Frequency Standards

Frequency Standards:

Precision Frequency Standard : Most high precision frequency standard in use utilise a geometric transverse (GT) cut quartz crystal as the frequency controlling resonant elements having changes in frequency of less than 1 part in 10 million per day. This GT quartz has the outstanding characteristic of exhibiting little variation in frequency over a temperature range of 0 — 100°C. The bridge stabilised Meacham oscillator given in Fig. 19.2 is a constant frequency oscillator of very high stability. It consists of a Wheatstone’s bridge and an amplifier section. One of the four bridge arms is a thermally controlled resistance element, two of the arms are fixed resistances, and the fourth is a frequency controlling sensitive element, i.e. crystal. Because of the presence of a thermally sensitive element having a large positive temperature coefficient of resistance (a small tungsten filament lamp of low wattage rating), the output of the bridge is amplitude stabilised and relatively insensitive …


Primary Standard : Primary Standard – One arrangement is a carefully designed quartz crystal oscillator operating in the range 50 to 100 kHz, having a low temperature coefficient, constant amplitude output and voltage regulated power supply. The crystal oscillator has a long time (several months) frequency stability of a few parts in a 100 million, without readjustment. A typical arrangement for comparing the frequency of a primary standard with the period of rotation of earth is given in Fig. 19.1. Here the frequency of the crystal oscillator is 100 kHz. Frequency dividers are used to reduce the frequency until an output frequency of 1 kHz is obtained, to drive the electronic clock. The clock keeps correct time when supplied with exactly 1 kHz frequency. Time indicated by the clock is checked periodically with the observatory time as broadcast on radio channels, i.e. the radio time signal. The primary standard must be provided with very accurate …


Updated: April 30, 2020 — 9:34 pm