Circuits and Systems, 2011, 2, 365-371
doi:10.4236/cs.2011.24050 Published Online October 2011 (
Copyright © 2011 SciRes. CS
Single-Stage Vernier T ime-to-Digital Converter with
Sub-Gate Delay Time Resolution
Chin-Hsin Lin, Marek Syrzycki
School of Engineering Science Simon Fraser University, Burnaby, Canada
E-mail:, mar
Received June 1, 2011; revised June 22, 2011; accepted Ju ly 1, 2011
This paper presents a single-stage Vernier Time-to-Digital Converter (VTDC) that utilizes the dynamic-logic
phase detector. The zero dead-zone characteristic of this phase detector allows for the single-stage VTDC to
deliver sub-gate delay time resolution. The single-stage VTDC has been designed in 0.13 μm CMOS tech-
nology. The simulation results demonstrate a linear input-output characteristic for input dynamic range from
0 to 1.6 ns with a time resolution of 25 ps.
Keywords: Vernier Time-to-Digital Converter, Dynamic-Logic Phase Frequency Detector
1. Introduction
The Vernier Time-to-Digital Converter (VTDC) is a cir-
cuit that has been commonly used to provide on-chip
timing measurement with fine resolution. These circuits
are being implemented in PLL-based frequency synthesis
systems [1,2], for on-chip PLL jitter measurement [3,4],
and for time-of-flight measurement units in particle
physics and medical imaging, such as Positron-Emission
Tomography (PET) imaging [5]. In all these applications
the adaptation of the Vernier method allows to achieve
sub-gate delay time resolution. Despite this fine time
resolution characteristic, the conventional VTDC still has
some disadvantages due to the linear structure of the
Vernier Delay Line (VDL), since the length of the VDL
determines the measurement range of the VTDC. Hence,
increasing the measurement range will increase the chip
area and the power consumption of the circuit. Moreover,
since the measurement accuracy of VTDC depends on
the matching of the delay cells, the mismatches in the
VDL delay cells lead to differential non-linearity (DNL)
and integral non-linearity (INL) errors. Although careful
layout techniques can help to minimize these mismatches,
these problems cannot be completely eliminated in the
design of VTDC. In order to improve the time resolution,
the TDC architecture has evolved from multistage VDL
[3,6] to 2-dimensional [7] and 3-dimensional [8] de-
lay-space scheme, and to the Δ-Σ architecture [9], lead-
ing to a dramatic increase in circuit complexity. However,
the increased complexity makes the circuits more suscep-
tible to process variation. In order to eliminate the prob-
lems caused by the large structures of VDL, single-stage
VTDC designs have been proposed [4]. In a single-stage
VTDC circuit, the linear VDL has been replaced by a
single Vernier stage that consists of two triggerable
oscillators featuring different oscillation periods, Ts and
Tf (Figure 1).
The input signals of the single-stage VTDC, START
and STOP, are used to trigger oscillators. When the
START signal arrives at the single-stage VTDC, the
slow oscillator is triggered and starts to oscillate with a
period of Ts. On the arrival of the STOP signal, the fast
oscillator is activated to oscillate with a period of Tf, and
the counter starts to count the number of its oscillations.
After both oscillators have been triggered, the phase dif-
ference between signals STs and STf is initially equal Tin.
Since Tf is smaller than Ts, the phase difference between
STf and STs gets reduced each cycle by an oscillation
period difference of (TsTf), and the signal edge of STf
gradually catches up with STs. When these two signal
edges are coincident, the phase detector signal will dis-
able the counter. The input phase difference, Tin, can be
determined using the following equation:
Figure 1. Concept of a single-stage Vernier Time-to-Digital
Converter [4].
