Since its inception, the oscilloscope is one test and measurement product that has seen a lot of transformations to deal with the needs of new technological advancements. “Recent advancements in analogue and digital electronics with growth in telecom, medical electronics, automotive and other sectors have brought revolutionary changes in the scope,” says Vivek Mantri, country manager, industrial segment, Scientech Technologies Pvt Ltd. Requirements have elevated measurement parameters from MHz to GHz, when it comes to bandwidth of the scope. He adds, “Right from an engineering student analysing his basic astable multivibrator to engineers testing and verifying complex designs, the oscilloscope is a useful tool.”
But, there are often some features and methods that are unused or missed-out while testing designs. In this attempt, we try to cover some important functions of the oscilloscope that would help you effectively use it and efficiently test your designs.
Use memory depth feature for finer signal analysis
While considering an oscilloscope, the obvious parameters that you look for are bandwidth, acquisition rate and sampling rate, but one feature that is often overlooked is memory depth. To achieve highly-accurate calculations, memory depth is said to be one of the top features of oscilloscopes today. This feature allows better monitoring, triggering and analysis of signals, especially applicable on those signals with high-frequency components. Obtaining extended time periods with the capability to zoom into the signal, without loss of data, allows an engineer to verify the hardware design in a swift and proper manner.
Some digital oscilloscopes provide the ability of encapsulating waveforms in long-memory mode. For standard oscilloscopes, memory depth ranges from 1kpt to 16kpt (pt is point). With the long-memory feature, oscilloscopes can capture complex signals in great detail over extended time periods. This allows test engineers to inspect effects of high frequency within the captured waveform.
Use peak-detect mode to calculate small pulses
When the sampling rate of a digital oscilloscope is not high enough to fully capture details of a signal, you can use peak-detect mode to help capture the envelope or narrow-pulse information that may be lost when using a normal-acquisition mode. To know the difference between normal mode and peak-detect mode, use an arbitrary waveform generator to output a signal. For example, the signal could be 1kHz in frequency, with 5V peak-to-peak amplitude and 10ns pulse width. Connect this signal to channel one of your oscilloscope and adjust the time base of the oscilloscope to 1ms/div, vertical scale to 1V/div and memory depth to 140kpt.
The acquisition, or acquire mode, on your oscilloscope is usually set to normal mode and in that mode, there are some pulses with varying amplitudes that are seen, but the display is not stable. By changing the mode to peak-detect mode, the amplitude of pulses is fixed and a stable view is available. Therefore to accurately measure narrow pulses using peak-detect mode on your scope, first configure the vertical and time-base scale to capture the signal. Then, choose peak-detect mode to get the exact information about narrow-pulse or peak features within the signal.
Use ground spring probes to achieve signal and noise quality
Most common factors that need to be tested for verifying the quality of every design, especially embedded systems, are data-transmission rate, signal quality and timing specifications. It is important to analyse the overall accuracy and system-level timing delays of an embedded design. Another common issue with testing and verification is noise, which can come from a variety of sources, including electromagnetic interference (EMI), bandwidth problems and bad grounding.
For noise-related issues, alligator clip ground strap, which connects on the probe, is normally used to establish a ground connection. Although connections are made correctly, noise could still be an issue, in which case it would be advisable to use the ground spring instead. Comparatively, the ground spring connects closer to the tip of the probe and significantly reduces the loop area of the connection, thereby improving the noise and signal quality.
If ground noise still persists, detaching your oscilloscope from ground could help. By isolating the rest of the device or the device under test, ground loops are eliminated.
Use precision probe for measurements at high bandwidth
Oscilloscopes have reached very high bandwidth levels today. For instance, Teledyne LeCroy introduced the first 100GHz real-time oscilloscope, and Keysight oscilloscopes offer bandwidth up to 63GHz. “But, you need to be very careful while taking measurements at high bandwidths, even if it is 16GHz or 20GHz, as the microwave effects from cables, connectors and other peripherals tend to distort signals,” informs Sanchit Bhatia, application engineer, Keysight Technologies India Pvt Ltd. The response waveform might not be absolutely clean. “One possible method to obtain a noise-free response would be to first make measurements on a network analyser and then use the response of the cable and de-embed that in the oscilloscope,” adds Bhatia. [De-embedding is an attempt to make the port as ideal as possible, similar to a calibration of a radio frequency (RF) measurement system with probes or connectors used for measurement].
A precision cable, or precision probe, can be used in order to get rid of the anomalies caused by a regular cable or probe, and hence bypass the process of using an analyser. Bhatia says, “In our oscilloscopes, we have a built-in simple, mini network- analyser feature, which measures the response of the cable, or probe, by generating a high-speed pulse, and the insertion loss of the cable is measured up to double bandwidth of the scope.” He adds, “The measurement performed by the scope is used to compensate for the loss with calculations done by the software. This unique feature can be used to measure the insertion loss in the time domain, which is ultimately converted into frequency domain.”
