DiagnoSYS Subway Electronics Functional Test Solution

DiagnoSYS has incorporated the ZTEC ZT450 PXI modular oscilloscopes into a functional test solution for subway car electronics validation. DiagnoSYS is a global company that leverages industry standard hardware and software to create flexible and scalable turnkey test solutions. ZTEC’s flexible software drivers and powerful built-in measurements proved to be a great match for DiagnoSYS’s test system.

For this subway electronics test system, DiagnoSYS used PXI which is mounted in a 19-inch rack. Within this system, the ZTEC ZT450 PXI is used mainly to test the functionality and accuracy of the train’s speed sensing equipment. This equipment consists of a microwave Doppler box that generates a 26 GHz signal which is focused onto the train track. The Doppler box then receives the reflected signal and is able to sense the frequency shift between the generated and received signal. Using the Doppler principle--where the perceived frequency (reflected) wave changes in proportion to the speed of the object that is generating the wave—the Doppler box generates a differential sine wave with a frequency and amplitude in proportion to the speed of the object at around 2 KHz. This signal is then sent to the train’s speed sensor or digital speedometer which displays the speed of the subway car.

Reactor Measurement System Incorporates High-Resolution ZTEC PXI Oscilloscopes

Sandia National Laboratories (SNL) and PrimeCore Systems have developed a pulse diagnostic measurement system for the SNL Annular Core Research Reactor based on PXI instrumentation and leveraging ZTEC ZT410 PXI modular oscilloscopes. The power and flexibility of the PXI platform and the ZT410 PXI 16-bit, 400 MS/sec instrument enabled SNL and PrimeCore to replace and decidedly improve an aging system based on 8-bit benchtop scopes, amplifiers, and long cable runs.

Oscilloscope Measurement Fundamentals: Frequency Domain Measurements (Part 3 of 3)

This is part three in a three part series in which we will examine oscilloscope measurements such as the ones available in hardware within the ZTEC family of modular oscilloscopes. Many oscilloscope users take advantage of only a small fraction of the powerful features available to them. In addition, selecting the right measurement from a catalog of possibilities and accurately interpreting the results can lead to confusion and mistakes. This series of articles is intended to help users understand oscilloscope measurements more completely in order to avoid common pitfalls. Digital storage oscilloscopes vary greatly among vendors in terms of form factor (stand-alone, PXI, VXI, PCI, etc), resolution (8-bit, 12-bit, 16-bit, etc), acquisition rates (1 MS/sec, 1 GS/sec, 40 GS/sec, etc), functionality (advanced triggering, deep memory, self-calibration, etc.), and more. One aspect that separates true oscilloscopes from most PC-based, modular digitizers is the ability to make measurements in hardware on an onboard processor. The available measurements also differ from one oscilloscope to another, although this paper will cover a large segment of them. In addition, the algorithms used to complete the measurements may differ slightly among vendors. This paper will focus on the measurements and algorithms used in ZTEC modular oscilloscopes, but most of these concepts are universal. Oscilloscope measurements can be sorted into the following three categories: • Vertical-Axis • Horizontal-Axis • Frequency Domain Part three of the series will focus on frequency domain measurements.

Frequency domain measurements involve translating a time-domain waveform with a fast Fourier transform (FFT), and then measuring the noise and distortion characteristics in the frequency domain. Frequency domain measurements provide magnitude and phase characteristics versus frequency.

Frequency Resolution and Accuracy

Using the FFT to quickly transform a signal into its frequency components is powerful, because it reveals signal characteristics that can’t be seen in the time-domain. The FFT used within ZTEC oscilloscopes returns complex IQ data which is then converted to magnitude and phase data. Figure 1 shows the result of calculating the FFT of a signal and a few of the measurements.

ZTEC oscilloscopes provide four FFT windows that can be applied as well. Windows are used to increase the spectral resolution in the frequency domain. The Rectangular Window provides the best frequency and worst magnitude resolution. It is almost the same as no window. The Blackman-Harris Window provides the best magnitude and worst frequency resolution. The Hamming Window provides better frequency and worse magnitude resolution than the Rectangular Window. It provides slightly better frequency resolution than the Hanning Window. The Hanning Window provides better frequency and worse magnitude resolution than the Rectangular Window.

