IEEE Circuits and Systems Magazine - Q2 2020 - 18

	

dF = - dfs Fc = - x $ Fc,
fs

Fnew = Fc + dF (6)

By changing current value Fc, frequency fs can be
adjusted. If fx is desired new frequency: dfs = fs - fx and
dfs /fs = (fs - fx) /fs = x where x represents a small change,
then (6) can be used to calculate new value Fnew . Most
likely, slightly adjusting fraction r can produce the desired result. In experiment, frequency granularity (resolution) of sub-ppb range has been demonstrated [43],
[53]. Frequency switching can be done very quickly
(two clock cycles after the command is received). These
two distinguished features of arbitrary frequency generation and instantaneous frequency switching are extremely valuable for the task of clock synchronization.
It provides flexibility to system developer since these
two features make it possible for higher communication

Frequency
Tuning

Voltage

XO

fo

(Analog)

Variable Cap.
Diode
MOS Varactor
Array

fs

(a)
On/Off
Selection
XO
LCO

Frequency
Tuning
fo

(Digital)

SwitchedCap. Array

fs

(b)
Divide
Ratio

(Digital)

Frequency
Tuning
MEMS

fo

Frequency
Tuning

Voltage

XO

fo

Fractional -N
PLL

fs

(c)
(Analog)

A /D + DSP +
DVCO

fs

(d)
Control
Word
XO
MEMS
LCO

(Digital)

Frequency
Tuning
fo

TAF-DPS

fs

(e)
Figure 9. Frequency-tunable sources. (a) VCXO, (b) DCXO,
(c) MEMS, (d) DSP Synthesizer, and (e) TAF-DPS DCXO.
18 	

l-ayers to access frequency adjustment capability when
necessary. Moreover, to make clock frequency more stable, compensation for environment (e.g. temperature)
can also be applied as shown.
D. Comparison of Sources in Suitability
for Synchronization
Frequency stability of a source is defined as the degree
to which the oscillator generates a constant frequency
throughout a specified period of time. In many cases, there
is however demand for frequency-tunable source whose
frequency can be quickly and precisely adjusted through
a control variable. This feature is especially useful for developing new algorithms to meet the T
- AACCS challenges.
Practically speaking, frequency adjustability and stability
are two requirements that contradict each other. Therefore, tunable frequency source is a special class of its own.
The performance of such sources is judged by resolution,
pulling range, monotonicity and linearity. Figure 9 depicts
several types of frequency-tunable sources that will be
studied for their suitability as on-chip syntonization tool
for network time synchronization.
Figure 9(a) is VCXO (voltage-controlled oscillator)
where variable capacitor diode or MOS varactor array is
incorporated inside crystal oscillator circuit. Frequency
tuning is accomplished through an analog voltage. A
voltage change leads to a change in capacitance value
which in turn leads to a change in oscillation frequency
[54]-[56]. Figure 9(b) shows the case of DCXO (digital
controlled oscillator) where the resonant circuit's capacitance is realized by switched capacitor array. The array is
controlled by a digital word which turns on or off each individual capacitor. The oscillation frequency is therefore
controlled through a digital word [57]-[59]. Besides crystal, the resonator can also be LC tank (LCO).
In recent years, thanks to its lower power, smaller
form factor, higher reliability and lower price, microelectromechanical oscillator (MEMS) becomes a popular solution that is replacing crystal oscillator in some
markets [60]. There are two application drivers where
quartz addresses with mechanical processes but MEMS
addresses electronically: a) providing a wide range of
application frequencies, and b) trimming resonator
frequency over production tolerances. The frequencytuning of MEMS oscillator is achieved not through adjusting its resonant frequency but by a post-processing
electrical circuit, which is typically a fractional-N PLL as
shown in figure 9.c. The desired frequency adjustment is
represented as divide ratio and is fed into the PLL [61].
Another approach for tuning frequency is based on
digital signal processing. In [62], two crystals are used
as temperature sensor. The difference in their f vs. T
(temperature) characteristics is converted into digital

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IEEE Circuits and Systems Magazine - Q2 2020

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