Time-Resolved Measurement Using Commercial Modular Boxcar Integrators Alexander Scheellne University of Illinois, Urbana. IL 61801
Transient phenomena are of interest in a variety of branches of chemistry. Two common measurements needed for studying transients are the instantaneous value of a time-varying signal and the total signal from one event out of a train of such events. For exwple, one can learn much about energy exchange phenomena by measuring the change in the lifetime of a fluorescent excited state of a molecule as the concentration of some other species with which the molecule can exchange energy is changed. This requires that the temporal decay of the fluorescence he measured. A device that can sample . .partionsof thesignal produced by a phohmultiplier that ol~senes the decav is a boxcar intecrator 11 ).This device looks at a shon portion or time slice within a repetitive signal and averages the signal value within the time slice over many repetitive events. The position of the time slice is then mobed ;md the signril at this new time is averaced again. There are two efiects ofthis measurement procedure: (i)noise and fluctuations from one event to the next are averaged ox smoothed, allowing extraction of signals that would otherwise he unobservable due to high noise and (2) the apparent time behavior of the high-speed signal is slowed or aliased. Because one can look a t a narrow time slice for as long as desired, a DC meter or slow chart recorder can be used to measure the signal level during the time slice under observation. Measurement of the total sienal from one event out of a train is useful in studying fluctuations among the events. A soark discharee (2-3) can be used for elemental analvsis of alloys by iirst sampling atoms from an rlectrode made oi the allov and then exciting those atoms. Suhseauent lieht emission fro; the excited stares can be used to identify (by emission wavelength) and to quantify (by emission intensity) the elemental composition of the electrode. By measuring the light emitted bv the material sampled during a sinale mark, composition df regions of the ailoy only :fraction i f a square millimeter in area can he determined, and sample heterogeneity can be mapped. Since a spark in general will last less than 100 ws, it is clear that rapid signal acquisition and integration is essential. There are a large number of electronic devices available for s. hoxcar integrator sysmeasuring surh s ~ ~ n a l(:ommerrial tems! ran cost in exress of .i10,000, but thry giw tlme resolution frum 1 ns to 1 ms. Diei1~l11scillosrooes'~"aresimilarlv useful and allow real time measurements in addition to inteerated measurements. Such hardware is bevond the budeet " " of many undergraduate institutions. An increasingly popular method of data acquisition is the use of a microcomputer with appropriate interfaces to measure short duration phenomena. When used directlv with analoe-to-dieital converters.' direct measurement response is l i m i 2 to 5 0 i ~ zBy . interfacing the comouter to a s a m.~ l i n eoscillosco~e. . eieahertz bandwidths are possible (4). However, unless a second-hand sampling oscilloscope can he obtained, this is a prohibitively expensive approach. A series of modules that can be used for eated inteeration. boxrar integration, timing, and analog data proressing is available from Evans Asstriates."t is from these circuit cards that a very inexpensive transient measurement system can
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Journal of Chemical Education
be fabricated. The manufacturer claims time resolution of 30 ns can be realized: without much care in eneineerine 50 ns can he readily achieved. Control circuitry can;ange f;om a dual monostable multivihrator (TTL art 74221) on a breadboard to a microprocessor system. ~ o r t i of k some large system can be in use while other parts are being designed and fabricated. The dual channel, digitally scanned boxcar integrator to he descrihed here was assembled hv a high school student with no previous experience in elecironi&. A wire list for the complete assembly can be obtained from the author. Desien andconstruction~ofsuch a system allows learning of tLe principles of analog and digital electronics, the skills of circuit debugging, and thk utility, upon completion, of a rapid and flexible time-resolving data-collection instrument. Duplication of the device pesinted here would cost $3000, onG 25% of the cost of a fully assembled commercial instrument of similar capability. However, a single channel gated integrator using a monostable timer and a digital multimeter for readout could be assembled for less than $300. The full boxcar svstem is described to show how the modules can be configur& into a research grade instrument; depending on the availability of microcomputers and designer interest, many other arrangements can be imaeined which would he simpler to assemble or he more germane to other situations. Description of Clrcult Cards Four Evans cards will he described: (1) type-4130 eated integrator, (2) type-4145-2 digital timer, (3) G e - 4 1 4 6 timer extender, and (4) tm-4122 ratiometer. It is hest to treat them as isolated components and then to link them together, much as one designs suhruutines for a computer then forges a prugram hy linking the suhroutines. This separation u,iH aid in clariiying the runrept of mudular elcctronir components and assist in seeing howone module may he useful independently of the others. Gated htegrator The 4130-gated integrator can be used either to accumulate incoming signals during a period when a logic signal (gate) is held at 0 V or to average a series of signals from successivegate intervals. In either case, a reset may be used (aseparate input pin that is mounded or set to 0 V brieflv) to clear all nrevious informati& from the card (output resets to 0). 1f thkre is no connection of feedback from the output to one of the four inputs, accumulation or gated-integration is the function performed by the 4130. Addition of a resistive feedback loop, as shown in Figure 1, results in boxcar integration. The output tracks a step change in the input after the gate has been open a time (expressed in seconds) equal to the product of the feedhack resistance (in ohms) times the integration capaci-
' Princeton Applied Research Corp., Division EG&G Inc.. Princeton,
NJ.