Like some of the vertical- and horizontal-axis measurements discussed previously, the accuracy of the FFT can be improved by analyzing longer waveforms. Due to the nature of the calculations, the resolution is limited to half of the resolution of the onboard processor. In the case of the ZTEC ZT4611 oscilloscope, which uses a 64-bit processor, the accuracy would be limited to 32 bits of resolution. The FFT algorithm is binary in nature, so for the best performance it is wise to select a waveform size that is equal to 2^N.

Frequency Domain Measurements

Once a signal has been converted to the frequency-domain, five valuable measurements can be performed as explained in the following paragraphs. All of these measurements assume that the input signal is a perfect single-frequency sine wave and that all other frequency components are assumed to be harmonics or noise. All except the ENOB (bits) are expressed in decibels relative to carrier (dBc). THD is the only negative value.

The Signal-to-Noise Ratio (SNR) is the ratio of the RMS amplitude of the fundamental frequency to the RMS amplitude of all non-harmonic noise sources. SNR does not include the first nine harmonics as noise. In Figure 1, the SNR would be computed by dividing the magnitude of the fundamental by the sum of the magnitudes of all of the other frequency components, excluding the 2nd through the 10th harmonics. SNR is commonly used when only the narrow-band around the fundamental frequency is of concern and the harmonics will not have an effect on the system under test.

The Total Harmonic Distortion (THD) is the ratio of the RMS amplitude of the sum of the first nine harmonics to the RMS amplitude of the fundamental. In Figure 1, this would be calculated by summing the magnitudes of the 2nd through the 10th harmonics and then dividing that by the fundamental magnitude. THD is a concern when using active components such as amplifiers and mixers where the harmonics need to be minimized to reduce distortion.

The Spurious-Free Dynamic Range (SFDR) is the ratio of the RMS amplitude of the fundamental to the RMS amplitude of the largest spurious signal. This spurious signal can be a harmonic or noise frequency component. In Figure 1, the SFDR would be computed by dividing the magnitude of the fundamental by the magnitude of the 2nd harmonic, since it is the largest spurious signal. SFDR is used when there is a dominant spurious signal in relation to the other noise and distortion components.

The Signal-to-Noise and Distortion (SINAD) is the ratio of the RMS amplitude of the fundamental to the RMS amplitude of the sum of all noise and distortion sources. This is equivalent to the sum of the SNR and THD. In Figure 1, this would be calculated by dividing the magnitude of the fundamental by the sum of the magnitudes of all of the other frequency components, including harmonics and noise. SINAD is used in broad-band applications where all harmonics and noise will affect the signal.

The Effective Number of Bits (ENOB) is another way of expressing SINAD. It provides a measure of the input signal dynamic range as if the signal were converted using an ideal ADC. For instance, the ENOB of an 8-bit oscilloscope is often somewhere in the 6-7 bit range due to the noise and distortion affecting the instrument. The ENOB is calculated using the following equation:

High-Speed ADC Test Example

The specifications and test procedures of a high-speed Analog to Digital Converters (ADCs) are generally expressed in the frequency domain. The frequency measurements on a ZTEC oscilloscope can be used to mimic a more expensive spectrum analyzer to complete these tests. One test that is often used is a two-tone or multi-tone distortion test. This is completed because intermodulation distortion can occur when the ADC samples a signal composed of more than one sine wave. Figure 2 shows the FFT of an acquired ADC data record undergoing a two-tone test. Once the FFT is created, measurements such as THD and SINAD can be used to characterize the performance of the ADC.

This concludes the third and final installment of "The Fundamentals of Oscilloscope Measurements". Hopefully, these articles have provided our readers with a little deeper understanding of the waveform measurements available from an oscilloscope. This understanding can help users leverage the power of oscilloscopes more effectively and avoid potential pitfalls.

ZTEC Instruments in Biologically Inspired Acoustic Systems (BIAS)

Approaching the Capability of Bats and Dolphins

Bats and dolphins have developed very sophisticated means of object detection, location and characterisation. Their capabilities in the generation, reception and processing of acoustic signals go way beyond man’s current understanding. Current technology drivers in most scientific, military and industrial fields include increasing resolutions, lowering power consumption and improving material assessment and characterisation. Existing solutions often result in current technologies merely being driven harder, but new, novel technologies can be developed from a fresh view of the way bio-acoustic systems solve similar problems.

How bats and dolphins achieve such high levels of object detection, location and characterisation has been the focus of much research. One aim being to develop similar, ‘bio-mimetic’ systems, but it is still unclear the effort that is required to approach the capability of bio-acoustic systems. Recent experiences suggest that new research should be ‘bio-inspired’ and investigate the way in which energy is delivered to and returned from the source of investigation.

