ARBITRARY WAVEFORM GENERATOR SIGNALS

Used to test various circuit topologies, an arbitrary waveform generator (or AWG) can be used as general-purpose function generator as well as a waveform generator. Let’s first address the difference between a signal generator, function generator, and an arbitrary waveform generator.

Signal generators produce high-fidelity sine wave signals that range from low frequencies to many gigahertz (GHz). Features of signal generators include attenuation, modulation, and sweeping.

Function generators are lower-frequency instruments which produce sine, square, pulse, triangle, and ramp waveforms from direct current up to a few megahertz (MHz) and usually cover a wide range of voltage.

Arbitrary waveform generators, which we’ll be focusing on below, are high-flexibility signal sources that can generate any arbitrary waveform constructed from point-by-point in digital memory, and these constructed waveforms are converted into analog signals with the AWG’s digital-to-analog converter (DAC), which operate at clock rates up to a few gigahertz (GHz). Because AWGs have built-in algorithms that generate standard functions, they can stand in as an ordinary function generator.

Types of Arbitrary Waveform Generator Signals

There are four categories of waveforms that an arbitrary waveform generator can create: standard and advanced functions, arbitrary waveforms, and waveform sequences. The standard functions category consists of pulse, ramp, sine, square, and triangle waveforms which are used in such applications as baseband, audio, sonar, ultrasound, and video components as well as circuits. Frequency response characterization, digital logic generation, device linearity characterization, and direct current-offset signal generation tests can be performed with an arbitrary waveform generator.

The majority of arbitrary waveform generators feature advanced functions including multi-tone, cardiac, noise, and much more that are used by specific industries for unique applications — e.g., cardiac and haversine signals are commonly used in medical device tests. Due to the abrupt transitions in the signal, standard pulse waveforms excite the device under test with extensive harmonic content; and other kinds of pulse waveforms have smooth transitions which shape the harmonic content for certain applications. Some examples are sinc and exponential pulses.

Sinc pulses, which are shaped with bandwidth-limited frequency spectrums, are used to characterize or excite communications channels that have limited bandwidth capability. Exponential pulses can simulate various physical phenomena, e.g., a resistor-capacitor charging circuit.

DIGITAL STORAGE OSCILLOSCOPES: CHOOSING THE RIGHT BANDWIDTH

Because there are hundreds of models available with different specifications at a wide variety of prices, choosing the right oscilloscope can be intimidating and confusing for many engineers and technicians. In this article we’ll look at a few aspects of digital storage oscilloscopes that are of particular importance in order to help you avoid a costly mistake.

Some things to consider before we get into the specifics of bandwidth are: where you’ll be using the oscilloscope, how many signals you’ll need to measure at one time, the minimum and maximum signal amplitudes you’ll be measuring, the highest frequency signal you’ll potentially measure, whether your signals are repetitive or single-shot, and whether you need to view signals in the frequency domain (i.e. spectrum analysis) and the time domain simultaneously.

Bandwidth

Bandwidth is the maximum signal frequency that can pass through the front-end amplifiers and the oscilloscope's bandwidth must be higher than the maximum frequency you’d like to measure. However, sufficient bandwidth isn’t the only consideration when making sure that a digital storage oscilloscope can capture a high-frequency signal accurately.

Oscilloscope manufacturers seek a certain type of frequency response when designing their instruments and this response is called the maximally flat envelope delay, or MFED. This type of frequency response provides superb pulse fidelity with very little undershoot, overshoot, and ringing. Because a digital storage oscilloscope is made up of amplifiers, attenuators, analog-to-digital converters, interconnects, and relays, maximally flat envelope delay response cannot be entirely realized — think of it more as an ideal.

Please note that the majority of oscilloscope manufacturers define bandwidth as the frequency at which a particular sine wave signal will be attenuated to seventy-one percent of its actual amplitude, i.e. the trace will be twenty-nine percent in error of the input signal.

Your signal will contain high-frequency harmonics if your input signal isn’t a pure sine wave; e.g. a twenty megahertz signal viewed on a twenty megahertz bandwidth oscilloscope will show up as a distorted and attenuated waveform. A good rule of thumb here is choosing an oscilloscope with a bandwidth five times higher than the maximum frequency signal you’d like to measure. The caveat here is that oscilloscopes with high bandwidths are more expensive, meaning you may have to compromise with regard to your oscilloscope’s bandwidth. Some oscilloscopes have bandwidths that aren’t available for all voltage ranges which is why should carefully read through the oscilloscope’s data sheet.

STEPPER MOTORS: THE BASICS

Found in both industrial and commercial settings as a result of their affordability, reliability, durable construction, and high torque at low speeds, stepper motors are brushless synchronous electric motors that work by converting digital pulses into mechanical rotation. Every revolution of a stepper motor consists of a division of steps and the motor is sent a pulse for each step. A stepper motor’s position can be controlled without feedback because stepper motors are able to take only one step at a time and each step is the same size. When pulse frequency increases to a certain point the step movement will turn into continuous rotation with the speed directly related to the frequency of pulses.

