This paper describes a method for measuring very small transmission loss of impulsive signals. The correlative
fluctuations within input and output signals are eliminated effectively by the analog simultaneous differencer. The difference and normal signals are alternately sampled and accumulated by the digitizer and
the microcomputer, that is, the microcomputer 2-channels
box-car integrator is realized. The difference and normal accumulated data are transferred to the personal computer, which calculates the ratio of the difference to normal accumulated data, i.e. the estimated attenuation in nepers. By the experiments of electrical and optical impulse transmissions, the minimum measurable attenuations become respectively 2.7×10(-4)NP and 6.0×10(-4)Np. Even the latter value is the smallest so far as the authors know

Sano, Hiroya

Tanada, Yoshihiro

Okayama University Scientific Achievement Repository

Memoirs of the School of Engineering, Okayama University Vol. 14.2, February 1980Microcomputer Filtering System to Measure Very SmallTransmission Loss of Impulsive SignalsYoshihiro TANADA* and Hiroya SANO*(Received February 4,1980)SynopsisThis paper describes a method for measuring verysmall transmission loss of impulsive signals. The cor-relative fluctuations within input and output signalsare eliminated effectively by the analog simultaneousdifferencer. The difference and normal signals are al-ternately sampled and accumulated by the digitizer andthe microcomputer, that is, the microcomputer 2-chan-nels box-car integrator is realized. The differenceand normal accumulated data are transferred to the per-sonal computer, which calculates the ratio of the dif-ference to normal accumulated data, i.e. the estimatedattenuation in nepers. By the experiments of electricaland optical impulse transmissions, the minimum measur-able attenuations become respectively 2.7 x lO-4NP and-46.0 x 10 Np. Even the latter value is the smallest sofar as the authors know.1. IntroductionMeasurement of transmission factor on electricsignals is one of the most fundamental matters for bothcommunication and instrument technology. The latestequipment for this purpose is inclined to have adVanced,functions such as automated measurement, multipie out-* Department of Electronics2324 Yoshihiro TANADA and Hiroya SANOput-modes and high accuracy by the aid of microcomputer. On the otherhand, both communication and instrumentation by laser light have reacheda stage of practical use, though the equipment for measuring trans-mission factor on optical signals is still complicatedly handled. Theuse of microcomputer may bring its functional improvements.Usually, photoelectric signals in this equipment are detected bytwo analog box-car integrators or lock-in amplifiers (1) ,(2). But anaccurate transmittance is not always obtained from the detected signals,because the correlative noises between the incident and transit lightsdo not vanish and also those signal detectors generate drift noises.The authors develope the improved signal detector for measuring thevery small absorption loss of pulse laser light through a short pathof dilute gas(3),(4). The detect~r is composed of the analog simulta-neous differencer and the microcomputer 2-channels box-car integrator.2. principle of Signal DetectionFig.l shows the transmission ofmedium such as glass, liquid or gas.related to the transit light power Pt-a,Pt = Piea light beam through an opticalThe incident light power p. is. .. 1by the following equation(1)where a, is the optical attenuation. In the practical measurement ofthe attenuation, the intensity of light is modulated and the corres-ponding signal detector is used in order to suppress noise disturbances.For example, a lock-in amplifier corresponds with the light at 50% dutyfactor and a box-car integrator at much smaller duty factor.In the measurement of this attenuation, light emitted by the pulsedye-laser has 10-ns pulse width and 100-ms repetition period (at the10- 7 duty factor), and its average power in watts is expressed asp = WiT (2)where W is statistical-average energy of a lightpulse in joules and T isperiod in seconds. Byadjusting the power level,p becomes p. = W./T and1 1accordingly Pt = Wt/T.Though the energy of eachlight pulse fluctuates,Optical r·ledi urnLight BeamFig.l Model of Optical Transmission.Microcomputer Filtering of Impulsive Signalsthere exists a strong correlation between the incident energy Wik andthe transit energy Wtk at the k-th pulse shot. The correlative energyfluctuations or the correlative noises could be eliminated by thefollowing equation for the estimated attenuation in 2-times nepers25M-lIn (kloWik/Wtk ) ( 3)where M is the pulse number. But the analog or digital operation ofEq.