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Frequency and Time Standards - An Introduction.

1. Time/Frequency Standards
   1. What is a Time/Frequency Standard
   2. Significance of Time/Frequency Standards
2. Important Types of Time/Frequency Standards
1. Cesium Standard
2. Rubidium Standard
3. Crystal Standards
3. Comparison of Time/Frequency Standards
4. Definition of Terms Used
5. Manufacturers of Time/Frequency Standards
6. References

1.  What is a Time/Frequency Standard:

1. Coordinated Universal Time (UTC): We know that the time is related to frequency. Hence, the Time and Frequency standards are interdependent. The frequency of Cesium atom is used for the purpose of establishing Time standard. Cesium is one of the most stable frequency generators under laboratory conditions. International Bureau of Weights and Measures (BIPM), France computes the frequency using coordinated atomic (Cesium) clocks (about 250 of them), which in turn is used for the computation of UTC. Cesium oscillator is used as the primary standard for time. According to the International System of Units (SI), a second was defined as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyper-fine transition of cesium-133 atoms in their ground state undisturbed by external fields.  Thus,

1 second = 9,192, 631,770 cycles of the standard Cs-133 transition

2. Significance of Time/Frequency Standard: Precise time/frequency measurement is essential for telecommunications, broadcasting, military, navigation, and other scientific experiments. For example. a small time error of “one second” at a speed of 17,580 miles per hour or 7860 m/s (this is the speed at which the space shuttle travels in orbit) of space shuttle, the target (say, docking with the space station) is missed by about 7.8 kilometers! Of course, for applications where the primary standard of frequency is not required, you have secondary and tertiary standards available. The secondary, and tertiary standards are traceable to the primary standard.

2. Important Types of Frequency Standards

1. Cesium Standard (Primary standard)
2. Rubidium Standard (Secondary standard)
3. Crystal Standards (Tertiary standard, and below)

1. Cesium (caesium) Standard: The cesium standard is the primary standard of time/frequency. It is the standard maintained by over 250+ atomic stations around the world for keeping the UTC.  The commercially available primary standards have an accuracy of ±1 in 10 ¹¹.
2. Rubidium Standard: Rubidium is known as the secondary standard for time/frequency. The standard is calibrated using primary standard. Commercially available rubidium standards have a stability of about ±5 in 10¹¹. Note that both Cesium and Rubidium are atomic standards and exhibit extremely high accuracy and ageing characteristics.
3. Crystal Standards: Normally, crystal standards provide stability of the order of 1 in 10⁸. The crystal standards are comparatively cheap, and available in various forms depending on the accuracy level required. As a result, crystal standards are widely used in test and measurement equipment, and telecom. Quartz is primarily used for making crystal oscillator standards. The material exhibits high Q, and higher stiffness (resulting in smaller Capacitance value, and low loss).

Crystal Oscillators may be further sub-divided into three broad categories. These are:

a.       XO

b.      TCXO

c.       OCXO

XO is short for Crystal Oscillator. Ordinary crystal oscillators exhibit high rates of inaccuracy, and ageing. These are normally used in applications that do not require high accuracies, or in conjunction with more stable oscillator source such as a TCXO. TCXO stands for Temperature Controlled Crystal Oscillator. TCXOs’ exhibit relatively more stable accuracy, and ageing characteristics. TCXOs are widely used in telecom equipment for providing stable source of frequencies. OCXO stands for Oven Controlled Crystal Oscillator. OCXOs are considered to be the most stable among crystal oscillators. Oven oscillators take a few minutes to warm-up and the power consumption is typically few watts at room temperature.

3. Comparison of Frequency/Time Standards (Cesium, rubidium, and crystal):

Important parameters that characterize a frequency/time standard are:

a.       Frequency Accuracy

b.      Ageing

c.       Phase Noise

d.      Warm-up time

e.       Temperature Stability

Green: Excellent

Red: Poor

Note that the values shown above are only typical, and you need to consult the manufacturer product datasheet before making any purchase. As seen in the table above, crystal oscillators exhibit good stability, but poor accuracy. Further, crystal oscillators require initial burn-in (this  to accommodate initial frequency drift associated with crystal oscillators). 

Definition of terms:

Frequency Accuracy: This is the degree of conformity to a specified value of a frequency. It is usually represented by the offset from the ideal frequency with zero uncertainty.

Ageing: Ageing is the process during which the frequency changes permanently. Ageing occurs primarily due to stress relief, and is structurally related. For quartz crystals, ageing is quicker during the initial period. For this reason, quartz crystal oscillators are aged before final manufacture and shipment.

Phase Noise: Phase noise is the random frequency fluctuation of the signal. Phase noise is normally measured at 1Hz, 10Hz, 100Hz, 3kHz, etc. from the carrier. Lower values are desirable (say –100dBc is better than –80dBc). The oscillator phase noise is a significant parameter, since it used to modulate the signal frequencies and ultimately affect the purity of the transmitted/received signal.

Warm-up time: Warm-up time is the amount of time an un-powered electronic device at room temperature takes to stabilize at its higher operating temperature once it has been powered on.

Stability: Oscillators frequency stability is defined as the measure of the degree to which an oscillator maintains the same value of frequency over a given time. Atomic frequency sources, such as cesium and rubidium are very stable. The stability of crystal oscillators can be improved by using TCXO’s and OCXO’s.

Manufacturers of Frequency/Time Standards:

A. Cesium Standards:

• Symmetricom, Inc. www. symmetricom.com

B. Rubidium Standards:

·        Symmetricom, Inc., USA www. symmetricom.com

·        Novatech  Instruments, Inc., USA www.novatech-instr.com

C. Crystal Oscillators:

·        Symmetricom, Inc., USA www. symmetricom.com

·        Novatech  Instruments, Inc., USA www.novatech-instr.com

·        Bliley Technologies, Inc., www.bliley.com

·        Spectracom Corporation, NY, USA, www.spectracomcorp.com

D. Hydrogen Maser Standards

·        Quartzlock (UK) Ltd. www.quartzlock.com

·        Symmetricom, Inc., USA www. Symmetricom.com

References:

http://tf.nist.gov/timefreq/general/precision.htm#Anchor-29509

http://www.ieee-uffc.org/freqcontrol/quartz/vig/vigtoc.htm

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