Capturing The Essence Of An Oscilloscope

Analog and digital oscilloscopes continue to improve in performance, serving as invaluable members of any RF/microwave design test bench.

Oscilloscopes are among the workhorse test instruments on any RF/microwave engineer's test bench. Tey are versatile in their signal analysis capabilities, presenting onscreen images that even novice users can interpret to better locate problems with a circuit design. Oscilloscopes are available in a wide range of shapes, sizes, and performance levels from a variety of manufacturers, and knowing what is available can often simplify the process of matching a new oscilloscope to a measurement need.

Oscilloscopes have been at work longer than most instruments, with origins that can be traced back more than 100 years. The original analog oscilloscope is based on the cathode-ray-tube (CRT) technology developed by Karl Braun in 1897. By controlling the voltages across the CRT, he was able to display two-dimensional imagesthe essential function of an oscilloscope. The first commercial dual-beam oscilloscope was introduced in the late 1930s by the British firm, AC Crossor; the instrument saw a great deal of service in testing and maintaining the radar systems in use during World War II. After the war, Howard Vollum and Jack Murdock, created an oscilloscope that could sweep across a given time span and trigger on desired test signals, showing waveforms in a more controlled fashion than the Crosser scope. The partners would later start Tektronix, which would become a leading supplier of analog, and later digital, oscilloscopes among many other test instruments. Another leading oscilloscope company was founded by Walter LeCroy after, in 1963, he had developed the first digital storage oscilloscope (DSO). LeCroy had previously researched ways to digitize electronic signals on behalf of the Swiss research center CERN.

From this brief history, it is apparent that the earliest oscilloscopes were fully analog instruments. It was not unusual to see a setup with an analog oscilloscope and a Polaroid camera to capture a screen image. An analog scope relies on the functionality of a CRT to show images on a display based on horizontal (x) and vertical (y) input voltages. The horizontal direction is controlled by the scope's timebase, while the deflection of the signal in the vertical direction is influenced by the level (amplitude) of the input signal, thus showing the behavior of a signal as a function of time. Typically, amplifiers are used to boost the input voltages fed to the CRT.

As digital signal processing became available, notably high-performance analog-to-digital converters (ADCs), developers of oscilloscopes were able to digitize input signals and process them in the digital realm. They could then be converted back to analog signals by means of a digital-to-analog converter (DAC) and shown on a CRT or other display device. Currently, three types of digital oscilloscopes are available: digital storage oscilloscopes (DSOs), digital sampling oscilloscopes, and digital phosphor oscilloscopes (DPOs).

A DSO incorporates a firststage vertical amplifier prior to an ADC, which samples input signals at a given sampling rate. Those signal samples are stored in memory within the instrument to serve as the points of a waveform, or saved as an actual waveform record. Through the use of DSP circuitry, each signal can be processed to filter or enhance parts of the waveform prior to the digital data being sent to the oscilloscope's display memory and display. This type of oscilloscope is particular good at capturing a transient event in single-shot mode.

For capturing repetitive highfrequency, high-speed signals, a digital sampling oscilloscope collects samples from a number of successive waveforms and builds an image that represents the effective average of those many sampled waveforms. By operating in this fashion, a digital sampling oscilloscope can capture waveforms with speeds that are far greater than the basic sampling rate of the instrument by using interleaving techniques. In general, this type of oscilloscope samples signals prior to the vertical amplification to ensure maximum bandwidth, with no limiting by the vertical amplifiers.

The DPO is a relatively recent development that takes advantage of parallel processing to capture spurious and transient events that may be part of an analyzed waveform. In contrast to one signalprocessing chain in a DSO, where there is a delay between triggered signal captures during which time some signal events can be lost, a DPO uses two or more processors so that during the delay between triggers on one signal chain, the other processor is capturing signal events. This allows for seamless signal capture without the lost events of a DSO. Although the name implies a screen with some form of phosphor, the DSO architecture can actually be implemented with a number of different display technologies.

Selecting an oscilloscope is a matter of matching an instrument's measurement capabilities to the requirements of an application. Oscilloscopes can be compared by a number of key specifications, including bandwidth, input signal sensitivity, and the number of channels. For example, most oscilloscope manufacturers specify bandwidth at their instrument's 3-dB points, but this should be ascertained before making a fair comparison of instruments from different manufacturers. When considering bandwidth, the type of signal to be evaluated must also be considered. For example, the second or third harmonics of a fundamental-frequency analog signal would require an instrument with two or three times the bandwidth of the fundamentalfrequency signal to be tested. If studying digital signals, oscilloscopes are usually chosen with about 10 times the bandwidth of the digital signal rate, such as a 1-GHz bandwidth to study 100-Mb/s digital signals, given the fast rise times of the digital signals. The number of channels may seem like an obvious choice, since most oscilloscopes offer either two or four channels. But for digital oscilloscopes, it is important to note the speed and bandwidth of the individual channels. The performance of the individual channels must be adequate for an application requiring high-performance levels across multiple channels.

