zyxwvutsrqp I E E E TRANSACTIONS ON COMMUNIChTION TECHNOLOGY DECEMBER S65 1967 Concise Papers zyxwvutsrqp zyxw zyxw zyxwvut zyxwvutsrq zy Experimental Verification of a Model of the Oscillating Limiter ENRICO BOZZONI AND UMBERTO MENGALI REFERENCE : Bozzoni, E., and Mengali, U. : EXPERIMENTAL VERIFICATION OF A MODEL OF THE OSCILLATING LIMITER,' University of Pisa, Pisa,Italy. Rec'd 3/6/67; revised 5/23/67. Paper 19TP67-968, approved by the IEEE Communication Theory Committee for publication without oral presentation. IEEE TRANS. ON COMMUNICATION TECHNOLOGY, 15-6, December 1967, pp. 865-867. ABSTRACT: Work done to verify the results of an analysis of the oscillating limiter published previously is reported upon in this paper. To this purpose experiments were made with an oscillating limiter, the characteristics of which are given, to check the nonlinear distortion produced on the baseband signals. The baseband equivalent circuit and nonlocking behavior of the oscillating limiter were also tested. Finally, anetwork for compensating the linear distortion produced in the message modulation by the oscillating limiter is shown, as well as experimental results. further stage is added to keep constant the level of the output of the limiter. Figure 2 shows the amplitude of the voltage at the output of the bandpass filter (when the feedback loop is open) vs. the amplitude of the voltage at the input of the limiter. Figure 3 illustrates the modulation suppressing properties of the limiter. The ordinates are the ratio between the amplitude modulation index of the inputof the limiter and the ratioof rms value of the envelope to thatof the carrier at the output of the filter; the abscissa is the modulation frequency. The filter is a simple double tuned amplifier. The amplitude and phase characteristics of the cascaded limiter and filter are given in Figs. 4 and 5. The center frequency is f~ = 30 MHz, the bandwidth a t - 3 dB is 520 kHz, and themean group delayis 1.3 MS. TRANSFER FUNCTION OF THE BASEBAND EQUIVALENT CIRCUITOF THE OSCILLATING LIMITER With reference to the block diagram of Fig. 1, let e t r ( t ) = Etr cos [wet - $.(t)l Efb COS [mol - dt)] and KEYWORDS: Detection, Distortion, Experimental Data, Feedback, Filters, Frequency Modulation, Phase Modulation. INTRODUCTION In arecentpaper['] the performance of the oscillating limiter driven by exponentiallymodulated signals has been investigated. This investigation led tothestudy of anequation relating the message modulation of the signal applied to the oscillating limiter to that of the signal at its output. This equation is a finite nonlinear differential and, assuch,inherently shows that the action of the oscillating limiter may give rise to nonlinear distortion in the message modulation. The conditionsunder which thisequation may be substituted witha linear one have been determined. Finally, the distortion affecting the message modulation at the oscillating limiter's output, when the aforementioned conditions are not fulfilled, has been given throngh graphs. In the following, the results of an experimental verification of the major conclusions reached in the preceding paper are reported and, furthermore,anetworkcapable of compensating the linear distortions will be shown. efb(t) = be the voltage a t the input and output of the oscillating limiter, where $ ( t ) and ~ ( t are ) the instantaneous phase fluctuations due to the modulation and wo is the angular frequency of oscillation of the circuit when it is not driven by external signals. Subject to the conditions that 1)limiter is ideal 2) amplitude response of filter is flat over band B occupied by first spectral zone of signal a t output of limiter 3) filter's group delay7 is constant in B, the characteristic eqostion of the baseband equivalent is['] sin [ ~ ( t ) $(t - T)] + K sin [ ~ ( t ) ~ ( -t T)] = 0 (I) in which K = Efb/Eif. There arecnses in which (1) may be linearized in the form d t ) - $(t - 7) + K[dO- d t - T)] = 0. (2) When (2) holds, the effect of the oscillating limiter on the message is taken into account through the transfer function DESCRIPTION O F OSCILLATINGLIMITER EMPLOYED FORi\lEASUREMENTS Figrlre 1 is a block diagram of the oscillating limiter. The limiter itself is realized with three cascaded st,ages. The