ins f
TTT (1)
where CNT is the number of oscillation cycles counted by
the counter. The performance of the single-stage VTDC
surpasses the conventional VTDC in measurement accu-
racy, chip size, and power consumption. However, the
measurement resolution of a single-stage VTDC is lim-
ited by the phase detectors’ performance. The resolution
of a single-stage VTDC cannot be smaller than the mini-
mum detectable phase error of its phase detector. The
measurement range of the single-stage VTDC is also lim-
ited by the detection range of the phase detector. Al-
though the use of single-stage VTDC alleviates the com-
ponent mismatch problems in the conventional VTDC,
the single-stage VTDC can be still unable to achieve
sub-gate delay time resolution due to the limitations of
the adopted phase detector [4]. This paper presents a sin-
gle-stage VTDC with a dynamic-logic phase detector that
has an extended phase detection range and zero dead-
zone characteristics, allowing for reliable sub-gate delay
time resolution.
This paper is organized as follows. Section II de-
scribes the single-stage VTDC with classic register type
phase detectors. The design details of the single-stage
VTDC with a dynamic-logic phase detector proposed in
this work are presented in Section III. The simulation
results are in Section IV and Section V provides conclu-
sions and a summary.
2. Single-Stage Vernier Time-to-Digital
Converter with Classic Regis t e r Type
Phase Detector
A single-stage VTDC is the VTDC that utilizes two
triggerable oscillators with a precise oscillation freque-
ncy difference to replace the VDL in the conventional
VTDC. In the previously reported implementation of the
single-stage VTDC [4], the classic phase detector with
two D-type registers and an AND gate has been utilized
in to control the timing measurement process (Figure 2).
The phase detector keeps track of the history of the phase
difference between two oscillators and stops the measur-
ement process once STf begins to lead STs. On the first
rising STf edge after the rising edge of STs, the output of
the first register Q1 goes high. On the following rising
edge of STf, the second register keeps the value of Q1
and switches Q2 to high. When the signal edge of STf
catches up with STs, the output QB1 rises and switches
the output of the AND gate to generate the Phase De-
tected signal, as shown in Figure 3. The Phase Detected
signal is fed to the counter where it stops the time meas-
urement process.
Figure 2. Single-stage VTDC with a classic two-register
phase detector [4].
Figure 3. Timing diagram of the correct phase detection [4].
The classic two-register phase detection mechanism
relies on the proper operation of D-type register. How-
ever, if the time difference between the rising edges of
Clock and Data signals violates the setup time constraint
of the registers, the outputs of the registers will not be
correct. This problem is generally referred as meta-sta-
bility [10]. In order to prevent the meta-stability, the
Data signal should be held steady for certain amount of
time before the Clock event, and the minimum value of
this time constraint is called a setup time. The meta-sta-
bility is likely to happen in the classic two-register phase
detector, when the STf signal catches up with STs signal
and the phase difference between these two signals is
smaller than the required setup time. The unpredictable
outputs of the registers will further cause the phase de-
tector unable to stop the measurement process accurately.
Therefore, the requirement for the non-zero setup time in
the classic two-register phase detector is equivalent to
the dead-zone characteristic of the phase detector. Due to
the dead-zone characteristic, the single-stage VTDC de-
signed with a classic two-register phase detector will
feature a serious limitation on the time measurement
resolution. The single-stage VTDC with the two-register
phase detector built in 0.18 μm CMOS technology has
been reported to achieve only a 54.5 ps measurement
resolution [4].
Hence, it becomes evident that any further improve-
ments in the time resolution of the single-stage VTDC
would require the using of the phase detector that is not
limited by the meta-stability error and thus eliminates the
dead-zone in the phase detector.
Copyright © 2011 SciRes. CS
C.-H. LIN ET AL.367
3. The Single-Stage Vernier Time-to-Digital
Converter w ith Dy na m ic -L ogic Phase
3.1. The Architecture of the Single-Stage VTDC
with the Dynamic-Logic Phase Detector
The new single-stage VTDC (Figure 4) exploits the con-
cept of the single-stage Vernier circuit, similar to the cir-
cuit proposed by [4]. In order to further improve the
measurement resolution of the single-stage VTDC, a dy-
namic-logic phase detector with zero dead-zone is pro-
posed in this work. The dynamic-logic Delayed-Input-
Pulse Phase Frequency Detector (DIP-PFD) is known to
have zero dead-zone and an extended detection range [11].