There are a number of reasons to use a stepper motor: 1) open-loop control simplifies the motor and makes control less expensive; 2) the motor’s rotation angle is directly proportional to the input pulse; 3) you need precise positioning and reliable repetition of movement; 4) stepper motors have wide operational ranges because the speed is commensurate to the input pulses’ frequency; 5) they have excellent response times with regard to starting, stopping, and reversing; 6) good stepper motors are accurate within three to five percent of a step; 7) when the windings are energized stepper motors have full torque at complete standstill and, when the load is coupled, you can achieve remarkably low speed synchronous rotation; and 8) because stepper motors don’t have contact brushes, they have an exceptionally high mean time between failure.

The three types of basic stepper motors are variable reluctance, permanent magnet, and hybrid — hybrids combine the strengths of variable reluctance and permanent magnet engines. Hybrid stepper motors have toothed stator poles, a permanent magnet rotor,typically  two hundred rotor teeth, and most hybrid motors rotate at under two degrees each step. As a result of their high static and dynamic torque and their ability to run at quite high step rates, hybrid stepper motors are commonly used in a number of different applications including computer disk drives, printers, machine tools, and much more.

Hopefully this brief overview helped you to understand a little bit more about the operation of stepper motors, their characteristics, and some common applications where stepper motors are employed.

USING SHUNTS IN CONJUNCTION WITH DIGITAL PANEL METERS

In the medical world a shunt is a means of diverting, e.g., blood from its typical path to another route. Shunts work in a similar manner in the realm of electronics, allowing current to bypass one point in a circuit and travel to another point.

An example of this would be Christmas lights. Because they are set in series, when one bulb fails the entire circuit goes out. Modern lights prevent this with shunts: each bulb has its own shunt in order to pass the current along, so to speak, when a filament fails.

Circuits are commonly tested for current, resistance, and voltage. Some devices — e.g. ammeters, voltmeters, and ohmmeters — individually measure these parameters. Other devices, like digital multimeters, can test several parameters at once. Digital panel meters are used to measure, display, and record a circuit’s current, resistance, and voltage; but because they are continually subjected to the parameters which they measure, they are prone to damage. However, using panel meters in conjunction with shunts mitigates the risk of damage.

When you combine shunts and panel meters you end up with shunt-resistive circuits which enable you to circumvent predetermined levels of current surrounding a piece of electronic testing equipment. Using a shunt and panel meter together (i.e., a shunt resistor) reduces the excess flow of current through your instruments and helps keep them better protected.

Moreover, combining a shunt and digital panel meter can extend the range of the shunt, which is known as a meter shunt. When you divide current between parallel shunt resistors you increase the range of an ammeter by adding another parallel resistor. Let’s say your meter can only read between zero and one milliamperes but your test requires full-scale detection of one hundred milliamperes. In this situation you can use a shunt to take on the difference between the detection capabilities of your meter and the desired level of full-scale detection. With regard to our example, your shunt would have to be able to handle the remaining current (i.e., ninety-nine milliamperes).

Shunts have myriad uses in electronics testing, but their chief utility comes from their ability to protect delicate equipment and boost the capabilities of panel meters, making shunts exceedingly useful for those who do serious electronic testing.

FUNCTION GENERATOR OVERVIEW

Sometimes a device will not properly respond to electrical impulses and other times a device won’t respond at all. In these situations a function generator is handy for diagnosing the problem. Function generators test the response of various circuits to electrical signals and impulses by releasing voltage patterns of differing frequencies and amplitudes, which are controlled by the operator of a function generator. Function generators are frequently used in conjunction with an oscilloscope, which displays electrical waveforms.

Waves

Function generators allow you to choose the shapes of the output which replicate several mathematical functions: a square wave moves abruptly from high to low voltage; a sine wave gently curves from from high to low voltage; triangle waves move from high to low at a fixed rate (i.e. diagonal lines); and sawtooth waves rise slowly and drop quickly.

Reason to Use a Function Generator

Function generators are a very easy and convenient means of testing electrical devices. You can use a function generator when a device or part of an electrical system malfunctions or stops functioning altogether. You test the device or system’s response to electrical activity by replicating its ordinary impulses using the function generator’s waveforms. When you send a device/system an electrical impulse like the one which it would usually have and the device/system either distorts or fails to accept the signal, you know that your device/system is defective.

Function generators are also used for measuring feedback from musical equipment — amplifiers, microphones, electric instruments, etc. — because the waveforms imitate situations when the device you’re testing will be affected by electricity, enabling you to draw conclusions as to what’s not working right.

Function Generator Use

The first step to using a function generator is (you guessed it) powering it on. Next you’ll connect your lead and ground to your oscilloscope in order to check the controls and make sure the generator is configured to use the waveform you need. After you have the waveform you want you’ll connect the signal lead to the device you want to test and the ground lead to the device’s ground (some devices have a negative lead; connect your ground lead to the negative lead in this case). After everything is set up you can then send impulses to your device. Not only can you control the waveform you send the device, you can also control the volume of the wave as well.