New Research: The BIAS Project

Researchers from the NERC British Geological Survey, the Universities of Southampton, Edinburgh, Leeds, Strathclyde, Leicester and Fortkey Ltd have formed the BIAS consortium to undertake research into the tools / techniques used routinely by nature that hitherto have not been embraced by man. Key challenges for a new approach include breaking the quarter wavelength barrier that currently limits spatial resolution, and the development of new signals for improved power efficiency and material property characterisation. The project will establish an experimental programme to be undertaken at three ultrasonic laboratory facilities:

• firstly, a waterborne, high frequency ultrasound facility focusing on medical physics applications
• secondly, a waterborne, low frequency ultrasound facility focusing on the physical characterisation of materials for geological applications,
• and thirdly, an airborne, low frequency ultrasound facility focusing on the characterising the effect of aspect angle on echo patterns.

Novel Ultrasonic Systems for New Coded Waveform Generation and Acquisition

A new study of the phase and magnitude of the echo-reflected wave is key to understanding the achievements of bats and dolphins. The Ultrasonics Research Laboratory at the British Geological Survey has commissioned Alba Ultrasound Ltd., Glasgow, UK to provide very wideband piezo-composite transducers to generate a new family of coded waveform signals to be used in this study. The bandwidths and efficiencies will be further improved when combined with new transducer matching networks commissioned by Blacknor Technologies, Portland, UK. The generation and detection of these new coded waveforms requires systems with very long memory lengths and high sampling frequencies at high dynamic ranges offered by ZTEC instrumentation.

ZTEC’s ZT530PXI-01 Arbitrary Waveform Generator has the voltage output capability to directly drive Alba Ultrasound’s wideband transducers. The ZT530PXI-01 offers sampling rates of up to 400MSs-1 at voltage levels of 20V peak to peak unlike many other competing products, which only offer 3V peak to peak.

ZTEC’s ZT530PXI-01 Arbitrary Waveform Generator can provide up to 4MSamples and the ZT410PXI-51 Digital Oscilloscope can acquire up to 16MSamples. Long memory lengths are vital for the generation and detection of signals such as wideband linear chirps. The linear upsweep above comprises 10, 000 samples at 10MSs-1 with a start frequency of 200kHz and an end frequency of 800kHz within 1ms, and is succeeded by a tail of 30, 000 points. The 200kHz to 800kHz upsweep was acquired with the ZT410PXI-51 with 125, 000 samples at 100MSs-1.

Step quantisation is a phenomenon that occurs when digitising analogue signals and can be seen as the staircase structure on the signals in the above figure. Step quantisation can introduce phase distortion to the signals, which can add unwanted phases for example when driving power amplifiers. Step quantisation can be reduced in the signal emitted from the ZT530PXI-01 using the interpolation feature to provide a smother signal input into the power amplifier.

Signal discrimination techniques can be much improved even if the transmitting transducer technology is bandwidth limited. For example, Barker code sequences can be sent to a transmitting transducer, each of which initiates a wave packet mainly containing damped oscillation at the resonant frequency of the transducer. The multiple reflections when returned to this transducer produce a composite signal that is the result of the convolution of all the echo returns from the various reflectors. Not only is the transmitted signal of long duration, but also the detection of pulse echoes requires a very long acquisition window. The ZT410PXI-51 with its combined 16MSample memory and very high sampling rate offers the capability of very high sampling rates of long duration events. This allows time delay measurements to very high resolution during pulse compression. The example below shows the deconvolution of an 11-point Barker code emitted from a piezoelectric transducer from a complicated pulse train comprising multiple echoes.

Bat and Dolphin Signals: Generation and Detection

Many bat species emit frequency modulated (FM) or combined FM and constant frequency (CF) signals, in pulse lengths ranging from 0.3 to 300 ms, with frequencies ranging from around 10 to 200 kHz. The figure below shows a complex chirp signal emitted from bat, and the time-frequency spectrogram shows that two FM signals are emitted, and, while one is delayed from the other, there is still some pulse overlap between the two signals.

The majority of dolphins emit clicks comprising a few cycles that don’t tend to change much in duration or shape, but where the frequency band appears to be related to signal intensity. High intensity signals tend to have centre frequency of 100 kHz or above and low intensity signals tend to have centre frequencies around 30 to 60 kHz, as shown below.

