Square-wave Frequency Modulation Research Articles

MODERNIZATION OF THE KHARKIV MICROWAVE SPECTROMETER: CURRENT STATE

Subject and Purpose. Results are presented of the recent considerable upgrade implemented at the Kharkiv microwave spectrometer. The upgrade has been aimed at extending the operating frequency range and increasing the utmost accessible spectral resolution of the spectrometer. Methods and Methodology. In order to extend the frequency range we have designed and constructed new BWO-based oscillator units, also providing for possibility of frequency tripler application. Construction of a new absorbing cell of enlarged diameter allowed us to considerably improve the spectral resolution for Lamb-dip measurements. Results. Owing to the upgrade, the spectrometer has become able to cover the frequency range from 34 to 420 GHz, with a gap from 183 to 200 GHz. The spectral resolution in the Lamb-dip observation mode has been increased by a factor of two. In addition, the functionality of the spectrometer has been significantly improved via modernization of several of its subsystems. Conclusions. The new upgrades of the spectrometer systems have permitted extending the operational frequency range and increasing the utmost accessible resolution by means of reducing the time-of-flight line broadening in the Lamb-dip measurements. In addition, application of square-wave frequency modulation with accurately known modulation parameters, in combination with careful modeling of the distortions caused by reflections in the absorbing cell, has allowed us to significantly improve the accuracy of line frequency measurements.

SQUARE-WAVE FREQUENCY MODULATION IN MICROWAVE SPECTROSCOPY

Subject and Purpose. The frequency modulation (FM)combined with lock-in detection, the technique which is used in microwave spectroscopy for enhancing the sensitivity of measurements, as well as the effects due to standing wavesinthe measuring absorption cell can lead to distortions in the spectral line shapes observed. To ensure the highest possible accuracy derivable from the experimental data, these distortions needto be taken into account. A way of improving theaccuracy is to approximate to the experimental line contour with a theoretical line shape that would account for the observable distortion effects.The relevant literature sources suggest examples of theoretical expressions for the line shape in the case of a sinusoidal frequency modulation. This work has been aimed at derivingsimilar expressions for the case of a square-wave frequency modulation that shall allow increasing the accuracy of measurements. Methods and Methodology.The square-wave-FM signalsare obtained with the aid of a direct digital frequency synthesizer thatcan provide switching between two frequencies known to a high accuracy. This technical solution permits generating FM signals with precisely specified parameters. Results. A closed-form expression has been suggested, based on numerically evaluated line shape derivatives, whichallows taking into considerationthe basic types of distortions encountered in the spectral line records. The cases that have been considered concern a variety of experimental conditions, including sub-Doppler measurements with Lamb-dip observations. Conclusions. The approach that has been proposed allows one to properly take into account the distortions of spectral line shapes resulting from the use of a square-wave-FM signal, as well as those due to standing wave effects in the spectrometer cell. As has been found, application of this approach to experimental spectra with a variety of modulation parameters permits reducing the errors of frequency determination to ±0.001MHz, provided the signal-to-noise ratios are reasonably high.

Impact of inflection points on the performance of resonator integrated optic gyro

Resonator integrated optic gyro (RIOG) is a high-accuracy inertial rotation sensor based on the Sagnac effect. Instead of square-wave frequency modulation and sine-wave phase modulation, triangle-wave phase modulation eliminates the pulse error in the serrodyne wave phase modulation and reduces the backscattering noise, and is widely applied in RIOG. However, the attributed to the inflection points in triangle modulation waveforms induce output random noise, which would degrade the performance of the RIOG. In this work, the influence of such fluctuations on the performance of RIOG is theoretically analyzed and simulated. By analyzing the frequency response of averaging filter and lock-in amplifier, Pectinate filter is proposed to suppress the random noise. The experiment results from the established RIOG experimental setup demonstrated that the random noise caused by discontinuous frequency-modulation is effectively eliminated and the gyro bias stability is greatly improved from 1.96 to 0.11°/s.

