] provided by the selector 440. The transmission gate array 410 includes N transmission gates 420o to 420N-I- Each of the N transmission gates 420o to 420N-I has an output. The N outputs of the N transmission gates 4200 to 420H-I are connected together to form the circuit output. The connected N outputs have an equivalent output capacitance. |0026] The transmission gate 420J of N transmission gates 420o to 420N-I includes an n-device 422j, a p-device 424j, and an inverter 426j. The n-device 422j and the p-device 424j are n-channel Metal Oxide Semiconductor (NMOS) and p-channel Metal Oxide Semiconductor (PMOS) transistors, respectively. They are connected together at terminals 427j and 429j. The terminals 427j of all the transmission gates 420o to 420N-1 (j = 0, N-l) are connected together to provide the circuit output. The terminal 429j is connected to pattern input PI[j] through inverter 450j. The p-device 424j has a gate controlled by select signal SEL[j]. The inverter 426j is connected between the gates of the p-device 424j and the n-device 422j so that when p-device 424j is turned on, the n-device 422j is also turned on, forming a connection between the terminals 427j and 429j. Similarly, when the p-device 424j is turned off by the controlling select signal SEL[j], the n-device 422j is also turned off, isolating the terminal 427j from the terminal 429j. The use of both the n-device and p-device as transmission gate provides a full voltage swing from ground up to the Vcc supply voltage level. [0027] The amplifier circuit 430 has a gain and is connected to the N transmission gates 420o to 420N-I at the circuit output to reduce the output capacitance by an amount approximately equal to the gain. The amplifier circuit 430 generates the output signal of the DTC 120. The amplifier circuit 430 includes an n-device 432, a p-device 434, and an inverter 436. The inverter 436 is connected to the circuit output to generate the output signal. The n-device 432 and the p-device 434 are connected together to form a feedback transmission gate. This feedback transmission gate is coupled across the inverter 436 to provide a feedback resistance. The n-device 432 and the p-device 434 are set in the on state, e.g., the gate of the n-device 432 is connected to the supply voltage VCc and the gate of the p-device 434 is connected to ground. [0028] The selector 440 generates the N select signals SEL[0:N-1] based on the N tap signals TAP[0:N-1] from the synchronous delay line 110 (Figure 1). The selector 440 includes N logic circuits 440o to 440N-1 connected in cascade to generate at most one active signal of the N select signals SEL[0:N-l]. In other words, the selector 440 activates at most one of the N select signals SEL[0:N-1] at a time. Each of the logic circuits 440j includes an inverter 442j and a NAND gate 444j. The input to the inverter 442j is connected to the tap signal TAP[j+l]. The input to the last inverter 442N-i is connected to the tap signal TAP[0]. Referring to the waveforms of the tap signals shown in Figure 2, when a tap signal TAP[j] goes high, the next tap signal TAP[j+l] remains low for a period equal to Tp/N. This fact together with the manner in which the N logic circuits 4400 to 440N-I are connected allows at most only one select signal to be active during any Tp/N time period. [0029] When a tap signal TAP[j] goes high, the corresponding select signal SEL[j] goes low, turning on or activating the transmission gate 420j. During this time, the other select signals are high, turning off the other transmission gates. Therefore, only input bit Pl[j] of the pattern inputs PI[0:N-1] is selected to be transferred to the circuit output which is inverted by the inverter 436 to become the synthesized output signal. Since only one transmission gate in the transmission array 410 is turned on or activated at a time, only one pattern input bit is selected at a time. [0030] The amplifier circuit 430 can reduce the output capacitance by an amount approximately equal to the gain provided by the inverter 436. Since the frequency of the output signal depends on the output capacitance, a reduction of output capacitance leads to an increased operational frequency. An analysis based on equivalent circuits for the DTC 120 without and with the amplifier circuit 430 can be used to show how the amplifier circuit 430 can reduce the output capacitance [0031] Figure 5A is a diagram illustrating an equivalent circuit 500 at the DTC without the amplifier circuit used in one embodiment of the invention. The equivalent circuit 500 includes a resistor 510 and a capacitor 520. [0032] The resistor 510 has a resistance R which represents the combined series resistance of transmission gate 420j (Figure 4) on the activated pattern input bit Pl[j] and the internal resistance of the input inverter 450j. The activated pattern input bit PI[j] is the bit that corresponds to the last SDL tap signal that has gone high. All other bits are de-activated at that time. The output of input inverter 450j is connected either to the power supply V,x or to ground Vss, depending on the logic level at its input. [0033] Note that the signal IN as shown in Figure 5A corresponds not to the input to the DTC 120 (Figure 1), but, rather, to the supply to which the inverter 450j connects the activated transmission gate 420j. [0034] The capacitor 520 has a capacitance C which is the combined parasitic capacitance of all the transmission gates 420o to 420N-I in the DTC 120. This parasitic capacitance consists of the portion of the gate capacitance between the source/drain nodes connected to the circuit output and the gates of the devices, combined with the diffusion capacitances of the source/drain nodes connected to the circuit output and with the interconnect capacitance on the circuit output. [0035] The resistor 510 and the capacitor 520 form a low pass filter on the input. This low pass filter has a transition time T proportional to the time constant RC of the filter. The larger the number of transmission gates 4200 to 420N-I in the DTC 120, the larger the equivalent capacitance C, and therefore, the larger the transition time T. [0036] The transfer function of the equivalent circuit 500 is the transfer function of a simple one-pole low pass filter as given below: (Equation Removed) 2 [0037] Figure 5B is a diagram illustrating an equivalent circuit 525 of the DTC with the amplifier circuit 430 according to one embodiment of the invention. The equivalent circuit 525 includes a resistor 530, a capacitor 540, an amplifier 550, and a feedback resistor 560:' [0038] The resistor 530 has a resistance Rl and represents the combined series resistance of the activated transmission gate and the corresponding inverter driving the transmission gate. The resistor 530 corresponds to the resistor 510 in Figure 5A. The capacitor 540 is equivalent to the capacitor 520 in Figure 5A and has a capacitance C. The amplifier 550 provides a negative gain -A. The feedback resistor 560 has a resistance R2. 