Recent research has shown that using this range of frequencies dolphins can detect differences in the wall thickness of metal cylinders of around 250 microns, or 1/50th of the wavelength. We cannot achieve anything like this at present but we hope to redress this balance with our new research programme.

Contributed by
David Gunn and Peter Jackson, British Geological Survey, Keyworth, Nottingham, UK
Said Assous and Clare Hopper, Leicester University, Leicester, UK

This work is being under taken as part of the Biologically Inspired Acoustic Systems (BIAS) project, which is funded by Research Councils UK via the Basic Technology Programme.

For more information, please visit http://www.biasweb.co.uk.

Oscilloscope Measurement Fundamentals: Horizontal-Axis Measurements (Part 2 of 3)

This article is the second installment of a three part series in which we will examine oscilloscope measurements such as the ones available in hardware within the ZTEC family of modular oscilloscopes. Many oscilloscope users take advantage of only a small fraction of the powerful features available to them. In addition, selecting the right measurement from a catalog of possibilities and accurately interpreting the results can lead to confusion and mistakes. This series of articles is intended to help users understand oscilloscope measurements more completely in order to avoid common pitfalls. Digital storage oscilloscopes vary greatly among vendors in terms of form factor (stand-alone, PXI, VXI, PCI, etc), resolution (8-bit, 12-bit, 16-bit, etc), acquisition rates (1 MS/sec, 1 GS/sec, 40 GS/sec, etc), functionality (advanced triggering, deep memory, self-calibration, etc.), and more. One aspect that separates true oscilloscopes from most PC-based, modular digitizers is the ability to make measurements in hardware on an onboard processor. The available measurements also differ from one oscilloscope to another, although this paper will cover a large segment of them. In addition, the algorithms used to complete the measurements may differ slightly among vendors. This paper will focus on the measurements and algorithms used in ZTEC modular oscilloscopes, but most of these concepts are universal. Oscilloscope measurements can be sorted into the following three categories: • Vertical-Axis • Horizontal-Axis • Frequency Domain Part two of the series will focus on horizontal-axis measurements.

Horizontal-axis measurements involve analyzing the horizontal time axis of the applied signal, and include measurements such as Period, Frequency, and Rise Time. The value returned is usually in time, but can also be expressed as a ratio, radians, or in Hertz.

Horizontal Resolution and Accuracy

The horizontal-axis resolution is limited by the sample rate of the onboard clock. A board with a 1 GS/sec acquisition rate can only achieve a time resolution of 1 / (1 GS/sec) = 1 nsec. Much like the vertical axis, the horizontal-axis accuracy can be reduced by high- and low-frequency errors.

High-frequency errors consist of clock jitter or phase-noise, but these are usually minute when considering that clocks used on most oscilloscopes have errors of 100 parts per million (ppm) or less. An error this small is insignificant when compared to the accuracy of the vertical axis. Occasionally, when completing horizontal-axis measurements, it may appear that clock jitter or phase-noise is causing incorrect readings. However, it is usually the lack of vertical-axis accuracy or noise that causes the incorrect time measurement. This will be further discussed later in the Edge Measurement section.

Low-frequency errors can be a problem and consist of drift associated with temperature, aging, etc. Annual factory calibrations must be completed to guarantee the accuracy of the clock over a long period of time.

Horizontal Waveform Measurements

The majority of the horizontal-axis measurements are fairly straight forward. They are shown in Figure 1. The Period measures the average time for a cycle to complete using the entire waveform in the capture window. The Frequency is the inverse of the period and is measured in Hertz. The Positive Pulse Width measures the time from the first rising edge to the first falling edge, while the Negative Pulse Width does the opposite. The Positive and Negative Duty Cycles are then calculated by taking the ratio of their corresponding Pulse Widths to the Period. All of these measurements are calculated based on the Middle voltage level which is simply halfway between the High and Low values. The time of the first maximum and minimum levels can also be retrieved using the Time of Maximum and Time of Minimum measurements.

When acquiring Period and Frequency measurements their accuracy can be very much affected by the sample rate. Both of these measurements are calculated by counting the number of samples that occur between Middle crossings. If a 10 MHz signal is being sampled at 100 MHz, this will result in exactly ten samples per period. The samples at the zero crossings may be very near the borders. If one is missed, this results in only nine samples being detected which returns a Period of 9 * (10nsec) = 90 nsec and a resulting Frequency of 11.1 MHz. This resolution is obviously not very good. It could be improved by acquiring long waveforms to capture many cycles and average out the resolution error. Another solution would be to sample the signal at 1 GHz or greater. Overall, for more accurate Frequency and Period measurements, it is best to sample at a far greater rate than the signal and capture many cycles. Cycle Average and Cycle Frequency measurements can be used to measure only the first cycle if desired. Also, the gated methods described in the vertical-axis section can also be employed. All of these methods are still susceptible to the resolution errors described above.

