This article discusses the use of precision oscillators in synchronized timing solutions, covering:
- Network Time Protocol
- Oscillators For Reference Clock Redundancy
- Types of Oscillators: Rubidium, OCXO, TCXO, Quartz
The Network Time Protocol, or NTP as it is more commonly known, is a computer protocol developed to distribute accurate time information across a computer network. Developed in the 1980′s by Dr David Mills of the University of Delaware, NTP is a client-server based rule aiming to provide computer time synchronization across the Internet and other local computer networks. NTP works in a hierarchical mode consisting of primary time servers, secondary time servers and time clients. Stratum 1 servers (the primary server), communicate directly to an external reference clock, this can be GPS or Radio time and frequency broadcasts, to synchronize its internal system time. Both dedicated and Internet-based Stratum 1 NTP time servers are utilised to synchronize computers and computer networks worldwide.
Oscillators for Reference Clock Redundancy
The NTP time server relies on a constant flow of timing information being available from the external reference clock. Should this flow of information be interrupted, the potential for timing errors is introduced and the time server begins to drift away from correct time. A solution for this is found with the use of a high precision, back-up frequency generator which serves to maintain correct time, and extend holdover. There are various precision oscillators utilised by NTP servers for this purpose.
A crystal oscillator, for example, is an electronic circuit that utilises the mechanical resonance of a vibrating crystal of piezoelectric material. This generates an electrical signal with a very precise frequency. The most common type of piezoelectric resonator utilised is the quartz crystal, and as such, oscillator circuits built around them are typically called ”crystal oscillators”.
The stability of an oscillator is determined by a quality factor, or Q factor, which is calculated by dividing resonance frequency by resonance width. Influencing its stability, the Q factor is an integral characteristic of an oscillator. A more steady or stable oscillator is characterised by a high Q factor. The high Q rating means the oscillator will stay close to its natural resonance frequency. Consequently back-up oscillators which feature a high Q factor are the best option for NTP server systems.
Types of Oscillators
Rubidium Oscillators offer a solution at the top end of the scale providing an extremely reliable and highly stable timing reference for extended holdover. Characterised by a high Q rating rubidium oscillators have a quality factor in the range of 10^7 (Q), and operate at 6834MHz, the resonance frequency of the rubidium atom. The high Q rating does, however, mean the rubidium option is typically a more expensive solution.
Oven Controlled Crystal Oscillators (OCXO) are available which feature a quartz crystal placed in a well-insulated electronically controlled oven. Temperature is continually monitored and adjusted by sensors inside the oven and a heating element is utilised to minimise oscillation variance and improve accuracy. This technique increases the stability of the crystal by a factor of 10+. OCXO devices typically feature a quality factor (Q) in the region of 10^6.
With a Q factor in the same region of 10^6, Temperature Controlled Crystal Oscillators (TCXO) are typically smaller than their Oven Controlled counterpart and as such, less expensive, so offer an ideal solution. A TCXO incorporates a minute sensor which continually adjusts the frequency of the crystal oscillation to compensate for fluctuations in temperature, resulting in the stability of the crystal increasing by a factor of 2 or 3. Continued improvements in the manufacture of TCXO’s has increased the level of performance further and resulted in reductions in size and price, making the use of TCXO’s an attractive proposition for even the most cost-sensitive budgets.
Quartz is the most commonly recognised crystal oscillator. Demonstrating a lower quality (Q) factor of 10^4, quartz crystal oscillators are common in numerous consumer devices ranging from watches and clocks to computer systems. Not recognised as providing a very stable oscillation, fluctuations in temperature speed up or slow down a quartz oscillator, which is why the techniques of Oven Control and Temperature Control were developed to reduce environmental influences and increase stability.
Stratum 1 NTP Time servers often incorporate precision oscillators which provide a stable timing reference, independent from the external reference clock. Higher Q factor oscillators are more stable, but are generally more expensive and alternatives exist which provide a good level of stability at a more competitive price.