Chapter 3 A New Ultra-Wideband Monocycle Pulse Generator
3.4 Comparison
Table 3-2 shows the comparison between our work and other proposed ones.
From Table 3-2, we may observe that our pulse generator owns the characteristics of low ringing level and adjustability.
TABLE 3-2
COMPARISONOF THIS WORKTO OTHER PULSE GENERATORS
Ref. [6] [15] [16] [17] This work
Simulation/
Measurement Measurement Measurement
Peak-to-peak
Waveform Monocycle Monocycle
*Ringing level is defined as ⎥
⎦
**The pulse duration is defined as the time interval between the 10% points of the positive and negative peaks.
Chapter 4
An Adjustable CMOS Ultra-Wideband Pulse Generator
4.1 Introduction
Generally there are four kinds of pulse generation methods listed as follows. First, several Gaussian-like impulses can be utilized to shape a fifth derivative of the Gaussian pulse [9]. Second, a second derivative Gaussian pulse can be generated by utilizing tanh(x) or other mathematic operations [10, 11]. Third, researchers utilize digital gates to compare two square waves with time difference to form a Gaussian impulse which further passes the filter to form the monocycle pulse [12-14]. Four, the pulse is generated from the step-recovery diode (SRD) and this pulse generator should be fabricated on PCB [5, 6].
We present an adjustable CMOS ultra- wideband (UWB) pulse generator using the second-order transient circuit to generate the adjustable monocycle pulse with good symmetry and low ringing. The rise time of the square wave can be adjusted to change the amplitude of the pulse for different requirements. This pulse generator is simulated and measured on TSMC 0.18μm CMOS technology. Our simulation shows that the pulse generator produces the monocycle pulse whose peak-to-peak amplitude ranges from 50 mV to 220 mV and 500-ps pulse duration.
4.2 Circuit Operation and Analysis
This chapter presents a new CMOS UWB monocycle pulse generator. This pulse generator is composed of a square wave shaping circuit, a current starving cell, a pulse shaping control circuit, and a second order transient circuit as shown in Fig. 4-1.
The circuit schematic was shown in Fig. 4-2. The square wave shaping circuit can decrease the rise time of the square wave and the current starving cell is used to adjust the rise time of the signal in order to have an adjustable amplitude of the monocycle pulse. In order to lower the effect of the process variation, the effective resistance of M13 is tuned for keeping a good symmetry of the pulse. The pulse shaping control circuit can change the symmetry of the monocycle pulse waveform. Finally, we use a current mirror to drive the second order transient circuit that can generate monocycle pulses.
Fig. 4-1 Block diagram of the propose monocycle pulse generator.
Fig. 4-2 Schematic of the proposed monocycle pulse generator.
4.2.1 Square wave shaping circuit and current starving cell
The square wave shaping circuit is composed of two CMOS inverters. As shown in Fig. 4-3, the CMOS inverters are implemented to decrease the rise time of the square wave. So the square wave Vi_1 with fixed rise time is obtained. The fixed rise time of the square wave is beneficial for the input of the current starving cell.
The current starving cell can further control the rise time of the square wave in order to make pulse amplitude adjustable. We use the voltage Vc to control VGS of M9 and M10 which can change the current of M7 and M8. Therefore, the rise time can be varied to create different pulse amplitudes.
20 40 60 80 100 120 140 160 180 200
0 220
-1 0 1
-2 2
time, nsec
Vi, VVi_1, V
Fig. 4-3 The effect of the two CMOS inverters.
4.2.2 Pulse shaping control circuit
The pulse shaping control circuit is composed of M11, M12, and M13. We treat M11 as a capacitor whose capacitance was about 30 fF in TSMC 0.18μm CMOS technology. When the voltage Vi_3 is negative where M12 and M14 is at off state, the output node Vo does not generate pulse. When Vi_2 is at the rising edge of the square waveform, M12 turns on and the voltage of Vi_3 in turn drives M14 on. The voltage of Vi_3 is increasing with Vi_2 and the monocycle pulse can be generated as shown in Fig. 4-4. Furthermore, because M13 can be regarded as a variable resistor, we can adjust Vgg to get a symmetric waveform of monocycle pulse for various process variation.