Therefore it is a promising candidate to improve the time
measurement resolution of the VTDC. The proposed sin-
gle-stage VTDC (Figure 4) is composed of two trigger-
able ring oscillators, the dynamic-logic phase detector,
and the counter. The functional blocks of the single-stage
VTDC are described below.
3.2. Triggerable Voltage-Controlled Ring
The triggerable voltage-controlled ring oscillators are
built similarly to the circuit proposed by [6]. However,
for the simplicity, we do not use Phase-Locked Loops
(PLL) to stabilize the oscillator frequencies. The trig-
gerable ring oscillators have been designed using voltage
controlled delay cells and NAND gates, as shown in
Figure 5. Instead of generating different oscillation fre-
quencies by using PLLs [6], the different oscillation fre-
quencies are generated by using voltage-controlled delay
cells. This solution is sufficient for the proof of the con-
cept and also provides a degree of controllability to the
time measurement resolution.
The NAND gate is used to accommodate the triggering
signal, START or STOP. When the triggering signal is
high, the NAND gate operates as an inverter closing the
feedback loop, and creating the conditions for oscillations
to take place. When the triggering signal is low, the
NAND gate deactivates the feedback loop and stops the
circuit from oscillating. The schematic of the triggerable
voltage-controlled oscillator is shown in Figure 6.
The slow and fast triggerable voltage-controlled oscil-
lators have the same architecture. The different oscilla-
tion frequencies are achieved by applying different con-
trol voltages. The control voltage of the fast oscillator is
connected to the VDD allowing a fast oscillation fre-
quency. Meanwhile, the control voltage of the slow os-
cillator is connected to a lower externally controlled vol-
tage, producing a slower but tunable oscillation frequ-
ency (Figure 7). The slow oscillator features a tunable
oscillation period of 4.47 ns down to 2.61 ns that de-
pends on the control voltage within the 0.5 V to 1.2 V
range, while the fast oscillator has a steady oscillation
period of 2.61 ns. In order to achieve a measurement
resolution of 25 ps, the oscillation period of the slow
oscillator is kept at 2.635 ns by applying the control
voltage around 1.1 V.
Figure 4. Single-stage VTDC with dynamic-logic phase de-
Figure 5. Concept of the triggerable voltage-controlled os-
Figure 6. Schematic of the triggerable voltage-controlled
Figure 7. Oscillation periods of the voltage-controlled oscil-
lators versus control voltage.
Copyright © 2011 SciRes. CS
3.3. Dynamic-Logic Phase Detector
The dynamic-logic phase detector is a two stage phase
detector constructed with a Delayed-Input-Pulse Dy-
namic Phase Frequency Detector (DIP-PFD) [11] fol-
lowed by a C2MOS register (Figure 8). The dynamic-
logic phase detector compares the phase difference be-
tween the STs and the STf signals and generates the Start-
of-Conversion signal (SoC) to control the measurement
The first stage of the dynamic-logic phase detector is
the Delayed-Input Pulse Phase-Frequency Detector
(DIP-PFD) proposed in [11]. It has been chosen because
of its zero dead-zone and extended detection range char-
acteristics. The PFD (Figure 9) compares the phases and
frequencies of the input signals, andgenerates the UP and
DN error pulses based on the phase and frequency dif-
ference between the input signals.
In this DIP-PFD, when the STs and STf signals are low,
the U1 and D1 nodes are precharged high. When STs
rises, the UPb node is discharged, producing the UP
pulse. On the arrival of the rising edge of STf, the DNb
node is pulled low, generating the DN pulse. When both
outputs UP and DN are high, the U1 and D1 node will be
pulled low, causing the UPb and DNb nodes to go high.
