## Test & Measurement Questions

• #### What are the differences between analog and digital oscilloscopes? +

Analog oscilloscopes use high gain amplifiers to present the waveform on a CRT (cathode ray tube) screen. The entire signal processing taking place in an analog oscilloscope is in analog form. In a digital oscilloscope (DSO), an extra step is used before the signal is shown on the screen. That is the conversion of the signal into a digital stream by an analog to digital converter. Therefore, CRT type screens are no longer necessary, greatly reducing the complexity of the design. In theory, an analog oscilloscope can offer higher bandwidth. However, along with the progress in analog to digital conversion technology and digital signal processing, digital oscilloscopes have been rapidly improving, now offering impressive bandwidths. In addition, digital processing allows offering features that are not easily possible with analog technology. For example, signal manipulation and complex mathematics operations are standard features of a DSO from most manufactures.
• #### What is the difference between the real-time sampling rate and the effective sampling rate? +

The real time sampling is converting the analog signal to digital bits at a fixed rate. There is no relationship between the sampling frequency and the frequency of the signal being converted. The time between the digital samples is exactly the same and equal to the sampling time. The digital representation of the original analog signal will then have missing sections between the consecutive samples. When the sampling frequency is much higher than the analog signal frequency (>100,) the digital construction of the signal is an accurate model of the input. However, as the input frequency increases, the constructed digital signal shows signs of discrepancy due to the missing information between the samples. For example, when the sampling frequency is exactly equal to the analog signal frequency, there is only one point of digital information available to construct the signal from. This will result in a DC line. Clearly, the digital model at higher input frequencies is far from the original signal.
The effective sampling rate is referred to a sampling technique for periodical signals. In this technique, each time the analog input is sampled, the sampling signal is delayed by a predetermined amount (ΔT) with respect to a fixed point of the input. Therefore, the time resolution between the consecutive samples is effectively ΔT, yielding a sampling rate of 1/ ΔT that is independent of the frequency of the sampling signal. The tolerance of ΔT over the consecutive samples determines the accuracy of the reconstructed model.

• #### What makes a good oscilloscope? +

The answer depends on the intended application. It comes down to performance to price ratio. There are a few key specifications that determine the quality of an oscilloscope. The bandwidth and the sampling rate of the digitizer (for digital oscilloscopes) are the most important specifications of an oscilloscope. The real-time sampling rate of an oscilloscope determines its time resolution. If the signal being analyzed is periodical, the time resolution is equal to the effective sampling time. It can be considerably smaller than the time resolution obtained from real time sampling. The voltage and timing accuracy, the frequency response (flatness,) and performance stability of an oscilloscope are also very important.
• #### What are the characteristics of a good oscilloscope probe? +

There are a few specifications, which must be considered before choosing an oscilloscope probe. A passive oscilloscope probe is essentially made up a resistor parallel with a capacitor between the probe tip and the BNC connector and of course, signal wiring. Assuming properly matched signal wiring, this parallel combination and the oscilloscope input impedance form a voltage divider at the input of the oscilloscope. Therefore, the accuracy of the measurement depends on the accuracy of the impedance of the probe. Some probes have an adjustment potentiometer that allows optimization of this impedance in relationship to input impedance of the oscilloscope. Most oscilloscopes offer a test signal to verify the performance of the probe and make the necessary adjustment.As the frequency of the signal increases, the probe parasitic capacitance and wiring inductance play a major role in the measurement accuracy as they reduce the bandwidth. That makes the probe bandwidth an important specification when testing high frequency signals. Another important feature of a probe is its common mode rejection ratio, CMRR, particularly at high frequencies. Consider a signal in a noisy environment, a probe with a poor CMRR distorts the signal before it presents the signal to the oscilloscope input. An active probe can offer some performance enhancements. However they have their own limitations too. When possible, a coaxial cable offer a simple alternative.
• #### How can I measure the level of a signal in the presence of noise or a larger signal? +

Averaging techniques reduces the noise level associated with as signal by a factor of √(the number of averaging). Averaging essentially filters the signal. In a time domain measurement, such as measuring the signal level by a DVM or an oscilloscope, this process is simply the addition of several occurrences of the event and dividing the sum by the number of additions. Clearly, as the frequency of the signal increases, the timing jitters and stability of the measuring equipment becomes more important as the sampling of the signal will not be consistent over consecutive cycles. When a spectrum analyzer is used to measure the signal parameters, averaging can still be used to reduce to noise level. However, the measurement is limited by the number of FFT points. Increasing the number of the FFT points will increase the resolution of the measurement. Making signal measurements in the presence of larger signal requires filtering the signal by a notch filter. The filter’s cutoff frequency is tuned to match the frequency of the larger signal, making the indented signal the dominant signal at its output.
• #### What is the difference between a USB (PC) oscilloscope and a stand-alone oscilloscope? +