Phase measurements make most sense when acquiring two or more waveforms to determine how many radians or degrees a waveform is shifted in relation to another. However, the phase can be measured on a single periodic signal. This can be confusing, but it is simply calculated by comparing the starting point of the waveform to the rising edge Middle crossing. Figure 2 shows one signal with a positive 90 degree (1.57 rad) phase shift and another with a 270 degree (4.72 rad) phase shift.

Edge Measurements

A subset of Horizontal-Axis measurements is Edge Measurements. All of these measurements are made in relation to the Reference High (REF HIGH), Reference Middle (REF MID), and Reference Low (REF LOW). These references are user-selectable and are different than the High, Middle, and Low levels discussed in the previous sections, which are not user-selectable. By default, the REF HIGH, REF MID, and REF LOW are set to 90%, 50%, and 10% of the Amplitude (High – Low). However, all of these percentages can be adjusted to suit the application’s needs, or input in terms of absolute voltages.

With a firm understanding of the references, the meaning of the edge measurements becomes clear. They are shown in Figure 3. The Rise Time (RTIMe) measures the relative time for the leading edge of a pulse to rise from the REF LOW to the REF HIGH. The Fall Time (FTIMe) measures the same thing on the falling edge. The Rise Crossing Time (RTCRoss) is the absolute time when the waveform rises above the REF MID, measured from the start of the waveform. The Fall Crossing Time (FTCRoss) measures the same thing on the falling edge. All four of these measurements are edge selectable, meaning that the user can choose which number edge to characterize within the capture window.

One possible problem when taking edge measurements are inaccurate crossings due to noise on the vertical axis. Figure 4 shows a signal with and without vertical noise and how that could affect a horizontal measurement. The noisy signal crosses the voltage thresholds at slightly different points than the smooth signal, causing a shorter Rise Time Measurement. Another problem with a noisy signal is the potential for false crossings. This occurs when noise causes a signal to dither near the crossing points in several recorded crossings. Both of these problems can be avoided by either oversampling and averaging or by using the Smooth function before taking the measurement to reduce the noise. The algorithms used on ZTEC oscilloscopes incorporate hysteresis at the crossings which helps avoid detecting false crossings. This does result in a minimal detectable edge, however.

Relative vs. Absolute Measurements

Much like the distinction made between absolute and relative voltage measurements made in the vertical-axis section, there are absolute and relative time measurements as well. For example, the Period of a waveform compares two points on the same waveform, so it’s often unnecessary to relate this to a real-world or absolute time. Therefore, this is considered a relative time measurement. An example where the absolute time would be important is measuring the Time of Maximum (TMAX), which returns the timestamp of the first maximum voltage level in relation to the start of the acquisition.

Oscilloscope Measurement Fundamentals: Vertical-Axis Measurements (Part 1 of 3)

This article is the first installment of a three part series in which we will examine oscilloscope measurements such as the ones available in hardware within the ZTEC family of modular oscilloscopes. Many oscilloscope users take advantage of only a small fraction of the powerful features available to them. In addition, selecting the right measurement from a catalog of possibilities and accurately interpreting the results can lead to confusion and mistakes. This series of articles is intended to help users understand oscilloscope measurements more completely in order to avoid common pitfalls. Digital storage oscilloscopes vary greatly among vendors in terms of form factor (stand-alone, PXI, VXI, PCI, etc), resolution (8-bit, 12-bit, 16-bit, etc), acquisition rates (1 MS/sec, 1 GS/sec, 40 GS/sec, etc), functionality (advanced triggering, deep memory, self-calibration, etc.), and more. One aspect that separates true oscilloscopes from most PC-based, modular digitizers is the ability to make measurements in hardware on an onboard processor. The available measurements also differ from one oscilloscope to another, although this paper will cover a large segment of them. In addition, the algorithms used to complete the measurements may differ slightly among vendors. This paper will focus on the measurements and algorithms used in ZTEC modular oscilloscopes, but most of these concepts are universal. Oscilloscope measurements can be sorted into the following three categories: • Vertical-Axis • Horizontal-Axis • Frequency Domain Part one of the series will focus on vertical-axis measurements.

