General information

 

Frequency and time intervals which are the parameters of AC electrical signal, including information, - one of the types of measurements that are most common. This is due to first of all a very high precision instruments frequency measure devices unattainable for other MI. In addition, the devices due to the frequency response - a characteristic, which largely depends on undistorted communication.

No less important is the control of the frequency stability, such as receiving and transmitting devices. Since the frequency is related to the rate of change of the phase voltage signal, then clearly, frequency control is needed and to account for the phase distortion, especially at very high frequencies.

And finally, verification, validation and calibration of other MMT (Means of Measurement Technic) occurs at certain points in the frequency range, which makes it necessary in the course of these operations to accurately measure the frequency.

The range of frequencies used in communications technology, extending from Hz to tens of GHz. If we exclude the industrial current frequency, the whole spectrum can be divided into five ranges: infrasonic frequencies

– low 20 Hz, sound frequencies – from 20 Hz till 20 kHz, high frequencies – from 20 kHz till 30 MHz, ultrahigh frequencies – from 30 till 300 MHz and super-high frequencies – more 300 MHz The upper limit of the frequency band is used in development of science and technology continuously improves and now exceeds 300 GHz.

The frequency of electromagnetic modes conveniently express through the length of the flat wave in free space λ and period T so these values are simple dependencies:

 

, ,

(1)

 

Where f – frequency, Hz;

с – the speed of propagation of electromagnetic oscillations, m/s;

λ; – length of wave, m;

Т – period of hesitation, the periodic signal characterizing time interval at which repeated his instant values, s.

The speed of propagation of electromagnetic oscillations depends on the environment in which they are distributed:

 

,

 

Where μ_а – absolute magnetic permeability;

μ_а = μ_0·μ;

ε_а–permittivity; ε_а = ε₀ε;

For vacuum μ₀ = 4π•10-7 H/m; ε₀ = 8,852·10^-12F/m, then с₀ = (299792,5 ± 0,3) km/s.

Here μ and ε relative dielectric permittivity and magnetic environment, respectively.

For example, the speed of propagation of electromagnetic vibrations in the cable connection is dependent on the μ_а, and ε_а the dielectric.

For the characteristics of electric oscillations can be measured frequency f, the period T, or the length of the flat wave in free space communications technology in λ is almost always measured frequency, more rarely-period oscillations, and only on ultrahigh frequencies are measured and the frequency and wavelength. Most often measure the average frequency of the account.

 

ƒ = N / Т count,

(2)

 

Where N – number of periods of fluctuation,

Т _count – time accounts (time dimension).

Measurement of frequency used when measuring scales, grades and signal generators, radio devices with smooth adjustment of the range; definition of resonance frequencies of oscillatory circuits and Resonators; determining bandwidth filters and networks are presented; measurement and control of frequency deviations from its nominal value assigned to this device – radio station, generation equipment system seals etc. widely used without-searching and without-adjusting radio-frequency multichannel compression, satellite and mobile communication has been possible as a result of the development of methods for the accurate measurement of the frequency.

In general, the measurement error of frequency setting in absolute Δƒ, or, more often, in relative terms ∆f/f. Permissible error amount is determined by the instability of the measured frequency must be less than her at least 5 times. For example, if the relative frequency instability of the radio station is equal to 10^-5, the relative measurement error shall not exceed 2·10^-6. Reduction, the needed accuracy of measurements must be calibrated using an even more accurate device error which, in this case, should not exceed 4∙10^-7. Error of measurement of low frequencies, of course, be much more – (1... 2) %, except for the frequencies used in tonal telegraphy and data transfer.

 

To measure the frequency using of MI, which are classified in following way:

F1 – standards of frequency and time;

F2 – resonance frequency counters;

F3 – adding electronic frequency counters;

F4 – frequency counters and bridging capacitor, heterodyne;

F5 – frequency converters and Synchronizers frequency signal;

F6 – frequency synthesizers, frequency multipliers and dividers;

F7 – receivers of standard frequencies and time signals and frequency comparators (phase, temporary).

In the frequency measuring instruments, as a rule, highly stable crystal oscillators are used as a reference measure for which distinguish short-term and long-term instability. Short-term instability of such generators is due to thermal noise and the elements of the crystal oscillator and fractional noise transistors. In addition, the short-term volatility affects the instability of supply voltage and vibration. Typical values of the relative standard deviation of the mean frequency increase from 10^-10 for an averaging time of 1s and 3·10^-9 for an averaging time of 1year.

Long-term instability is determined primarily quartz resonator aging and changes in its mechanical properties under the action of destabilizing factors: humidity, pressure, vibration, and radiation exposure. To reduce the destabilizing action of humidity and pressure crystal placed in a vacuum tank. Typical limiting values of the relative error of frequency reproduction due to the long-term instability, ranging from 10^-8 per day to 5·10^-7 per year of operation, and some devices even smaller.