RICHLAND COLLEGE School of Engineering Business & Technology Rev. 0 W. Slonecker Rev. 1 (8/26/2012) J. Bradbury

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1 RICHLAND COLLEGE School of Engineering Business & Technology Rev. 0 W. Slonecker Rev. 1 (8/26/2012) J. Bradbury INTC 1307 Instrumentation Test Equipment Teaching Unit 8 Oscilloscopes Unit 8: Oscilloscopes Objectives: 1. To explain the operation of an analog oscilloscope. 2. To explain the operation of a digital sampling oscilloscope (DSO). 3. To explain to operation of vertical signal coupling, V/Div. gain, and position settings. 4. To explain to operation of horizontal time/div. sweep, position, and trigger settings. 5. To understand the function of a 10x probe and explain probe compensation. Analog Oscilloscopes Figure 1. The above drawing depicts the main components of an analog oscilloscope. A portion of the analog information applies to digital sampling oscilloscopes, so read this section carefully to get the terminology. Notice that the cathode ray tube display uses electrostatic beam deflection with the plates inside the glass envelope. The CRTs used in television sets use magnetic beam deflection with the deflection yoke external to the tube. Whereas TVs constantly perform a raster scan covering the whole face of the CRT, with the signal controlling only the intensity of the beam, oscilloscopes scan only horizontally, allowing external signals to control vertical beam position. Oscilloscopes can also accept an external horizontal signal, but normally operate in a horizontal time sweep mode using an internal sweep generator. In both TVs and oscilloscopes a phosphor screen emits light when the electron beam strikes it. It is important that the phosphor continue to glow after the beam moves on, but it must not retain the glow too long. Oscilloscopes have an intensity control adjustment that increases the energy of the beam in order to increase the intensity of the phosphor glow. This is like the brightness control in TVs. With analog oscilloscopes, in order to capture a waveform, you had to take a picture. Many high quality analog Page 1 of 6

2 oscilloscopes had Polaroid camera adapters which took a screen picture of a displayed waveform. The camera shutter could be activated by a delayed trigger to photograph a single-sweep event. The Screen Graticule The rectangular grid etched on the CRT screen cover plate is called the graticule. The graticule provides a reference for measuring vertical and horizontal displacement of the trace. When the vertical (Volts/Division) and horizontal (Time/Division) controls are in the CAL (calibrate) positions position, the signal amplitude and time characteristics can be measured directly from the graticule. Most scopes have special 0%, 10%, 90%, and 100% horizontal dotted lines used to measure rise and fall times of a waveform. Standard risetimes are measured as the time between the 10% crossing of the trace and the 90% crossing when the base of the rise is set on 0% and the top of the rise is at 100%. Care must be taken to assure that you are not measuring the oscilloscope risetime and not the actual signal risetime. Risetime = 0.35/Bandwidth A 100Mhz oscilloscope has an inherent risetime of 0.35/ or 3.5 nanoseconds. Signal risetimes greater than about 10 nanoseconds will be accurately displayed on a 100MHz scope. Vertical Control The voltage of the input signal is amplified and applied to the vertical deflection plates to control the vertical position of the beam spot on the screen. It isn t complicated. The gain of the vertical amplification is calibrated in volts per graticule division. Higher input voltage causes higher beam deflection. A position control permits the waveform trace to be adjusted higher or lower on the graticule. Usually, position adjustment begins by setting the vertical input to ground and positioning the sweep line on a graticule which will represent 0 volts thereafter, then switch back to the desired input coupling. The input can be set to ground, AC coupled, or DC coupled. If you are trying to see small AC voltage changes on a voltage with DC offset, it is best to use AC coupling. The operator needs to keep in mind how the input coupling is set when the displayed trace is interpreted. Dual Channel Oscilloscopes Dual channel oscilloscopes provide amplifiers and display for two inputs. The signals are normally displayed on alternate horizontal sweeps. For very low sweep rates, an internal chopping circuit rapidly switches between the signals, like two dotted lines. At low sweep rates the operator sees two continuous traces, but at high sweep rates the chopped nature of the display can be observed. Horizontal Sweep The normal application of oscilloscopes is to display the voltage of a signal over an interval of time. To do this, the electron beam is swept across the CRT screen at a constant rate while the input signal controls the vertical beam position. When the beam reaches the right side of the screen it is blanked (cut off) and quickly returned to the left side to begin the next trace. It is very important that: 1) the sweep rate is calibrated precisely so that time can be accurately measured across the screen; 2) the sweep starts (triggers) at exactly the same place on the waveform for each sweep. Sweep generators are made to produce very constant, selectable sweep rates that are started by a trigger signal from the trigger generator. The trigger generator accepts an input signal, usually one of the vertical channels, and generates the trigger at a settable voltage level and voltage direction (rising or falling). In this way, the display produced by each sweep overlays the previous display exactly and refreshes the signal display so that it does not fade on the screen. Page 2 of 6

