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 means for making comparisons, since 0.01s in one day represents more than 1 part in 10,000,000.
Secondary Standards of Frequency
Secondary frequency standards are ordinarily based on carefully designed crystal oscillators. When the utmost in frequency precision is not required of the secondary standard, it is possible to relax the design in certain respects, such as less precise temperature control to the crystal, use of crystals at frequencies lying outside the optimum range of 50 — 100 kHz, etc.
The crystal oscillator frequency used for a secondary standard frequency can be maintained constant for long periods without readjustment, to within a few parts in a million.
Practical Frequency Standards
Frequencies are categorised as either primary or secondary standards.
The unit of time universally accepted is the mean solar second, which is defined as 1/86400 of the average time required for one complete rotation of the earth around the sun. However, this rotation is not constant. There is a continuous reduction of angular velocity amounting to an increase in length of the day of 0.00164 s per century. Secondly, there are cyclic seasonal variations in the length of the day.
The primary standards of frequency consist of eight precision frequency standards.
The 100 kHz outputs of these standards are automatically compared with each other and the time determined by the US Naval Observatory. The National standards of frequency are made available by continuous broadcast from radio stations on a total of eight radio carrier frequencies of 2.5, 5, 10, 20, 25, 30 and 35 MHz.
Two standard audio frequencies, 440 Hz and 4000 Hz, are broadcast on these radio carrier frequencies. The accuracy of each of the radio carrier frequencies as transmitted, is better than one part in 50 million.
In each case, however, if the accuracies of the received carrier frequencies of this order are required, it is necessary to make repeated measurements over a long interval in order to take into account the fluctuations in the transmitting time of the received signal. These fluctuations are a direct result of any changes in the propagation medium, i.e. changes in ionospheric density and level cause the frequency of the received signal to exhibit small variations about the transmitted signal frequency.
These variations, known as the doppler effect, are related to geographic location, season, sunspot cycle, and other factors, and necessitate time averaging measurements over a long interval, or application of some corrective technique.
Radio Signals as Frequency Standards
The US National Bureau of Standards conducts regular transmission that includes continuous transmissions on several carrier frequencies. The carrier frequencies, modulated frequencies and the time interval associated with these signals are derived from a primary standard that has an accuracy that is better than 1 part in 50 million.
A shift in the height of the ionosphere leads to the Doppler effect, which causes the received frequency to differ from the transmitted frequency, causing an error. However, the maximum error introduced is very slight, if the received frequency is averaged over an extended time interval, the doppler effect will cancel out.
Signals from broadcast stations are particularly good secondary standards, because they are required to maintain their assigned frequency to within 20 Hz, and commonly maintain it to within a few parts/million.