Test Equipment, Calibration and Scientific
Labs
Introduction
- Atomic frequency standards, kept in metrological laboratories, are being used
as ¡°primary¡± frequency and time standards for the calibration of secondary
time/frequency standards. Secondary time standards may be used for local
calibration of laboratory instruments, seismic data collection, or as a basic
ingredient of the calibration services. These users may be interested only in
standard frequency dissemination, thus releasing stringent accuracy requirement
on the primary source, but usually they ask for traceability to international
standard. Such traceability should be recognized by international accreditation
bodies. Under following heading you will find more information about the few of
the test equipment, calibration and scientific labs' applications:
Calibration Laboratories
- In the last decade most countries have established dedicated networks of
calibration laboratories in order to transfer the traceability from national
standards to instruments used at production level. The accredited calibration
laboratories must ensure the traceability of their metrological standards to the
national primary standards. In case of time and frequency quantities, the
calibration laboratories have their own frequency standard – cesiums or
disciplined rubidium, and the Traceability is ensured by means of different
techniques that range from Radio coded signals, Telephone time codes, LF
broadcasts, television broadcasts and navigation satellite broadcasts. Frequency
sources used as reference in calibration laboratories usually are rubidium
frequency standards, GPS disciplined.
- Calibration labs need a frequency reference and not a ¡°time tagging
reference¡±, i.e. they must calibrate frequency or time interval standard, not
dating devices. The relative uncertainty on the knowledge of the frequency is at
the level of 10-13 at best.
Considering the transmission path and the
uncertainty sources depicted in the Figure 8.2-1, some requests may relaxed.
Indeed in frequency comparison systems all the ¡°fixed¡± or stable delays may
disregarded.
- Over the past years, most of the calibration laboratories bought their own
frequency standards, usually an atomic one based on the rubidium or cesium atom.
To ensure traceability, then they had to buy also a comparison system as well.
Therefore the devices and the amount of money were twofold. Now, with the
availability on the market of rubidium frequency standards disciplined to GPS,
the market of stand alone frequency standards has dramatically
decreased.
High Resolution
Counter
- A frequency counter is an electronic instrument, or
component of one, that is used for measuring frequency. Since frequency is
defined as the number of events of a particular sort occurring in a set period
of time, it is generally a straightforward to measure it.
- Most frequency counters work by using a counter which accumulates the
number of events occurring within a specific period of time. After a preset
period (1 second, for example), the value in the counter is transferred to a
display and the counter is reset to zero. If the event being measured repeats
itself with sufficient stability of frequency and this frequency is considerably
lower than that of the clock oscillator being used, the resolution of the
measurement can be greatly improved by measuring the time required for an entire
number of cycles, rather than counting the number of entire cycles observed for
a pre-set duration (often referred to as the reciprocal technique).
The
internal oscillator which enables the frequency counter to measure time is
called the timebase, and must be calibrated very accurately. If what is to be
counted is already in electronic form, simple interfacing to the instrument is
all that is required.
- More complex signals may need some conditioning to make them suitable for
counting. Most general purpose frequency counters will include some form of amplifier, filtering and shaping
circuitry at the input. Other types of periodic events that are not inherently
electronic in nature will need to be converted using some form of transducer.
For example, a mechanical event could be arranged to interrupt a light beam, and
the counter made to count the resulting pulses.
Frequency counters
designed for radio
frequencies (RF) are also common and operate on the same principles as lower
frequency counters.
- Often they have more range before they overflow. For very high frequencies,
many designs use a high-speed prescaler to bring the signal
frequency down to a point where normal digital circuitry can operate. The
displays on such instruments take this into account so they still read true. If
the measured frequency is too high for any prescaler, a mixer and a local oscillator can
produce a suitable frequency to measure.
- The accuracy of a frequency counter is greatly dependent on the stability of
its timebase. Highly accurate circuits are used to generate this for
instrumentation purposes, usually using a quartz crystal oscillator
within a sealed temperature-controlled chamber known as a crystal oven or OCXO (oven controlled
crystal oscillator). For higher accuracy measurements, an external frequency
reference tied to a very high stability oscillator such as a GPS disciplined rubidium oscillator may be
used. Where the frequency does not need to be known to such a high degree of
accuracy, simpler oscillators can be used. It is also possible to measure
frequency using the same techniques in software in an embedded system. A CPU for example,
can be arranged to measure its own frequency of operation provided it has some
reference timebase to compare with.
High
Performance Synthesizers
- A frequency synthesizer is an electronic system for
generating any of a range of frequencies from a single
fixed timebase
or oscillator. They
are found in many modern devices, including radio receivers, mobile telephones, radiotelephones, walkie-talkies, CB radios,
satellite receivers, GPS systems, etc.
- Coherent techniques generate frequencies derived from a single, stable master
oscillator. In most applications, crystal oscillator
are common, but Rubidium frequency sources can be used in high performance
requirements.
Special Test
Equipment
- With greater expansion of wireless cells the need for rubidium standards for
field service and calibration is growing. For example, new handset designs must
meet the standards expected by the consumer and that means carrying out earlier
and more comprehensive development, design verification and regression testing.
In some of the solutions rubidium frequencies standards are required as
frequency references.
Space
Exploration
- How does NASA know where a spacecraft is in deep space? The spacecraft's
precise range, velocity and angular position are determined by the aid of highly
stable frequency standards. The range is determined using information from the
propagation time of microwave radiation between an antenna on Earth and the
spacecraft. The velocity is determined from the "doppler," i.e., by comparing
the phase of the incoming carrier signal with that of a reference signal
generated from the ground station frequency standard. The angular position is
determined by very long baseline interferometry (VLBI) in which widely separated
stations (in California, Spain and Australia) simultaneously receive signals
from the spacecraft. Differences between times of arrival coupled with knowledge
of the baseline vectors joining the station antennas provide direct geometric
determination of the angles between the baseline vectors and the direction to
the spacecraft. Hydrogen masers (see chapter 6) provide the best stability
(~10-15) for the propagation times of interest, which typically range from
minutes to hours. VLBI is also used for high resolution angular measurements in
radio astronomy.
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