Tektronix. Inc., Beaverton, OR. Nicolet Instruments, Madison. WI. Cyborg Inc., Newton, MA. Evans Associates, Berkeley, CA.
A zero adiustment circuit is provided on the 4130, but it bas been found that the adjustment is inaccessible and overly sensitive. Thus, the zero trim circuit shown in Figure 1 has ---r---.
If the input exceeds 5V, a light-emitting diode can be used to indicate overvoltage. This feature has not been found to be useful. To run the boxcar, very few items are needed in addition to the components in Figure 1: (1) a f15, +5 V power supply, (2) a card-edge connector to which to attach leads, (3) a reset button, and (4) a gate signal. The gate must be time-locked to the physical process of interest and of a width commensurate with the time resolution desired. In general, the time lag from the start of the monitored event to the opening of the gate must be adjustable. The output of the integrator can be observed by any convenient voltage monitor. This may range from a d'Arsenval voltage meter or digital volt meter to a chart recorder, oscilloscope, or computerlanalog-to-digital converter, depending on what is available.
Figure 1.Diagram far assembly and use of model 4130 Evans gated integrator card as a boxcar integrator wia AC or DC input coupling, variable gain, and variable time constant. tance (in farads). The board comes with a capacitance of 0.01 pF, s o t h a t when the feedback resistance has its minimum allowable value of 1000 R (the card is normally shipped with 10,000 Q), the gate needs to be open only 10 ps to acquire data. Gate-open time can be either as a single time interval or as many shorter intervals; if 50 ns resolution is desired, the gate must be open 200 times to accumulate a signal. In addition to settine accumulation-time remonse. the feedback resistance helps dtttermine the wltage gain of the system. Overall gain is c.qlt;~lto feedback resisti~ncedivided by input resistance. Although Figure 1 shows the arrangement in.use. it has heen found that when a totd feedhnck reststance -., -. of 10 Mfl is employed, it is impossible to zero the output. The rectaneular box below the 4130 card in Figure 1 is a double-pole, double-throw switch to allow e i t h e r k c or DC c o u ~ l i n eof the inout sirnal. Note that the input resistance netkor; is not simply aswitched resistance ladder but also includes adiustable capacitance. Without these tuning capacitors stray wpacitanre i n thr input cirruitry slows duwn the response of the interrator frum its inherent rise time of 30 ns to a rise time of nearly 2 ps for 1MQ input resistance. With the tuning capacitors flat response is maintained. However, the input impedance is reduced by these capacitors. The shunt on the 90 kfl resistor is 15 pF, and the measured circuit capacitance is 33 pF. This gives an impedance of only l ca~acitoron 4800 fl at 1 MHz. Similarlv. the ~ a r a l l eshunt the 900 ki1 input resistor is 1unal;lr to 3 pF, and the parallel capacitor combination to gn~utidis npproxitnatelv 200 pF. T h ~ rrsults s in an impe(lauce of XJ!?at 1 MHL.(:l~arl?,high freauencv sinnals can be accuratelv monitored onlv if they come fro& lo; impedance sources. Thus photomulti&ers can be monitored onlv after a re-amplifier stage is used for current-to-voltagi conversion. ~
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Digital Timer The model 4145-2 digital timer can generate delays ranging from 450 ns to 1ms with 10 ns resolution and less than 5 ns jitter. This stability, combined with the discrete, easily manipulated digital data format, makes this card useful not only for controlling integration timing hut also for general timing tasks in the laboratorv. In contrast to analoe" delavs such as those derived from monostable multivibrators, one does not need an oscilloscone to set the timine of these cards. e x c e ~ t ns claimed by the for initial calibraiion (the offset of manufacturer is accurate within 10 ns). The delav can be triggered by positive or negative edgesof T T L signals. If a 50-fl driver is oroducina the triaaerine ~ u l s e sit, is essential to terminate the trigger signal-through 50 f l to ground or ringing and overshoot will prevent triggering. In addition to a pulse at the end of the delay, a pulse at the start of thedelay and a pulse that lasts the duration of the delay are produced. While soldered wires on the card allow a choice of positive or negative going pulse polarity (normal-on or normal-off behavior), the polarity as received is correct for use with the 4130 card. The 4145-2 is programmed in binary-coded decimal (BCD) format. Five digits are controlled using 20 input lines that are considered asserted when grounded. Thus programming can be done electronically or mechanically through single-pole, sinele-throw switches. BCD thumbwheel switches. with the common connection grounded, are convenient for this purpose and are available through any electronics supply house. A crude "scanning" of a boxcar window is possible using only the thumbwheel switches for setting the delay time and the delayed output pulse to trigger the gated integrator. One can manually reset the switches to choose the observation time. The output pulse length is no longer than 100 ns, so, if coarser resolution with the gated integrator is desired, a pulse stretcher such as a monostable or another delay generator (using its pulse duration output) is needed. More automatic (but more complicated) scanning approaches are discussed in the section below. that gives an example assembly of a boxcar integrator.