An experimental study of intermodulation effects in an atomic fountain frequency standard

The short-term stability of passive atomic frequency standards, especially in pulsed operation, is often limited by local oscillator noise via intermodulation effects. We present an experimental demonstration of the intermodulation effect on the frequency stability of a continuous atomic fountain clock where, under normal operating conditions, it is usually too small to observe. To achieve this, we deliberately degrade the phase stability of the microwave field interrogating the clock transition. We measure the frequency stability of the locked, commercial-grade local oscillator, for two modulation schemes of the microwave field: square-wave phase modulation and square-wave frequency modulation. We observe a degradation of the stability whose dependence with the modulation frequency reproduces the theoretical predictions for the intermodulation effect. In particular no observable degradation occurs when this frequency equals the Ramsey linewidth. Additionally we show that, without added phase noise, the frequency instability presently equal to 2×10-13 at 1 s, is limited by atomic shot-noise and therefore could be reduced were the atomic flux increased.

Evaluation of the accuracy of the optically pumped caesium beam primary frequency standard of BNM-LPTF

The primary frequency standard LPTF-JPO employs a caesium thermal beam, and optical pumping techniques. We present the main parts of the design. The short-term stability is reported, σy(τ) = 3.5 × 10−13 τ−1/2, where τ is the averaging time. The general expression for frequency shift for a square-wave frequency modulation is given. We discuss the determination of each frequency shift, and we present new methods to evaluate some of them. A relative combined standard uncertainty of 6.4 × 10−15 (1 σ) is obtained.

Single-fibre DC-connected wideband analogue data link

The choice of modulation technique plays a critical role in determining the performance of wideband DC-coupled analogue fibre data links. This paper describes a modified version of square wave frequency modulation (SWFM) that overcomes the restricted DC-performance arising from the poor thermal characteristics of key components employed in standard SWFM techniques. The new approach employs local feedback within both the transmitter and receiver, resulting in a DC-connected 9.6MHz single-fibre system whose measured longterm drift performance of 100 mV is equivalent to that of previous two-channel systems using global feedback. At the centre of this new fibre system is a novel low-drift frequency-to-voltage converter based on pulse counting rather than traditional SWFM demodulation principles.

BIDIRECTIONAL TRANSMISSION OF ADDRESSED AUDIO SIGNAL THROUGH GI MULTIMODE OPTICAL FIBRES

This paper describes two types of bidirectional addressed audio signal transmission systems where the square-wave frequency modulation (SWFM) and intensity modulation (IM) are used to obtain the optical SWFM signals generated from LEDs at 850 nm. The first one is an addressed audio signal transmission network of tree structure. The sender and auxiliary receiver in the centre are connected to four terminals, each consisting of a receiver and an auxiliary sender, through pairs of optical fibres and an optical splitter used as an optical signal distributor. The second one is a new type of full-duplex one-to-one communication system. The sender and auxiliary receiver in one terminal are connected to the receiver and auxiliary sender in the other terminal through an optical fibre and a pair of directional couplers. Although LEDs in the second system deliver optical SWFM signals at the same wavelength of 850 nm at the same time, the communications are satisfactory because there is no interference from one LED to the other. In both the first and second experimental systems, the audio signal, address, and SWFM carrier occupy 10 Hz to 20 kHz, 65 kHz to 710 kHz, and 6 MHz, respectively. The SNR and distortion were 62 dB and 0.4 per cent, respectively.

