Topics in..
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Chemical Instrumentation
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Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These articles are intended to serve the readers of THIS JOURNAL by calling attention to new developments i n the theory, design, or availability of chemical laboratrm~instrumentation, or by presenting useful insights and explanations of topics that are of practical importance to those who use, or teach the use of, modem instrumentation and instrumental techniques. The ediW invites correspondence from prospective contributors.
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Oscilloscopes in Chemistry-Part
One
Joseph E. Nelson, Chemtrix, Inc., Beoverton, Ore. 97005 INTRODUCTION One may say that the oscilloscope is a logical extension of the strip-chart recorder with the X-Y recorder as a. natural transition. B y comparison the horizontd Xaxis of the strip-chart recorder is produced by the paper movement and the vertical Y-axis by the pen movement across the paper. The resultant display on the paper is a chart or graph. The X-Y recorder is similar except that the paper remains stationary while the pen moves across the paper in both coordinates. If one now imagines that the oscilloscope screen is a piece of graph paper and the electron beam is a pen the relationship is immediately apparent. One might suggest that the comparison is not complete since the waveform or graph drawn by the oscilloscope beam disappears almost immediately. Even this shortcoming has been overcome by the recently introduced storage tube oscilloscopes that have the ability to retain the written image for indefinite lengths of time. Most nonstorage type oscilloscopes can present a visual fixed display if the signal is repetitive. The block diagram of Figure 1shows the esserltial working parts of an oscilloscope. The high and low voltage power supplies have been omitted since they will not be discussed here. The working components < f irnl~rtanveto the rbcn~istarc: I the I I I ,I l l I T , 2 rhrn~ndifi+r.3 t h c t ~ n t c l ~ x a r . n r d1 thr triggerLcirciit. ' Of these, thk one' most, likely to be unfamiliar is the trigger circuit. I t is this circuit which has been largely
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Figure 1. The moior component sections of a modern orcillorcope.
responsible for the vast improvements that have occurred in oscilloscope design in the past twenty years. The trigger is discussed in detail below.
THE CATHODE-RAY TUBE The cathode-ray tube can be considered the end result of a series of transductions. Thus, suppose one start,s with a sudden change in temperature and uses s thermistor to convert this to a change in voltage. Within the oscilloscope the voltage change is amplified and applied to the cathode-ray tube where it is converted to a visual display on the screen of the tube. This action of the CRT starts with the electron gun where a stream of electrons is produced (See Fig. 2). This gun, located within the narrow neck of the tuhe, contains an indirectly heated cathode which produces a cloud of electrons. These electrons are then attracted down the neck of the tuhe to the screen by a potential of several thousand volts between the screen and the electron gun. By use of various elements within the gun it is possible to focus the beam to a small dot where i t strikes the screen.
Joseph E. Nelson received his early training in biochemistry at MIT, and worked in this field for the U. S. Army during World W a r 11. Binee then he has had more than twenty years experience in electronic instrumentation, including most recently six years with Tektronix. Ino., on oscilloscope development. During this period he developed n singlesweep polarographic analyzer using a storage oseilloseope. He is now President and Directov of Research at Chemtria. Inc.
At this point the dot on the screen is similar to a resting pen on a recorder, waiting for commands. These commands me applied to the electron beam by two pairs of deflection plates contained within the tube neck and located about midway hetween the screen and the electron gun. m e n voltaxes of sufficient a m ~ l i t u d eare applied to the pair of deflection plates an electrostatic field is develoned hnt,wean ... . .... them. If the applied voltages are unequal the steady stream of electrons will he deflected toward the more positive plate. Because of the field between the plates, this type of deflection is called electrostatic. I n Figure 3 the voltages shown represent the potentials applied to the two horizontal deflection plates of the cathode-ray tube. Note that in position 1 the voltage applied to both plates is the same and the I t I remain* at CeutQr etrect,. Position 2 ahow 200 v on the left plate aud ICII r t 8 r . 1l.c rwht r h t e . n difl~rrnveof 40 v which resulted'in d o t movement of 2 em toward the more positive plate. These data. provide the deflection factor of the CRT. I n this instance i t required 40 v of difference between the plates to move the beam 2 cm or 20 v to deflect 1 cm. This ~
Figure 2. The internal structvre of an eledrostatic deflection cathode-ray tube.
The screen consists of a thin layer of cdmium or zinc sulfide which fluoresces momentarily when struck by an electron. Thus, a steady stream of electrons striking the screen will muse a dot of fluorescence. If the construction of the tuhe is symmetrical, this dot will occur directly in the center of the tube. Later in the text the subject of different screen materids will be discussed in detail.
