Test & Measurement

Type

There are three main kinds of oscilloscopes in use today: Digital Storage Oscilloscopes (DSOs), Mixed Signal Oscilloscopes (MSOs), and USB oscilloscopes. DSOs, as the name implies, store signal data digitally, and can do so for an indefinite amount of time. The storage abilities of DSOs make them ideal for capturing intermittent faults or observing signal change over an extended period of time, such as recording temperature variation over the course of a day.

MSOs have both analog and digital inputs, typically two or four analog channels and 16 or 18 digital channels. An MSO combines all the measurement features of a DSO with some of the measurement capabilities of a logic analyzer, and as such, is a wise choice for applications such as circuit debugging.

Portable and budget friendly, USB scopes have made quite the impact since first hitting the market about a decade ago. As USB scopes derive power from the USB drive of a PC or laptop, they carry the distinct advantage of immediate upload and transfer of data. Despite relying on the computer for processing and display, USB scopes can be comparable to many bench models in terms of signal analysis.

Bandwidth

Bandwidth refers to the maximum frequency the oscilloscope can accurately analyze. Technically, an oscilloscope can process signals of higher frequencies than its listed bandwidth, however, much of the waveform detail will be lost. To avoid this, multiply the maximum frequency in your signal by 5 to determine your minimum bandwidth requirements.

Sample Rate

When discussing oscilloscopes, the sample rate determines how much of the waveform detail is captured per second. The higher the sample rate, the better your scope will represent the signal under test. Typical sample rates range from 20 MS/s (megasamples/second) on a basic scope to upwards of 5GS/s for a mid-range, depending on the model. Sample rate is especially important when analyzing high speed digital signals. To ensure your scope is up to the task, as with bandwidth, follow the 'five times rule' and choose one with a sample rate at least 5x the maximum frequency of your signal.

Channels

The number of channels a scope can read at one time determines the types of analyses it can perform. Comparing the signal timings in your waveforms, for example, would be an easy task for a 2-4 channel DSO. However, when analyzing a mixed signal design where more than four signals need to be captured, an MSO with 2-4 analogue channels plus an additional 16-18 digital channels would be the instrument of choice.

Additional Features

After establishing your basic scope needs, it's time to think about extras. A scope with integrated spectrum analyzer, function generator, or synthesizer capabilities can handle a wide variety of tasks, while touch screen controls make for a more user-friendly interface. Various software bundles can further increase the flexibility of your device, and scopes that can execute advanced mathematics functions allow for more in-depth signal analysis. Bear in mind, however, that while additional features can mean additional convenience or functionality, they usually also mean additional cost. Fortunately, a wide variety of oscilloscopes are available from basic models suitable for student or hobbyist use, to high end models designed with the professional in mind.

Linear Power Supplies

Linear power supplies are relatively simple in construction and regulate voltage via a single, low-frequency transformer. The design simplicity of these power supplies means they have a fast response time, low noise generation, and excellent regulation. That being said, the larger electronic components of a linear design mean the supply itself can be bulky and ill-suited to a small work area. Because they are always dissipating power, the average efficiency of a linear power supply is 30-40%, with a maximum efficiency of 60% depending on the model chosen.

Switching-Mode (SMPS)

These power supplies run AC mains input through various filters, rectifiers, and switching transformers at high frequencies until the desired DC voltage is produced. The smaller, lighter, and less expensive transformers and capacitors necessary for working with such high frequencies mean a smaller, lighter, and less expensive device. In addition, the average SMPS has an efficiency of 75%, though in some models up to 95% efficiency can be achieved.

Before purchasing an SMPS, consider your voltage needs carefully, as some models will struggle to produce low voltages or may be unable to operate at all due to lack of adequate current. Refer to the manufacturer's specifications for voltage minimums. The main drawback of an SMPS is the electromagnetic interference (EMI) and noise resulting from the high frequency switching.

Channels and Features

Often, multiple components must be run simultaneously, each with different power requirements. Many benchtop power supplies offer 2 and 3 channel outputs, with 4-6 channels available on some models. Different channels will usually have different voltage capacities, though some power supplies offer an identical voltage range across all channels. Newer models can come equipped with remote control activation, channel specific automatic shut-off after a user specified period of time, or programmable voltage and current rise and fall times to protect low impedance loads from damage.

Voltage Range

When discussing digital multimeters, voltage range refers to the minimum and maximum voltages the meter can detect. Some models can detect voltages as low as .01mV up to 1000V. Such a broad range, while practical for the professional electrician, would be unnecessary for most diy and home applications as 240V is the highest voltage usually encountered in the home.

