Test and measurement instrument development is never static

Article By : Arthur Pini

Instrument developments add to the versatility and usefulness of new test and measurement innovations.

Test and measurement instruments evolve over time. The banner improvements come in performance, things like measurement bandwidth and dynamic range. These always make the trade press headlines, but less apparent improvements involve secondary specifications, like number of channels, I/O interface speed, and combined functionality. This technology comes on more slowly with an almost unheralded pace.

Eight analog input channels

For decades, high frequency oscilloscopes offered two or four channels. Dual channel oscilloscopes were the practical limit for analog scopes. Digital oscilloscopes eliminated the cathode ray tube and its display limitations and moved to four channels. This limit stood for several decades until the size of the components provided room for additional channels. Over the past several years, eight channel oscilloscopes have been added to the product lines of three of four major oscilloscope suppliers. Having eight available analog channels opens up a whole range of measurement applications that do not require multiple passes in the measurement process.

Probably the most common measurement that requires more than four channels is three phase power. Three-phase electric power is the most common type of poly-phase AC electrical distribution system. It finds common application in electric power generation, transmission, and distribution, and is used to power large motors and other heavy electrical loads.

A three-phase device, such as three-phase motors, are connected in either WYE (four wire) or DELTA (three) configurations. A basic three-phase power determination for a wye connected device requires the measurement of three phase voltages (Va, Vb, and Bc) and three phase currents (Ia, Ib, and Ic). The power per phase for each power phase is computed by multiplying its voltage by its related phase current and the result is the instantaneous power in each phase. The mean value of the instantaneous power is the real power component. The sum of all three phase-power readings is the total real power of the device. This measurement is referred to as the three-wattmeter power measurement. In order to make this measurement using external differential voltage probes and current probes to measure the voltages and currents, it will require six channels.

An example of a three-phase power measurement on an eight-channel scope using a Teledyne LeCroy WaveRunner 8038HD is shown in Figure 1. This is one of the possible eight-channel oscilloscopes that are available from the major oscilloscope suppliers. The WaveRunner 8000HD series oscilloscopes have eight 12-bit channels. The three measurements of the phase voltages are made using differential probes. They are shown in the top three display grids. There are an additional three channels for the phase current measurements, made using current probes, that appear in the three grids immediately below the phase voltage measurements.

oscilloscope screenshot of a three phase power measurementFigure 1 For this three phase power measurement made on an eight-channel oscilloscope, six of the eight channels are used to measure three phase voltages and three phase currents simultaneously. The power contribution from each phase are summed to read the total power. (Click to enlarge)

This oscilloscope has 12 dual function math traces that are used to mathematically operate on any analog waveform. As an example, the product of the individual phase voltage and current are computed and appear in the third row from the top. Additionally, the phase power contributions are summed in the center grid of the bottom row. Generally, the number of math channel should be greater than the number of acquisition channels for instances such as this.

Parameter measurements of the RMS voltage and RMS current of each phase along with the mean phase power and the total power appear below the waveform displays. There is also a total of 12 measurement parameters available, like the math traces, and the number of measurement parameters should exceed the number of acquisition channels.

Displaying eight acquired waveforms and additional derived signals make the oscilloscope display quite busy, as seen in this example. Having multiple display formats and the ability to add labels to each display grid help clarify these displays. This example used a display format with 12 grids. Display grid configurations of one, two, four, six, eight, 12, 16, or 20 grids are supported, along with some application specific grid layouts.

Other applications that can benefit from having up to eight analog channels are automotive measurements, medical electronics, and nondestructive testing. Each of these applications uses multiple sensors and gains from being able to display them simultaneously. All the major oscilloscope suppliers offer specialized software supporting the most common applications, such as multi-phase power and motor drive analysis.

Multifunctional instruments

Another interesting development has been the introduction of instruments with combined functionality; instruments with internal signal sources matched to their measurement capability. This permits a single instrument to be used for stimulus-response testing. Many oscilloscopes have added function generators as an internal feature. Some of these generators offer arbitrary waveform generator nodes. This development continues from other instrument suppliers with an increase in the number of source and measurements channels per instrument.

As an example of this multichannel/multifunction development, consider the hybridNETBOX from Spectrum Instrumentation. This standalone LXI instrument offers up to eight digitizer channels for measurement and up to eight channels of arbitrary waveform generation as the signal source. The hybridNETBOX family offers six models with 2, 4, or 8 channels of 40, 80, or 125 MS/s digitizers and matching arbitrary waveform generators (AWGs), both instruments sharing a common clock and trigger. A DN2.806-08 hybridNETBOX is shown in Figure 2.

photo of the hybridNETBOX multifunctional instrumentFigure 2 The DN2.806-08 hybridNETBOX is a top-of-the-line instrument with eight 16-bit digitizer and eight 16-bit AWG channels clocked at a maximum sample rate of 125 MS/s linked to a PC via an LXI Ethernet link. Source: Spectrum Instrumentation

The front panel of this instrument has eight, 16-bit AWG outputs and eight 16-bit analog digitizer input channels. Both the digitizers and the AWG have a maximum clock rate of 125 MS/s and 512 MSample of memory per channel.

The AWG channels can produce almost any waveform with signal amplitudes that go up to ±3V into 50 Ω, or ±6V into high impedance. Additionally, there are four digital I/O connectors that can be used as marker outputs that are fully synchronous with the analog channel waveforms. It is a feature that enables precise control of other devices that may be connected in a test system. The final two connectors in the AWG section allow for an external clock and trigger input.