3 Quality instruments provide a delayed sweep capability. The operator sets up a subsection of the displayed waveform to be magnified and displayed. The width of the subsection as well as its position along the display is adjustable. It is usually designated in the display as a bright segment. After setting up the desired delay parameters, the operator enables the delayed display and the selected segment fills the screen. Delayed sweep is useful for examining the fine details of complex waveforms when it is difficult to trigger near the interesting part. An external horizontal input is provided that bypasses the sweep and trigger generators. The external horizontal input behaves like the vertical inputs. The signal is amplified and applied to the deflection plates. This capability adds versatility to the instrument for applications like differential phase measurements. Some instruments also have a Z axis input in the back of the instrument. This input accepts a signal that modulates the beam intensity to give the illusion that the brighter parts of the display are closer and the dimmer parts are further away (hence Z axis). In my experience it is rarely used. Oscilloscope Bandwidth The bandwidth of an oscilloscope determines how quickly an oscilloscope can respond to an instantaneous change in input voltage. Faster responding scopes cost more money since it is harder to design for very wide band operation. Scope rise time relates to scope bandwidth as rise time= 0.35 Bandwidth. A 50MHz oscilloscope has a rise time of 0.35/( ) = 7 nanoseconds. Probes Oscilloscopes are basically voltage input instruments so they require a high impedance input. Generally, the standard is 1MΩ with provision for switching to 50Ω for RF work. Times-ten probes (10x) are used to increase the input impedance of the device to 10MΩ but do so at the expense of a reduction of the signal amplitude at the input to the scope. The V/Div setting must be multiplied by 10 when a 10x probe is used. (The 10mV/Div setting becomes 100mV/Div when a 10x probe is used. Modern analog scopes are able to sense the probe type and illuminate the appropriate scale setting. The operator needs to be aware of the effect of the probe in case automatic probe sensing doesn t work. My experience has been that it can t be trusted. This is probably due to the fact that probes get mixed up and new instruments don t automatically sense old probe types. The probe shown in Figure 1 is typical of a 10x probe. The probe is essentially a 9MΩ series resistor paralleled by a small variable capacitor. An adjustment hole on the probe accommodates a small flat screw driver blade for capacitor adjustment. The instrument provides a connector which outputs a calibration square-wave with accurate amplitude and very fast transitions. The probe capacitor is adjusted while watching the display of the square wave as shown below: C too high C too low C just right The probe capacitance compensates for the effect of scope input capacitance and probe cable capacitance that forms an RC time PROBE TIP PROBE R1 9MOhm 9MOhm C1C2 SCOPE INPUT C1 C2 30pF 30 pf 1MOhm Page 3 of 6

4 constant with the 9MΩ series probe resistor. The probe/input circuit is shown in thee figure on the right. The circuit is analyzed for balance like a bridge. The capacitive reactance ratio should equal to the resistor ratio for balance. Xc 1 Xc 2 = R1 1 2π fc 1 = 2π fc 2 1 2π fc 1 =C2 C1 = R1 2π fc 2 C 2 = C 1 R1 =30 pf 1M Ω =3.33 pf 9M Ω Digital Sampling Oscilloscope Figure 2. Keep in mind that the digital oscilloscope was initially introduced to replace analog oscilloscopes, providing the same functions but with waveform memory in microchips. While the analog scope captured and displayed input signals continuously, the DSO samples the input signals with a fast A/D converter. After sampling, the DSO stores the samples in semiconductor memory from which the display can read them out and present them on a screen. Most DSOs today use a liquid crystal screen with a high pixel density. Pixel means picture element. LCDs displays are not as bulky or heavy as CRTs, do not require lethal high voltages, and use only a fraction of the power that analog scopes need so they can run off batteries. LCD screens are very good at displaying text information as well. The disadvantage of LCD displays is that they require external lighting or else screen background illumination. Some DSOs intended for workbench use employ modern raster-scan CRT displays. These displays are bright and have good resolution, but also require high-voltage electron guns and are heavier than LCDs. The horizontal and vertical raster scanning is done like a TV picture tube rather than like an analog oscilloscope. Since a DSO is a programmable digital device, a text area at either the bottom of the screen (Agilent) or the extreme right of the screen (Tektronix) provides labels for buttons. The buttons are multi-use; that is they represent different functions determined by the mode set by the operator. The screen-edge labels indicate the button functions. This is a significant advance over an instrument that requires panel Page 4 of 6

