Timing is everything

An accurate clock mechanism is a fundamental
element of any modern system design either serving as a reference
or for enabling synchronisation to be established. Technological
progression in both wireless and wire-line communication sectors is
placing huge pressure on manufacturers to produce timing devices
capable of keeping pace with ever-higher performance benchmarks.
This article looks at the challenges being faced and how more
sophisticated devices are emerging as a result.

In communication and broadcasting, use of a
highly stable clock for either reference and synchronisation
purposes is advised. This is normally taken care of by a precision
crystal oscillator (XO) device — in most cases taking the
form of an oven-controlled crystal oscillator (OCXO), typically
with a frequency range from 10 to 40 MHz (see Fig. 1).

However, as communication infrastructure moves
into the new IP-based era, with Long Term Evolution (LTE) mobile
networks and 10/40-Gbit Ethernet optical lines being deployed, as
well as high-definition (HD) broadcasting becoming increasingly
commonplace, far larger quantities of data will be transferred.
This will depend on implementation of complex modulation techniques
beyond the scope of conventional OCXO technology.

Due to significant increases in the subscriber
base and the bandwidth required per individual subscriber, more
transfer channels will be needed. However, as the available
frequency range for the different kinds of technologies is limited,
tighter tolerances are imperative. With tighter tolerances the gap
between the channels can be reduced, so the bandwidth for each
channel can be expanded or with the same bandwidth, more channels
can be squeezed frequency range.

The increase in data transfer rates also calls
for a reduction in bit error rates. This means the stability of the
clock source must be improved, so that the impact of jitter is
reduced and phase noise performance can be lowered. Next-generation
communication systems will need to specify higher-performance
reference clocks. This can be achieved through a phase-locked loop
(PLL), but this has the disadvantage that it simultaneously
decreases system performance. So to maintain higher resolution,
more advanced OCXO technology is now proving to be the more favored
approach. Furthermore, the need to fully use all available board
space is leading to greater use of compact, surface mount
packaging. At the same time demands are being placed on devices to
have more rugged construction, with wider operational temperature
ranges.

Crystal stability

The key item for consideration when looking to
ensure OCXO stability is the characteristics of the crystal at the
heart of its construction (see Fig.
2). Crystal stability is defined by:

Fig. 2: The key to ensuring OCXO stability
is in the characteristics of the crystal at the heart of the
OCXO’s construction.

1. Aging
stability: Typically a 10 MHz OCXO will see its stability
impinged upon by around 50 ppb/year, with high-end OCXO devices
only witnessing a deterioration of perhaps 20/30 ppb/year. This
parameter is very important in relation to the overall system
stability for a long period of operation.

2. Short-term
stability: For periods of 1 s up to 100 s, short-term
stability is of prime importance. To reach good short-term
stability, a crystal with a high quality factor (Q-factor) is
necessary. This depends on the crystal mode, frequency, package,
and various other factors associated with its production. A
third-overtone crystal reaches higher Q-factors compared with the
fundamental mode at the same frequency. For a fifth overtone at the
same frequency, the Q-factor is also better, but resistance levels
will also increase. It is therefore very challenging to create
low-frequency crystals in a fifth overtone. Also the
crystal‘s high resistance can hamper the oscillator
circuit’s ability to maintain stable oscillation under
all operational conditions.

For high-performance OCXOs, an SC-cut (stress
compensated-cut) crystal is usually specified. The oscillation-mode
third overtone is preferred compared to a fundamental mode, thanks
its higher stability in all cases because of the blank thickness.
The blank thickness is inversely proportional to the frequency of
the crystal, so a highly stable crystal should have thick blank
(see Fig. 3).

Fig. 3: The blank thickness is inversely
proportional to the frequency of the crystal, so a highly stable
crystal should have thick blank.

At higher frequencies (above 50 MHz),
fifth-overtone SC-cut crystals are the best choice for attaining
high stability, due to the fact that, for third-overtone crystals,
the Q-factor decreases and also the aging will get worse compared
with a fifth overtone. So fifth-overtone crystals generally exhibit
a greater degree of optimization for higher frequencies, but this
needs innovative crystal design and fabrication to deliver crystals
with really tight tolerances.

Use of higher overtones, like seventh or ninth,
though theoretically possible, is very hard to realize as it is
difficult to fabricate crystals for this. In addition, the
oscillator design is very complicated because of the high
resistance and low pullability of these crystals. Another important
factor is temperature stability. For OCXOs, this is predominantly
defined by the heating circuit and heating control of the
oscillator circuit. For the crystal, it is very important to have a
tight adjustment tolerance at the turnover point because the
pullability of the fifth-overtone crystal is less than the third
overtone.

Currently OCXO designs are mainly based on a
third-overtone SC-cut crystal. To move to higher frequencies using
a fifth-overtone SC-cut crystal means that certain, more
forward-thinking manufacturers are looking to employ completely new
circuit concepts so that the position where oscillation occurs can
be moved. Also, because of the higher crystal resistance, the gain
has to be improved to guarantee startup under all conditions. For
such next generation designs the phase noise performance also has
to be improved. IQD Frequency Products has set a goal to reach
values near the carrier in the range of the company’s
current 10-MHz IQOV-90-series and improve noise floor values (at
offset frequencies around 100 Hz away from the carrier). In this
frequency range, the phase noise is determined by the crystal, so a
very good crystal with strong Q-factor will help to reach similar
values. Far away the phase noise is determined by the power supply
and output stage, so here filtering is needed. For the temperature
stability the main issue is to have efficient thermal coupling
between the crystal and heating circuit. For an OCXO the internal
heating temperature must be 10 to 20°C higher than the
maximum ambient temperature to guarantee a stable operation at high
temperatures. This is due to the unregulated part of the power
dissipation caused by the oscillator circuit, power supply and
output stage.

It is clear that to cope with the exacting
demands of next-generation communication systems, OCXOs need to
evolve. They must offer higher frequency levels, tighter
stabilities, improved phase noise and jitter characteristics, wider
operational temperature ranges, and more compact form factors.
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