Index

Measuring and testing

    Measuring instrument interfaces

    IEEE 488 is propably the mostly adopted communication bus and protocol used in electronic measusing equioment. In 1977, the IEEE adopted the bus structure and communication protocol that it named IEEE 488. Some others call it GPIB (general-purpose instrumentation bus). The bus's original name was HPIB (Hewlett-Packard instrumentation bus). Until the advent of the HPIB, no standardized methods existed for interfacing instruments with computers. IEEE 488 remained for more than two decades the industry's primary standard for enabling instruments and computers to talk with one another.IEEE 488 standard did a good job of defining the communications hardware, it initially gave short shrift to interfacing's software aspects. More than a decade elapsed before the evolution of the necessary software standards, particularly SCPI (standard commands for programmable instruments).IEEE 488 was not the only interface used. RS-232 ports have became popular on slower instruments. The two top contenders for the instrument-interfacing standard of the future are Ethernet and USB. You can find one or both in many instruments. Scopes that offer communication ports other than IEEE 488 are becoming increasingly common. The current and most likely future leader in replacing IEEE 488 is Ethernet. USB will also play a major role. The most obvious reasons for turning to computer-standard interfaces in place of IEEE 488 for instruments are cost, size, cable length of instrument networks, and increasing difficulty of installing specialized peripheral controllers in newer PCs. For test instruments, an advantage of an Ethernet connection over a USB or IEEE 488 connection is Ethernet's much greater allowable cable length. Ethernet LANs.even using gigabit-per-second Ethernet technology.can span thousands of feet. USB and IEEE 488 are limited to tens of feet. Don't be fooled by the new protocols' high nominal bit rates; instrument interfacing usually involves short messages. In such service, IEEE 488 can be significantly faster than protocols that at first appear to be much faster than IEEE 488.Using an instrument as a Web server is a new aspect in interfacing. Web-server technology is particularly well-suited to instruments that connect to Ethernet networks and that use TCP/IP (Transfer Control Protocol/Internet Protocol).

    Oscilloscopes

    Every scientist, engineer, and technician involved in any form of electronics has used an oscilloscope. Scope displays of amplitude as a function of time provide intuitive and easily interpreted pictures of signals. Oscilloscope is one of the most important test instruments foravailable engineers. It is useful for very many electronics measurement. The main purpose of an oscilloscope is to display the level of a signal relative to changes in time. You can use an oscilloscope to analyze signal waveform, get some idea of signal frequency and many other details.Scopes are ment for looking at the qualitative aspects of the signal (like signal waveform, esitence of signal, etc.).

    For making quantitative measurements, a scope is "usually" a bad choice (for example multimeter is more accurate tool to measure DC voltage levels than a scope). It is quite typical for the scope to be out by a percent or two or three but if you're counting on that kind of accuracy, you're using the wrong tool. Deviations as high as ~3% or more are considered "in-cal", and in uncalibrated scopes this can be much worse.

    Traditional oscilloscopes used a CRT screen and were completely analogue devices. Those analogue oscilloscopes are still very usable devicesnowadays. Analogue oscilloscopes work very well as general testing instrumentfor viewing repetitive signals. Many simple and cheap analogue oscilloscopes have typical bandwidth of 20 MHz. Some better ones go to 100 Mhz or higher in bandwidth. Even a 20MHz analogue scope will produce some response at a higher frequency but of course it will be at a lower level because it is outside of the calibrated specified bandwidth.

    Digital oscilloscopes are digital versions of that analogue instruments. Digital oscilloscopes sample signals using a fast analog-to-digital converter (ADC). The digitized signals aresotred to the scope memory and shown on the scope screen or at computer screen. The benefit of the digital technology is thatthe waveforms can be captured to memory and then analyzed, immediatlyor later, in many ways. Digital oscilloscopes can be used to capturerepetitive signals as well as transient signals.

    Oscilloscope bandwidth is generally listed as the -3-dB point in oscilloscope frequency response. Traditionally, oscilloscopes have exhibited a Gaussian frequency response. A Gaussian response results from the scope design's combining many circuit elements that have similar frequency responses. Analog oscilloscopes achieve their frequency response in this manner, thanks to chains of amplifiers from the input BNCs to the CRT display. (Analog oscilloscopes used the input signal to directly deflect the electron beam in a CRT. This architecture required amplifying the input signal by three orders of magnitude and driving the large capacitive load that the CRT deflection plates presented.) The properties of Gaussian-response oscilloscopes are fairly well-taught and well-understood throughout the industry. In a Gaussian-response oscilloscope, the oscilloscope's rise time is related to the oscilloscope's bandwidth by the familiar and commonly used formula, rise time=0.35/bandwidth. (Rise time is measured from the pulse's 10 to 90% amplitude points. Bandwidth is defined as the frequency at which the response is down 3 dB relative to dc. The theoretical relationship for a Gaussian system is rise time=0.339/bandwidth, but the industry has settled on 0.35/bandwidth as a practical formula.) Another commonly used property of Gaussian systems is the overall system bandwidth, which is the rms value of the individual bandwidths. You can calculate it using the familiar relationship, system bandwidth=1/(1/BWPROBE2+1/BWOSCILLOSCOPE2)0.5. "System bandwidth" refers to the bandwidth you achieve with a combination of an oscilloscope probe and oscilloscope. Oscilloscope probes are often designed to have sufficiently higher bandwidth than the oscilloscope bandwidth, so that the above formula is usually not necessary.

