CC BY
12
Y^K 621.317.3
Frequency Instability Measurement Device Based on the Pulse Coincidence Principle
Verveyko A. I.1, Lappo I. M?, Arkushenko P. L.\ Yusukhno S. I.3,
1 State Research Institute of tests and certification of arms and military equipment of the Armed Forces of Ukraine
2Chernihiv Collegium 3Chernihiv National University of Technology
E-mail: irinal-appo&i. ua
Context. The task of rapid and accurate measurement of the dynamic characteristics of modern signal sources with a frequency output, in particular, the short-time frequency instability function, calls for refining measurement techniques with account of the requirement to improve their met.rological characteristics, reduce test time, and automate measurements by using information-and-measurement systems. Objective. The goal of the work is to develop a method of measuring the short-time frequency instability function using the principle of pulse packet coincidence and experimental investigation of measurement devices based on this principle.
Method. A method was developed for measuring the short-time frequency instability function based on the principle of packet coincidence of regular independent pulse trains. The developed method has advantages over the best version of the method based on the period-time interval-code (PTC) conversion when working with the same initial value of the investigated frequency and when working with the same value of the averaging interval.
Results. Analytical relationships were obtained for basic met.rological characteristics. A comparative analysis was carried out for the met.rological characteristics of the developed method and the method using period-time interval-code conversion. Acceptable met.rological characteristics are inherent, to the short-time frequency instability function (SFIF) measurement, method based on the period-time interval-code technique. The difference of investigated and reference intervals form the measurement, interval, which is filled with pulses of the investigated or reference frequencies.
Conclusions. Stand-alone and virtual measurement, devices were developed, and experimental studies of standard oscillators were carried out.. The features of measurement, devices were specified and the ways of their further improvement, were described. Further development, of the measurement, device can involve an increase in the number of measured signal source with frequency output. (SFO) parameters, in particular, changes in short-time frequency instability due to the action of destabilizing factors, and the characteristics and time of frequency setting. This calls for developing a controlled source of destabilizing factors and synchronizing its operation with the measurement, device.
Key words: short-time frequency instability: stand-alone measurement, device: virtual measurement, device: Lab VIEW: packet, coincidence of pulses
DOI: 10.20535/RADAP.2019.76.29-36
Introduction
Analysis of the current state and trends in the development of instrumentation is indicative of insufficient accuracy and short-time frequency instability function (SFIF) measurement challenges. This deficiency is being eliminated by developing new methods and tools for SFIF measurement. In this connection, along with the development and refinement of conventional measurement devices, more effort is being put into developing virtual measurement devices (VMDs) that help streamline the process of performing intricate measurements.
The object of research IS 3. stand-alone SFIF measurement device developed on the PLD Emulator console and a virtual measurement device based on the Virtual Instruments technology by National Instruments.
The subject of research is methods for SFIF measurement and instrumentation based on these methods.
Known methods and measurement devices based on them demonstrate insufficient accuracy of measuring SFIF with specified speed and their design is challenging.
The purpose of the work is to develop a method of measuring SFIF using the principle of pulso
packet coincidence arid experimental investigation of measnrement devices based on this principle.
1 Problem statement
Modern electronic systems comprise a wide variety of different signal sources with a frequency output (SFO) differing in their purpose and functionality that are generally signal generators, quartz oscillators or frequency synthesizers. Improvement of the systems and extension of the tasks performed with their help have translated into more stringent requirements for the specifications of such systems, namely, dynamic characteristics (short-time frequency instability, readiness time, time and rate of transfer from one frequency to another, etc.). Moreover, in many respects, the achievement of the required characteristics is possible as a result of the improvement of the SFO, which, in turn, requires the development of some qualitatively new measuring equipment for the measnrement of overall and specific dynamic characteristics.
2 Review of the literature
Methods of measnrement of short-time frequency instability and the measnrement devices based thereupon are being improved continuously.