Vertical-axis measurements analyze the vertical component of the applied signal. These measurements most often describe a signal in terms of a voltage level. However, they can also correspond to current, power, or any other physical phenomena converted to voltage via a probe or transducer. Some common vertical-axis measurements include Amplitude, Peak-To-Peak, Average, and RMS measurements.

Vertical Resolution and Accuracy

The resolution and accuracy of an oscilloscope can affect measurements greatly, so it’s important to understand these limitations. An oscilloscope with an 8-bit analog-to-digital converter (ADC) has 2⁸ (256) levels available while a 16-bit ADC has 2¹⁶ (65536) levels. Thus, a 16-bit oscilloscope has 256 times more resolution than an 8-bit oscilloscope. Since only finite levels are available to represent the signal, there is a quantization error of 1 least significant bit (LSB). To find the minimum detectable voltage change (code width), divide the input range by the number of levels. Figure 1 depicts a 16-bit oscilloscope digitizing an 8 V_pp square wave with a 100 mV ripple voltage. In this case, the oscilloscope’s code width is (10/65536) 150 uV which allows it to produce a good representation of both the large and small signals. An 8-bit oscilloscope’s code width would be only (10/256) 39 mV, so it could not show the 100 mV component adequately. Changing the input range setting to 250 mV_pp would improve the performance, reducing the code width to (0.25/256) 1 mV.

The dynamic range of an oscilloscope refers to how well the instrument can detect small signals in the presence of large signals and is expressed in decibels (dB). It is limited by the quantization error and all other noise sources such as background noise, distortion, spurious signals, etc. The equation for computing the dynamic range is:

V_max is the maximum voltage that must be acquired and Vres is the minimum resolution that can be seen. A good rule of thumb is that every bit of resolution equals 6 dB of dynamic range. An 8-bit instrument’s theoretical maximum dynamic range is 48 dB, but it is significantly less once all limitations are considered.

Accuracy refers to the oscilloscope’s ability to represent the true value of a signal. An oscilloscope with high resolution, does not necessarily translate into giving an accurate result. Accuracy and resolution are related though, because the achievable accuracy of an instrument is limited by the resolution of the ADC.

The factors that reduce the accuracy of an oscilloscope can be mostly lumped into high- and low-frequency errors. Noise is generally the cause of high-frequency errors, while low-frequency errors are caused by drift stemming from temperature, aging, bias currents, etc. High-frequency errors can usually be removed by oversampling and averaging. Low-frequency errors often require the calibration of the instrument, either internally or through a factory calibration.

Relative vs. Absolute Measurements

An oscilloscope’s accuracy is often specified in terms of gain accuracy and offset accuracy. Gain accuracy is related to how well it handles high-frequency noise and can be called its relative accuracy. Offset accuracy is related to how well it handles the low-frequency issues and can be referred to as absolute accuracy. Figure 2 shows a real and measured 1 V_pp sine wave. Notice that the measured Amplitude is 0.99 V which equates to a gain error of 0.01 V or 1%. The measured signal is also offset 0.02 V for a 2% offset error.

Vertical-axis measurements can either be relative or absolute in nature. Relative measurements compare two voltages within the same signal. Amplitude is an example of a relative measurement because it returns the difference between the high and low voltage. The Amplitude of a 1 V_pp sine wave will be the same when it is centered at zero or has an offset of 5 V. Therefore, relative measurements are unaffected by the offset error. Absolute measurements are a representation of their real-world value and are affected by gain and offset errors. The Average measurement is an example of an absolute measurement.

Amplitude vs. Peak-To-Peak

Two vertical-axis measurements that are often confused are Amplitude and Peak-To-Peak. This is understandable because they are identical for all types of signals, except a pulse signal. Figure 3 shows the difference between the Amplitude and Peak-To-Peak (PTPeak) for a pulse signal. Peak-To-Peak returns the difference between the extreme Maximum and Minimum values, while the Amplitude returns the difference between where the pulse settles at the top (High) and bottom (Low) of the signal. The other measurements shown--Rise Overshoot (ROV), Rise Preshoot (RPR), Fall Overshoot (FOV), and Fall Preshoot (FPR)--are only valid when measuring pulses.