    Most oscilloscopes are built so that the signal input connector is BNC connector. The input impedance in the connetion is typically around 1 megaohm in typical normal oscilloscopes and 50 ohms in many high speed oscilloscopes (check what you have from scope manual). The connector ground side (outer shield) is normally connected to the equipment case ground which is generally wired to mains ground through mains connector. This means that the grounds of all channels are genrally connected together and then wired to mains ground (unless you power your scope through safety isolation transformer which isolated your scope from ground). Oscilloscopes are intended to be operated with their chassis at ground potntial. There are good technical and safety resons for this. If you are measuring some mains powered device, it is a very good idea to power the device through an isolation transformer. When working with mains powered equipment, the equipment you measure should be isolated from mains voltage for safety reasons.

    When doing the meausrement the right grounding is important for meaningful results. A good oscilloscope probe has a removeable ground lead, that allows the user to ground it to circuit board or not depending on what is needed in that specific meaurement. In general case the measurements are made better and more accurate with the ground lead connected. If you do not connect the ground lead then the display will show allthe noise the probe cable picks up (cable acts like antenna that picks up noise nearby). If you want rid of this you connect the ground lead to the low of the circuit you are trying to monitor. The oscilloscope ground lead will eventually find its way back to the mains earth of the oscilloscope.If you are trying to make measurements, you must have a reference against which to measure. Without that, "Pissing against the wind" comes to mind, as acomparison. There are some potential dangers when the circuit ground is at a potential with respect to oscilloscope ground then current will flow in the oscilloscope through the measuring cable shield. If the potential on the circuit is direction connection to mains then there will be a bang and possibly some damaged measuring hardware / circuit. Remedies are:

    • a) Double insulated oscilloscopes with no ground connection
    • b) Battery powered osciloscopes
    • c) Differential input oscilloscopes
    • d) Differential input adapter for your oscilloscope
    • e) Isolating transformers
    Using the option e is by far the cheapest and most commonly used, although not always the best. There are also some special oscilloscopes (expensive ones) with inputs that are not connected to ground (usually referred as differential inputs). This kind of scope can be safely connected to almost any electronics circuit. You can get the same performance with a normal scope also if you use a differential proble connected to a normal oscilloscope. In some cases the battery powered small oscilloscopes are very handly because those devices are completely floating.

    If you want to make accurate measurements, you need to have your oscilloscope calibrated. A calibrated scope will allow you to make considerably more accuratetime/voltage measurements, will show square waves as true step-functions(even at the highest sweep rates) and not some sort of distortedrepresentation, and most importantly it will trigger reliably on signals.There's a whole lot of difference between a calibrated and un-calibratedscope, but you wouldn't usually know it unless you have a source of precision calibration signals to compare against. Once calibrated, an instrument should be re-calibrated within 2-3 years since the adjustments can in fact vary a surprising amount over time (the time interval could vary somewhat depending on scope type and needed calibration accuracy). A scope requires significantly more maintenance than simpler measurement instruments like a multi-meter or signal generator. CRT based oscilloscopes are complex instruments. Much more complex than almost any other piece of test instrumentation and the circuitry is not selfadjusting (for the most part). Most common analog oscilloscopes require a fair amount of specialty calibration equipment and a thorough calibrationtakes at least 1/2 day and often longer (there can be up to 50 separate adjustments tha can be made on older scope, this is labor intensive process to get them right). Most scope problems are revealed in the calibrationprocedure in which the tech can choose to either ignore or repair. Sometimes the repairs are trivial, sometimes not. Becauses the cost of maintaining older oscilloscopes accurately many so-called "working" units find themselves on the surplus market.