In addition, digital oscilloscopes include specifications for sampling rate and vertical bit resolution. For example, sampling rate, which is the number of digital samples captured per second, contributes to the amount of detail possible in a captured waveform, with higher/faster sampling rates providing greater resolution. Although most oscilloscope manufacturers tout the speed of their fastest sampling rates, it can be useful to know an instrument's slowest possible sampling rate when studying slowly changing waveforms. Although the Nyquist theorem states a sampling rate that is two times the highest frequency components of a signal to be studied, this can lead to missing certain glitches and random events. A safer rule of thumb is to apply a factor higher than 2, such as 2.5 or 3.0, when matching maximum scope sampling rate to a measurement application.

The vertical resolution indicates how many levels are used to represent the x-axis on an oscilloscope's screen. An instrument with vertical resolution of 8 b, or 28 levels of resolution, provides 256 levels of resolution for a signal under analysis. If signals under study are characterized by extremely wide dynamic range, a higher level of vertical resolution may be required.

DSO manufacturers typically specify a sampling rate that might be the interleaved total of the multiple channels. A two-channel unit with specified sampling rate of 200 Msamples/s, for example, may have a maximum sampling rate of 100 Msample/s for each channel. In terms of sampling rates, many manufacturers also quote an extremely high rate for repetitive sampling, such as 1 Gsamples/s or more by means of interleaving for an instrument with a sampling rate of 100 Msamples/s for each channel. Repetitive sampling can accurately capture signals that vary little from waveform to waveform, but the normal sampling rates should be applied when studying signals with rapidly timevarying waveforms.

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An important part of an oscilloscope is often overlooked: the test probes. These can be active or passive. Passive probes are more common and less expensive than active probes, although active probes can provide outstanding performance for challenging applications, such as measurements of extremely low signal levels. Passive probes usually are differentiated by an attenuator factor, such as a 10X probe, with a factor of 10 attenuation to measured signals. A 1X passive probe provides no attenuation, although it may not be the ideal choice in probes when measuring high-voltage signals. A 10X probe has a built-in attenuator with higher level of impedance than a 1X probe. The 10X probe presents a lower level of capacitance to a circuit under test than a 1X probe, reducing the high-frequency loading of the circuit by the probe. An active oscilloscope probe incorporates one or more integrated circuits (ICs) to assist an oscilloscope in capturing challenging signals, such as low-level or fast-rise-time signals. It features an extremely low capacitance and low loading of the circuit under test, for accurate measurements on sensitive circuits. Recognizing the importance of oscilloscopes on the test bench, global instrument supplier Rohde & Schwarz entered the market last year with several series of instruments, including the R & S RTM DSOs with 500-MHz bandwidth and sampling rates to 5 GSamples/s, such as the compact model R&S DTM 1054 (Fig. 1). Designed for versatility and universal use, these oscilloscopes lean on low-noise ADCs to achieve minimum input sensitivity of 1 mV/div and show waveforms on an easy-to-read 8.4-in. thin-film-transistor (TFT) display. Units are available with two and four channels and supported by an extensive line of active and passive probes.

Another leading supplier of oscilloscopes, Tektronix, last year boosted the sampling rates of its DSA70000C sampling oscilloscopes and DPO70000C DPOs to 100 GSamples/s to provide the benefits of five-times oversampling of signals across a 20-GHz bandwidth. The instruments were also equipped with a more stable timebase for more accurate pulsed radar measurements. The WaveMaster 8Zi-A digital oscilloscopes from LeCroy provide sampling rates to 120 GSamples/s at bandwidths to 45 GHz in two- and fourchannel versions (click here for a review). These scopes, like the Infinium 90000 X-series oscilloscopes (Fig. 2) from Agilent Technologies, with bandwidths to 32 GHz and sampling rates to 80 GSamples/s, make use of low-noise indium-phosphide (InP) front-end electronics for outstanding signal sensitivity over broad bandwidths.

While traditional oscilloscopes have been housed in benchtop or rack-mount enclosures, it is sometimes useful to have oscilloscope functionality that can be brought to a remote site. For example, a number of manufacturers offer analog and digital oscilloscopes for portable use, typically with bandwidths around 100 MHz for general-purpose low-frequency troubleshooting. The dual-channel D7510 series of DSOs from HC Protek feature 100-MHz bandwidth with 100 MSamples/s sampling rate and equipment sampling rates to 10 GSamples/s. Fluke's ScopeMeter instrument line includes the battery-powered, model 199C DSO with 200-MHz bandwidth and 2.5-GSamples/s sampling rate. It also provides all the capabilities of a digital multimeter (DMM) for testing versatility. The new 860FC from Protek (Fig. 3) is also a combination dualchannel DSO and DMM, with 60-MHz bandwidth and sampling rates as high as 100-MSamples/s for both channels and 200 MSamples/s for single-channel operation. It can achieve equivalent sampling rates as high as 2.5 GSamples/s and offers a sensitivity range of 5 mV/div to 100 V/s with 8-b resolution. The battery-powered tester can connect to a computer via USB 2.0 interface for simple firmware updates.

In addition, a growing number of products are being made available as portable units or in circuit card format that relies on a personal computer (PC) for some processing and display functions. Acquiris has been a long-time supplier of PCI-based instruments. Its model DP240 PCI digitizer is essentially a two-channel DSO with 1-GHz bandwidth and 2-GSamples/s sampling rate. Gage Applied Sciences also offers a two-channel PCI DSO as its model 82G digitizer with optional 1-GHz bandwidth and 1-GSamples/s sampling rate.

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