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Volume 45, Number 9, September 1968
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Chemical instrumentation
3
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Figure 3. A graphic reprerentdon of the roltage opplied to the horizontal deflection plates, and the resulting dot position on the CRT screen; the deflection sensitivity is 20 V/cm.
then, is the deflection factor of the horizontal plates: 20 v/cm. I t is very possible that the deflection factor of the vertical plates in this same tube will be considerahly different than that of the horizontal plates. Aside from the general interest in knowing how things function, there is amore important reason why the chemist should he aware of deflection plate characteristics. This concerns the application of signals directly to the plates. Most modern day CRT's contain deflection plate connections directly accessible on the neck of the tube. To use this crtpability, one simply removes the internal connections and carefully tapes them to prevent electrical shorts, then clips on the new signal connections. In many applicsi tions only the vertical plates will be connected to external signals wit,h the internal horizontal time base amplifier used for horizontal deflection. When electrical signals are passed through attenustors and amplifiers on their way to the CRT screen they frequently suffer slight distortion and time (phase) shift. In most csses, especially where high frequency oseilloscapes are used, these effects are negligible. However, if only a low frequency instrument were available it is possible that the observed signal would be a. poor replica of the original. In this ease one might benefit by connecting directly to the oscilloscope plates. Or, if same other type of higher-frequency amplifier is available this could beused to precede connection to the plates. The objection to this method concerns the high voltages required to deflect the beam. Typically for a 6 cm sine wave the change would need to be 120 v with a CRT having a 20 v per cm deflection factor. One further aspect of CRT operation that can be useful is called Z - u i s modulation. The Zaxis in this case refers to a change in the intensity of the beam produced by varying the potential between the cathode and a control grid. Usudly, the beam is either on or off; a short pulse applied to the cathode can momentarily turn off the beam and serve as a time mark on the screen. Conversely, one can intensify the trace at precise points by using a. pulse of the correct polarity. This can he especially useful to coordinate some exterior event with the waveform shown on the oscillo(Continued on page A6S9)
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Chemical instrumentation scopes. For example, if a rising voltage is causing a heating effect in some circuit component that is being followed on the oscilloscope screen, the trace could be intensified at a point in its scan that corresponds to some preset voltage level. Any temperature lag at that point would he immediately apparent. Trace intensification is also used on digital readout oscilloscopes to identify the point at which the measurement is being made. To close the discussion on CRT's, the chemist should also he aware of screen persistency. Different compounds or phosphors continue to fluoresce or persist far differentlengths of time, after beingstruck by an electron, that extend from several milliseconds to as long as 30 sec. The choice of persistency should be determined by the type of experiments planned. Slow-scan work should use a long persistence CRT phosphor. A long persistence CRT used in a repetitive scanning test can give the impression of a stationary image. Two examples are: (1) a. csthode-ray polarographic analyzer in which the scan is ~ynehronizedto repeat every 7 sec. If a screen with 10 sec of persistence is used the resulting palamgram will almost appear stationary. (2) where a spectrofluorometer is monitored with an oscilloscope, the X-axis corresponds to wavelength, and the Y-axis fluorescence, so that a continuous repetitive scan will portray a plot of a fluorescence spectrum.
ELECTRONICS The vertical amplijer, also called the Yaxis amplifier, receives the signals to he measured and converts them to a form that can he displayed by vertical deflecbion of the electron beam of the CRT. I n its simplest form it consists of a fixed gain amplifier with a calibrated variable sttenuator a t itsinput and the vertical plates of the CRTatits output. This isshownin Fig. 4. The signal to he displayed is applied to the vertical amplifier through approprishe probes or cables. Generally this aignd must be in the form of a voltage or current that has been transduced or changed from another form such as temperature, pressure or vibration. For example, light intensity cannot he applied directly to s vertical amplifier but must first be converted to a voltage signal through a phatomultiplier or photosensitive cell. A second example is the change in current through a thermistor as a function of temperature change. I n polarography, of course, the quantity of interest is current through the cell which can be applied directly to a current amplifier and does not require a transducer. Of particular importance to the chemist who requires an accurate replica of s varying quantity is the faithfulness of reproduction of the instrument being used. Most chart recorders and even X-Y recorders are quite limited in this respect. The sensitivitv control of the vertical ooscillosco~e a m p h e r is calibrated in volts per division of deflection. That is, a 10-v attenustor (Cmlinued on page A640)
Chemical Instrumentation setting will require that a 110-v aignd m u t be connected to the input to produce one division of vertical deflection as measured on the CRT graticule. (A ClZT screen is usually divided into 8 or 10 vertical divisions and 10 horizontal divisions by a scribed plaatic graticule). Because of this, the calibrated attennator is usnally called the volts per division switch and contains switch positions that extend from the most sensitive to the least sensitive in a. repeated sequence, i.e., 10, 5, 2, 1 v/division. I n many amplifien the most sensitive position of the volts per division switch also repre sents the sensitivity of the fixed gain ampliier and the actual attenuation in this case is unity. When the mulied sienal ia considerahlv attenuator probe can be used to reduce the or t h of its original value. signal by Convenely, if the signal level is considerably below the most sensitive position additional amplification is required.