    The oscilloscope probe used to establish a connection between the circuit under test and the measuring instrument. A probe can be any conductor used to establish a connection between the circuit under test and the measuring instrument. This conductor could be a piece of bare wire, a multimeter lead or a piece of unterminated coaxial cable. These "simple probes," however, do not fulfill the essential purpose of a probe; that is, "to extract minimal energy from the circuit under test and transfer it to a measuring instrument with maximum fidelity." There are many different kinds of probes that suit to different applications:

    • The bare wire can load the input amplifier with its high capacitance and inductance or even cause a short circuit; multimeter leads are unshielded and are often susceptible to stray pickup
    • The unterminated coax will severely capacitively load the circuit under test (100 pF per meter typically). Also, the unterminated coax is usually resonant at certain frequencies and does not allow faithful transfer of the signal to the test instrument due to reflections.
    • A simplest probe type is is "x1" probe that just consists of probe tip, grounding conductor and low capacitance coaxial cable to the oscilloscope. Typically the oscilloscope at probe setting "x1" it loads the circuit being measured with the full capacitance of probe + probe cable + oscilloscope input. The unterminated coax will severely capacitively load the circuit under test. Typical capacitance of "x1" probe is tens of picofarads. For DC measurements the input resistance is the same the resistance of the oscilloscope input (typically 1 Mohm on traditional CRO-type oscilloscopes, 50 ohm on some high frequency models).
    • Attenuating Passive Voltage Probes are the most commonly used probes today. The "x10" setting gives you reduced sensitivity and reduced capacitace (the load capacitance is around one tenth of "x1" setting). This means a typical input capacitance of around 15-20 pF. The 10X passive voltage probe presents a high impedance to the circuit under test at low frequencies (approximately 5 MHz and lower). Their main disadvantage is a decreasing impedance level with increasing frequency (i.e., high input capacitance).
    • FET probes include active components (field effect transistors or other active devices) rather than passive components. The FET input results in a higher input impedance without loss of signal, i.e., low input capacitance (typically less than 1 pF) and high input resistance values (typically higher than 20 kohms). Since FET probes have a 50 ohm output impedance, they can drive a 50 ? cable so they can be long cables between the probe and oscilloscope. Downside of FET probes are that they are typically expensive and need operating power to work (either supplied by oscilloscope using properietary methods or powered with batteries).
    • Several high voltage probes are available, and they typically provide 100X or 1000X compensated dividers. Because of the larger attenuation factors required for high voltage applications, the input capacitance is typically reduced to approximately 3 pF.
    • 50 ohm Divider Probes provide the lowest input capacitance (typically less than 1 pF for high frequency signals) and are used with high frequency, 50 ohm input scopes. The simplest 50 ohm divider probe consists of just one 1 kohm or 2.2 kohm resistor that is placed between the signal connection on the circuit and the 50 ohm ciaxial cable going to the oscilloscope.
    • Current probes provide a method to measure the current flowing in a circuit. Two types of current probes are available, the traditional AC only probes and the "Hall Effect" semiconductor type. AC only current probes use a transformer to convert current flux into AC signals. Combining a "Hall Effect" device with an AC transformer provides a frequency response from DC up to many MHz range. Because of its "non-invasive" nature, a current probe typically imposes less loading than other probe types. The AC current probes can be just passive devices, while the models with "Hall Effect" device need some operating power (typically provided by local battery on the probe).

    Proper probe selection will extend and enhance an instrument's performance, while imprudent probe selection often reduces your system's performance. When making measurements make sure not to exceed the maximum allowable input ratings of the oscilloscope input ports. This will prevent costly damage and provide reliable measurements. Rememeber also not to exceed the input voltage ratigns of oscilloscope probes as well, because this can damage the probes and cause severe safety risk to the person using those probes.

    A proper oscilloscope probe grounding is essential requirement to get meaningful measuring results with normal oscilloscope probes. The measured the current must always form a loop. The signal beign measured cannot exit the measured circuit and go to the oscilloscope input without having a path through which it may return. If you are measuring a "floating" circuit, then the return would go through a parasitic capacitance directly between the oscillator and the scope. This capacitance varies depending how the devices are positiones, which means that the position of the probe cable will have an effect on the shape of the signals you see on the scope! Another nasty artifact of a no-ground probe arrangement is the resonance associated with the combination of the rather large inductance of cable, and the input capacitance of the probe. This resonance is called a probe resonance and can cause considerable measurement errors. A short, explicit ground connection made between the scope ground and the equipment under test shunts those capacitances and inductances, eliminating their influence on the measured result and pushing the probe resonance up and out of the band of interest. All good probes come with short, tiny ground attachments to prevent such problems. For single-ended measurements, don't depend on mysterious ground connections. Always use a good, short ground connection.

    Oscilloscopes are used for very many different kind of measurements. In telecommunication and data communications applications you can often see results of eye diagram and eye pattern measurement. An eye pattern is an oscilloscope display in which a pseudorandom digital data signal from a receiver is repetitively sampled and applied to the vertical input, while the data rate is used to trigger the horizontal sweep. System performance information can be derived by analyzing the display. An open eye pattern corresponds to minimal signal distortion. Distortion of the signal waveform due to intersymbol interference and noise appears as closure of the eye pattern.