Parallel transmission of vehicle mirror control signals by a single optical fiber

Describes a vehicle mirror control signal transmission system consisting of a transmitter, a single optical fiber, and a receiver terminal. The transmitter in the mirror controller to be operated by the driver controls the mirror functions, and the receiver terminal consists of a multifrequency receiver, a data processor, mirror control circuits to control an inner rearview mirror, and a pair of outside door mirrors. Multifrequency signals at 135, 235, 335, and 435 kHz are encoded to obtain 4-bit binary data that specifies the instructions to control these mirrors. The 4-bit multifrequency signal identified as 4-bit binary data is formed in accordance with the frequency division multiplexing (FDM) scheme, fed to the voltage-controlled oscillator (VCO) to obtain the square-wave frequency modulation (SWFM) signal in accordance with the SWFM scheme, and then transmitted from the transmitter to the receiver terminal through a single optical fiber. After the 4-bit binary data is reproduced from the 4-bit multifrequency signal, the mirrors are controlled by the data processor to accomplish their operating functions satisfactorily.

Squarewave frequency modulation techniques

Transmission of high-quality analogue sourced signals over copper or optical fibre links and networks demands a critical choice of modulation format to achieve both high performance and to avoid unnecessary bandwidth overhead penalties. Pulse time modulation (PTM) techniques are being increasingly employed to overcome many of the problems associated with traditional analogue and digital methods. Among the family of pulse time modulation techniques squarewave frequency modulation (SWFM) is particularly well suited to the transmission of high-quality wideband instrumentation and video signals, such as high definition TV, due to its attractive combination of performance and simplicity. This paper presents theoretical and practical aspects of SWFM and describes a number of experimental SWFM systems capable of transmitting signals from DC to 30 MHz. >

A tuning and testing frequency synthesizer

In this paper we describe the theory and application of a newly developed synthesizer that is especially suitable for cavities suffering instabilities and drift effects. The synthesizer presented here uses square-wave frequency modulation as its principle of operation. The synthesizer consists of two main phase-locked loops that are interconnected so that their outputs are alternately available. Spurious signals in the output spectrum have been measured to be more than 77 dB below the carrier at 405.782 kHz.

Optical fibre transmission of video and audio signals using square wave frequency modulation

The authors report the design, construction and performance of a square wave frequency modulation (SWFM) technique for point-to-point transmission of video and voice channels over 1 km of fiber. The results indicate that the SWFM system is a bandwidth-efficient and low-cost alternative to more complex and expensive digital systems requiring far greater bandwidth. >

Analogue intensity modulation schemes for optical fibre HDTV transmission

The major analogue intensity modulation schemes are assessed and compared in relation to their ability to provide high-quality transmission of high-definition television (HDTV) signals on single-mode optical fibre. The practical investigation and novel implementations for subcarrier frequency modulation (SCFM) and square wave frequency modulation (SWFM) schemes are then described prior to discussion of the relative performance attributes of these transmission systems. Both modulation techniques proved satisfactory for high-quality HDTV transmission over single-mode fibre with the balance of the measured performance and the necessary system requirements not decisively favouring either method.

Effects of Servo Loop Modulation in Atomic Beam Frequency Standards Employing a Ramsey Cavity

Some kind of modulation needs to be imposed on the mi- crowave signal to acquire lock in all passive atomic frequency stan- dards. In this way a frequency-discriminating dc signal is obtained by synchronous detection, which allows control of a slave oscillator. Fre- quency stability and frequency biases of the latter under closed-loop operation depend on the type and characteristics of the used modula- tion. The case of atomic beam standards using a Ramsey cavity is stud- ied, and different modulation schemes are considered under the aspects of efficiency (obtainable stability) and insensitivity to systematic effects. Theoretical results for the dependence of these on modulation type and parameters and on microwave power are reported and discussed. schemes and their sensitivity to some systematic effects are given as a function of modulation type and parame- ters, and are calculated numerically for an existing atomic beam tube. In particular, the effects of sine-wave modu- lation and slow square-wave frequency modulation on short-term stability and on various biases are discussed. A perfect realization of these ideal schemes will be as- sumed, and system imperfections like second harmonic distortion will be overlooked in the scope of this paper. 11. SYSTEM ANALYSIS Whatever the particular coherent detection system may be, signal processing of the modulated output Pod from the atomic resonator can be ideally divided in two steps: a filter section with a time-dependent output IF synchron- ous with the modulation, and a __ phase-sensitive detector with a time-independent output Pd.