(3) involves some difficulties for the guarantee of accuracy andspeed. The very small attenuation 0«1 as through a short path ofdilute gas can be practically well estimated by the following equation(4)The component of the difference energy at the (2~+1)-th pulse shot isobtained by the analog circuit and also the component of the incidentenergy at the 2~-th pulse shot, and the digitized values of thosecomponents are accumulated by the microcomputer. The first and lastoperations are to be called the analog simultaneous differencing andthe digital box-car integration respectively. Both operations cansuppress the correlative noises and the quantizing noises on digiti-zation.3. Hybrid Signal Detecting SystemFig.2 shows the block diagram of the system for detecting the verysmall attenuation and Fig.3 shows the signal waveforms illustrating theoperation of the system. In this case, the pulse width of the photo-electric currents from the PIN-type photodiodes are about 20ns, and toonarrow to difference and digitize. Therefore, the photoelectric Currentsare to be integrated and re-formed by the charge-to-voltage converterswith the transfer coefficient Kp or K'p and the relaxation time T = 2vs.Then the photoelectric currents resulting from the incident and transitlight powers at the time t>kT can assume the respective impulses asandi k(t) = Q'ko(t-kT) ~Q'k = K,W' kP l l l l (5)i' pk (t) (6)""a>-<oUl:r;.:8'IIIi5..Ul~::r:::j'o'<IIII}.PersonalComputerx50InvertingAmplifierNormal Pulse Smoother,x5000 Ix 1: .l:- _1_ I_ ~:.lO":'lQ9 JDifference Pulse Smoother1_-InvertingPream-plifierInvertingOutputAmplifier Ii I____JI~ ~Channel Selector- - - - - - - - - - - 1 Digitizerxl0 rU-~--UTI. ISample A-DHolder Converter0- -ldBDigitalController.{,.1IIII y O--l5dB x14 I1 -'Charge-to-Voltage Converter-~----------~~--I0, -15dB 1, Inverting IPream- I-.,I---+--_.......:~Photo- ' ~,. " plifier IDiode I - - - - - - - - - - - - - - - - - -'IncidentLight~TransitLight ~TriggeringLight ~'- --- ./From Digital ControllerFig.2 System for Measuring Photoelectric Signals.Microcomputer Filtering of Impulsive Signalswhere o(t) is Dirac's delta function as271 (7)and Qik and Qtk are the electric charges from the respective photo-diodes with the responsivities Ki and Kt . The corresponding outputsof the charge-to-voltage converters show the identical waveforms asfollows:Vpk(t) = (QikKp/T) exp {-(t-kT)/T}V'pk(t) = (QtkK'P/T) exp {-(t-kT)/T}Photo-Diode J IOutput 0 T 2TNormalSignal (SUB)~ (ADD) I LSAMPLESELECT (CH-A) (CH-B) I (CH-A)START --fl nFig.3 Time Chart of the Box-Car Integration.(8)(9 )Next, those waveforms pass thrclUgh the normal and difference pulsesmootherswith the identical relaxation times T and the respectivelow-frequency voltage gains Kn and Kd, to change into the normal signalwaveforrtl(10)and the difference signal waveform1~ Kd(QikKpQt!!~) (t-kT) exp {-(t-kT)/T}1~ ~ KiKpKd{Wik-Wt~l+£)}(t-kT) exp {-(t-kT)/T} (II)where K' is adjusted in advance so as to satisfypK K' = K.K (1+£); £«1t p ~ P (12)and the gains Kn and Kd commonly involve the gain of the following28 Yoshihiro TANADA and Hiroya SANDchannel selector. The normal and difference signal waveforms rise upslowly below the slew rate of the amplifiers, never to be distorted;K_ is selected to be larger than K , typically Kd = 10K, where theannquantizing noises do not exert influence on the difference signals somuch.The normal and difference signals at each pulse shot are sampledand accumulated by the digitizer and the microcomputer, as mentionedbelow. At first, while the channel selector passes the normal signalon the channel A (CH-A), the digitizer samples its peak value att = T+2£T and the microcomputer stores the value in the addition modeand a little later after the normal signal vanishes they sample andstore the off-set value in the subtraction mode. Thus, the componentof the normal signal is obtained with the suppression of electric driftnoises. Secondly, while selecting the channel B(CH-B~the component of thEdifference signal is obtained from ,its peak value at t = T+(2£+l)T andthe off-set value at its vanishing. With alternation of the channelselector, the respective components are accumulated in the microcomputer.Hence the 2-channels box-car integrator is realized. Fig.4 shows thecircuit diagram of the digital controller.When finishing the accumulation, the microcomputer displays theaccumulated. data on the LED and transfers them to the personal computer,in which the ratio of the differnce to normal dataPhoto-DiodeT-FF T-FFOS SOnsQpCK_05 Q6CL CIJFig.4 Digital Controller.