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Time Extender The 4146 timer extender is an additional circuit card that is connected by four wires to the 4145-2 and provides four additional digits of delay. Thus a set of 4145-2 and 4146 cards can generate delays up to 10 s with 10 ns resolution. Ratiometer In many signal-processing situations, functions of an initial measured voltaee must he comnuted before full interoretation of data can be completed. FO; example, in absorption specVolume 61 Number 12 December 1984
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EVANS
X-Y OUT TRIM .
REF OUT
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Flgve 2. Arithmetic function model employing 4122 Evans ratiometer.
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tronhotometrv. - , the loearithm of nhotocurrent is needed to compute absorbing species concentration. In an Arrhenius plot of reaction rate as a function of temperature, the activation energy is computed from the slope of a line whose abscissa is the inverse of temperature, so the operationy = 11x is useful in automating collection of such data. The 4122 ratiometer nrovides three functions: for two inputs identified as x and ,; outputs are simultaneously prohded as xly, lxlyl, and loglolxly 1. Further, a precision, l-V reference is provided, so that the functions lly, Ix 1, loglolx I and -logloly I are easily computed. In Figure 2, an arrangement is shown where the additional function x - y can he added with little difficulty (the 20 kCl trim pot is the only component in the x-y circuit that costa more than $1.00). The 741-op amp was chosen hecause i t is commonly available and inexpensive; many other pin-compatible circuits that give faster response and greater accuracy (for example the TL-081) are availahle. While the intent of this module is to combine the outnuts of two eated integrators, i t is clear that many other sign.al processing situations can convenientlv use this module for data reduction. The cost of this card is iess than many modular logarithmic amplifiers. Example Assembly of Boxcar Integrator The above modules can he combined into a full boxcar intemator in a variety of ways. I t is clear that, with current microprocessor technology,-the various delay settings, reset pulses, and control pulses could be provided by interfacing 6a microcomputer.~tthe time the boxcar repoked here was built, the author was unfamiliar with microprocessor interfacing hut was familiar with TTL logic. Thus all logic was hard-wired, with ideas for useful functions derived from use of a PAR-162 analog scanning boxcar6 and various digital delay generators. The choice between using TTL and microprocessor control need not he permanent hut can be based on accessible hardware and teaching goals. Cost of either apnroach is comnarahle. ' Good enginkeriq practice dictates that the power supply for dieital loeic be filtered or "decouoled" at each intecrated circuit. This can he accomplished by placing a capacitor hetween the nower sunnlv and mound at each intemated circuit. Chips witi decoupii&capa&ors fabricated as part of the IC package are just becoming availahle (5).The approach used here was to use bus strips with built-in capacitance7 to supply the +5 V required by T T L and decoupling capacitors for the *15 V used by analog components. Only two additional decoupling capacitors were installed on the digital control circuit board (which contained 60 TTL IC's). The gated integrators and 4122 "postprocessor" receive control information and data through coaxial cable connec-
'Sea footnote 1.
'Eldre Components Inc., Rochester, NY.
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Journal of Chemical Education
tions on the boxcar's front panel. The trigger and delay pulses for the digital delays also are accessihle at the front panel, but controls for scanning including delay time values are all generated on the separate circuit hoard. The circuit card also contains several monostable multivibrators to use as "pulse stretchers," the 741 op amp for the x-y postprocessor function, and a digital to analog convertor. The DAC is used to produce a voltage proportional to the delay currently set for the scanned digital delay. This allows use of an analog x-y recorder for data output, with absolute synchronization between scan time and pen position. Its function will he clearer after the overall scanned delay circuit is described. Figure 3 shows a hlock diagram of the delay-generating and scanning hardware. It can he seen that the Evans cards are fed the appropriate delays over data buses in such a way that any source of TTL level signals could feed these huses. Reengineering for control by microcomputers could he performed by cutting any of the huses now in place and replacing them with the appropriate interface chips. Independent of the hardware implementation, scanning of the time delays involves the following steps: (1) set initial delay time, (2) set final delay time, (3) set the time increment for each step of the scan, (4) set the dwell time at each scan step, (5) load these parameters into the control logic, (6) start scan. In addition, useful functions include (1) the ability to reset to the initial conditions, (2) the ability to pause in the middle of a scan but without resetting to initial conditions (pause, then resume), (3) a choice between a single scan and repeated scans, (4) a stop command to interrupt scanning. The hardware shown in Figure 3 implements these functions as follows. The counters (74192 IC's) contain the BCD numbers that represent the current delay of the 4145-214146 timers. One set of thumbwheels (TW-1) sets the initial time, while the other (TW-2) sets the end of the scan. The counters are loaded at the beginning of a scan, then count as each additional delay increment is needed. Equivalence test chips (7485's) detect when the number in the counter equals the value set on the second set of thumhwheels (TW2). A total of 9 series counters with thumhwheels are employed. 7404 inverter buffers are used to set logic levels correctly. The control logic hlock in Figure 3 combines trigger information, equivalence status, and function selection (from a set
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