Pendulum Instruments; FAQ

FAQ

Wander - Wander is slow beatings or irregularities in time. Wander variations occur over a period greater than 0.1s (10 Hz). Wander of the synchronization clock means that the clock is out of phase, and is the main cause for synchronization problems in the transport networks. Wander, which only partly can be filtered out in the network nodes, accumulates in the network and may even cause a total loss of synchronization. Incorrect synchronization in transport networks may cause severe transmission problems. Voice calls (fixed or cellular) will be lost, fax machines will misprint, and data will be lost or frequently re-transmitted. The network operator may loose customers (=money).

Jitter - Jitter is a "faster" form of wander. Jitter variations occur over a period of time less than 0.1s (10 Hz). Jitter may cause bit errors, and retransmission of data.

Synchronization - Synchronization can be described as "perfect timing" of signals, that is the receiving end of a communications link must have exactly the same clock frequency as the transmitting end. Imagine bringing together the best musicians in the world, without calling in a conductor to synchronize (conduct) the ensemble. You would never get the best out of the musicians. And a communications network consisting of the best network elements in the world would never have the optimal performance characteristics without synchronization of the signals.

Calibration - Calibration is the process of verifying the uncertainty of an instrument by comparison with an internal or an external calibration standard. The purpose is to find out the deviation of the instrument to be able to correct measured values. Imagine using a wristwatch that you never compared (calibrated) to Greenwich time, or another approved national time reference. Then you would never know if the displayed time was accurate or not.

Hopping frequencies - Hopping frequencies can be described as a dynamic frequency change where an information-bearing signal is being forced to change frequency on a regular basis (several times per second). This is to avoid overhearing, and is used by the military. It is also used to make communication safer, by avoiding shadow effect at certain frequencies, and is used in Wireless LAN's, Bluetooth, etc.

Modulation - Modulation is the process where some characteristics of a wave (the carrier wave) is made to vary in accordance with an information-bearing signal wave (the modulating wave). If the frequency of the carrier wave varies in accordance to the modulating wave it is called frequency modulation or FM. If the phase of the carrier varies, it is called phase modulation.

Last modified: 2007-10-04