    Many modern digital oscilloscopes allow you to show you signal waveforms and even store the recorded signal for later inspection. Old analogue oscilloscopes lacked the ability to store the picture on the screen, unless you took a picture of the screen with a normal film camera (not very convient, camera settings needs to be right). If you happen to have an old analogue oscilloscope and need to store the waveform on the screen, then you might be able to use modern inexpensive digital camera connected to computer instead of old traditional film camera. You can for example have an usb pc camera mounted on a tripod at the?oscilloscope screen, focus close for a sharp picture,?camera output cable into the USB port. With the bundled software installed on your computer (Windows 98se, 2000, or never), you can view the image on your computer screen and save the image on the oscilloscope screen to you hard disk (for example to be included to your laboratory documents later). You see it all in real time (well almost...) and if you are recording it all as well, then you have the option of playback, editing and splicing the info/displays later for whatever purpose?or archiving etc.?It work, usually well. This could be an useful trick for those technicians out there with limited funds and equipment. Digital cameras and webcams are nowadays quite cheap compared to a modern digital oscilloscope.

      Differential measurements

      Most oscilloscopes can perform only single-ended voltage measurements; that is, measurements of signals referenced to earth ground. Wiring within the probe connects the probe's reference lead to the shell of the BNC. When you plug the probe into the scope, the reference lead becomes electrically common with the scope's chassis. The power cord's ground conductor connects the chassis to earth ground. In most oscilloscope applications the inability to make anything except single-ended measurements poses no problems. But oscilloscopes' single-ended inputs present challenges when you try to view signals that are not referenced to ground. A common example is the voltage across the switching device in an off-line switching power supply. Another type of signal that you must measure differentially is a balanced signal.

      Simplest way of doing differential measurements is to use two normal 10X probes conencted to two oscilloscope inputs and the "minus" operation to show the difference of signals between them. The normal 10X probe has a typical accuracy of ?1% and gives a differential measurement accuracy (when using two probes) of two parts per 100. Using this 10X probe, the common mode rejection ratio of a scope and probe combination would be no better than 50:1.

      True differential measurements are safe and accurate way to measure signals that are not ground referenced. To make those measurements you need a differential probe. Unlike a conventional scope probe, a differential amplifier ijn differential probe has an input that is only implicitly referenced to ground. As the name implies, a differential measurement produces a waveform that represents the difference in voltage between the two inputs. Ground does not enter into the measurement.Differential amplifiers ignore potentials that are equal in amplitude and phase and appear on both inputs. This characteristic is known as "common-mode rejection" (CMR). An ideal differential amplifier totally rejects the common-mode component.The other key feature of a differential amplifier is balanced input impedance (both inputs have identical impedance to ground, typically high impedance). A true differential probe has typically adjustments and electronics to provide common mode rejection ratios of 10,000:1 and higher.

      Building oscilloscope probes

      A probe can be any conductor used to establish a connection between the circuit under test and the measuring instrument. This conductor could be a piece of bare wire, a multimeter lead or a piece of unterminated coaxial cable. These "simple probes," however, do not fulfill the essential purpose of a probe; that is, "to extract minimal energy from the circuit under test and transfer it to a measuring instrument with maximum fidelity." Attenuating Passive Voltage Probes are the most commonly used probes today. They provide a convenient and extremely rugged, yet inexpensive, way to acquire signals from your device under test. FET probes include active components (field effect transistors or other active devices) rather than passive components. The FET input results in a higher input impedance without loss of signal.

      RF and EMI probes

      RF probes allow you to examine high frequency RF signals (much higher that your scope frequency response) on your oscilloscope screen. The RF probes generally form a some kind of rectifier / peak sampler, which allows you to see the signal strenght as the signal which connects to scope input. This allows you to quite easily measure signal amplitude and look at the moduleation (AM modulation). Rapidly changing voltages and currents in electrical and electronic equipment can easily result in radiated and conducted noise. Electromagnetic interference (EMI) can be difficult to locate and correct in electronic equipment. A miniature EMI "sniffer probe" and an oscilloscope can help to locate and identify magnetic-field sources of EMI. Typical EMI probles consist of some form of elecrical field sensing circuit (voltage proble) and some form of small coil (H-field probe).

      Video signal measurement accessories

      Typical oscilloscope does not usually sync well enough to video signal to be as such a convient instrument (when compared to special videomeasurement tools). With suitable accessories (usully special sync circuits), a normal oscilloscope can be used as a very nice video signal analyzing instrument.