Accuracy and Resolution

Accuracy, expressed as a percentage, describes the difference between a known (standard) signal value and the measurement reading of the meter. Typically a handheld DMM will provide accuracy levels of around 0.5%, while bench multimeters can provide accuracy of 0.01% or better. If a highly accurate machine is required for your application be sure to purchase a DMM that can be calibrated from time to time to maintain the accuracy, prolonging the usability of the device. Resolution, given as either digits or display count, refers to the smallest increment the meter is able to display, and a higher number means a more precise reading. Both accuracy and resolution should be taken into account when considering a DMM, and for applications such as observing power consumption in a low-power device, high accuracy and high resolution are invaluable.

True RMS

True RMS (root mean squared) is the value of an AC waveform expressed as its equivalent DC measurement. A meter that is not explicitly stated to be a true RMS device can be assumed to be an "average responding" meter, so called because of how the AC value is calculated. Able to measure non-sinusoidal waves, they are not able to do so with the guaranteed accuracy of a true RMS meter. While not a requirement for every application, True RMS instruments are of vital importance when working with HVAC systems, computers, electronic ballasts, variable speed motor drives, and some audio equipment.

Many DMMs can also test for resistance, current, frequency, and even temperature. However, no single meter can do all things, and sometimes a more specialized device is required. Most DMMs, for example, cannot measure capacitance, and not all meters that can, do so well. Look instead for an LCR meter, an instrument crafted specifically to measure inductance (L), capacitance (C), and resistance (R). Additional specialty meters include dedicated ammeters, EMF meters, voltmeters, and milliohm meters.

Handheld meters share many features with bench models, but are portable and lightweight, making them a good choice for household, job site, or hobbyist use. The small size of handheld DMMs make them ideal for any work bench where space is at a premium.

Bench meters are generally more accurate than handheld varieties, with some models having an accuracy of 0.0018%. Bench meters can also have higher resolution and can allow for connection to multiple devices at a time for a streamlined benchtop measurement station.

Test Equipment Accessories

Optimizing the performance of many precision test and measurement instruments such as oscilloscopes, digital multimeters, and function generators begins with the right accessories. A wide assortment of clips, connectors, leads, probes, adaptors, modules, and jacks add versatility to your device, maximizing its potential and allowing for easy transition between projects.

Keeping your equipment safe is important. Available in a variety of materials and sizes, covers, holsters, pouches, and cases (hard or soft) protect your investment from dust and other potentially hazardous environmental factors. If your project necessitates travel, an instrument case with handles or straps makes transport easy and convenient.

Other accessories include cables, oscillators, and test sockets. Test sockets are an important part of prototyping and testing various electronic components. Contacts can be beryllium copper, phosphor bronze, or brass, and plated with tin, silver, or gold, depending on the requirements of your project.

Bandwidth

One of the most important factors to consider when purchasing a signal generator is bandwidth, also called frequency. Bandwidth denotes the maximum signal oscillation the device is able to accurately produce, and can be listed by the manufacturer as either a range or a maximum value. This only applies to sine waves however; for other waveforms, the maximum frequency output will be less. Both function generators and AWGs generally have a top frequency output of between 10-20MHz. If a higher frequency is needed, consider an RF or Microwave generator as the peak signal output of these devices is often above 6GHz, and in some models, above 20GHz.

Amplitude and Impedance

Amplitude is the output magnitude of a waveform measured in volts. In general, function generators can produce 10-20volts peak-to-peak. If an amplitude greater than 20Vpp is required, as would be the case when working with MEMS devices, invest in a quality amplifier. Output impedance is usually around 50&ohms;, though for working with with audio circuits, look for a device with 600&ohms; impedance.

Sample Rate and Memory

In essence, sample rate, measured in samples per second (Sa/s), describes how quickly data is transferred from memory to the generator's analogue-to-digital converter (DAC), and determines how accurately a signal can be displayed. Generally speaking, sample rate should be a minimum of 5x the highest frequency component. Bear in mind that faster, more complex waveforms will need higher sample rates, i.e. more points of data to be stored in the memory. As such, be sure your generator has enough memory to handle your frequency and sample rate requirements.

Additional Features

Other factors to consider include number of channels, resolution, and modulation (AM, FM, PM etc.). Regardless of the signal generator you choose, a variety of convenience features are available to enhance the utility of your device. Large, thin-film-transistor (TFT) LCD screens allow for sharp image contrast, while push button controls create a more user-friendly interface. Many models have USB connectivity for quick software and data sharing access, software bundles for additional waveform options, or remote control operation.