A USB oscilloscope is essentially a data acquisition device that connects to your computer through one of its USB ports. The data is then processed and shown on the screen of the computer. Some manufacturers use the power of the computer to perform digital signal processing on the data that could range from FFT to application specific data manipulation. On the other hand a stand-alone oscilloscope integrates all the necessary components into one package. Therefore, you do not need a computer for it to work. Clearly, for a comparable performance a stand-alone oscilloscope is more expensive. Also, since the power supply of a stand-alone oscilloscope is not limited to the power provided by the USB port, they can theoretically offer a better performance than the USB type. Some USB oscilloscopes inconveniently come with an external power supply for this exact reason. Although traditionally USB Oscilloscopes have not matched the performance of the stand-alone models, recently design breakthroughs have resulted in some very impressive products.
• #### What is oscilloscope bandwidth? +

The bandwidth of an oscilloscope specifies the usable frequency range of the oscilloscope. The signal processing circuitry of an oscilloscope acts like a low pass filter, limiting the frequency behavior of the device. Therefore, for a proper analysis, the oscilloscope bandwidth must be considerably higher than the frequency of the signal under test.
• #### What is rise time? What is oscilloscope rise time? +

A signal takes a certain amount of time to change from its lowest value to its highest. Consider a 1 Volt peak to peak square wave changing between -.5V to +.5V. The time it takes to make the transition from -0.5V to +0.5V is the rise time of the square wave. An oscilloscope can effect this rise time, and make it appear slower. Assuming the signal has a sufficiently small rise time (considerably faster than the oscilloscope rise time,) the rise time measured on the oscilloscope screen is called the rise time of the oscilloscope. The rise time of an oscilloscope is directly related to its bandwidth. It indicates how the oscilloscope responds to fast changing signals. Therefore, for a proper analysis the rise time of the oscilloscope must be considerably smaller than the rise and the fall times of the signals under test.
• #### What is coupling? What is oscilloscope coupling? +

A signal contains both a DC component, and also an AC content. For example, consider a 1 volt peak to peak sine wave riding on a 1000 Volts DC voltage. When the analysis of the AC of the signal is of the interest, placing a capacitor in series with the signal can block its DC competent. In the previous example, the capacitor only passes through the 1 Volt sine wave. This is called AC coupling. Most oscilloscopes offer both AC and DC couplings. The DC coupling is a direct connection of the signal to the input of the oscilloscope. The AC coupling places the described capacitor in series with the signal to block its DC component. The DC coupling is a useful feature when the signal’s DC component is high.
• #### What is a good oscilloscope for a hobbyist? +

The application determines the required specification of the oscilloscope. Therefore, the first step to choose an oscilloscope is to know what it is going to be used for. Once that is known, contacting manufactures about their products that could serve the application is the next step.
• #### What is a good oscilloscope for ham radio project? +

For most ham radio projects an oscilloscope with a 100 MHz bandwidth is sufficient. Other features of the oscilloscope should be also looked at. One feature, which can be particularly useful, is the capability of the oscilloscope to perform spectral analysis.
• #### What is a good oscilloscope for audio projects? +

There are a vast number of projects, which could be considered “audio”. Tuning an instrument such as a piano and modulating a voice signal are two examples of audio projects. In general, a 100 MHz bandwidth oscilloscope equipped with a spectrum analyzer meets the requirements of the majority of audio projects.
• #### What is a good oscilloscope for lab experiments? +

For a laboratory environment, similar to other applications, the specification of the oscilloscope is determined by the requirements of various testing that the oscilloscope will be used for.
• #### What is an oscilloscope input impedance? +

The input impedance of the oscilloscope is the amount of impedance load placed on the signal, which is being tested. For most oscilloscopes it is about 1 M-Ohms parallel to a few picofards.
• #### What is jitter in an oscilloscope? +

In general, jitter is the timing uncertainty and noise associated with a clock. In an oscilloscope, it could cause noise and triggering uncertainty, particularly for high frequency signals.
• #### What is oscilloscope memory depth? +

The memory depth of an oscilloscope determines the amount of real time data, which could be saved by the oscilloscope.
• #### What is oscilloscope resolution? +

