Metrology-4-Time Flashcards
Explain the Time Calender part 1
In Babylonia, again in Iraq, a year of 12 alternating 29-day and 30-day lunar
months was observed before 2000 B.C., giving a 354-day year.
What did the Mayans of Central America rely on to establish 260 and 365 day calenders?
The sun, moon and planet Venus
When did the Mayan culture flourish from and what did this indicate about their beliefs?
Flourished from around 2000 B.C. until about 1500 A.D. They left celestial-cycle records indicating their belief that the creation of the world occurred in 3113 B.C.
Their calendars later became portions of the great Aztec calendar stones.
What type of calendar has other civilisations like our own adopted?
A 365-day solar calendar with a leap year occurring every fourth year.
When and who initiated clock-making as opposed to calendar-making?
5000 to 6000 years ago great civilisations in the Middle East and North Africa.
When and what was the first portable timepiece to measure the passage of hour come into use?
1500 B.C, Egyptian shadow clock or sundial.
This device divided a sunlit day into 10 parts plus two twilight hours in the morning and evening.
What are the two basic components that clocks must have?
- A regular, constant or repetitive process or action to mark off equal increments of time. early examples include movement of sun, sand in glass( hour glass) oil lamps.
- A means of keeping track of the increments of the time and displaying the result.
Our means of keeping track of time passage include the position of the clock hands and a digital time display.
Who made the first pendulum clock?
Dutch scientist, Christiaan Huygens.
What was the first pendulum clock regulated by?
A mechanism with a natural period of oscillation
What was the error of Huygens’ pendulum clock?
Less than 1 minute a day, his later refinements reduced his clock’s error to less than 10 seconds a day.
When and who improved the pendulum clock’s accuracy to 1 second a day?
1721, George Graham
How did George Graham improve the pendulum clock’s accuracy?
He compensated for the changes in the pendulum’s length due to temperature variations.
Who refined and added new methods of reducing friction to the pendulum clock?
John Harrison
What did john Harrison do by 1761?
He built a marine chronometer with a spring and balance wheel escapement that won the British government’s 1714 prize offered for a means of determining longitude to within one-half degree after a voyage to the West Indies.
What did the marine chronometer do on on board a ship?
It kept time on a rolling ship to about one-fifth of a second a day, 10x better than required.
When and who developed a clock with a nearly free pendulum which attained an accuracy of a hundredth of a second a day.
1889, Siegmund Riefler
When was the standard of time changed to quartz crystals?
1930s and 1940s
This improved time keeping performances far beyond that of pendulum and balance-wheel escapements.
What is Quartz clock operation based on?
Piezoelectric property of quarts crystals
Explain the piezoelectric property of quartz crystals?
If you apply an electric field to the crystal, it changes its shape, and if you squeeze it or bend it, it generates and electric field.
What do you do to generate a constant frequency electric signal that can be used to operate an electric signal that can be used to operate an electronic clock display?
Put in a suitable electronic circuit, this interaction between mechanical stress and electric field causes the crystal to vibrate.
Why were quartz crystal clocks better than pendulum clocks?
They had no gears or escapements to disturb their regular frequency.
Why can no two quartz crystals be precisely alike, with the same frequency?
The crystals rely on mechanical vibration whose frequency depends critically on the crystal’s size and shape.
Why do quartz clocks continue to dominate the market in numbers?
Their performance is excellent and they are inexpensive.
Whose timekeeping performance substantially surpasses that of quartz clocks?
Atomic clocks
Explain Atomic Clocks?
Atoms/molecules have resonances; each chemical element and compound absorbs and emits electromagnetic radiation at its own characteristic frequencies. These resonances are inherently stable over time and space.
Explain what significance there is for an atom of hydrogen or cesium today being exactly like one million years ago or in another galaxy ?
This is a potential pendulum with a reproducible rate that can form the basis for more accurate clocks.
When was the first Atomic Clock built and what was it based on?
1949, ammonia
Who built the first atomic clock?
NIST
What was the performance like for the first atomic clock based on ammonia and what happened as a result?
its performance wasn’t much better than existing standards and attention shifted almost immediately to more-promising, atomic-beam devices based on caesium.
When did the NIST complete it6s first caesium atomic beam device?
1957
When was the caesium atom’s natural frequency formally recognised as the new international unit of time?
1967
What was the definition of the second based on?
Caesium atom’s natural frequency
What is the definition of the second?
The second is the duration of 9 192 631 770 periods of the radiation
corresponding to the transition between the two hyperfine levels of the ground
state of the caesium 133 atom.
How is the unit of time realised?
Using a caesium-beam atomic clock, based on the vibration states of the caesium-133 atom.
To what uncertainty does the caesium-beam system allow the SI unit of time to be realised to?
Between 1 part in 10^13 and 10^14.
What are secondary standards provided by?
Rubidium gas cell resonator controlled oscillators or quartz oscillators.
What short term and long term stability do Rubidium oscillators provide?
Short term stability of five parts in 10^12 (over 100s).
Long term stability of 1 part in 10^11 (month).
What short term and long term stability do quartz oscillators provide?
Short term stability of five parts in 10^12 (over 1s).
Long term stability of 1 part in 10^8 (month).
What can the quartz crystal resonator in a timer be?
Uncompensated, temperature compensated or oven stabilised.
What is the frequency stability of quartz oscillators affected by?
Ageing, temperature, variations in supply voltage, and changes in power supply mode.
To what accuracy do uncompensated oscillators give?
Sufficient accuracy for 5-6 digit measurements in most room-temperature applications.
To what accuracy do compensated oscillators give?
Sufficient accuracy for 6-7 digit measurements
What oscillators have better ageing performance and are suitable for 7-9 digit instruments?
Oven stabilised oscillators
What do single shot time interval measurements using a 10MHz clock have a resolution of?
+/- 100ns
Resolution will be either n or n+1
Number of counts = Time (T) * Frequency of the oscillator= Tf
What is trigger error?
The absolute measurement error due to input noise causing triggering which is too early or too late.
TE = +/- (Peak-to-peak noise voltage) / (Signal slew rate)
What factors are the accuracy of counters and timers limited by?
System resolution, trigger errors, systematic errors and time-based errors.
What is systematic error caused by in timers and counters?
Differential propagation delays in the start and stop sensors, amplified channels or by errors in the trigger level settings of the two channels.
How do you remove systematic errors in counters and timers?
Calibration
What are time-base Errors caused?
Deviation on the frequency of the crystal frequency from its calibrated value.
