Standard Frequency Signal Research Articles

Instrument for checking quartz-crystal oscillators with standard frequency signals in the long-wave and very-long-wave ranges

1) The quartz-crystal oscillators of a local time service having an average degree of stability can be checked without reducing the frequency of the signal being checked and the standard signal to a single nominal value, thus substantially simplifying the design of the frequency-comparison instrument. 2) By employing a narrow-band quartz-crystal filter tuned to the standard frequency in the signal-amplifying channel it becomes possible to receive on a loop antenna the standard-frequency signals in the long-wave and very-long-wave ranges at distances of three thousand kilometers or more from the transmitting stations, thus making it practical to use these signals over the entire territory of the USSR. 3) An instrument based on the method realized for processing and recording the frequency-error signal of an oscillator being checked with respect to its nominal value makes it possible to determine with a high degree of accuracy the magnitude and sign of this departure over a range of ±25 Hz which is needed in order to find the correction for quartz-crystal oscillators in the various types of instruments involved in the absolute measurements of physical quantities.

Standard Frequency and Time Signal Emission and Its Utilization

Standard Frequency and Time Signal Emission and Its Utilization

A transportable working frequency standard

The use of GSVCh standard reference frequency signals transmitted by radio, in combination with transportable RECh radio broadcasting stations, can play a decisive role in maintaining consistency and accuracy in frequency measurements under any operating conditions of radio engineering equipment, as confirmed by the results of experimental operation, certification, and confirmation of the transportable working frequency standard at VNIIFTRI.

The French quadristatic incoherent scatter facility

The original Nançay‐St. Santin incoherent scatter facility based on the radio astronomy antenna at Nançay and a CW transmitter located at St. Santin (300 km south) was improved in many ways between 1965 and 1970: doubling of the transmitted power to 140 kw, lowering of the receiver noise level down to 100 K, monitoring of the antenna and data acquisition through the use of a computer, rejection of the parasitic tropospheric echoes.However, the most important improvement brought to the system is the addition of two new receiving stations 100 km east and west of the transmitter. Each of the new stations is equipped with a three‐beam Cassegrain antenna (25‐m parabola) aimed to simultaneously intersect the vertical transmitted beam at three different heights. Both new receivers have wide band (38 MHz at 3 db) gaseous helium cooled parametric amplifiers and on‐line computers. While one of the two new stations is equipped with filter banks for the purpose of spectral analysis, a different choice has been made for the other station. For the latter the data acquisition will imply multibit correlators, for both the ionic spectrum and plasma line measurements.Most of the improved electronic equipment designed for the new stations has also been implemented in the Nançay receiver. The frequency and time synchronization between the four sites is now accomplished by employing a single 5‐MHz frequency standard in each station monitored by standard time and frequency signals from MSF and/or HBG stations.The observation procedure consists of tilting the different receiving antennas so as to gather the signal scattered from the ionospheric vertical column illuminated by the transmitter. For calibration purposes a standard ionosonde is operating at St. Santin.The main objective of the quadristatic system is to determine completely the ion drift vector in order to study the electric fields and the neutral tidal winds. Other capabilities offered by the new system are a determination of the plasma line and a higher data acquisition rate.The first observations with one of the new stations were performed in December 1972.

A step in time Changes in standard frequency and time signal broadcasts

An improved Coordinated Universal Time (UTC) system has been adopted by the International Radio Consultative Committee (CCIR).* It was implemented internationally by the standard frequency and time broadcast stations on January 1, 1972. The new UTC system eliminates the frequency offset of 300 parts in 1010 between the old UTC and Atomic Time, thus making the broadcast time interval, the UTC second, constant and defined by the resonant frequency of cesium atoms. The new time scale will be kept in synchronism with the rotation of the earth within ±0.7 second by step-time adjustments of exactly 1 second, when needed. A time code will be added to the disseminated time signals that will permit Universal Time to be obtained from the broadcasts to the nearest 0.1 second for users requiring such precision.