The ability of an oscilloscope to resolve small voltages is called "oscilloscope resolution". Modern digital oscilloscopes convert the signal from analog domain to the digital format before they are processed and presented on the oscilloscope screen. In these oscilloscopes, an analog to digital converter performs the conversion process. The number of bits of the analog to digital converter is referred to as the resolution of the oscilloscope, and is determined by the number of bits of its analog to digital converter. It indicates the accuracy of the data and the fineness of the vertical scale of the oscilloscope. It determines the smallest signal that can be ideally resolved by the oscilloscope. For example, a 10 bit A/D converter can resolve 1 part in 1024 ( 2^10). That is to say for a 1 volt range, the ADC can resolve just about 1 mV. In turn, an oscilloscope with a 10 bit resolution ADC as its converter, in its one volt range, has about 1 mV sensitivity. This oscilloscope is specified as a 10 bit resolution instrument.
• #### What is an oscilloscope sample rate? +

The sample rate of an oscilloscope is the clock frequency of its analog to digital converter. It limits the highest signal frequency, which could be properly analyzed by the oscilloscope. According to the Nyquist criteria, in any sampling system, the maximum signal frequency is limited to the half of the sampling frequency.
• #### What is a USB oscilloscope? +

A USB oscilloscope is a device, which uses a USB port to transfer the data to a computer. The data is then processed by application software and shown on a graphical user interface.
• #### What is a bench-top oscilloscope? +

A bench top oscilloscope is a standalone device which is usually too big to be portable.
• #### What is an oscilloscope and what does it do? +

An oscilloscope is a tool that graphically shows the real time variations of a signal applied to its input. It is used to monitor signals. An oscilloscope is often the primary tool to debug systems and circuit boards.
• #### What is the difference between a 10x & 1x oscilloscope probe? +

A 10X oscilloscope probe is a probe with a 9M Ohms series resistance. Since the input impedance of an oscilloscope is usually about 1M Ohms, this forms a resistor divider that reduces the sincerity of the oscilloscope by a factor of 10. The 1X probe does not use a series resistance. 1X probes are usually used for high frequency signal analysis since they do not affect the signal under test.
• #### How does an oscilloscope work? +

With the advent of high speed analog to digital digitizers, oscilloscopes have undergone a major design revolution. Up until early 1980s the vast majority of oscilloscopes were analog. An analog oscilloscope deign consists of a front end analog signal processing circuitry to adjust for various voltage settings and a cathode ray tube to display the signal. With respect to the current technology, they were bulky, expensive, and consumed a great amount power. The digital revolution changed all that. The design of a digital oscilloscope is centered around an analog to digital converter. Once the signal is digitized, it is processed by an FPGA or a microprocessor before being displayed.
• #### How much oscilloscope bandwidth do I need? +

Generally, the bandwidth of an oscilloscope must be at least 1.5 times bigger than the highest frequency component of signals, which are going to be monitored. Just be aware that a square wave signal has high frequency components regardless of its period. Therefore, the rise time of the oscilloscope must also be sufficiently small for the intended applications. Clearly, having a margin of safety is important.
• #### How can I measure jitter on an oscilloscope? +

One way to measure the timing jitter of an oscilloscope is to provide a low jitter signal to one of the inputs of the oscilloscope and while triggering on that channel build a history of that signal on the monitor. The horizontal thickness of the signal over time indicates the jitter of the oscilloscope.
• #### What is a spectrum analyzer? +

While an oscilloscope displays a signal with respect to time, a spectrum analyzer plots it with respect to frequency. In theory, Fourier theorem states that a signal is composed of a number of sinusoidal signals. Analyzing the amplitude, frequency, and phase of these sinusoidal signals is referred to as the frequency spectrum analysis of the signal. To extract these parameters, the signal is filtered, digitized, and Fourier transformed to the frequency domain. The amplitude and the phase spectrum of the signal can then be plotted based on these parameters. A spectrum analyzer performs the described operation on its input signal and provides the spectrum on its display.
• #### What is the noise floor on a spectrum analyzer? +

The noise floor of a spectrum analyzer is what the spectrum analyzer displays when there is nothing connected to its input. It is an indication of the smallest signals, which can be properly displayed by the analyzer.
• #### What is spectrum analyzer resolution bandwidth? +

The resolution bandwidth of a spectrum analyzer is the smallest frequency changes, which can properly be resolved by the analyzer. This feature directly relates to the number of FFT points performed on the signal.
• #### What is spectrum analyzer reference level? +

The reference level, in an spectrum analyzer is the highest voltage level, which can be properly analyzed in that setting.
• #### What is spectrum analyzer software? +

Spectrum analyzer software refers to the program used to analyze the signal. It performs the various features offered by the device.
• #### What is spectrum analyzer video bandwidth? +

The bandwidth of a spectrum analyzer is the frequency range for which the spectrum analyzer does not affect the amplitude of the signal under test.
• #### What is spectrum analyzer span? +

