Rubidium Oscillators Additional Precision for NTP Serve (Part 2)

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Continued…

However, there are some occasions when a time server can lose connection with the atomic clock and not receive the time code for a prolonged period of time. Sometimes this may be because of downtime by the atomic clock controllers for maintenance or that nearby interference is blocking the transmission.

Obviously the longer the signal is down the more potential drift may occur on the network as the crystal oscillator in the NTP server is the only thing keeping time. For most applications this should never be a problem as the most prolonged period of downtime is not normally more than three or four hours and the NTP server would not have drifted by much in that time and the occurrence of this downtime is quite rare (maybe once or twice a year).

However, for some ultra precise high end applications rubidium crystal oscillators are beginning to be used as they don’t drift as much as quartz. Rubidium (often used in atomic clocks themselves instead of caesium) is far more accurate an oscillator than quartz and provides better accuracy for when there is no signal to a NTP time server allowing the network to maintain a more accurate time.

Rubidium itself is an alkali metal, similar in properties to potassium. It is very slightly radioactive although poses no risk to human health (and is often used in medicine imaging by injecting it into a patient). It has a half life of 49 billion years (the time it takes to decay by half – in comparison some of the most lethal radioactive materials have half-lives of under a second).

The only real danger posed by rubidium is that it reacts rather violently to water and can cause fire

Rubidium Oscillators Additional Precision for NTP Serve (Part 1)

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Oscillators have been essential in the development of clocks and chronology. Oscillators are just electronic circuitry that produces a repetitive electronic signal. Often crystals such as quartz are used to stabilise the frequency of the oscillation,

Oscillators are the primary technology behind electronic clocks. Digital watches and battery powered analogue clock are all controlled by an oscillating circuit usually containing a quartz crystal.

And while electronic clocks are many times more accurate than a mechanical clock, a quartz oscillator will still drift by a second or two each week.

Atomic clocks of course are far more accurate. They still, however, use oscillators, most commonly caesium or rubidium but they do so in a hyper fine state often frozen in liquid nitrogen or helium. These clocks in comparison to electronic clocks will not drift by a second in even a million years (and with the more modern atomic clocks 100 million years).

To utilise this chronological accuracy a network time server that uses NTP (Network Time Protocol) can be used to synchronise complete computer networks. NTP servers use a time signal from either GPS or long wave radio that comes direct from an atomic clock (in the case of GPS the time is generated in a clock onboard the GPS satellite).

NTP servers continually check this source of time and then adjust the devices on a network to match that time. In between polls (receiving the time source) a standard oscillator is used by the time server to keep time. Normally these oscillators are quartz but because the time server is in regular communication with the atomic clock say every minute or two, then the normal drift of a quartz oscillator is not a problem as a few minutes between polls would not lead to any measurable drift.

To be continued…

Dealing with Time across the Globe

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No matter where we are in the world we all need to know the time at some point in the day but while each day lasts for the same amount of time no matter where you are on Earth the same timescale is not used globally.

The impracticality of Australians having to wake up at 17.00 or those in the US having to start work at 14.00 would rule out suing a single timescale, although the idea was discussed when the Greenwich was named the official prime meridian (where the dateline officially is) for the world some 125 years ago.

While the idea of a global timescale was rejected for the above reasons, it was later decided that 24 longitudinal lines would split the world up into different timezones. These would emanate from GMT around with those on the opposite side of the planet being +12 hours.

However, by the 1970’s a growth in global communications meant that a universal timescale was finally adopted and is still in much use today despite many people having never heard of it.

UTC, Coordinated Universal Time, is based on GMT (Greenwich Meantime) but is kept by a constellation of atomic clocks. It also accounts for variations in earth’s rotation with additional seconds known as ‘leap seconds’ added once of twice a year to counteract the slowing of the Earth’s spin caused by gravitational and tidal forces.

While most people have never heard of UTC or use it directly its influence on our lives in undeniable with computer networks all synchronised to UTC via NTP time servers (Network Time Protocol).

Without this synchronisation to a single timescale many of the technologies and applications we take for granted today would be impossible. Everything from global trading on stocks and shares to internet shopping, email and social networking are only made possible thanks to UTC and the NTP time server.

European Time Synchronisation with DCF-77

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The DCF 77 signal is a long wave transmission broadcast at 77 KHz from Frankfurt in Germany. DCF -77 is transmitted by Physikalisch-Technische Bundesanstalt, the German national physics laboratory.

DCF-77 is an accurate source of UTC time and is generated by atomic clocks that ensure its precision. DCF-77 is a useful source of time that can be adopted all over Europe by technologies needing an accurate time reference.

Radio controlled clocks and network time servers receive the time signal and in the case of time servers distribute this time signal across a computer network. Most computer network use NTP to distribute the DCF 77 time signal.

There are advantages of using a signal like DCF for time synchronisation. DCF is long wave and is therefore susceptible to interference from other electrical devices but they can penetrate buildings that give the DCF signal an advantage over that other source of UTC time generally available – GPS (Global Positioning System) – which requires a open view of the sky to receive satellite transmissions.

Other long wave radio signals are available in other countries that are similar to DCF-77. In the UK the MSF -60 signal is broadcast by NPL (National Physical Laboratory) from Cumbria while in the USA, NIST (National Institute of Standards and Time) transmit the WVBB signal from Boulder, Colorado.

NTP time servers are an efficient method of receiving these long wave transmissions and then using the time code as a synchronisation source. NTP servers can receive DCF, MSF and WVBB as well as many of them also being able to receive the GPS signal too.