To determine a short-time frequency instability, often times a method of comparison to a reference frequency is used. The simplest method is an electronic counting method [1,2] that has a relatively low measnrement accuracy. In order to increase the accuracy, a two-channel frequency measnrement method [3] is used. Although, these two methods have several major drawbacks: quite a high labor intensity, inability to visualize the functions of a short-time frequency instability in real time and design complexity-
A combined method using an oscillator and frequency counter measuring the period [4] and time interval [5] IS 3. better method. A reference source is used as a comparison oscillator. Its disadvantages include low accuracy, a relatively high labor intensity of setting time averaging, and inability to visualize the functions of a short-time frequency instability.
Short-time frequency instability can also be measured using a phase or frequency demodulator [6]. When measuring an effective output signal voltage of the phase demodulator, a mean square value of the phase fluctuation is assessed. If a differentiating circuit is active at the output of the phase demodulator, then the output voltage will be directly proportional to the frequency fluctuations. To assess short-time frequency instability, a low frequency filter with a rectangular transmission characteristic needs to be included to the circuit upstream the voltmeter. But the accuracy of measnrement, in this case, is relatively low.
There IS 3. method of measuring the phase shift between periodic signals of arbitrary duration based on the principle of coincidence of pulses in packets [7]. Its drawbacks include the need to generate minimum length pulses, complexity of implementation and retrieval of information regarding SFIF, especially at short times of averaging.
A method for measuring the SFIF based on the transformation of "period-time interval-code" (PTC) has good metrological characteristics, where a measuring interval is generated ES ct difference between the tested and the reference frequencies [8,9]. There are four options for implementing the PTC transformation based method. Its disadvantages include a low frequency and period resolution in some cases, and implementation complexity.
3 Materials and methods
The development of the SFIF measnrement method was based on the principle of packet pnlse coincidence (PPC) suggested in [10,11]. The method assumes the performance of the following operations.
Regular independent trains of pulses of the reference signal (Fig. lb) and investigated signal (Fig. la) are formed, with the period Tx(t) of the latter changing according to the law
Tx(t)= Tn ± ATx(t),
(1)
where Tn is a initial value of the investigated signal pulse repetition period; ATX (t) is a change in the initial value of the investigated signal pnlse repetition period.
The difference of the pnlse repetition periods of the investigated and reference signals AT should be less than the duration of the pulses of the investigated and reference signals th = th0 = rHX. In this case, the pulses shall coincide in packets (Fig. lc). Thereat, the number of pulses in a packet Nn is determined, as follows from analysis [10,11] and Fig. 1, by the relationship
Nn
2t„
AT- 1'
(2)
where AT is a difference of pulse repetition periods of the investigated and reference signals.
Obviously, the averaging interval of the investigated frequency pulses ry shall coincide with the repetition period of the coincidence packets.
Function (2), with account of (1) and analysis of Fig. 1, takes the form
2t„
ATpn ± ATx(t).
1
(3)
where Nnj is a number of pulses in a packet on j-th averaging interval; rH IS cl duration of pulses of the investigated and reference signals; ATx(t)j is a mean value of change of the period of investigated oscillations
on the j-th averaging interval; ATpn is a initial value of the difference of pnlse repetition periods of the investigated and reference sigrictls, TQ is ct reference signal pulse repetition period; ATpn = Tn — T0 should be chosen with account of the required averaging interval of the investigated frequency pnlses and the maximum change of the period of investigated oscillations during the test time.
Function (3) yields
ATpn ± Tx(t). =
2tu
Nu, + 1
hn
has the form
NnlATœ(t)j = ATC NnlATœ(t)j =0 '
hr =
2tu ■ ATC
(ATPn + ATC) + ATp.
AT,
pc
2tu- AT,
pn
IJUUL
'JUULl
hJ
Fig. 1. Coincidence of independent regular pnlse flows
It can be shown that relationships (4)-(6), depending on the parameters of the laws of variation of the frequencies of the investigated and reference signals, take the form
where fpn is an averaging frequency; fpn = fa — f„; fa IS 3. reference signal frequency; fn IS 3. initial investigated frequency; Afx(t) ■ is a mean value of frequency-change of investigated oscillations on the j-th averaging interval; Afc. is a investigated frequency increment; hf is a frequency response of the method; Afpc is a frequency resolution of the method.