Distribution of standard frequency and time signals

This paper reviews the present methods of distributing standard frequency and time signals (SFTS), which include the use of high-frequency, low-frequency, and very-low-frequency radio signals, portable clocks, satellites, and RF cables and lines. The range of accuracies attained with most of these systems is included along with an indication of the sources of error. Information is also included on the accuracy of signals generated by frequency dividers and multipliers. Details regarding the techniques, the propagation media, and the equipment used in the distribution systems described are not included. Also, the generation of the signals is not discussed.

NBS Standard Signal Broadcasting Stations

An increasing need for greater precision in broadcasting standard time and frequency signals has led the National Bureau of Standards to construct new transmitting facilities for standard broadcast stations WWVB and WWVL at Ft. Collins, Colo.

Worldwide VLF standard frequency and time signal broadcasting

Recent studies and measurements have shown that the phase stability of the signals in the VLF region is very much higher than in the HF spectrum. This fact, along with its excellent coverage characteristics, has caused considerable interest in employing this .medium for the wide distribution of standard frequencies and time reference. Basic limitations in stability of the received signals are discussed, including path phase distortion, carrier-tonoise and envelope delay variations as related to precise synchronization of clocks, and highly accurate frequency calibrations. Also included is a discussion of the present services of standard frequency and time signal stations throughout the world at HF, LF, and VLF.

Aircraft Motor Generator with Secondary Standard Frequency Output

The paper describes an experimental ac motor alternator assembly which produces an output frequency exactly equal to a low-power secondary standard frequency signal source, such as a tuning fork or crystal oscillator. The frequency-controlled system employs a synchronizing type of control in contrast to the more generally employed error-detecting systems. As a result, no error exists between the output frequency and the secondary standard. The control system requires only low-level signal power from the secondary standard reference. The main synchronizing power is drawn from the unregulated feeder line. No mechanical devices are needed for frequency correction or synchronization. The synchronizing system is completely electrical, requiring no mechanical or hydraulic compensating devices. The mechanical components employ only a single moving party composed of rotor shaft and two bearings. The electrical energy supplied to the rotor is magnetically induced across air gaps so that no sliding contacts or brushes are used. Transient loads applied to the system do not produce any frequency transients; in other words, transient and steady-state frequencies are identical. Characteristics of a 500-va experimental unit are presented.

Fade-Canceling Zero Beat Indicator for Reception of Standard Radio-Frequencies

A simple circuit is described which enables a local secondary frequency standard to be set quickly to a definite zero beat with a received short-wave standard frequency signal, without the usual uncertainty due to amplitude fading of the short-wave signal. In principle, two identical receivers, both fed by the same antenna, are used. One receiver picks up WWV, for example, and a weak signal from the local standard. The other picks up WWV either alone or together with a second weak signal from the local standard which is 180° out of phase with the first. The outputs of the detectors of the two receivers are subtracted, leaving only the beat note, which is recorded on a strip chart. In practice, one unmodified AM receiver is used on a time-sharing basis by means of a chopper. The band width of the device may be made much less than one cycle per second, and therefore, it is particularly useful when the WWV signal is reasonably stable in phase but far too weak for an audible zero beat to be achieved. Alternatively, the device may be regarded as a simple means of observing fast Doppler shifts of a received standard frequency signal. A schematic diagram and sample recorder charts are included.

Adjustment of High-Precision Frequency and Time Standards

High-precision frequency and time standards are becoming more and more widely used in many technical fields. The basic equipment used by the Central Radio Propagation Laboratory of the National Bureau of Standards in providing and broadcasting standard frequency and time signals is discussed. Mention is made of the manner in which it is adjusted and the corrections which may be applied to make use of the ultimate accuracy that may be expected in using these signals for measuring and calibrating similar equipment. Several methods are given for checking the frequency of precision oscillators and precision clock performance. Suggestions are given as to methods of recording and evaluating performance data for such standards. Expected improvements in constancy and accuracy and possible future changes in the types of standards used in physical time measurements are considered.