first consist,s of an amplifier whose gain is automatically cont,rolled witha time constant of the order of one second, and which serves the purpose of maintaining the level needed for the best, performance of t>hefollowing stage. The latter is a modu1at)iou sxppressitg circuit in which the envelope of the voltage at the plateof a pentode is detected, amplified, and fed to the suppressor of the same tube with the appropriate phase. Since this circuit gives rise to a variat,ion of the level of the carrier at, it,s output when the depth of modulation at its input varies, a 1 T h e work reported in this paper was carried o u t a s a p a r t of the research by theConsiglioNazionale programonsatellitecommunicationsestablished delle Ricerche (Italian National Research Council). Figure 6 shows the graphs of IW(0)I for some values of K together with experimentally obtained values. As may be seen, the agreement is excellent in the whole range of frequencies in which the hypotheses 2) and 3) are satisfied. NONLINEAR DISTORTION zyxwvu When the conditions under which (1) is linearizable do not occur, the oscillating limiter gives rise to non!inear distortion in the message modulation. The amountof such distortion has been c a l ~ u l a t e d ~ ~ la function as of the frequency modulation index m, of the ratio N between the period of the modulating sinusoid and the group delay T, and of the value of K. I n Figs. 7 and 8 some analytical results are compared with the corresponding experimental ones. 866 zyxwvutsrqpon IEEE TRANSACTIONS ON COMMUNICATION TECHNOLOGY DECEMBER 1967 Fig. 1 . Block diagram of csci1l:tting limiter. zyxwvutsrqpon Fig. 2. Amplitude Etb of voltage a t outpui, of bandpass limiter vs. amplitude Eif of voltage at input (no feedback). Fig. 7. K-3 Fig. 8. Fig. 3. zyxw Continuous line gives calculated total distortion, dots experimental results. el 511 Continuous line gives calculated total distortion, dots experimental results. Modulation suppressing characteristics of bandpass limiter. zyxw zyxw zy zyxwvutsrqpo zyxwv zyxwvutsrqp Fig. 9. Fig. 4. Calculated waveform of dppldt for continuous wave excitation with frequency 39 KHz off center, K = 4. Amplitude response of bandpass limit,er. Markers are 0.5 MHz apart. Fig. 11). Fig. 5 . Phase characteristic of bandpass limiter. Oscillogram of dppldt obtainedin conditions of Fig. 9. Time base 20 &3/div. CONCISE PAPERS zyxwvutsrqponm 867 zyxwv zyxw zyxwvutsrqpo z z zyxwvutsr zyxwvut zyxwvuts The Quatrix-A Proposed Electrooptical Position or Angle Sensor BURTON J. ASKOWITH REFERENCE : Askowith, B. J. : THE QUATRIX-A PROPOSED ELECTROOPTICAL POSITION OR ANGLE SENSOR, Martin Marietta Corp., Orlando, Fla. Recd. 3/22/67;revised 8/30/67. Paper 19TP67-975, approved by theIEEEData Communication and Telegraph Systems Committeefor publication without oral presentation. IEEE TRANS. ON COMMUNICATION TECHNOLOGY, 15-6, December 1967, pp. 867-868. Fig. 11. Block diagram of compensating network. Fig. 12. [ W ( n ) M . ( n )1 obtainedexperimentally for some values of K . For comparison, the corresponding gra,phs of IW(n)I are also drawn. NONLOCKING BEHAVIOR It has been that when the exciting signa' e l l has aconstant , oscillating limiter will or will not lock according frequency w / ~ T the to whether Klsin (w - oo)7\ 5 I. KEYWORDS: Coding, Data Display, Data Storage, DataTransmission Systems,Detection, Digital Signals, Optical Communica(4) tions, Telephone Systems, Television Systems. Equation (I) has been used to determine the waveform of d q / dt when the sign > applies in (4). I n Fig. 9 is shown the waveform of dpp/dt pertaining to a case studied in Boszoni and Mengali;['I Fig. 10 shows the oscillogram of &/dt obtained experimentally. COMPENSATION FOR LINE.411 DISTORTION Under the conditions necessary for (3) to hold, the oscillating limiter originates linear distortion in the message modulation. When the waveform of the message must be preserved, it is therefore indispensable that compensation be supplied. This may be realized most easily a t baseband by means of acircuithavingatransfer function of the form M ( n ) = C(1 + K - Ke-jn') (5) in which C is an arbitrary real constant. A system having a transfer function of the type [1/(1 + K ) ] (1 + K - Ke-jnr) (6) is shown, as a block diagram, in