The conversion characteristic (7) has a complex functional dependence on Afx(t)-. However, since during the measurement of SFO parameters the following
(4) conditions are always satisfied:/« > Af x(t)-, fn > j1
pn 5
Nn > 1, it can be written as
Hence, at the j-th averaging interval, the number of pnlses in the respective coincidence packet will uniquely define the mean deviation of the period of investigated oscillations for ATpn.
Let us investigate the transient response (TR) of the method. It is its response to a step change of the measured parameter. The expression of TR for the period found from condition
(fpn ± Afx(t)j)
2tu ■ f,
Nn
with an error not greater than
Ó--
fn ■ {fn/Nn fpn + Afmax.) {fn Afmax) ■ {fn + fpn)
(5)
where Afmax is a maximum change of investigated SFO frequency.
Let us conduct a comparative analysis of the developed method and the method based on PTC conversion, the best version of which demonstrates a frequency resolution of
where hT is a period response of the method; ATC is a increment of investigated period. The method resolution for period ATpc found from condition hx = 1 takes the form
AT2
AfpePTc =
fn
fc
PTC
(6)
h_n_
where fa ptc is a reference signal frequency of the PTC conversion method; ry is a averaging interval for investigated frequency pnlses.
Fig. 2 shows the results of calculating the resolution of the methods.
An analysis of resolution relationships and of Fig. 2 shows the following: the developed method has advantages over the best version of the method based on PTC conversion; tv > (fa — fn)/(2tu ■ f^) when working with the same initial value of the investigated Vfoptc/(2tu ■
"LJLJ frequency; - fn
>
when working
with the same valne of the averaging interval.
4 Experiments
The suggested principle was nsed to develop a standalone SFIF measurement device whose block diagram is shown in Fig. 3.
The precision one-shot multivibrator and pnlse shaper generate pnlses of the investigated and reference frequencies, with the duration of the pnlses being
fpn ± Afx{t) =
/ _\ ii^qucii^icD, wiuii une uuiauiun ui une puioco lucin^,
2tu ■ if« ± Afx(t) . j ■ (fn + fpn) respectively tux mid tu0. The AND gate output is a
Nn i + 1
h f =
■ Afc ■ {fpn + fn
{fpn - Afc) ■ f
pn
Af
fr
pn
pc
2tu ■ {fp n + fn) 2 + fp
(7)
(8)
(9)
packet of coincidence pnlses that set the first counter to zero and are input to the second counter to count their number. The flip-flop is set to "1" with the first pnlse of the coincidence packet and reset to !'0" if the packet has no pnlses and if five reference frequency pnlses were input to the first counter. This improves measurement device noise immunity. The flip-flop output signal leading edge enables a write signal to write the number
y
T
t
T
c
2
of pulses in the packet from the counter to the register. The write signal is delayed and then sets the counter to "0". Hence, this generates data on frequency instability for adjoining time intervals to improve measnrement accuracy.
The code of the number of pulses in the packet and the codes of the durations of pulses and reference frequency values are input to calculation units to determine frequency instability.
The measnrement device uses the standard algorithm of determining the short-time frequency instability (SsFl) with the formula SSFI = (/max - fmin)/frated, where frated is a rated (mean) frequency during tests: frnax is a maximum frequency measured on the averaging interval: fmiri IS 3. minimal frequency measured on the averaging interval. DAC converts calculation results to voltage to visualize frequency change functions by using any commercial recorder.
5 Results
The frequency instability measnrement device was developed using the PLD Emulator console [12] whose digital components design is based on programmable logic integral circuits (FPGA) by Altera and the digital-analog converter by Analog Devices.
The following features were revealed by analysis of the measnrement device structure and its experimental investigation:
- the measnrement device is distinguished by straightforward design duo to the small bit capacity of the second counter, register, and DAC because frequency deviation is measured for a difference value, which is significantly smaller than the reference frequency:
- frequency deviation is converted to voltage, making it possible to use any commercial oscilloscopes for visualization of results:
- investigation of different signal sources presents difficulties in changing the parameters of tu0, Tm and T0 measurement settings, resulting in increased test run time.