Use software packages for performing different kinds of analyses
Naresh Narasimhan, country marketing manager, Tektronix India, says, “Engineers are increasingly customising their general-purpose oscilloscopes via software packages that are now available with many oscilloscopes in this class.” For instance, software applications, such as various serial decode packages, vector signal analysis (VSA) software, power application and offline viewing and analysis software, allow users to customise and use their general-purpose oscilloscopes in a very specific manner.
“SignalVu VSA software, from Tektronix, that runs on the company’s various oscilloscopes series, has been enabling engineers to easily characterise and validate wideband and microwave spectral events,” says Narasimhan. He adds, “SignalVu-PC VSA software helps one to easily validate wideband designs.”
The latest generation of oscilloscopes are also coming with Windows 7 embedded operating system (OS) platform today. Bhatia says, “With Windows 7 embedded OS comes the advantage of writing advanced software, thereby providing high analysis capabilities, such as jitter analysis or mathematical functions. New user interfaces are also powering Windows based scopes, which help users customise by creating a large number of windows, grids and mathematical functions on the screen of the oscilloscope.” These windows can be resized as per requirements. He adds, “For instance, some people prefer having very big measurement windows and a small waveform window, and vice-versa.”
Also, for ease of use, the actual vertical scale is printed on the oscilloscope’s screen. Normally, if the user has switched the scale to one volt/division and the waveform is seen across three divisions each from the central axis, it is interpreted as six volts by looking at the divisions and scaling, informs Bhatia. He says, “With scaling numbers printed on the screen in time and voltage, it makes it quick and easy for the user to interpret data.” Such advanced and powerful OSes are slowly trickling down to low-end oscilloscopes as well.
Use digital trigger for precise measurements
The trigger is a key element in the scope. It allows capturing events of interest to provide a detailed analysis and ensuring a stable display for repeating signals. Traditional digital oscilloscopes implement analogue triggers, which means acquisition and trigger paths are different.
“The industry’s first digital trigger is implemented by Rohde & Schwarz in their RTO and RTE series of scopes. In a digital trigger, the trigger signal is derived from samples of analogue-to-digital (A/D) converters. Therefore the digital trigger processes an identical signal that is acquired and displayed,” shares Srinivasa Rao Appalla, manager, product support and applications, Rohde & Schwarz. He adds, “The digital trigger offers very low trigger jitter in real-time, combined with high-acquisition and analysis rates in RTO and RTE scopes. It enables precise measurements due to high trigger sensitivity at full bandwidth and adjustable digital filter for the signals.”
Fig. 1 shows a block diagram of digital trigger implementation.
Use advanced maths operators for better power analysis
Modern oscilloscopes have powerful analysis and software capabilities; in particular, they have a lot of maths functions. Add, subtract, multiply, divide, differentiate, integrate, fast Fourier transform (FFT), square-root, absolute value, logarithms, filters and many more transforms and operators are available. You can apply these operators on captured data, and it shows you the result not only in stop mode but real-time as well. Some oscilloscopes also allow users to enter and edit formulae to actively perform advanced calculations. It is practical in various applications such as power analysis by using operators (such as integral to find out power consumption of a microcontroller).
Some other useful features and suggestions
Here are some extra features and suggestions that you might find to be helpful.
Making the most of triggers. The time-out-trigger mode can help with the challenging task of capturing signals that contain different pulse widths. Also, the slope-trigger function in an oscilloscope, which triggers on the positive or negative slope of a specified time interval, is useful for capturing saw-tooth and triangular-shaped signals.
Directly manipulate with fingers. “In our 6000X series scope that offer bandwidth starting from 1GHz up to 6GHz, we have moved to a capacitive-enabled, multi-touch gesture-enabled display that provides similar capabilities of modern-day smartphones and tablets,” informs Bhatia. For example, a user can use the multi-touch feature to effectively change the horizontal or vertical scaling by using two fingers and gesture for zooming out or in and even drag a waveform around.
Consider five times the measured signal’s bandwidth. Correct sampling is essential for making accurate measurements and it also completely eliminates errors from your design. A general rule for analogue signals is considering five times the bandwidth of the signal you wish to measure. Coming to high-frequency signal elements, it would be advisable to set your scope to attain five to ten times over sampling.
Make the most of those probes. Also, appropriate probing and comprehending the correct use of ground reference and differential signals is important to go error-free. If your data lines are not grounded, make necessary adjustments, keeping in mind the effect of ground loops and noise on measurements. Make full use of probe techniques and advanced noise-cancelling features on your oscilloscope to curb noise. If required, utilise differential probes for superior measurement quality.