(13)To AnalogSwitchTo MicrocomputerSAMPLETo A-DConverterMicrocomputer Filtering of Impulsive Signalsis calculated. Where the optical medium does not exist, by anothercalculation, is obtained the following value2913' = -(K /K )sd nso the estimated attenuationis calculated.4. Algorithm of Box-Car Integration(14)(15)The microcomputer is composed of the 8-Bit CPU (8080A), the 0.75-kByteROM, the l-kByte RAM, the parallel I/O port and the interface for thecassette magnetic tape (CMT). The I/O port is divided into the 8-BitPort A, Port B and Port C. Port Ais ~ssigned for data outputs, Port Bfor data inputs, the upper 4 Bits in Port C for control signal outputs(FLAG etc.) and the lower 4 Bits in Port C for control signal inputs (STARTand CONTROL). Some measurement programs are recorded on the CMT,from which the program for a certain measurement is loaded on the RAM.This measurement is explained according to the flow chart in Fig.5.If the START signal (see Fig.3) is detected, four sets of data are readthrough Port B in order, when the addition and subtraction on the CH-Aand then those on the CH-B are executed. Generally, A-D converter worksas limitter for the over~evel input. Here, if the output codes of the8-Bit A-D converter in use show 11111111 (for-l.27v) or 00000000 (for1.28v), the mark of over-level is indicated in the LED and the flow isstopped promptly. If the input level is not over, the addition andsubtraction repeat with the binary double precision (of 16 Bits) andtwo sets of accumulative data continue to be stored in each pairregister.When the number of the accumulative repetition reaches the number(~256) which is previously stored in another register, each accumulateddata of binary number are converted into those of 4-digit decimalnumber and displayed on the LED. A negative number is indicated by •mark in the LED. Thus, both accumulated data take the numbers withinthe range from -9999 to +9999 and if either data are out of this range,they become ineffective together with the error marking.The accumulated decimal data are transferred to the personal30 Yoshihiro TANADA and Hiroya SANOcomputer by 1 digit. The order of a transfer becomes the following.Receiving the data-request signal CONTROL, the microcomputer puts 1-digit data on the data bus of Port A and at the same time the FLAGsignal on Port G, to be informed to the personal computer. Then thepersonal computer reads I-digit data.On the execution of this program, 0.35ms is required through aset of addition and subtraction on the CH-A and CH-B, and 0.77ms, fromthe end of accumulation to the data display. Therefore, the minimumperiod of the repetitive pulse signals measurable by this program isexpected to be 0.35/2 = 0.175ms.NFig.5 Flow Chart of the Box-Car Integration.5. Performance of the Signal Detecting SystemAt first, the quasi-signals with 10-ns pulse width and 100-msrepetition period, generated from an electric pulse source, are measuredwhere the pulse distributor and the variable attenuator of very smallloss are used as supplement and the instrument parameters of Kd/Kn=10 and M = 100 are chosen. The characteristic of measured vs. givenattenuation shows a fine linearity and the minimum measurable attenu-Microcomputer Filtering of Impulsive Signals 31ation 2.7x 10-7NP is recognized, which originates in the non-correlativesystem noises.oNext, using the 4500-A pulse light emitted by the pulse dye-laserand the beam splitter and the optical attenuator, zero attenuation ofthe empty glass-made cell for gas is measured. Its standard deviationleads to the minimum measurable attenuation 6.0 x 10-4NP • Therefore,it is shown that the non-correlative noises generate at the greatermagnitude in the optical than electronic system. However, even thelatter measurable value is the smallest so far as the authors know.6. ConclusionThe filtering system for measuring the very small transmissionloss of impulsive signals is developed, which is composed of the analogsimultaneous differencer and the microcomputer 2-channels box-carintegrator. The minimum measurable attenuation by the system shows anorder of 10-4Np , which is the smallest so far as the authors know.This filtering system can be applied to the measurements of the verysmall attenuation in both optical and electrical signal transmissions.AcknowledgementThe authors wish to acknowledge the experimental assistance ofMr.HUYNH Tam-To, the former postgraduate student of Okayama University,the present engineer of Ikko Electric Company in Takamatsu.References(1) H.Yoshinaga(Ed.): "A handbook of applied spectroscopy", Asakura(1973) •(2) K.Asai, H.Kuroiwa and T.Igarashi: "Measurement of atmospheric ozoneusing long-pass differential· absorption with cw CO2 laser", Trans.IECEJ, J60-C(1977), 10, 743.(3) H.Sano, Y.Tanada, R.Koga and T.Ono: "Method of narrow pulsed laserlight detection applied to the monitoring of air pollutions",J.Illum. Eng. Inst. Jap., 61(1977),3,159.(4) Y.Tanada, T.Huynh, R.Kogiand H.Sano: "Precision measurement ofimpulsive photoelectric signals with a microcomputer", Trans.IECEJ,J62-C(1979), 6, 471.

Microcomputer Filtering System to Measure Very Small Transmission Loss of Impulsive Signals

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