To reduce test run time, the process of excluding information about the initial value of the investigated frequency from measnrement results should be automated. This can be done, in particular, by using virtual measnrement devices.
A virtual measnrement device (VMD) is a measnrement device based on a universal computer with additional software (an application and a driver) installed and efficient technical equipment [13]. The term "virtual" is usually applied to two VMD aspects:
- first, they are not commercial products in the sense of off-the-shelf ones, but rather a temporary item intended for solving specific measnrement problems:
- second, VMD control and display members are represented as graphic images on a computer screen, and a VMD is controlled using typical input devices: keyboard, mouse, and a touch screen [14, 15]. The SFIF virtual measnrement device is built around a PPC converter whose structure in Fig. 3 is shown with a dash-dotted line and a computer. Usually, three programming techniques are used for developing VMD computer programs [13,15]:
- textual or textual graphic (Pascal, Delphi, LabWindows/CVI, Measurement Studio, Visual Basic, Visual C/'C— packages) that nso elements of visual textual programming foensod, primarily, to experienced programmers:
- object-oriented graphic (In Tonch and Trace Mode packages) using graphic images of the objects of an automated industrial process control system as programming elements:
- fnnction-oriented graphic (LabVIEW, LabVI-EW/'DSC, Agilent VEE, DASYLab, DIAdem, ZETLAB, and Hypersignal packages) using the functional logic principle of designing (drawing) and graphic presentation of program algorithms.
Using textual programming for each specific project, though perhaps being the most optimal one from the view-point of solving a definite problem, loses its advantages because the problem has to be solved each time almost from scratch, involving big time and material costs. Duo to this, preference is given to dedicated software, in particular, graphic programming.
National Instruments is the developer of the virtual instruments technology a breakthrough concept that changed the approaches to and technique of developing data acquisition systems and measnrement control. Its Lab VIEW CAD package became do facto an international standard. It offered and patented a new graphic based programming language G. Working with familiar concepts (functional block, connection, chart), a development problem can be solved fast and, what is important, with a visual representation without getting lost in the maze of programming. According to most conservative estimates, development with programming language G can reduce a project lead time by at least 4-10 times [13,15].
The computer program developed in LabVIEW consists of two interrelated parts: a front panel and a block diagram (Fig. 4) [16,17].
10"1
10
S3
^ 10"4
10"5
10"7
f„=10 Hz
fopic=10s Hz
iu=2*1( "6 |sec Tu=10"6 |sec
Tu=4*10"6 |sec
33
0.1
(a)
1 10 Tn. usee
(b)
Method based on period-time interval-code conversion Method based on the pulse coincidence principle
Fig 2. Dependence of resolution of methods for measuring short-time frequency instability on: (a) — averaging
interval and (b) — initial value of investigated frequency
Fig 3. Block diagram of a stand-alone device for measuring frequency instability
The front panel accommodates control members, buttons, graphic indicators, and other control elements. They are the tools the user works with to input data. The indication elements display the program output data. The elements are input with a mouse and keyboard, with action results being displayed on the monitor screen.
The SFIF virtual measurement device has five indication elements on the front panel. The most interesting indication element is oscilloscope Chart, which ensures automatic Y-axle scaling. This enables visual representation of the frequency measurement without having to have to perform different adjustment of the settings during testing.
The front panel elements represented on the block diagram are shown as terminals, via which data flow from the user to the program and back. The block diagram describes the VMD operation logic: data acquisition from communication interfaces, mathematical
treatment, computation of related quantities, (icltcl transfer to indicators, and saving the results.
Basic functional components of the VMD block diagram:
- VICA Read and VICA Write ensure data exchange through RS-232. Use of RS-232 is dictated by the fact that the VM is based on PLD Emulator [12];
- Array Max & Min and Mean determine the maximum, minimal and mean period of investigated oscillations in an array with a user specified dimension.
Fig. 5 shows the VMD architecture.