50 100 150 200 250
0 300
-1 0 1
-2 2
time, nsec
Vi_2, VVi_3, V
Fig. 4-4 The voltage waveform at node Vi_2 and Vi_3.
4.2.3 Second order transient circuit
We apply a pulse voltage to M14 for driving the current mirror M15 and M16.
An impulse voltage is generated from the inductor L1. Generally the monocycle pulse have some ringing. We therefore use a resistor Rx to form the second order transient circuit operating in over-damping mode which can decrease the ringing of the pulse.
Fig. 4-5 is the equivalent circuit diagram of the output stage of Fig. 4-2 where we let L1=L, C1=C, and R=Rx+RL. The transient response on the over-damping mode is deduced as follows.
Fig. 4-5 Diagram of the second order transient circuit.
Let the form of v(t) be e . We can apply the Kirchhoff’s law to the RLC loop st and obtain the following equation as
0 1 ) 1
( 1 =
+ +
R sC
V sL (4.1)
Then, the characteristic equation of the transient circuit can be obtained as, 1 0
)
2 +( + =
s LC L
s R (4.2)
Finally we can derive s from (4.2) as
2
ω is the oscillation frequency and
L R
=2
α is defined as the damping coefficient of the circuit.
When α > ωo,the value of s is negative and makes the energy stored in the inductor and the capacitor discharges to R in the exponential decay manner. This condition is called the over-damping response of the second order transient circuit which can reduce the ringing of the monocycle pulse.
4.3 Simulation Results
The whole circuit simulations are completed in Agilent ADS®. We can observe the effect of the over-damping response in Fig. 4-6. There exists some ringing of the monocycle pulse as shown in Fig. 4-6(a) and the ringing of the monocycle pulse can be decreased by the over-damping response in Fig. 4-6(b). The voltage Vc can be adjusted to change the amplitude of the monocycle pulse whose peak-to-peak amplitude ranges from 50 mV to 220 mV in Fig. 4-7 and 500 ps of the pulse duration as shown in Fig. 4-8. We will discuss how process variation, temperature variation and voltage variation impact the symmetry of the monocycle pulse and how Vgg can adjust the symmetry of the monocycle pulse.
155 156 157 158 159 160 161 162 163
154 164
-100 -50 0 50 100
-150 150
time, nsec
Vo_NoRx, mV
(a) Rx=0Ω (Have ringing)
155 156 157 158 159 160 161 162 163
154 164
-50 0 50
-100 100
time, nsec
Vo_WithRx, mV
(b) Rx=100Ω (Low ringing)
Fig. 4-6 The ringing of the pulse is decreased by the over-damping response.
155 156 157 158 159 160 161 162 163
154 164
-100 -50 0 50 100
-150 150
time, nsec
Vo1, mVVo2, mV
Fig. 4-7 The amplitude of the monocycle pulse is adjustable and the voltage Vc equals -2V(Vo1) and -2.42V(Vo2) respectively.
158 160 162
156 164
-50 0 50
-100 100
time, nsec
Vo_500ps, mV
Fig. 4-8 The duration of the monocycle pulse is 500 ps.
4.3.1 Process variation
Usually process variation will impact the performance of circuit. As shown in Table 4-1, process variation will change the symmetry of the monocycle pulse.
Therefore, the voltage Vgg can adjust the symmetry of the monocycle pulse for the compensation of process variation.
Process variation: TT、FF、SS、FS、SF Simulation tool: Agilent ADS®
Simulation condition: TSMC 0.18μm CMOS technology Input square wave repetition rate: 10MHz
Output monocycle pulse duration: 500 ps Temperature: 25 ℃
TABLE 4-1 PROCESS VARIATION
Process variation Simulation results
Vgg=0.503V
Symmetrical waveform
FF
Vgg=0.503V
Asymmetrical waveform
FF
Vgg=0.445V
Symmetrical waveform
SS
Vgg=0.503V
Asymmetrical waveform
SS
Vgg=0.51V
Symmetrical waveform
FS
Vgg=0.503V
Asymmetrical waveform
FS
Vgg=0.445V
Symmetrical waveform
SF T=25℃
156 158 160 162 164 166
154 168
Vgg=0.503V
Symmetrical waveform
Input signal
4.3.2 Temperature variation
As shown in Table 4-2, temperature variation will also change the symmetry of the monocycle pulse.