      Building measuring accessories

      • Calibrate scope jitter using a transmission-line loop - Digital-clock-period jitter is the variation in the period of a clock cycle compared with a nominal (average of many cycles) clock period. To accurately measure period jitter using an oscilloscope, you must subtract the oscilloscope jitter from the measured jitter. However, oscilloscopes rarely have a jitter specification, so you must determine the oscilloscope jitter. One method of measuring oscilloscope jitter is to use the oscilloscope to measure the jitter of a pulse generator with known jitter. The ideal generator for measuring oscilloscope jitter would have zero jitter. This article shows a circuit for generating a calibration signal with near-zero timing jitter.    Rate this link
      • Coax connectors make low-cost test pieces - you can construct low-cost small test pices like filters, attenuators and terminators using coaxial panel jacks without pc boards or enclosures, design idea from    Rate this link
      • Counter Circuit Improves Oscilloscope Triggering - this prescaler circuit, when plugged into the scope's external trigger input, can provide reliable, low-jitter triggering for both older and modern oscilloscopes    Rate this link
      • Delay line upgrades vintage scope - Vintage triggered-sweep oscilloscopes find use in many applications. However, they have no internal delay line, so they can't display the pulse that triggers the sweep. Moreover, early laboratory scopes contain delay lines having insufficient delay to display such pulses during a uniform portion of the sweep. With such oscilloscopes, the true pulse shape remains a mystery. You can circumvent these limitations if you add an external delay line and equalizer. The scope can then display the exact trigger-point trace. The instrument then becomes easier to use, and the measurements become more trustworthy.    Rate this link
      • Matching pads - This article describes some impedance matching circuit for measurements.    Rate this link
      • Multiplexer creates mixed-signal scope input - using two multiplexer ICs and some TTL logic), you can view eight analog or digital (or some of both) signals on the oscilloscope    Rate this link
      • Simple circuit provides timebase calibration - inexpensive and quick way to check the timebase speeds and linearity in vintage oscilloscopes    Rate this link

    Using PC as a measurement instrument

    In those early years of computer-based measurement and automation, the desktop computer, linked by the General Purpose Interface Bus (GPIB), played an auxiliary role; however, the increasingly powerful PC has changed all of that. Today, the PC can acquire, analyze, and present data at increasing frequencies, resolutions, and sampling rates.In the dim and distant past, engineers recorded measurements with pencil and paper - a slow and error-prone method. Today, 20 years after the introduction of the IBM PC, two types of instruments - inboard and outboard - take measurements and move data into a host computer. PC technology has become the backbone of automated test and measurement systems.Today virtual instruments are superseding the traditional kind by revolutionizing how measurements are made and the data shared. History of virtual instrumentation began over 15 years ago as PCs started coming into use in test and measurement as instrument controllers. The PC is now the most powerful and cost-effective approach to building instruments. Virtual instrumentation leverages the power, flexibility, and programmability of the computer and thus brings a wide variety of benefits. Laptop computers have further encouraged this trend with a form factor ideal for many portable applications. Even a basic normal modern PC can be used to do many different kinds of measurements with no extra hardware. The soundcard found in most PCs can be used for various applications, althrough those applications are limited to audio frequencies and have usually quite limited absolute accuracy (PC soundcards are not designedprecise calibrated measureemnt instruments). With suitable software and soundcard you can use your PC as a signal generator that gan generate different waveform signals. You can generate practically any waveform (within audio frequnecy band limits) if you use some suitable sample editor software or mathematics software to generate the signal waveform and then play it out through soundcard.With suitable software a PC with a soundcard can be used as a multi-purpose audio frequency signal analyser. You can for example use PC as audio signal oscilloscope, VU meter, spectrum analyzer, frequency response analyzer. PC can also used as a very convient recording device that can record and play back any audio signal.There are also special measuring instruments that can be connected to PC to expand it's capabilities. There are varieties that connect to PC bus or some PC interfacing port (like parallel or serial port). The oscilloscope products that connect to PC through a slow port(serial, parallel etc.) and can sample at high rates are generallyimplemeted in the following way:The device has a buffer memory in it. When the device starts sampling(manual start or automatic trigger), it then samples it's memory fullat the given sample rare. After the data is sampled to memory itis stransferred to the PC. And the process can start all over.What comes to the software that controls commercial PC based measurign instruments there is one software that is more popular than anything else in the field: LabView from National Instruments. Agilent has it's own VEE software competing on the same field. There are also measuring instrument manufacturer specific control software that is supplied with the instruments.