Over the past decade, widespread adoption of RoHS (Restriction of Hazardous Substances) standards has seen an increase in the variety of lead-free solder available. In addition to lead, solder can be an alloy of tin, silver, copper, or other metals in varying percentages. When the proportions of the components change, the properties of the solder change as well. For example, a solder containing a 60/40 mix of tin and lead will melt between 183-190°C (361-374°F). A 63/47 blend, however, is eutectic, meaning it melts at a precise point, 183°C (361°F), instead of over a range. The solder itself can take many forms such as pastes, pens, balls, and bars, though by far the most popular choice for hand soldering is wire imbued with flux.

Flux, an agent designed to remove and prevent oxidation from both the workpiece and the solder, is a crucial element in most soldering. Another important function of flux is easing the flow of filler into the joint, creating a cleaner finish and more stable bond. Rosin, an organic compound derived from pine trees, is considered the go-to flux for use in soft soldering as it is inert when solid and only mildly corrosive when heated. As some fluxes can be highly corrosive, to avoid damaging your work, choose the least acidic flux that can work for your application. If you prefer not to use flux core solder, flux pastes, liquids, and pens are also available.

Proper soldering cannot be achieved without the proper tools. Soldering irons, guns, and stations are all good choices for soft soldering depending on the project and the type of solder used. Soldering stations in particular are useful when precise heat control is needed, as, unlike guns, pencils, and irons, stations have variable heat settings. Soldering guns are ideal for working with heavier electrical connections, while soldering irons are lighter and are compatible with multiple tips for increased flexibility. For hard soldering, a butane torch is usually the tool of choice.

When the need to remove solder arises, be sure to have a healthy stock of desoldering supplies on hand. For desoldering near delicate electronic assemblies, a tweezer iron or hot air pencil/gun will quickly melt the undesired solder. A pump or vacuum is then used to remove the molten solder from the workspace. In lieu of a "solder sucker," desoldering braid can be used to absorb the molten solder. Other useful desoldering products include removal alloy, flux remover, tweezers of various sizes, and application brushes. A final note on safety. Due to the hazardous fumes released during both soldering and desoldering, adequate ventilation and air filtration is vital. Charcoal HEPA filters, when used with a fume extractor, help keep your workspace safe and healthy.

Have all personnel entering the workspace wear anti-static garments, footwear, and wrist bands. Monitor air quality closely; dry air promotes static build up. Use an air ionizer to neutralize charged surfaces, and if possible, keep the relative humidity between 40-60%. On tabletops, anti-static table mats are crucial, as are anti-static floor tiles or mats. Make sure all materials and items entering the EPA such as films, drinking cups, packing foam, and even waste receptacles, are slightly conductive to remove static charges gradually. Most importantly, keep a static monitor handy at all times to periodically check static levels in the workspace.

Adhesives come in a variety of mediums including gels, cements, aerosols, melt sticks (i.e. hot glue), and tapes. They can be conductive or non-conductive, blocking or allowing current to travel from one surface to another. Curing time—how long the adhesive and newly joined surfaces need to rest before the bond is completely set—will vary depending on the type and formula of adhesive used, and should be taken into account when selecting an adhesive.

Epoxies, like traditional adhesives, are useful for affixing one substrate to another. Unlike traditional adhesives, epoxy resins are excellent insulators, greatly reducing the risk of short circuiting sensitive electronic components. Epoxies are ideal for potting transformers and inductors. They also require a reactant, either itself or a separate agent, to facilitate proper curing. Though curing strengthens and hardens the epoxy against structural, chemical, or thermal damage—depending on the epoxy and reactant used—the chemical reactions taking place can cause high heat and should be monitored carefully.

Lubricants and protective coatings not only guard against corrosion, but in some cases, enhance the functionality of the component to which they are applied. A thermal transfer compound, while technically considered a lubricant, has for its primary purpose the maximization of contact between two surfaces (generally between a heat-sink and its respective part) by eliminating gaps and air bubbles between them. This results in more efficient heat transfer from one surface to another. Protective coatings can also be highly specialized, including conformal coatings designed to help electronics function in harsh environments, conductive coatings for shielding against electromagnetic/radio frequency interference, and connector coatings providing moisture protection and insulation to a variety of surfaces.

Cleaning products are among the most important chemicals to have on hand, as even trace amounts of dust or oil can wreak havoc on delicate electronic assemblies. Fingerprints can become faintly conductive when introduced to a small amount of moisture, and over time reduce circuit accuracy through current leakage. Minimize contamination risks by keeping items like contact cleaners, dusters, and wipes near your workbench at all times. Additional cleaners to consider are degreasers, epoxy and adhesive cleaners, flux removers, and isopropyl alcohol.