10039] The transfer function of the equivalent circuit 525 is given by: (Equation Removed) 3) where T=—R2— Ri (0040] Equation (3) may be simplified by assuming that Rl ~ R2 = R and A >>1. The simplified transfer function becomes: (Equation Removed) 4 [0041] Comparing equation (4) and equation (1) shows that in the equivalent circuit 525 the effective capacitance on the output signal is lower by a factor of A, the amplifier gain, than that of the equivalent circuit 500. Therefore, the effective bandwidth of the DTC with the amplifier circuit 430 is approximately A times wider than that of the DTC without the amplifier circuit. By using the amplifier circuit 430, it is possible to synthesize waveforms having significantly higher frequencies than waveforms synthesized by prior art DTC without the amplifier circuit. [0042] The negative sign for the gain A is due to the inversion caused by the amplifier, which has a negative gain. This inversion can be negated either by adding an inverter after the DTC or by inverting the pattern input PI[0:N-1] to the DTC!. i [0043] The amplifier 550 corresponds to the inverter 436 shown in Figure 4. The feedback resistor 560 corresponds to the feedback transmission gate formed by the n-device 432 and the p-device 434 in the amplifier circuit 430 shown in Figure 4. [0044] In practice, the gain A of the amplifier is generally on the order of 5 to 10. Therefore, the assumption A »1 may not be completely accurate. Nonetheless, the output capacitance is significantly lowered using this technique. The output swing may not be over the full 0-to-VCc range due to this rather low value of A. However, connecting a simple logic gate, such as another inverter, at the output of the DTC readily restores the output swing to the full 0-to-Vcc range. The resistances Rl and R2 may be adjusted by varying the sizes of the transmission gates in the transmission gate array 410 and the feedback transmission gates 432 and 434 in Figure 4. [0045] While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the spirit and scope of the invention. We Claim: 1. An apparatus for reducing output capacitance of digital-to-time domain converter for very high frequency digital waveform synthesis comprising: a synchronous delay line (110) to generate N tap signals in response to a reference signal; and a pattern generator (130) to generate N pattern inputs synchronizing with the output signal. a selector (440) to generate the N select signals based on N tap signals from a synchronous delay line (110). characterised in that it comprises N transmission gates (420o- 420N-I) having N outputs to transfer one of N pattern inputs to a first output based on an active signal from N select signals, the N outputs being connected together to form the first output and having an output capacitance; and an amplifier circuit (430) having a gain and coupled to the N transmission gates (420O-420N-I) at the first output to reduce the output capacitance by an amount approximately equal to the gain, the amplifier circuit (430) generating an output signal. 2. The apparatus as claimed in claim 1 wherein each of the N transmission gates (420O-420N-I) comprises: a first device (424) and a second device (422) coupled together at first (427) and second terminals (429) in provide the first output at the first terminal (427) and to receive each of the N pattern inputs at the second terminal (429), the first and second devices having first and second gates, respectively, the first gate being controlled by one of the N select signals. 3. The apparatus as claimed in claim 2 wherein each of the N transmission gates comprises: an inverter (426) connected between the first and second gates to turn on the second (422) device when the first device (424) is turned on by the one of the N select signals. 4. The apparatus as claimed in claim 3 wherein the inverter (426) turns off the second device (422) when the first device (424) is turned off by the one of the N select signals. 5. The apparatus as claimed in claim 2 wherein the first (424) and second (422) devices are p-type and n-type devices, respectively. 6 The apparatus as claimed in claim 1 wherein at most one of the N select signals is active at a time. 7 The apparatus as claimed in claim 6 wherein at most one of the N select signals is active for a period equal to Tp/N, Tp being a reference clock period. 8. The apparatus as claimed in claim 1 wherein the amplifier circuit (430) comprises: an inverter (436) coupled to the first output to generate the output signal; and a feedback transmission gate coupled across the inverter to provide a feedback resistance. 9. The apparatus as claimed in claim 8 wherein the feedback transmission gate comprises: an n-type device (432) and a p-type device (434) connected together to form a feedback resistor having a feedback resistance, each of the n-type and p-type devices being in the on state. 10. The apparatus as claimed in claim 1 wherein the selector (440) comprises: N logic circuits (440O-440N-I) connected in cascade to generate at most one active signal of the N select signals.

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1096-delnp-2004-abstract.pdf

1096-delnp-2004-assignment.pdf

1096-delnp-2004-claims.pdf

1096-delnp-2004-complete specification (granted).pdf

1096-DELNP-2004-Correspondence-Others (29-01-2010).pdf

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