The eight digitizer channels can handle a wide range of input signals as they have variable ranges that go from ±200 mV up to ±10V. They also have programmable offset and selectable input impedance (50 Ω and 1 MΩ). Single-ended and differential measurement modes are both available. There is also an external clock and trigger input, as well as two more digital I/O lines.

These instruments are intended for applications that require an excitation source before the signal measurements can be made. Testing devices like amplifiers, filters, receivers, and digital interfaces are typical applications. A common source response measurement is to verify the bandwidth of an intermediate frequency (IF) channel, as shown in Figure 3.

A single AWG-digitizer pair or an oscilloscope with an internal function generator are used to determine the frequency response of a 10.7 MHz IF channel. The AWG outputs a linear sinewave frequency sweep covering a frequency range from 9 to 12.5 MHz, as shown in the upper left grid of the display. The fast Fourier transform (FFT) of the excitation signal appears in the upper right grid. The IF response appears in the lower left grid. The IF frequency response is shown in the lower right grid, revealing a bandwidth of 400 kHz. This can be duplicated for up to eight channels simultaneously using this instrument.

oscilloscope screenshot of frequency response of an IF channelFigure 3 These results show the frequency response of a 10.7 MHz IF channel using a swept sine excitation signal.

Multifunction instruments can be used for developing and testing multiple input multiple output (MIMO) array systems. Nondestructive test (NDT) systems use ultrasonic waves to penetrate materials and generate responses which can show voids, cracks, and other defects. The technology uses phased arrays of transducers to transmit and receive ultrasonic pulses. The transducer array uses multiple transmitted signals to steer and focus the ultrasound pulses (Figure 4).

diagram showing how to control a trasmitted pulseFigure 4 Use a phased array of transducers to control a transmitted pulse.

By controlling the delay of each ultrasonic pulse applied to the individual transducers, the output pulses can be directed or focused. If all the pulses arrive at the same time the wavefront radiates in a broad parallel pattern, as shown in the leftmost diagram. If the pulses arrive at the transducers sequentially, the effect is to steer the wavefront in the direction of the increasingly-delayed pulses, as shown in the center diagram. Finally, if the pulses have minimum delay at the outside transducers and increasing delay at the inner transducers, the effect is to focus the wavefront on a central target area, as shown in the rightmost diagram.

Such signal generation is easily controlled using an AWG as the signal source. The ultrasonic pulse carrier frequency, amplitude, delay, attack time, and decay time can be easily controlled, as noted in Figure 5.

oscilloscope screenshot showing the generation of ultrasonic pulsesFigure 5 The generation of eight ultrasonic pulses with increasing decay is intended to steer the combined output from a transducer array. The amplitude, carrier frequency, duration, attack time, decay time, and delay are each individually controllable.

Eight AWG channels produce 40 kHz ultrasonic pulses with increasing delay steps of 25 μs. The equation used for a single instance of the pulse is shown in the inset at the lower left corner of the figure. The colored over score lines indicate the parts of the equation that effect the output pulse shape. The attack time is set by a ramp function, while the decay is controlled by a decaying exponential. Eight individual equations, one per AWG channel, are used to define each individual pulse.

After the pulses are transmitted, the transducer array is turned around to receive. Each transducer is connected to a digitizer input (Figure 6).

diagram showing a transducer in receive modeFigure 6 In receive mode, each transducer is connected to the input of a digitizer via a programmable delay function. The delay values are used to program the array steering and focusing, just as was done during the transmit mode.

Steering and focusing of the transducer array in receive mode is accomplished using programmable delays, just as was done in the transmit mode. This can be done either with external programmable delay elements or by using finite impulse response filters available in the digitizers.

Multifunctional instruments like the hybridNETBOX fall right into applications such as phased array signal processing, where multichannel signal sources drive transducer or antenna arrays and multichannel digitizers acquire the return signals.

Local area extensions for instrumentation (LXI)

No discussion of instrument evolution would be complete without mentioning changes that have occurred in the remote input/output/remote control interfaces. LXI is the current industry standard for local area network (LAN) connectivity and control for instruments and modular systems using Ethernet. LXI is a standard that defines communication protocols for bench top and modular instrumentation systems establishing interoperability between such instruments. Most instruments support Ethernet connectivity via Gbit Ethernet, 100Base-T, or 10Base-T Ethernet standards. The LXI standard is maintained by the LXI Consortium.

LXI is intended to replace the decades-old IEEE-488 interface with a higher speed serial data interface. In addition to the LAN interface using the RJ-45 connector and CAT5 LAN wiring, the LXI interface supports VXI-11 discovery to detect LXI instruments on the network. LXI instruments must also provide an interchangeable virtual instrument or IVI driver to support a standard application programming interface (API) for communications with the instrument.

Figure 7 shows the LXI interface homepage for the Spectrum Instrumentation hybridNETBOX, which identifies the instrument and its LXI characteristics.

screenshot of an info page for the hybridNETBOXFigure 7 This page shows information for an instrument that is a combination of an 8 channel digitizer and an 8 channel AWG.

Every LXI instrument has a discoverable homepage like this that can be addressed on the network using a browser searching on the instrument host name or IP address.

Most mid-range and higher instruments offer LXI or USB interfaces. The older IEEE-488 interface is often available as an option for adding such instruments to an existing test system.

Test and measurement instrument development is never static; there is always something new. Changes such as increasing channel count, multifunctional topology, and faster remote interfaces add to the versatility and usefulness of these new innovations. Regardless of who introduces such new concepts, they soon find their way into instruments from multiple sources as users demand worthwhile improvements.

This article was originally published on EDN.

Arthur Pini is a technical support specialist and electrical engineer with over 50 years experience in electronics test and measurement.

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