Oscillators
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Oscillators
About Oscillators and Frequency Standards
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Natural Quartz Crystals
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Understanding the characteristics of the complete range of frequency standards, based on technologies
from quartz crystal to atomic resonance, is a lifework in itself. Oscillators are fundamental
to virtually all modern electronic applications. Free-running, uncompensated crystal
oscillators (XO) are the workhorses in the vast majority of applications, e.g. microprocessor clocks.
Temperature-compensated crystal oscillators (TCXO) come into their own in more sensitive applications,
such as local oscillators in coherent, spread-spectrum receivers like those used in the GPS or in
digital cell phone systems. Crystal oscillators continue to offer the best short-term
stability/phase noise of any oscillator technology. This is why the output signal from any atomic
standard actually comes from a crystal oscillator that is frequency locked to the atomic resonance
frequency. The atomic standard provides long-term stability, while the quartz standard provides
short-term stability.
Ovenized Quartz Crystal Oscillators
When a single (OCXO) or double (DOCXO) temperature-controlling oven is wrapped around the crystal and
its oscillating circuitry, the frequency stability may be improved two to four orders of magnitude
relative to that of the TCXO. Such oscillators are used in laboratory and communications grade
applications and often have the means to adjust their output frequency via electronic frequency
control. In this way they may be "disciplined" to match the frequency of a GPS or Loran-C
reference receiver.
GPS-disciplined DOCXOs are the Stratum I Primary Reference Sources (PRS) for
many of the world's wired telecommunications systems. They are also widely deployed as GPS time
and frequency references for the basestations operating under the IS-95 standard for the Code Division
Multiple Access (CDMA) cellular mobile phone systems originated by Qualcomm. The sheer volume of
these basestation applications has profoundly affected the OCXO market by driving prices down and
consolidating vendors.
An unfortunate characteristic of most quartz-based oscillators is the
presence of sudden step changes in their output frequency. In the huge bulk of applications,
these steps are small enough to not be noticed. In precision applications, this is not the case.
The steps may be large enough to send a system out of specification for a significant period of time.
Read about Frequency Steps and our exclusive "No Frequency Steps"
guarantee.
Atomic Resonance Frequency Standards
The next level in stability is achieved when the resonance frequencies of certain atoms such as
rubidium, hydrogen or cesium are used to control the frequency of a crystal oscillator. Over the
last 20 years, rubidium-based frequency standard technology has matured to the point that they are
cost-effective replacements for the high-end, ultra-stable OCXO/DOCXO. Though comparable in size,
their short-term stability and power consumption are inferior to an ultra-stable OCXO, but the
systematic drift in frequency versus time is generally superior and the restabilization time following
a power interruption is much shorter. Problems with rubidium cell wear-out have been solved and
manufacturers now offer essentially unlimited warranties against cell failure.
Hydrogen-maser-based
frequency standards remain to this day as very large, exotically expensive oddities looking for an
application that requires the remarkable medium-term stability they are capable of. Typically they are
found only in national standards laboratories where they are used for very low-noise medium-term measurements
of other frequency standards. Hydrogen masers suffer from systematic frequency drift that requires
that they be referenced to a primary frequency standard for long term measurements, which is not a problem
for a national standards lab.
Primary Frequency Standards
Of the atomic frequency standards, only the cesium-based standards may be called primary standards,
meaning that they are autonomous absolute frequency references. These primary standards may
contribute to the world's timescale, Universal Coordinated Time (UTC), which is a "paper clock"
composed of an elaborate ensemble of the time from hundreds of primary standards scattered around the
world. All other frequency standards mentioned here exhibit varying levels of systematic
frequency drift over time and may not be classed as "primary".
Cesium-based frequency standards are
typically available in two performance grades depending upon the beam tube design: standard or
high-performance. They continue to be large, expensive units with a well defined wear-out
mechanism (cesium depletion) that periodically requires an expensive, beam tube replacement. The
high-performance tube has about one-third of the life of the standard tube. In addition,
prior to actual failure, anomalous frequency behaviors usually occur which are best detected by
monitoring via a co-located GPS or Loran-C based frequency reference. Since modern,
high-performance GPS-disciplined frequency standards like our Meridian
Rubidium products can match the long-term performance of the best standard tube cesium, at a much lower
cost, while delivering superior short term performance and life expectancy, it's pretty easy to make a
case for skipping cesium altogether in any practical application where an antenna may be installed.
Click here to see a chart comparing the stability performance of a Meridian GPS TimeBase vs. HP Cesium.
Short-Term and Long-Term Stability
Often the short-term behavior of an oscillator's frequency is as important as its long-term drift
and sensitivity to environmental stresses. Short-term stability is measured in the frequency
domain as phase noise, or in the time domain as 2-sample or Allan variance. Over observation
intervals less than one second, crystal oscillators outperform all other oscillator types for this
parameter. For this reason, the output of an atomic standard is taken directly from a crystal
oscillator that is frequency locked to the atomic resonance frequency.
Beginning at observation
intervals of 10 to 100 seconds, hydrogen masers take over and reign supreme until their frequency
drift component reaches a level that gives the advantage to cesium. Rubidium offers superior
short term stability to cesium for observation intervals less than about one day, which is why the
GPS now uses rubidium standards in the satellites. The system architecture and satellite orbit
periods allow the longer term instabilility to be measured and compensated effectively in real time.
Click here for an application note on oscillators used by EndRun Technologies.
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