VMD is based on a standard PC running under the Windows OS. OS UNIX, Linux, Mac OS, Microsoft Pocket PC, Microsoft Windows CE, and Palm OS can be used, and the measurement device can be set up on a laptop. The configuration files are developed with the Quartus CAD package and used for determining the schematic design of the primary convertor based on an Altera FPGA.
0
-2
0
-3
0
-3
10
-4
0
-6
0
-5
10
0.1
10
0.01
(a) (b)
Fig 4. Application software: a — front panel and b — block diagram
Fig 5. Architecture of virtual SFIF measurement device
The PC and the primary converter are connected via two channels: information is sent to the PC via a serial interface, and the parallel interface operating in the EPP mode serves for configuring the FPGA with the ByteBlasterMV programmer.
Test results for pulse generator G3-63 at To = 10-5 sec and tu = tuo = tux = 10-6 sec are shown in Fig. 6 including temporal function of frequency alteration. This function enables the increase in volume of the data regarding the parameters and characteristics of the generator tested, the sensitivity to the exposure to different destabilizing factor (input voltage, load, etc.), overall and specific dynamic attributes (transitional characteristic, temperature and frequency characteristic, time of frequency setting, etc.).
The analysis of Fig. 6 suggests that the second generator has low reliability due to the significant instability of its frequency.
6 Discussion
The developed method enabled improving the metrological characteristics of the SFIF measurement device by a minor schematic and design modification of measurement devices based on its principle.
Using the functional blocks included in CAD Lab VIEW, which were tested on many occasions by different development engineers, reduced the VMD lead time and improved its operational reliability.
(a)
Fig 6. SFIF measurement results for low-frequency signal generators G3-63: a
' No. 32344
(b)
serial No. 31116 and b — serial
Conclusions
References
The scientific novelty of obtained results is that the method for measuring SFIF based on the principle of packet coincidence of regular independent trains of pulses was developed. Its metrological characteristics are described. The practical significance of obtained results is that the standalone and virtual SFIF measurement devices were developed. Using GAD Lab VIEW and a reconfigurable FPGA for VMD design and operation has ensured marked advantages of the proposed measurement device over known ones: control of measurement device parameters was simplified; automatic scaling was provided for visualizing SFIF; the user can change the front panel configuration, the block diagrams of the virtual measurement device and the reconfigurable files during operation.
The downsides of the developed VMD are as follows: the need to input the reference signal frequency value and the investigated and reference frequency pulse durations on the block diagram, requiring that the VMD users have adequate skills. However, these deficiencies can be eliminated by placing control elements on the VMD front panel to input required values.
Further development of the measurement device can involve an increase in the number of measured SFO parameters, in particular, changes in short-time frequency instability due to the action of destabilizing factors, and the characteristics and time of frequency setting. This calls for developing a controlled source of destabilizing factors and synchronizing its operation with the measurement device. The resolution capacity can be increased by multiplying frequency deviation using standard instruments (a frequency comparator and synthesizer) according to a typical schematic diagram.
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Вим1рювач нестабшьност! частоти на принцип! збЫв 1мпульс1в
Вервейко А. /., Лаппо I. М., Аркушенко П. Л., К)суXIw С. I.
Актуальшсть. Завдаппя швпдкого й точного ви-м!рюваппя дипам!чпих характеристик сучаспнх дже-рел сигпал!в з частотпнм виходом, зокрема фупкцп короткочаспо! пестабглыюст! частоти (ФКНЧ), иотре-буе вдоскопалеппя метод!в вим!рюваппя з урахувашшм пеобх!дпост! полшшеппя i'x метролог!чпих характеристик. зпнжеппя часу проведения вииробувапь, можли-вост! автоматизацп вим!рювапь за рахупок застосува-ппя шформацшио-вгмрювальних систем. Мета роботп полягае в розробц! методу вим!рюваппя ФКНЧ па принцип! зб!г!в !миульс!в пакетами та експеримепталышх досл!джеппях вим!рювач!в па його основ!.