Figvro 4.
The chemist who uses an oscilloscope would, no doubt, like the displayed waveform on the CRT to be an exact representation of the applied signal. Yet, unless he is familiar with his instrument's ban& width he may be seeing a poor reproduc tion. Bandwidth, as applied to an oscilloscope vertical amplifier, means the sinewave frequency a t which the displayed signal is 70% (-3db) of the applied signal. This is measured by connecting a vanable freouenev oscillator of eanstsnt am~litude to 1 and the oscillator amplitude adjusted to produce sixdivisions of vertical deflection at a sine-wave frequency of 1 kilohertz. (Note: the designation hertz has now replaced cycles per second.) The oscillator frequency is then increased until the displayed wmeform reduces to 4.2 divisions (70% of 6 divisions). The frequency at which this occurs is the bandwidth of the amplifier. I n the case of an ac coupled amplifier the value described above would be the upper bandwidth. Since the amplifier does not, pass dc levels it also has a lower bandwidth which is measured in the same way. A good rule of thumb is to use
Component. of the vertical deflection system.
(Continued on page A648)
time value of each horizontal division on the oscilloscope CRT screen. This is accomplished by setting a Time per Division switch to some discrete value. For example, with this switch set to 1 seo, it requires 10 see for the beam to scan across the full
Chemical Instrumentation an am~lifierthat haa a bandwidth that is at least ten times the frequency of the signal of interest. Of even greater interest to many chemists is the response of the oscilloscope amplifier to a step function or square wave. Despite the square-sided appearance of a step function it still requires a. finite time to change from one voltage level to another. This time, designated risetime, is the time interval between the 10% and 90% amplitude .points on the rise of the step funotian. Figure 5 shows a step function with the period of risetime outlined. A step function cen also he used to measure amplifier bandwidth through the following relationship: The bandwidth times the riset,imeequals a, constant which is typically 0.35. For example, an oscilloscope amplifier with a bandwidth of 500 kilohertz would have a rise time of 0.35/500 kH = 70 rsec. This means that a faserise step function of 1 psec would appear to have a 70 met risetime when applied to this amplifier. In the same manner fast risetime signals of any kind would be considerably damped due to the low bandwidth of the amplifier. Thus, the displayed waveform may only he a very poor representation of the actual applied signal. From the foregoing one might conclude that s. wide bandwidth oscilloscope would he desirable in all applications. Unfortunately this is not tme. High-frequency
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Figure 5. The tvo points on o waveform h o t a e used to moosure risetima.
noise often obscures the signals of interest and the penalty ofwide bandwidth becomes apparent. A good example is the amplifier used with a dropping mercury electrode in polarography. To amplify the small currents requires high sensitivity. But the eolomn of mercury acts like an antenna and considerable noise pickup will be evident with a wideband oscilloscope amplifier. Because of this it is necessary to reduce the bandwidth to a point where small polaragraphic currents can bemeasured. Consequently, some amplifiers have a bandwidth control that permits adjuse ments of the bandwidth over a considerable range. When using operational amplifiers bandwidth can be reduced by a small capacitor placed across the feedback network. The lime base (X-axis) establishes the
to rrome value and then rapiily falls to zero. Due to the appearance of this wave it is called a sawtooth. The sawtooth is usually developed by integrating a constant current to produce the linear rise. Since the integrator rate of rise is related to both current and capacity, it can be changed to different values by changing the value of either the resistors or the capacitors involved in the integrator. This is actudly what takes place when the Time per Division switch is changed. Time base integrators have been designed to extend from as slow as 10 sec per division to as faat as 1 nanosecond per division. In order to relate these times to some better known reference, consider the elapsed time of 1 cycle of a. 60 Hz sinewave signal. The time period of 1 cycle is equal to the reciprocal of the frequency, or 16.6 milliseconds. 1/F which equals similarly, one can see that a time base of 1 nanosecond per division would be required to display 1 cycle of 100 megahertz sinewave signal on rt 10-division screen. The integrator output is applied to a horizontal amdifier from which it is then applied to thl Jhizontal deflection plates of the CRT.
(To be concluded in the Oclober issue.)