Temperature variation: 0℃、25℃、85℃
Corner case=TT
TABLE 4-2
TEMPERATURE VARIATION
Temperature
Vgg=0.503V
Asymmetrical waveform
T=0℃
Vgg=0.483V
Symmetrical waveform
T=25℃
158 160 162
156 164
-50 0 50
-100 100
time, nsec
All_PG_Monocycle_Final_ForReport_25C..Vo, m
m14 m14time=
All_PG_Monocycle_Final_ForReport_25C..Vo=64.70mV159.5nsec
Vc=‐2.3V
Vgg=0.503V
Symmetrical waveform(Typical)
T=85℃
156 157 158 159 160 161 162 163 164 165
155 166
-0.5 0.0 0.5
-1.0 1.0
time, nsec
All_PG_Monocycle_Final_ForReport_85C..Vo, m
Vc=‐2.3V
Vgg=0.503V
Asymmetrical waveform
T=85℃
156 158 160 162
154 164
-40 -20 -0 20 40 60
-60 80
time, nsec
All_PG_Monocycle_Final_ForReport_85C_Vgg045V..Vo, m
m16 m16time=
All_PG_Monocycle_Final_ForReport_85C_Vgg045V..Vo=62.84mV159.3nsec
Vc=‐2.3V
Vgg=0.45V
Asymmetrical waveform
4.3.3 Voltage variation
As shown in Table 4-3, voltage variation will also change the symmetry of the monocycle pulse and the voltage Vgg can also adjust the symmetry of the monocycle pulse.
Voltage variation: Vcc(1.8V) and Vss(-1.8V) ±10%
Corner case=TT
TABLE 4-3 VOLTAGE VARIATION
Voltage variation Simulation results
Vgg=0.503V
Symmetrical waveform
Vdd = 1.62V Vss = ‐1.62V
156 158 160 162 164 166 168 170
154 172
156 158 160 162 164 166 168 170 172
154 174
Vgg=0.45V
Symmetrical waveform
4.4 Fabrication and Measurement
This pulse generator is measured and fabricated on TSMC 0.18μm CMOS technology. Fig.4-9 shows the microphotograph of the proposed pulse generator whose size is 0.868mm * 0.457mm.
The measurement is made by using a 10-MHz square wave having 20-ns rise time and fall time respectively. Fig.4-10 shows the measured Gaussian pulse waveforms at different Vc. Change of Vc can result in the peak amplitude of the Gaussian pulse from 18 mV to 30 mV. The result also shows the 300-ps pulse duration with low ringing which results from the over-damping effect of the second order transient circuit.
If smaller value of capacitor C1 is chosen in simulation, the monocycle pulse will be asymmetric. Therefore, we instead use a large capacitor C1 to get a monocycle pulse with low ringing. However, from the measurement result, the monocycle pulse can not be generated for the large capacitor C1. A possible solution to modify this work is to connect a broad band amplifier which can amplify the Gaussian pulse and a high pass filter to differentiate the Gaussian pulse.
Fig. 4-9 The microphotograph of the proposed pulse generator.
40 42 44 46
38 48
-0.02 -0.01 0.00 0.01
-0.03 0.02
time, nsec
Vc_n2V2..voltage
(a)
40 42 44 46
38 48
-0.02 -0.01 0.00 0.01
-0.03 0.02
time, nsec
voltage
(b)
Fig. 4-10 Measured output waveforms of the pulse generator at different control voltages Vc.
4.4.1 Measurement with battery power supply
We use batteries and regulated power supply ICs, LM317 and LM337, to obtain pure DC sources. From this measurement, the amplitude of the tuning range of Gaussian pulse is increased. The following figures show the amplitude of Gaussian pulse for different control voltage, Vc. Table 4-4 lists the simulation and measurement results of the proposed pulse generator.
38 40 42 44 46
38 40 42 44 46
36 48
-0.005 0.000 0.005 0.010 0.015
-0.010 0.020
time, nsec
''Vc_n2V_Vgg_0.3V''..voltage
(d) Vc = -2V
38 40 42 44 46
36 48
-0.005 0.000 0.005 0.010 0.015
-0.010 0.020
time, nsec
''Vc_n2.2V_Vgg_0.3V''..voltage
(e) Vc = -2.2V
Fig. 4-11 Measured output waveforms with battery power supply of the pulse generator at different control voltages Vc.