    Transmission line measurements

    There are applications where you need to measure long cable lines that are used as transmission lines for various signals. There are many techniques related to transmission line measurements, because there are various factors that needs to be measured. Most commonly measured transmission line characteristics are the following:

    • Conductor and shield resistance
    • Insulation resistance
    • Capacitance between wire pairs and/or between conductor and shield
    • Characteristic impedance
    • System impedance mismatch (return loss)
    • Line attenuation
    • Amount of noise coupled to line

    Let's say you have a long cable with a problem. Part of the cable is buried under ground, some of it runs through walls and floors. You measure one end of the cable with an ohmmeter, and it reads about an ohm. So the cable is shorted. Hoping for the best, you cut off the connector and measure just the cable. Still reads about an ohm, so the short is somewhere else along the cable. But where? If you could locate the short, you could save a lot of time and money by repairing just that one spot, rather than pulling in a whole new cable. TDR to the rescue! You can use Time Domain Reflectometry to look at the characteristic impedance along the entire length of the cable.

    Cables used to carry high frequency electrical signals are generally analysed as a form of Transmission Line. The amount of capacitance/metre and inductance/metre depends mainly upon the size and shape of the conductors. The Characteristic Impedance depends upon the ratio of the values of the capacitance per metre and inductance per metre. To understand its meaning, consider a very long run of cable that stretches away towards infinity from a signal source. The result, when the signal power vanishes, never to be seen again, is that the cable behaves like a resistive load of an effective resistance set by the cable itself. This value is called the Characteristic Impedance, of the cable.

    Return loss (RL) is a measure of the reflected energy caused by impedance mismatches in the cabling system. Reflections create an unwanted disturbance signal or "noise" on the cabling link that potentially interferes with the reliable transmission over the link. As a noise source, return loss is measured and evaluated to assure that the reflected signal energy is sufficiently small in reference to the transmitted signal such that the reliability of the transmission is not negatively impacted. Return loss is an important characteristic for any transmission line because it may be responsible for a significant noise component that hinders the ability of the receiver when the data is extracted from the signal. It directly affects "jitter." Return loss is one number which shows cable performance meaning how well it matches the nominal impedance. Poor cable return loss can show cable manufacturing defects and installation defects (cable damaged on installation). With a good quality coaxial cable in good condition you generally get better than -30 dB return loss, and you should generally not got much worse than -20 dB.

    Return loss is especially important for applications that use simultaneous bidirectional transmission. Opens, shorts or less-severe impedance discontinuities have a way of showing up on cables in strange places - places you might never suspect. These can occur on coaxial transmission lines or twisted-pair lines. Such opens, shorts or other impedance discontinuities are called faults. The location of faults cannot be determined with simple ohmmeters. Even the existence of certain faults cannot be determined with an ohmmeter. Time domain reflectomer is an instrument often used ot locate such faults.

    Time Domain Reflectometry measurements (sometimes called Time Domain Spectroscopy techniques) work by injecting a short duration fast rise time pulse into the cable under test. The effect on the cable is measured with an oscilloscope. The injected pulse radiates down the cable and at the point where the cable ends some portion of the signal pulse is reflected back to the injection point. The amount of the reflected energy is a function of the condition at the end of the cable. If the cable is in an open condition the energy pulse reflected back is a significant portion of the injected signal in the same polarity as the injected pulse. If the end of the cable is shorted to ground or to the return cable, the energy reflected is in the opposite polarity to the injected signal. If the end of the cable is terminated into a resistor with a value matching the characteristic impedance of the cable, all of the injected energy will be absorbed by the terminating resistor and no reflection will be generated. Should the cable be terminated by some value different from the characteristic impedance of the cable the amount of energy reflected back to the cable start point would be the portion of the pulse not absorbed by the termination. Also any change in the cable impedance due to a connection, major kink or other problem will generate a reflection in addition to the reflection from the end of the cable. By timing the delay between the original pulse and the reflection it is possible to discern the point on the cable length where an anomaly exists. The cable type governs this signal propagation speed. For example normal Category 5 cable propagation speed is 66% the speed of light, and for most coaxial cables this value is between 66% and 86%.

    Other cable characteristics are usually easier to measure and can be done with more conventional instruments.

    Cable conductor resistance can be measured in installed cable by shorting the cable on one end (short center wire to shield on coax, short two wires in wire pair on twisted pair cable etc.), and then using a multimeter on the other end to read the resistance value.

    Cable capacitance can be measured with a capacitance meter by leaving one end of the cable not connected anywhere (all wired free) and connecting the meter to the other end of the cable.

    Cable insulation is typically measured with an insulation resistance meter. The cable is typically not connected anywhere (or connected to equipment that do not cause error in measurement and do not get damaged by measuring). Insulation resistance meter typically applies some quite high voltage DC (125V, 250V, 500V, 1000V) to the line between two wires and measure if there is any leakeage. The leakage current is measured and the result is converted to resistance (usually in megaohms to gigaohms range). The measuring voltage needs to be selected based on the ratings of the wiring (and equipment if such are connected). Low voltage telecom wiring and similar is typically tested with 125V or 250V voltage. Higher voltages are usually used when testing the insulation on the mains power carrying cables and some radio transmitter coaxial cable systems. The measurin voltage needs to be right for the intended application. Too low voltage might not reveal insulation problems, but too high voltage can damage wiring and equipment connected to it.

