The Network Time Protocol (NTP) is a standard Internet protocol for the transfer of time around a computer network. NTP was originally developed to synchronise time on Internet time clients. The protocol has a hierarchical structure, each level of the structure serves time to the level below. At the top of the hierarchy is a stratum-1 network time server that synchronises to an external time & frequency source, such as GPS.

NTP Servers utilise the Network Time Protocol (NTP) to provide an accurate timing reference to network time clients. Stratum 1 NTP servers synchronise to highly accurate external reference clocks such as GPS or Radio Time & Frequency 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 most common form of oscillator is a quartz crystal 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.

Temperature controlled crystal oscillators (TCXO) contain a sensor that adjusts the frequency of the oscillator to compensate for the effects of temperature on the crystal. 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. Combined, the heating element and sensor maintain the internal temperature of the crystal well above the ambient 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 based oscillators operate at the resonance frequency of the Rubidium atom, 6.834,682,612 GHz. 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.