5 space for each control function. DSOs generally duplicate all the functions of analog oscilloscopes and add many others. As with analog scopes, DSOs with wide bandwidth and faster response have higher cost. Math In addition to displaying waveform traces that look like analog scope traces, DSO can easily perform calculations on the stored digital information. It is simple to display parameters like peak-to-peak amplitude, RMS amplitude, period, and frequency calculated from the currently displayed record. Mid-trace Triggering A DSO can capture a single record of input samples, say 10 microseconds of data, and hold it on the display indefinitely. There is no persistence fading as with analog scopes since the trace is held in memory. DSOs are perfect for capturing the rare event, something an analog scope could not do at all. Another helpful feature is that the instrument can take and store samples continuously but only display them when the trigger level is exceeded. This means that it can retain and display samples taken before the trigger. When triggering on a glitch caused by some kind of circuit fault, it is very useful to see what happened just before the glitch occurred. The default position of the trigger is usually in the center of the display but can be moved to either the start or end of the display. Low Frequency Displays Analog scopes depend on signal repetition to continuously refresh the screen phosphor in order to see the signal. If phosphor persistence is too long, the display of very high frequencies becomes smeared. If phosphor persistence is too brief, the display of low frequency signals fades on the left before the trace makes it all the way across the screen. The display of rare events is impossible. Since a DSO stores a digital record of the trace and then displays it, it can easily trigger on glitches that occur infrequently and display the signal both before and after the glitch. In my opinion, this ability is the most useful feature of DSOs for troubleshooting circuits. Not only can the rare event be captured, it can be saved on a floppy disk. Archival Storage Most DSOs have floppy disk drives that permit waveform records and scope configurations to be saved and recalled at a later date. For example, a display screen can be written to a floppy as a.tif file that can be loaded from the file as a picture into a Word document and printed. Another floppy use is to store instrument configuration. A DSO has so many functions and options, accessed via multi-use buttons and knobs, that it is difficult and time consuming to set it up for a particular configuration. This problem is alleviated by the ease with which a set-up configuration may be saved on a floppy disk and reloaded for a later session. Downside to DSOs DSOs are wonderful instruments; light, rugged, packed with features like floppy disk storage of signal records and measurements. There are several downsides however. Cost is one. DSOs are more expensive than simple workbench analog oscilloscopes. Another downside is an artifact of sampling a signal. The operator must have some knowledge of sampling theory to avoid display problems. Sampling theory states that the sample rate of a system must be at least twice the highest frequency in the signal. Put another way; there must be no signal energy at a frequency higher than half the sample rate. This is due to aliasing, the technical name for energy fold-over. Page 5 of 6

6 Any signal energy that is above half the sample rate gets folded back and interferes with the desired signal. Usually, systems employ a sampling rate about 8 times higher than the highest signal frequency component just to be absolutely sure there will be no aliasing. It is mandatory that before sampling a signal, it should be band limited by a filter to suppress any frequencies that might alias. You may see some audio products offered with 8 or 16 times over-sampling. This specification is about signal purity. Because a DSO does not have unlimited sample storage, it cannot sample at the highest rate for very long before it runs out of memory. It depends on the time base setting to adjust its sample rate. It is possible to under-sample by viewing a high frequency signal with a low time/division display (hence low sample rate). When you do this what you see is not a true representation. Even knowing this, I ve been caught by it several times, mistaking the waveform displayed for a true representation. Another DSO problem also requires an alert operator. Since the DSO display screen is reading stored waveform samples from memory, the display does not necessarily blank out when the waveform trigger is lost as with an analog scope. The display may continue to show the last set of samples taken before the trigger went away. This can deceive an operator, who glances at the screen and thinks he is seeing a current picture, when the actual waveform may be a constant zero volts. The Agilent lab oscilloscopes will blink the Trig d label at the top of the screen if there is a loss of triggering. Page 6 of 6