The measurements shown in Figure 3 are computed on the oscilloscope processor using a histogram. Figure 4 shows how the pulse signal in Figure 3 is represented in an 8-bit oscilloscope histogram. The samples are sorted into one of 256 bins, each corresponding to a voltage range. The algorithms simply look for the bit value with the most points for the Low and High measurements and the absolute largest and smallest bit values for the Maximum and Minimum. This allows for an extremely fast computation, but the measurement’s resolution is limited by the quantization error (1 LSB) of the ADC. The accuracy also suffers due to a single sample’s susceptibility to noise.

Root Mean Squared (RMS) & Average

The Direct Current (DC) RMS, Alternating Current (AC) RMS, and Average measurements are methods of characterizing the vertical level and power using the entire waveform.

The Average function is the mean vertical level of the entire captured waveform. It can be calculated by taking the sum of all of the voltage levels and dividing that by the number of points as shown:

The DC RMS and AC RMS measurements return the average power of the signal. The DC RMS returns the entire power contained within a signal including AC and DC components. This can also be described as the heating power when applied to a resistor. The AC RMS is used to characterize AC signals by subtracting out the DC power, leaving only the AC power component. The equations for the RMS measurements are as follows:
Figure 5 shows these results on a 4 V_pp square wave with 0.5 V of offset.

All three of these measurements are capable of more accuracy than the Amplitude and Peak-To-Peak measurements described in the previous section. The reason for this is that every single point in the waveform is included in the calculation of the Average and RMS measurements. This naturally cancels out noise that may be present in the signal. Additionally, when measuring the Average or RMS values, the more points that are acquired in the waveform, the better the accuracy of the measurements become. The upper bound of the accuracy is determined by the number of bits in the onboard processor. Some oscilloscopes use a 16-bit processor, so these measurements are limited to 16 bits of resolution because the largest number that can be stored on the chip is 16-bits. However, the 64-bit processor on the ZT4611 modular oscilloscope allows users to attain up to 64 bits of resolution. The tradeoff for the higher accuracy is longer computations since more points must be analyzed.

When only a few cycles of a waveform is acquired, it becomes critical to acquire only the full cycles or otherwise the results contain an asymmetric error. Figure 6 shows the same signal as Figure 5 except that an additional (high) half cycle was acquired. The Average and RMS values are offset because of this.

There are a few ways to avoid this circumstance. The best way is to acquire a longer waveform that includes many cycles so that the offset is effectively minimized. This method requires more time and more onboard memory to store the waveform. Another way to solve the problem is to make use of the Cycle RMS or Cycle Average measurements. These calculate the RMS and Average including only the points from the first cycle of the waveform. The third way to solve the problem would be to use a gated measurement. Gated measurements allow the user to choose the points that are included. This can be done by selecting a start and stop time or a start and stop point. Both the Gated By Time and Gated By Points methods require the user to know the period of the waveform to solve the problem shown in Figure 6.

Interactive High Speed Measurements with the new ZTEC Plug-in for SignalExpress

Leveraging ZTEC modular oscilloscopes and National Instruments SignalExpress software, design and test engineers can now acquire and analyze high-speed measurement signals without any programming required. The new ZTEC Scope Plug-In for SignalExpress allows technical professionals to interactively acquire signals and take oscilloscope measurements simply by using the drag-and-drop capabilities of SignalExpress.

Figure 1: Combining the Power of ZTEC Modular Oscilloscopes and NI SignalExpress Software

Oftentimes, engineers and scientists simply need to acquire a high-speed signal for a rudimentary visual verification or take a few quick measurements to validate their design or unit under test. In these cases, it is too time-consuming and arduous to write an entire application for what is often a one-time test. At the same time, a bench-top oscilloscope or the soft front panel provided with ZTEC modular oscilloscopes may not provide enough functionality or flexibility to complete the entire required test set. Common application areas that fall into this gap include design modeling, design verification, design characterization, device validation, and automated test troubleshooting. National Instruments SignalExpress combined with the new ZTEC Scope Plug-in fills this void for high-speed interactive measurements.

A PC Oscilloscope That Looks and Works Like the Real Thing!

As the leader in modular oscilloscopes, ZTEC is continually looking for ways to pack even more benchtop oscilloscope features into our modular instrument products.  One key and often overlooked aspect of a benchtop oscilloscope is the well defined and intuitive user interface.  All technical professionals, at one point or another, have used a benchtop oscilloscope - turning horizontal and vertical adjustment knobs to capture a waveform of interest. 