TABLE 4-4
PERFORMANCEOF THE MONOCYCLE PULSE GENERATOR
Simulation Measurement
(Power Supply)
Measurement (Battery) Process
Technology TSMC 0.18μm CMOS
Pulse
Waveform Monocycle pulse Gaussian pulse Gaussian pulse
Pulse Width 450 ps 300 ps 270ps ~ 470 ps
(Vp-p) 53 mV ~ 214 mV 18 mV ~ 30mV 6 mV ~ 20mV
Chip Area 0.397 mm2
(0.868mm * 0.457mm)
4.5 Comparison
Table 4-5 shows the comparison of this work to other pulse generators. From Table 4-5, we present a pulse generator of low ringing level and adjustability.
TABLE 4-5
COMPARISONOF THIS WORK TO OTHER PULSE GENERATORS Ref. [12] [18] [19] This work
Simulation/
Measurement Simulation Simulation
Peak-to-peak
amplitude 22.97 mV 200 mV 110 mV 53 mV~
214 mV
Ringing level -24.9 dB N/A -17.8 dB -40.5 dB~
-26.6 dB
Pulse
duration 450 ps 250 ps 170 ps 450 ps Technology CMOS Bi-CMOS CMOS CMOS
Adjustable No Yes
Waveform Monocycle Monocycle
4.6 Discussion
4.6.1 Pulse shaping control circuit using RC high pass filter
We will discuss the pulse shaping control circuit which uses the RC high pass filter as shown in Fig. 4-12. The comparison between RC high pass filter and our work is shown in Fig. 4-13. The black color line is the response of RC high pass filter and the blue color line is that of our proposed circuit. We use a RC high pass filter as a pulse shaping control circuit. It can not quickly turn off M14, so the output waveform Vo is not a monocycle pulse but a Gaussian pulse. The simulation of our work shows
that the fall time of Vi_3 is too short in Fig. 4-13(a). Practically this waveform can not be realized. Similarly, since the voltage waveform Vi_3 of our work also has a very short fall time duration, our work is not able to turn off M14 quickly either. As a result, the voltage waveform VL is not a Gaussian pulse, and the resultant voltage waveform Vo is a Gaussian pulse rather than a monocycle pulse. Finally we propose several methods to modify this work.
Vdd Vdd
Fig. 4-12 Shaping control circuit using the RC high filter.
154 156 158 160 162 164 166 168 170 172
152 174
Vi_3, V_960829_test..Vi_3, V
(a)
158 160 162 164 166
VL, mV_960829_test..VL, mV
(b)
156 158 160 162 164
154 166
Vo, mV_960829_test..Vo, mV
(c)
Fig. 4-13 The comparison between RC filter (Black) and our work (Blue).
4.6.2 Methods to modify this pulse generator
Because the output waveform Vo on measurement is not a monocycle pulse but a Gaussian pulse, we propose some possible methods to modify this work. The first is to design a pulse shaping control circuit which can generate a short pulse to quickly turn on and turn off M14. For example, we can utilize a NAND gate whose input signals are two square waves with time difference to generate a short pulse for driving
M14. Moreover, since the output waveform Vo is a Gaussian pulse in the measurement, another method is to connect a wideband amplifier in our work which can increase isolation and connect a high pass filter to differentiate the Gaussian pulse and generate the monocycle pulse.
Chapter 5 Conclusion
Two new UWB monocycle pulse generators have been designed, fabricated and measured in this thesis. First, we fabricate a monocycle pulse generator using two BJTs and a second-order transient circuit. The monocycle pulse duration ranges from 600 ps to 780 ps with good symmetry and low ringing. Vcc can vary the peak-to-peak amplitude of the monocycle pulse from 120 mV to 280 mV. Second, we use TSMC CMOS process to fabricate a pulse generator. The inductor L1 is used to generate the impulse and differentiate it to get the monocycle pulse. The ringing of the pulse can be decreased by using the over-damping technique of the second order transient circuit. From the measurement results, Vc can vary the peak amplitude of the Gaussian pulse from 18 mV to 30 mV. The result also shows the 300 ps pulse duration with low ringing. The voltage-adjustable characteristic makes both the proposed pulse generators applicable in many UWB systems.
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