A spectrum analyzer graphs the variation of the amplitude of a signal on the vertical scale versus frequency on the horizontal scale. Span determines the width of the frequency for which the amplitude is plotted. It can act as a frequency magnifier for resolving finer frequency variations. Most spectrum analyzers provide the user with the ability to adjust this parameter to a desired value. Analog Arts demo application software helps getting a hands-on experience on various parameters of a spectrum analyzers.
• #### How does a spectrum analyzer work? +

In general in a spectrum analyzer, the signal under test is filtered, digitized, and Fourier transformed to the frequency domain. The amplitude and the phase spectrum of the signal are then plotted based on the parameters obtained from these analyses and plotted on the spectrum analyzer display.
• #### How to read spectrum analyzer graph? +

The vertical axis of spectrum analyzer is amplitude of the signal under test in volts and the horizontal axis is the frequency of the signal in Hz. So, at any given frequency the amplitude of the signal is its component at that location, which is shown on the graph.
• #### What is the difference between FFT and DFT? +

DFT or Discrete Fourier Transform is an algorithm that computes the Fourier transform of a digitized (discrete) signal. FFT (Fast Fourier Transform) is an optimized implementation of this transform. For a comprehensive explanation of FFT, you can review EDN FT: Equations and history article.
• #### What is aliasing? +

In a digitizing system, the sampling rate is a frequency limiting factor. According to Nyquist theorem, the highest frequency that can be accurately sampled is half of the sampling rate. Signals with frequencies higher than the Nyquist limit produce digital signals with false frequencies. This effects is called aliasing. To become more familiar with the term, please watch the video tutorialon this subject.
• #### What is a digital oscilloscope? +

The design of a digital oscilloscope is centered around an analog to digital converter. Once the signal is digitized, it is processed by an FPGA or a microprocessor before being displayed.

• #### What is a PC oscilloscope? +

A PC oscilloscope is a device, which uses a computer to process and display the signal under test. A PC oscilloscope can use any of the data I/O ports the computer to perform its task. A USB oscilloscope is an example of a PC oscilloscope.
• #### What is a mixed signal oscilloscope? +

A mixed signal oscilloscope is referred to an oscilloscope, which could display analog signals along with several channels of digital waveform. It can be thought of as a combination of an oscilloscope and a logic analyzer.
• #### What is a low cost oscilloscope? +

The price of an oscilloscope vary from a few hundred dollars to close to one hundred thousand dollars. The performance of the oscilloscope determine its price. Bandwidth of an oscilloscope is the the key specification that determine its price. For example, a 10 GHZ bandwidth oscilloscope can cost 200 times more expensive than a 100 MHz oscilloscope. Of course extra features, such as the number of oscilloscope channels also affect its price. Usually, USB type oscilloscopes offer a better price for the similar performance than standalone devices. Typically, they also provide better features. Clearly, their biggest draw-back is the need for a personal computer. In addition in some some cases, they might require an external power source. Before, the purchase, the applications for which the oscilloscope is required must be thoroughly reviewed to make sure the intended oscilloscope is capable of meeting the requirements.
• #### What is a high performance oscilloscope? +

A high performance oscilloscope is usually marked by its bandwidth. The higher the bandwidth of an oscilloscope , the higher performance it can be regarded.
• #### What does the 'trigger' feature do on an oscilloscope? +

“Trigger” feature of the oscilloscope synchronizes the display with a defined event such that the signal looks stationary on the screen. Without this feature, the signal moves from one screen refresh to the other, making it difficult to study the signal.
• #### What kinds of triggering modes oscilloscopes have? +

The most common triggering options provided by manufactures are various forms external event triggering and signal event triggering. The options may include rising or falling events and or single, normal, alternate, and auto triggering. Each one of these options defines a synchronization criteria by which the oscilloscope displays the signal.
• #### What is an arbitrary waveform generator used for? +

In general, there are times, when there is a need to generate a signal not provided as a standard option by signal generators, in these situations an arbitrary generator can offer a technique to produce the desired signal.
• #### What's the Difference Between a Function Generator and an Arbitrary Waveform Generator? +

A function generator provides a limited number of standard signals for the user. On the other hand, an arbitrary generator ideally gives him the ability to generate any desired signal.
• #### What is logic analyzer? What is logic analyzer used for? +

A logic analyzer is used to analyze digital waveforms. The shape of a digital signal varies between highs and lows or 1s and 0s. A logic analyzer shows the transition between with respect to time. Sometimes, it is important to see the relation between several digital signals to verify proper occurrences of the transitions. For example, the group of signals in a a logic protocol such as SPI or a digital data bus can be analyzed to be validated. The device is powerful tool to test and debug FPGA, micro-controllers, and any digital systems with multiple channels of data. To debug a digital system, a logic analyzer offers unique features and versatility.
• #### What is a pattern generator? What is a pattern generator used for? +