This condition will deactivate the UP and DN pulses and
reset the PFD. The difference in the pulse width between
the UP and DN signals is therefore equal to the phase
difference between STs and STf. Since the dynamic-logic
PFD uses its own output signals directly to reset itself,
there is virtually no dead-zone in this design. The DIP-
PFD was simulated with the ring oscillators presented in
the previous section. The outputs of the ring oscillators
have been connected to the DIP-PFD to verify the dead-
Figure 8. Dynamic-logic phase detector.
Figure 9. Delay-input-pulse ph ase fr e que ncy detector [11].
zone characteristic. The triggerable ring oscillators were
triggered by two rising signal edges with a phase differ-
ence of 500 ps. The phase difference between STs and STf
was gradually decreasing (by 25 ps in each oscillation
cycle), finally reaching zero in the 21st cycle. The simu-
lation result (Figure 10) shows that the DIP-PFD is ca-
pable of detecting phase error in a single picosecond
range. This makes the DIP-PFD a good solution to
eliminate the dead-zone of the phase detector in sin-
gle-stage VTDCs.
The second stage of the dynamic-logic phase detector
(Figure 8) is a C2MOS register. The error signals, UP
and DN, generated by the DIP-PFD are connected to a
C2MOS register (Figure 11). The register is used to
sample the UP signal by the DN signal as the clock. In
the case when the UP signal leads the DN signal, the
C2MOS register will generate SoC signal to enable the
counter clock. When STf catches up to STs, the UP and
DN signals will overlap each other. The Data input sig-
nal of the C2MOS register will change at the same time
as the Clock signal so that the output signal, SoC, will
fall to deactivate the counter clock (Figure 12).
However, since the required setup time of the C2MOS
register is sensitive to process variation, the SoC signal
may remain high for additional oscillation cycles. This
will result a minor time offset to the time measurement.
This time offset can be easily determined and removed
Figure 10. Zero dead-zone characteristic of the Delayed-
Input-Pulse Phase Freque nc y Detector.
Figure 11. C2 MOS register.
Copyright © 2011 SciRes. CS
C.-H. LIN ET AL.369
Figure 12. Timing diagram of dynamic-logic phase detector.
by measuring zero input phase difference [4]. Due to the
offset, the input phase different Tin calculation must be
modified. Taking offset into account, the measured input
phase difference Tin is equal:
CNT offset 
ins f
3.4. Counter
The SoC signal generated by the dynamic-logic phase
detector controls the operation of a counter, which counts
the number of cycles the measurement takes (CNT). A
6-bit counter has been designed with C2MOS registers,
as shown Figure 13. The counter is enabled by the SoC
signal and clocked by the STf signal. Each stage of the
counter divides the clock frequency signal by half.
Therefore, the output signal of each register oscillates
two times slower than its input clock signal. The final
output signal levels [Q0:Q5] can be interpreted as the
binary value of the CNT. The input phase difference, Tin,
can be calculated with the CNT by using Equation (2).
4. Simulation Result
The single-stage VTDC with the dynamic-logic phase
detector has been designed using the 0.13 μm IBM
CMOS technology using the 1.2 V power supply. The
Figure 14 shows the schematic diagram of the sin-
gle-stage VTDC with dynamic-logic phase detector.
The Figure 15 shows digital output as a function of
the input phase difference, simulated in the typi-
cal-typical (TT) technology corner. The control voltage
has been set to 1.10V to achieve the resolution approxi-
mately 25 ps. The output characteristics of the sin-
gle-stage VTDC with dynamic-logic phase detector are
linear within the time range from 0 to 1600 ps. These
results demonstrate that the circuit can correctly detect
the phase difference and control the measurement proc-
ess even at an oscillation period difference of 25 ps.
Figure 13. Block diagram of the 6-bit counter.