Метод. Розроблепо метод вим!рюваппя фупкцп короткочаспо! пестабглыюст! частоти па принцип! зб!г!в регулярпих пезалежгшх посл!довпостей !мпульс!в пакетами. Розроблепий метод мае переваги в пор!впяпш з кращим вар1аптом методу па баз! перетвормшя перюд-часовий !птервал-код при робот! з одпаковим початко-вим зпачеппям досл!джувапо1 частоти ! при робот! з одпаковим зпачмшям штервалу усередпеппя.
Результати. Отримаш апал!тичп! сп!вв!дпошеш1я для осповпих метролог!ч1шх характеристик. Проведено пор!впялышй апал!з метролог!чпих характеристик розроблепого методу й методу па баз! перетвормшя перюд-часовий штервал-код. Добрими метролог!ч1шми характеристиками волод!е метод вим!рюваш1я ФКНЧ па баз! перетвормшя перюд-часовий !птервал-код, в якому формуеться вим!рювалышй штервал як р!зпиця досл!джувапого ! опорного !птервал!в ! заповшоеться !мпульсами досл!джувапо1 або опорпо! частот.
Висновки. Реал!зовапо автопомпий й в!ртуалышй вим!рювач!. а також проведено експеримептальп! до-сл!джмшя стапдартпих геператор!в. Вказапо особливо-ст! вим!рювач!в ! шляхи ix подалыного удоскопалеппя.
Подальший розвиток вим!рювача можливий в папрямку зб!льшеш1я к!лькост! вим!рювапих параметр!в джерел сигпал!в з частотпим виходом, зокрема, змши короткочаспо! пестабглыюст! частоти в!д вплнву дестабгл!-зуючих фактор!в, характеристики ! часу встаповлмшя частоти. Для цього пеобх!дпо розробити керовапий дже-рело дестаб!л!зуючих фактор!в ! сипхрошзувати його роботу з вим!рпиком.
Клюновг слова: короткочаспа пестабгльшсть частоти: перетворювач па принцип! зб!г!в !мпульс!в пакетами: автопомпий вим!рювач: в!ртуалышй вим!рювач: ЬаЬУ1-
Измеритель нестабильности частоты на принципе совпадения импульсов
Вервейко А. И., Лаппо И. Н., Аркушенко П. Л., Юсухно С. И.
Задача быстрого и точного измерения динамических характеристик современных источников сигналов с частотным выходом (ИЧВ), в частности функции кратковременной нестабильности частоты (ФКНЧ), требует усовершенствования методов измерения с учетом необходимости улучшения их метрологических характеристик, снижения времени проведения испытаний, возможности автоматизации измерений за счет применения информационно-измерительных систем. Цель работы состоит в разработке метода измерения ФКНЧ па принципе совпадения импульсов пакетами и экспериментальных исследованиях измерителей па его основе. Разработан метод измерения функции кратковременной нестабильности частоты па принципе совпадений регулярных независимых последовательностей импульсов пакетами. Разработанный метод имеет преимущества в сравнении с лучшим вариантом метода па базе преобразования ПВК при работе с одинаковым начальным значением исследуемой частоты и при работе с одинаковым значением интервала усреднения. Получены аналитические соотношения для основных метрологических характеристик. Проведен сравнительный анализ метрологических характеристик разработанного метода и метода па базе преобразования период-времешгой иптервал-код. Хорошими метрологическими характеристиками обладает метод измерения ФКНЧ па базе преобразования период-времешюй иптервал-код (ПВК), в котором формируется измерительный интервал как разность исследуемого и опорного интервалов и заполняется импульсами исследуемой или опорной частот. Реализованы автономный и виртуальный измерители, а также проведены экспериментальные исследования стандартных генераторов. Указаны особенности измерителей и пути их дальнейшего совершенствования. Дальнейшее развитие измерителя возможно в направлении увеличения количества измеряемых параметров ИЧВ, в частности, изменения кратковременной нестабильности частоты от воздействия дестабилизирующих факторов, характеристики и времени установления частоты. Для этого необходимо разработать управляемый источник дестабилизирующих факторов и синхронизировать его работу с измерителем.
Ключевые слова: кратковременная нестабильность частоты: преобразователь па принципе совпадений импульсов пакетами: автономный измеритель: виртуальный измеритель: ЬаЬУ1ЕШ