    Line attenuation can be measured by connecting the signal source used in the application (or test instrument generating suitable signal) and signal receiver on other end (receiving equipment or terminating resistor). Then you just mesure the signal level on the transmitting and receiving ends (using a suitable multimeter or oscilloscope or similar instrument). The difference on those tells how much the cable attenuates the signal. In some applications you need to do measurement with different frequencies, recording how cable attenuates on different freuqncies. Some cable TV system measurements use a wideband noise source as the transmitter and a spectrum analyzer as the receiver (difference on the signal spectrum on the transmitting and receiving ends tells the attenuation on different frequencies).

    Amount of noise coupled to the line is measured with the indended equipment or suitable line terminators connected to the ends of the cable. If you use equipment they need to be turned off so that they do niot send anything to the line. Any signal that is now measured on the line is the amount of coupled noise.

    Cable wiring testers

    Proper testing of wiring system after installation is essentialto guarantee good operation later. The cabling system needs to bemeasured after installation and the results of those measurementsshould be documented for later use. Measurement is also usefulduring use when cabling problems are suspected.The most common cable fault is an open circuit, usually due toproblems close to or at the ends of the cables. A simple ohm metertest generally suffices. For multiplair cables where cable ends are many wires inside, a simplemultimeter is bothersome. For those applications multi-pair cabletestes which find showrt circuits and broken wires are a good choise.In some application you need to measure the cable length. Dependingon the cable characteristics you know and the measuremenet instrumentsyou have, you can use a multimeter (resistance measurement), RLC meter(capacitance measurement). time domain reflectometer (pulse tesing)or signal ateenuation testing (signal source and level meter)to measure the lenght of the cable you have installes somewhere.

      Multi-wire cable testers

      Engineers have long known how to test a cable for continuity by simply connecting all conductors in series and checking with an ohmmeter. This method is sometimes impractical, however, because it cannot check for short circuits (or you need to make very many test to measureresistance between very many wire combinations). To solvel thos problem on multi-conductor cables, there are specialcable testing instuments designed for this.

      • Cable tester is fast and cheap - This simple microcontroller based cable tester verifies the correct wiring of the cable, up to 8 conductor cables.    Rate this link
      • Simple method tests cables - Engineers have long known how to test a cable for continuity by simply connecting all conductors in series and checking with an ohmmeter. This method is sometimes impractical, however, because it cannot check for short circuits. This simple method solves the short-circuit detection problem. Connecting LED indicators at each shorting loop provides a visual indication.    Rate this link

      Cable test tone senders

      • How to Build a Signal Tracer and Injector - This audio signal tracer/injector will undoubtedly prove to be very useful for many routine servicing operations. The unit consists of an audible signal monitor for "listening" to the signals present in an electronic device (such as an audio system, receiver, amplifier, or tape deck) at circuit points inside these devices. It also includes an RF detector probe and signal generator.    Rate this link
      • Microphone Circuit Test Oscillator - 440 Hz tone generator for testing XLR microphone lines    Rate this link

    High voltage measurements

    DMMs may not be particularly forgiving of voltages on their inputsexceeding their specifications. You need special tools and proceduresto successfuly and safely measure high voltages.A simple high voltage probe for a DMM or VOM may be constructed from a pair ofresistors. This kind of devices are sold as ready made devices(for example Tektronix, Agilent and Fluke sell those).Follow safety precautions when working around high voltages.Usually some form of equipment protection should be considered whenworking with high voltages.

    Frequency measurements

    Frequency counter is a necessary instrument to check that certain circuit operated at thr right frequency. Frequency counter is an useful tool when you need to tune oscillators, measure some input signal frequency and when youplay with radio devices.Inexpensive frequency counters that will measure frequency well into the microwave range are available to the hobbyist today. A frequency counter is an excellent means of accurately determining the frequency of unknown signals, or to see if an oscillator or a multiplier stage in a receiver or transmitter is working. However, one must watch out as what is really being measured and exactly what the counter is "seeing".

      General information

      • Frequency Counter Measurement Techniques - Inexpensive frequency counters that will measure frequency well into the microwave range are available to the hobbyist today. A frequency counter is an excellent means of accurately determining the frequency of unknown signals, or to see if an oscillator or a multiplier stage in a receiver or transmitter is working. However, one must watch out as what is really being measured and exactly what the counter is "seeing".    Rate this link

      Prescaler circuits

      Prescalers are circuit which are used to extend the meausrement range of other frequency measuring circuits. If you have for example a frequency coutner which can count up 10 Mhz, then with suitable prescaler circuit you can extend the measurement range to higher frequencies. For example suitable 1:10 frequency prescale woudl extend the measurement range to 100 Hz. And prescaler with higher division factor will enable you to measure even higher frequencies.