New LabVIEW Drivers Make Programming For Your ZTEC Card a Snap!

ZTEC Instruments provides “plug-and-play” instrument drivers with all its modular instrument products.  Developed with National Instruments’ LabWindows/CVI, plug-and-play instrument drivers have many advantages.  Because they are based on industry standards, plug-and-play drivers provide an intuitive and flexible programmatic interface that can be used in a wide variety of situations.  For customers using LabWindows/CVI, the interface is ideal.

Ironically, plug-and-play instrument drivers are not the ideal solution for customers using National Instruments LabVIEW.  LabVIEW does provide a plug-and-play driver auto import utility.  However, the resulting VI library is lacking key usability features commonly associated with a LabVIEW interface.  With the release of LabVIEW 8, the imported VI libraries have become even more awkward and difficult to generate.  Due to the increasing difficulty of using plug-and-play drivers with LabVIEW, ZTEC has developed a more native solution.

ZTEC now offers LabVIEW specific drivers for its complete line of PXI and PCI modular oscilloscope products.  These new drivers, compatible with LabVIEW 7.1 or greater, are available for download at the end of this article for the ZT410PXI/PCI,  ZT450PXI/PCI,  ZT431PXI/PCI, and ZT530PXI/PCI. ZTEC is also committed to provide LabVIEW specific drivers for all new instruments.

Once installed, the instrument’s VI library will be available in the “Instrument I/O>>Instrument Drivers” subpalette (Figure I).  The subVIs contained in the palettes provide complete programmatic control of all the features and capabilities of the instruments.  The palettes are organized following National Instruments guidelines making their use intuitive for even beginner LabVIEW programmers. 

 

Figure I: ZT450PXI/PCI VI Pallets

On the main VI palette you will find the typical VIs, including “Initialize.vi” and “Close.vi”.  In between the initialize and close VIs, you will find subpalettes for instrument configuration, action, status, and data operations.  In addition, a SubVI Tree is provided (Figure II) to give the user an overview of the driver’s structure and capabilities.  Often, users will “drag-and-drop” subVIs directly from the SubVI Tree instead of using the palettes.

 

Figure II: ZT450PXI/PCI SubVI Tree

Each subVI is fully documented, including tip strip “bubbles,” control and indicator help, and Context Sensitive help (Figure III).  When possible, inputs are enumerated to help self-document your program and ensure valid input settings. 

 

Figure III: ZT450PXI/PCI Context Sensitive Help

Using the SubVI Tree and help system, it becomes very easy to program a custom application for a ZTEC modular instrument.  Using as little as 5 subVIs, you can configure your instrument, acquire, and measure an input signal (Figure IV). 

 

Figure IV: ZT450PXI/PCI Basic LabVIEW Example

To get you up and running in minutes, ZTEC also provides the ZScope soft front panel.  ZScope’s intuitive oscilloscope interface provides complete instrument control without the need for programming. ZScope auto-detects your hardware and can be automatically configured using the AutoSetup feature. Waveform data can be easily exported for use in other analysis applications and instrument settings can be saved for use at a later time.

Figure V: ZScope Soft Front Panel

ZTEC is committed to provide the functionality, technology, integration, and assistance you need to make your test and measurement integration projects a success. Providing a LabVIEW specific programming interface is yet another example of this commitment. With these new LabVIEW specific drivers it is now easier than ever to program your ZTEC modular oscilloscope with LabVIEW.

Please visit the ZTEC Support Center to download the current LabVIEW Drivers.

Programming as Easy as 1, 2, 3 (4 and 5)

For those developing automated test and measurement applications, an intuitive instrument driver is probably the most important tool.  ZTEC understands this fact and provides “plug-and-play” instrument drivers with all their modular instrument products.  These drivers provide complete access to all the instruments capabilities and can be used in most commonly used programming environments, including National Instruments LabWindows/CVI and LabVIEW.  To download the latest version of all ZTEC’s instrument drivers, please visit the ZTEC customer support center.

Programming a ZTEC modular instrument could not be easier when using the instrument driver.  In fact, in as few as 5 functions/VIs, a program can be written to self-configure an instrument, acquire a waveform, and perform a waveform measurement.  The LabWindows/CVI and LabVIEW driver are identical with a one-to-one map of each function to each VI.  The native interface (called “Functions Panels” for LabWindows/CVI and “VI Libraries” for LabVIEW) provides a consistent and familiar interface fore each environment.  A windows help file is also provided for driver documentation.