A pattern generator is a device, which can be programmed to generate any particular digital signal for testing purposes. It usually consists of several channels. This channels serve as the output of the generator. Each channel can individually be programmed fro a desired signal. The programmed signal or the pattern is stored in a buffer memory and clocked out at a user defined sampling rate. The application for a pattern generator varies from generating a single channel square wave to digital protocols such as SPI, and multi channel complex patterns. To get a better idea, you can download Analog Arts free demo application software. It provides a fully featured pattern generator, which one can use to become familiar with the instrument. Analog Arts video tutorial on pattern generators is also a good source of information.
• #### What is frequency? +

Frequency is the rate of change of a periodical signal with respect to time. A wave, by nature, goes through highs and lows or peaks and valleys. The distance between two consecutive peaks or two consecutive valleys is called the period of the wave. The frequency of the wave is the inverse value of its period and it is measured in Hertz or Hz.
• #### What is frequency measured in? +

Frequency is the rate of change of a periodical signal with respect to time. Its unit of measurement is Hertz. The distance between two consecutive peaks or valleys is called the period of the wave. The period is measured in time units. A period of 1 second means that wave repeats itself every second. The frequency of the wave is the inverse value of its period and it is measured in Hertz or Hz. 1 Hz (Hertz) = 1/Second. For a 1 second period, the frequency of the wave is 1 Hz. On the other hand if the period of a wave is 10 ns, its frequency is 100 MHz.
• #### What is a data logger? What is a data recorder? +

"Data logger" or "data recorder" are basically referred to the same device. A data logger/recorder is used to monitor and record slow varying signals such as the output of a sensor over long period of times. A typical application for this instrument is recording the temperature variations in an experiment.
• #### What is a TDR? +

TDR or time domain reflectometry refers to a measurement technique for measuring the length and the characteristic impedance of a cable. An electrical wave traveling through an open-ended cable reflects back to the source, when it reaches the open end. Based on this fact, a TDR tester sends a pulse through the cable under test, then the time it takes for the pulse to return back to the source is measured. This time is used to calculate the length of the cable based the velocity of the electrical signal through the cable. In such a set-up, the characteristic impedance of the cable can also be calculated based the amplitude distortion of the signal the source. That is to say if the output impedance of the source is not zero, the cable connected to this source acts like a resistor divider while the signal is reflecting back.
• #### What is a TDR cable tester or analyzer? +

A TDR cable tester, or analyzer, is a device that enables the user to make various measurements on the cable under test. TDR stands for time-domain-reflectometer. An electrical wave traveling through an open-ended cable reflects back to the source, when it reaches the open end. Based on this fact, a TDR tester sends a pulse through the cable under test, then the time it takes for the pulse to return back to the source is measured. This time is used to calculate the length of the cable based the velocity of the electrical signal through the cable. In such a set-up, the characteristic impedance of the cable can also be calculated based the amplitude distortion of the signal the source. That is to say if the output impedance of the source is not zero, the cable connected to this source acts like a resistor divider while the signal is reflecting back. Some TDR analyzers are also capable of measuring other important parameters such as capacitance, inductance, resistance and loss through the cable.
• #### What is a multimeter? +

A multimeter is an instrument that can measure several different electrical parameters; such as voltages, currents, resistance, and capacitance. Originally before the progress of the digital technology, all of the measurement processing was performed in the analog domain. The measurement was then shown on a mechanical analog panel, much like analog clocks. With the arrival of digital multimeters, the previous multimeters became known as analog multimeters. On the other hand, the new multimetrs were named digital multimeters, due to the fact that a digital multimeter uses an analog to digital converter to convert the measurement parameter to the digital domain or numbers. It then processes the information before it is shown on its digital display panel. A handheld voltmeter is an example of such an instrument.
• #### What is a digital multimeter? +

A multimeter is an instrument that can measure several different electrical parameters such as voltages, currents, resistance, and capacitance. Originally before the progress of the digital technology, all of the measurement processing was performed in the analog domain. The measurement was then shown on a mechanical analog panel, much like analog clocks. With the arrival of the digital multimeters, the previous multimeters became known as analog multimeters. On the other hand, the new multimeters were named digital multimeters, due to the fact that a digital multimeter uses an analog to digital converter to convert the measurement parameter to the digital domain or numbers. It then processes the information before it is shown on its digital display panel. A handheld voltmeter is an example of such an instrument.
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