Network Time Protocol (NTP) is a standard protocol for synchronising the time of client computers on the Internet and computer networks. The protocol operates in a hierarchical manner, each level or stratum serving the next level in the hierarchy. At the highest level of the hierarchical structure is a stratum 1 NTP server that synchronises to an external time and frequency reference. Many stratum 1 NTP servers reside on the Internet and are used for synchronising network time clients.

NTP Servers utilise the Network Time Protocol (NTP) to provide an accurate timing reference to network time clients. Stratum one Network Time Servers sync to precise external reference clocks such as GPS or Radio timing transmissions.

Many NTP servers utilise a high precision backup frequency generator to maintain time in the event of signal loss. This article describes the various precision frequency generators used by NTP servers and the price-performance issues.

Oscillator stability is inherently defined by their quality factor known as their Q factor. The Q factor of an oscillator is calculated by dividing its resonance frequency by its resonance width. The higher the Q factor the more stable the oscillator. Therefore, NTP servers are best served with backup oscillators with a high Q factor.

The quartz crystal is by far the most common form of oscillator. More than 2,000,000 quartz crystal oscillators are manufactured each year. Quartz crystal oscillators are used in all manner of timing equipment from clocks and watches to computer systems. Quartz crystal oscillators have a quality factor in the range 10^4. Environmental factors, such as temperature, can affect the resonance of quartz crystal oscillators. However, a number of techniques have been developed to minimise external environmental effects on the crystal.

TCXO's, or Temperature Compensated Crystal Oscillators, contain frequency compensation circuits that adjust for temperature variations. TCXO's provide an increase in the stability of the crystal by a factor of 2 or 3. Oven Controlled Crystal Oscillators (OCXO) utilise a technique designed to minimise temperature fluctuations that affect the resonance of a quartz crystal oscillator. The quartz crystal is encased in an insulated enclosure along with a small heating element and a temperature sensor. The heating element and temperature sensor maintain the crystals temperature at a constant level, well above the external temperature. Maintaining a constant temperature can increase the stability of the crystal by a factor of 10 or more. Typically, TCXO and OCXO devices have a quality factor (Q) in the range 10^6. Recent advances in TCXO manufacturing processes have dramatically improved size, performance and cost. TCXO's are now an attractive proposition for even cost-sensitive applications.

Rubidium oscillators operate at 6834 MHz, the resonance frequency of the Rubidium atom. Rubidium based oscillators are relatively expensive, but have a much higher stability than crystal oscillators. However, manufacturing and design advances mean Rubidium Oscillators are constantly becoming smaller and less expensive. Rubidium based NTP servers can provide a highly stable timing reference but are still relatively expensive. To summarise, NTP servers with oscillators that have a higher Q factor provide a more stable timing reference. However, stability often comes at a cost and a compromise between price and performance may need to be sought.