Next we compared the characteristics of the single-
stage VTDC with the dynamic-logic phase detector with
those of the single-stage VTDC with a classic regis-
ter-type phase detector (Figure 16) implemented in the
TSMC 0.35 μm CMOS as reported in [6]. The corre-
sponding time resolution of this design was 37.5ps, and
the characteristics offset estimated as 125 ps. [6]. The
single-stage VTDC with the dynamic-logic phase detector
has much smaller offset, in the order of 25 ps. We attrib-
ute the large offset of this VTDC [6] to the classic regis-
ter-type phase detector. The offset results from a substan-
tial dead-zone of the classic register-type phase detector,
when for small phase differences (smaller than the width
of the detector’s dead-zone) the VTDC is unable to pro-
duce a digital output. Using the dynamic-logic phase de-
tector clearly decreases the characteristics offset and
makes measurements of small phase differences possible.
The ability to vary the VCO’s oscillation frequency
can be used to control the resolution of the single-stage
VTDC with the dynamic-logic phase detector. Example
results (Figure 17) show that varying the control voltage,
one can achieve better resolution of the time measure-
ments and further minimization of the offset. The in-
creased resolution leads to a smaller input time range.
Hence, the control voltage adjustment can be used for
applications that require different time resolutions and
input time ranges.
We also observed that without the PLL-type stabiliza-
tion of the VCO, the single-stage VTDC characteristics
will vary with process variations. The results presented
in Figure 18 show significant variations in time meas-
urement resolution, and in the circuit gain and offset. To
diminish this effect while keeping the circuit architecture
simple, we postulate to compensate the process varia-
tions through appropriate adjustment of the control volt-
age, which varies the VCO oscillation frequency. The
compensated results are shown in Figure 19, demon-
strating that it is possible to obtain nearly identical output
characteristics in different process corners by adjusting
the control voltage. The characteristics in the TT corner
(Vc = 1.10 V, Figure 19(a), are almost the same as in the
FF corner (but for Vc = 1.06 V, Figure 19(b), and as in
the SS corner (but for Vc = 1.13 V, Figure 19(c). Hence,
the control voltage can be used not only for setting up the
parameters of the single-stage VTDC, but also for cali-
bration if necessary.
Copyright © 2011 SciRes. CS
Copyright © 2011 SciRes. CS
Figure 14. Schematic diagram of the single-stage VTDC.
Figure 17. Digital output characteristics of the single-stage
VTDC with the dynamic-logic phase detector in which the
control voltage has been used to set up different time
measurement resolution. Only the 0 - 200 ps fraction of the
entire time scale has been shown for simplicity. (A small
vertical offset in the digital output has been introduced for
better readability).
Figure 15. Digital output characteristic of the single-stage
VTDC with the dynamic-logic phase detector (TT corner).
Figure 18. Variability of the digital output characteristics of
the single-stage VTDC with the dynamic-logic phase detec-
tor as a function of process variations (in the TT, FF, and
SS process corners), for a constant control voltage, Vc = 1.10
V. Only the 0 - 200 ps fraction of the time scale has been
shown for simplicity. (A small vertical offset in the digital
output has been introduced for better readability).
Figure 16. Comparison of the digital output characteristics
of the single-stage VTDC with the dynamic-logic phase de-
tector with the characteristics of the single-stage VTDC
with the classic register-type phase detector from [6].
C.-H. LIN ET AL.371
(a) (b)
Figure 19. Digital output characteristics of the single-stage
VTDC with the dynamic-logic phase detector in the differ-
ent process corners compensated using different values of
the control voltage (a) TT corner Vc = 1.10 V; (b) FF corner,
Vc = 1.06V; (c) SS corner, Vc = 1.13 V.
5. Conclusions
This paper presents a single-stage Vernier Time-to-
Digital Converter with sub-gate delay time resolution.
By utilizing the dynamic-logic phase detector that elimi-
nates the dead-zone problem, the single-stage Vernier
Time-to-Digital Converter in this work has demonstrated
a linear digital output characteristic with a 25 ps time
resolution. The presented simulation results have con-
firmed the very important role of the phase detector qual-
ity for the performance of single-stage Vernier Time-
to-Digital Converters with sub-gate delay resolutions.
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