Changes in ionization and radio reception during the sunspot period 1944-1947.

If 12-month running means of critical frequencies are taken to eliminate seasonal effects, a close correspondence exists between critical frequencies for both the E- and F-layers and sunspots. Since the ultra-violet light of the Sun is assumed to be the major ionizing agent, this increase in ionization with sunspots during the night hours presents some interesting cosmic problems as to the cause and the maintenance of ionization on the unilluminated half of the Earth. Standardized measurements of field intensities of the Bureau of Standards standard-frequency signal, WWV, have been recorded at the Cosmic Terrestrial Research Laboratory at Needham during recent years for the 5-, 10-, and 15-Mc frequencies. Curves of night fields of WWV-5-Mc show that during the rise of sunspots from 1945 to 1947 the average values recorded between 22h and 02h rose from 300 microvolts to 900 microvolts when the Zurich sunspot-nurnber of 105 was reached; thereafter, field intensities decreased in value as sunspot-numbers continued to rise to a maximum of 150, a value reached last year. The decline in field strength with very high sunspot-numbers is attributed to attenuation introduced by increase ionization of the lower layers. Day fields, representing averages of observations between 10h and 14h showed a steady decline with increasing sunspots, a fact which is again explained on the basis of increasing absorption in the lower layers. The changes in the daytime field were from 225 microvolts when the sunspot-number was 20 to about 15 microvolts when sunspot-numbers were 150. For the 10-Mc reception day fields rose from a value slightly in excess of 90 microvolts, when the sunspot-number was 80, to values of 260 microvolts or more, when sunspot-numbers registered from 140 to 150. Night fields on the 10-Mc frequency, based on observations between 22h and 02h, rose from 10 microvolts, with sunspot-number of 80, to values in excess of 30. microvolts, with sunspot-numbers between 140 and 150. Observations on the broadcast band of Station WBBM, Chicago, 780 kc, received at Needham, a distance of 851 miles, show a general decline in field intensities from values in excess of 270 microvolts to values of but 45 microvolts as sunspot-numbers increased from 20 to a maximum value of 150.

The emission of special radio signals for the study of the ionosphere

The paper gives an account of the development and construction of the new transmitter now installed at the National Physical Laboratory for the emission of the special signals required in connection with modern research on the ionosphere. It also deals briefly with the methods adopted for the dissemination of standard-frequency signals for calibration purposes.The new installation has been designed to handle all the sending services required from the Laboratory over a continuous frequency range of 250 to 6 000 kilocycles per sec. (wavelength 1 200 to 50 metres). Owing to the rapid change in the emitted frequency required in many tests, a single oscillating circuit is employed and the oscillations are maintained by a push-pull system. A series modulator is provided for controlling the anode-circuit supply to the oscillator.The special emissions required for ionospheric research consist of frequency-change and pulse transmissions. In the former emission, the frequency is continuously varied over a small range by means of an entirely automatic mechanism which gives precision of both timing and frequency-change, while in the latter type of emission, pulses of waves may be emitted of short duration and at a repetition frequency synchronized with the a.c. supply mains.Two types of standard-frequency programmes are emitted. In one of these the output from the continuously-running 1000-cycle standard tuning-fork is used to modulate a suitable carrier wave; and in the other a standard radio frequency is emitted by using the transmitter as the final stage in an amplifier supplied by a quartz-crystal-controlled master oscillator.All the operations required in connection with these various emissions are effected by simple panel switchgear so as to render rapid changes in frequency and type of transmission possible, while a cathode-ray oscillograph equipment is incorporated for the observation of the modulation envelope.The whole installation was designed and constructed at the Laboratory for an input power of at least 4 kW, although at present the input is limited by supply conditions to 2 kW.