Fig. 11. The circuit originating the delay T has been realized with an allpass network built around a phase splitter.L31 Figure 12 provides the comparison of the calculated amplitude characteristic of W ( 0 )with that of M ( 0 ) . W ( 0 ) obtained experimentally for some values of K . It is worthwhile noticing that the value of M . W begins to depart appreciably from unity a t frequencies near the half of the bandwidth B f of the bandpassfilter. CONCLUSION Excellent compliance of the experimentalresults with those obtained on the basis of the models advanced in Bozzoni and Mengalir'l is shown. Also shown is a circuit that compensates for the linear distortionoriginatedin the message modulation bythe oscillating limiter. REFERENCES E. Rozzoni and U. Mengali "A general analysis of the performance of the oscillatinglimiterwithnoiseless'signals," I E E E Trans.CommunicationTechnologg. vol. COM-14, pp:, 578-588 October 1966. 121 E. J. Raghdady, Theory 'of feedbackaround the limiter," 1967 I R E Nat'l Conu. Rec.. vol. 5, pt. 8. pp. 176-202. Ial R. W . Calfee. "An active netxvyk equivalent to the constant-resistance lattice with delay circurt applications, I E E E Trans. Circuit Theory, vol. CT-10, pp. 532-533, December 1963. 111 ABSTRACT: This technique, called puatriz (for quasi-matrix), has potential application in numerous communication anddatahandling areas,sinceit provides high-density matrix-photocell capability at much lower cost. The method uses a few photographically-produced precision coding masks fitted over an equal number ( N ) of photocells. Thus, ten ( N ) typical high-detectivity photocells and ten coding masks will produce 21° (2N) binary combinations, behaving like a 32 by 32 matrix photodetector and using only 10 preamps. Twenty larger cells and preamps promise a million element matrix. Limitations are discussed. INTRODUCTION The quatrix is a novel method of angle sensing devised by t,he author. I t provides many of the advantages of a high-density photoelectric matrix detector withoutmany of the disadvantages, suchas excessive multicell and preamp cost. Its use as a densely packed discrete angle detector has been demonstrated and proved to be basically feasible. POTENTIAL APPLICATIONS AND CONCEPT DESCRIPTION Possible applications include: random-scan television, satellite tracking, high-density switchboards, optical-communication monitoring in space (laser beams), visual presentation, character reading, infraredtracking, analog-to-digital conversion, computerrandom read-in, read-out, data storage and display techniques, and many others. The quatrix technique employs a high-quality camera-type objective of medium focal length (typically 3 inches or more) to collect incident light from a source, distant or near, at any angle within the field of view of an on-axis photocell set in the focal plane behind the lens. This central on-axis cone is intercepted by several partially aluminized mirrors or beam splitters (Fig. 1) arranged to reflect or split,-off approximately equal-intensity convergent cones, each incident onthe geometric center of a photocell of the same type and configuration as the on-axis photocell. Thus, for a total of N photocells, there are N - 1 beam splitters. The photosensors are preferably, although notnecessarily, of the flatphotocathode variety, and should be large enough in diamet,er to subtend a substantial angular field of view (five or ten degrees or more). Flat photoconductors, photodiodes, and head-on photomultipliers are good examples. By using two such systems side-by-side or stacked (as in Fig. 2 ) , 2N photocells may be employed. Stacking permits use of only three mirrors with 20 cells, as in Fig. 2. Over each photocathode area is placed a thin, square matrix-like mask (Fig. 3), each mask different for each photocell, and each divided up into alternate lands and seas or opaque and transmissive portions. Thus, one mask [Fig. 3(a), (l)] consists of one opaque and one transmissive vertical column or broad slit; a second [Fig. 3(a), ( 2 ) ] consists of four columns, twoopaque and twotransmissive; a third [Fig. 3(a), (3)], of eight dternate columns. Similarly, an equal number of masks [Fig. 3(b), (1)-(3)] consists of two, four, eight, etc., alternate horizontal rows making up the square area of