A Transmission Scheme for Integrated Services on Broad-Band Internal Private Business Networks

Internal business networks are expected to present an economic area of application for broad-band communications. A star network with switches optimized for various services is envisaged. An asynchronous switch for video in square wave frequency modulation (SWFM) format can provide a practical solution for full bandwidth switched video services. A transmission scheme is described which combines a SWFM video service together with synchronous PCM data and voice services, asynchronous data up to 9.6 kbauds and network clocks at both 2 MHz and 160 kHz for ISDN, so that all services destined for one terminal site may be transmitted in binary format on cable or fiber. An experimental link is described which provides up to 10 Mbit/s data capacity together with the video service. The hardware is constructed with 74HC family CMOS. An analysis of the effects of data modulation on video quality is given. Measurements of video S/N and distortion are reported. Measured bit-error rate in a 64 kbit/s data channel is reported. The measurements relate to a coaxial cable link with link lengths from 200 to 400 meters.

Fiber Optic Video Transmission Employing Square Wave Frequency Modulation

Investigations are carried out on the performance of video transmission systems employing square wave frequency modulation (SWFM). A simple formula for the signal-to-noise ratio (SNR) in the SWFM video transmission systems is derived. The SNR performance is compared to that of frequency modulation (FM) and pulse frequency modulation (PFM). SWFM is shown to display the greatest receiver sensitivity when the fiber 6 dB bandwidth is less than 100 MHz. Video transmission experiments demonstrate the advantages of the SWFM, including 1) degree of simplicity in modulation/demodulation circuits as well as in optical transmitter and receiver circuits, 2) tolerance to nonlinearities in the system, and 3) insensitivity to the fiber baseband frequency-phase characteristics. Both the theoretical and experimental results have made clear the superiority of SWFM optical video transmission systems in application to CATV distribution systems and subscriber loop systems.

A Squarewave Frequency Modulation Servo System for Cesium Frequency Standard and its Influence on Frequency Stability

A Squarewave Frequency Modulation Servo System for Cesium Frequency Standard and its Influence on Frequency Stability

Digital frequency modulation in an NQR spectrometer

A digital system for a programmable frequency synthesiser is described. It performs a staircase frequency sweep with a step of 1 Hz and digitally programmed bidirectional square-wave frequency modulation in an NQR spectrometer with a frequency-locked super-regenerative oscillator. The system is particularly convenient when using digital averaging techniques. NQR spectra parameters are not influenced by frequency modulation even when extremely large frequency modulation deviations are used. Parameters such as the sweep rate and the amplitude of frequency modulation deviation are stable, accurately fixed and independent of the running frequency. NQR spectra of 35Cl in p-dichlorobenzene and 65Cu in Cu2O were plotted against frequency by an X-Y recorder.

Cesium-Beam Servo System Using Square-Wave Frequency Modulation

This paper describes a new modulation and servo system that is being tested in the NRC long-beam cesium-frequency standard. Square-wave frequency modulation is employed; it is introduced by switching between two separately generated frequencies. The synthesizer producing these frequencies and the problems caused by switching transients are described. Since in a laboratory standard systematic errors play a very important role these errors are discussed in the description of the servo system. The resulting systematic error as calculated from results of experiments on its components is less than 1 part in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> .

Frequency Shift Keying and Square-Wave Frequency Modulation

Some students have difficulty during initial discussions in understanding the differences between frequency shift keying and square-wave frequency modulation. This article describes a problem for student assignment which is designed to assist the student who lacks, to some degree, this understanding. The problem consists of a requirement for calculating the spectra for three different, but related, deterministic signals. The article describes the signals and gives a solution.