      • 3 GHz Prescaler - will take a 0.1 - 3 GHz signal and divide it by 1000 so you can measure frequencies outside the normal range of your frequency counter    Rate this link
      • VHF/UHF Prescaler - This prescaler is ridiculously simple. It consists of just one IC, a TV tuner prescaler, the Philips SAB6456A, which can divide by 64 or by 256. The device sensitivity is about 10mV RMS over the range 70 - 1000 Mhz, and the output is typically 1V p-p. The input resistance varies from 560 down to 30 Ohms, and the input capacitance, excluding the PCB, no more than 5pF.    Rate this link

      Frequency to voltage conversion

      Frequency to voltage converson allows you to convert input signal frequency to a voltag signal which can be fed to a normal digital multimeter imput, moving coil meter or A/D converter. Frequency to voltage converters are not usually as accurate as real frequency counter circuits, but they are still useful in many applications.

      • Frequency to voltage adapter - in pdf format, text in Finnish    Rate this link
      • F/V converter has high accuracy - This high-accuracy frequency-to-voltage converter (FVC) demonstrates how a synchronous, charge-balance, voltage-to-frequency converter (VFC) can function as a single-supply FVC given proper biasing and level shifting.    Rate this link
      • Idea for a car tachometer - A tachometer is simply a means of counting the engine revolutions of an automobile engine. In this suggested idea a NE555 timer is configured as a monostable or one shot. The 555 timer receives trigger pulses from the distributor points. Integration of the variable duty cycle by the meter movement produces a visible indication of the automobiles engine speed.    Rate this link
      • Pulse period to voltage converter - This circuit converts a square wave input signal into a voltage proportional to the time between edges (period) of the signal, not the frequency, the range is from 100uS to to 10mS, which produces a voltage from 100mV to 10 volts.    Rate this link

    Audio measurements

    Audio volume is the most commonly measured audio signal property. VU and dB meters both measure the audio power involved in recording and they both use logarithmic scales to report that power. When measuring electrical signals the following is true:

    • VU is short for "volume units" and it is a measure of average audio power. A VU meter responds relatively slowly and considers the sound volume over a period of time. Its zero is set to the level at which there is 1% total harmonic distortion in the recorded signal.
    • dB is short for "decibels" and it is a measure of instantaneous audio power. A dB meter responds very rapidly and considers the audio power at each instant. Its zero is set to the level at which there is 3% total harmonic distortion.
    Because of these differences in zero definitions, the dB meter's zero is roughly at the VU meter's +8.

    When measuring electrical signals decibel is the difference (or ratio) between two signal levels; used to describe the effect of system devices on signal strength. A signal strength or power level; 0 dBm is defined as 1 mW (milliWatt) of power into a terminating load.

    When measuring audio signal power (vibrations in air) the following measurements are made:The decibel (abbreviated dB) is the unit used to measure the intensity of a sound. On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB.What does 0 dB mean? This level occurs when the measured intensity is equal to the reference level. i.e., it is the sound level corresponding to 0.02 mPa. In this case we have equation: sound level = 20 log (Pmeasured/Preference) = 20 log 1 = 0 dB

    Sometimes the amount if noise needs to be measured.Most typically harmonic distortion needs to be measured.Harmonic distortion describes a nonlinear property of systemswhere the output of the system has added energy at frequenciesthat are at integer multiples of the frequencies input to thesystem. The traditional technique is to input a single frequency F into the system under test, then take the output, apply a filter thateliminates F, and measure everything that's left over. This is usually done with a twin-T, high-G notch filter centered on F. The problem with such a technique is that it measures EVERYTHING that's left over: not only the harmonic products of F at 2*F,3*F, 4*F and so forth, but all noise, uncorrelated components( line frequency noise, RF interference) and so forth. Nowadays computer techniques can be applied where a more detailed analysis can be made (usually based on FFT methods) where harmonic and non-harmonic componentscan be identified.

    "Standard multimeters" are not usually good instruments for audio measurements. Measuring audio (music) voltages on an AC voltmeter will give meaninglessresults as the voltmeter measures the average, over a fairly longintegrating time. This means that the level indicated will depend totally onthe programme content of the CD being played."Standard multimeters" (digital or analog) also often have a poor frequencyresponse and are not very useful for audio work for this reason. Most multimeters are designed for AC power line work and DC measurements, so perfomance up to 50-60 Hz or little bit over it is enough. To make any meaningful measurement, you need to us a CD with single frequency tones, and, unless you know that the meter measures well at higher frequencies, keep