Test and Measurement of passive components

Wednesday, June 12, 2013:From a process point of view, there is always some risk of instruments currently in use being actually out-of-specification. Let’s take a close look at the solutions, challenges and trends in test and measurement of passive components

By Ashwin Gopinath

An electronic component is any basic discrete device or a physical entity in an electronic system used to affect electrons or their associated fields. The most basic classification of electroni components—passive, active and electro-mechanical—is on the basis of energy. In layman’s terms, passive components are ones that cannot supply energy themselves, whereas a component like a battery would be called active as it truly acts as a source of energy. Let us now discuss the test and measurement solutions available for passive components.

Why test passive components
Most quality systems define actions that must be followed in case an instrument is found out-of-specificationduring calibration. Sometimes though, selection of test equipment to reduce the risk of out-of-tolerance condition affecting a process does not get enough credit. Cost is an important consideration, but appropriateness of the instrument to the process is critical. The instrument should meet the measurement requirements of the process not only on the day it is delivered new but also during the time period between calibrations. Unfortunately, from a process point of view, this means there is always some risk of instruments currently in use being actually out-of-specification.

Anjaneya Vara Prasad Gundu, head of process management, Cigniti Technologies, says, “Globally, the industry is actively working on designing, developing, building, enhancing and creating a good number of components/applications. With the best engineers, market competition and the right allocation of funds to research, there is no doubt that tonnes of newer components will be created and more exciting features will get added to the existing set of components in the market. However, what is quite important, yet compromised even today is ensuring some of the key attributes, viz, reliability, performance, maintainability, supportability, usability and performance, that end users are looking for in addition to low-cost components. The industry is maturing towards recognising these additional attributes that help it gain the customers’ confidence.

Solutions for test & measurement of passive components
Today, a variety of solutions for test and measurement of passive components are available in the market.

T. Anand, managing director, Knewron, explains, “Our solutions include simple testers such as Go-NoGo units or more sophisticated ones that could perform thorough testing and alignment of components/assemblies. All these solutions are either fully automated or semi-automatic, which depends upon the operating environment at the customer’s place. Some of our solutions also involve test-design for selected parts and then development of ATE for performing those tests.”

The most commonly used are Go-NoGo units, LCR meters, vector net-work analysers, spectrum analysers, data quality analysers and a number of other similar instruments. Let’s take a closer look at some of these solutions.

Go-NoGo gauge. A Go-No gauge is an inspection tool that is used to check whether a work piece is working within its allowed tolerances. The gauge derives its name from its use. It has two tests and the check involves the work piece having to pass one test (Go) and fail the other (No Go). It is one of the most basic tools, yet an integral part of the quality process that is used in the manufacturing industry to ensure interchangeability of parts in between processes.

The feature of the Go-NoGo gauge is that instead of returning a value, it returns a state. The state is either acceptable, meaning that the part is within tolerance and may be used, or unacceptable (and hence the part must be rejected).

LCR meters. An LCR meter is an electronic test equipment that is used to measure the inductance (L), capacitance (C) and resistance (R) of a component. In the basic form, the true values of these quantities are not measured; rather the impedance is measured internally and then that value is converted for display to the corresponding capacitance or inductance value. As long as the capacitive or inductive load doesn’t have a signifcant resistive component, the readings will be reasonably accurate.

In the advanced versions of the meter, the true inductance, capacitance and also the equivalent series resistance of the capacitors along with the Q-factor of the inductive components are measured.

Handheld LCR meters typically have selectable test frequencies ranging from 100 Hz to 1 kHz, 10 kHz, and 100 kHz for top-end meters. The display resolution and measurement range capability are different for different test frequencies.

On the other hand, bench-top LCR meters typically have selectable test frequencies of more than 100 kHz. They often include features that allow the user to superimpose a DC voltage or current on the AC measuring signal.

 Vector network analyser. Network analyser is a popular solution for test and measurement of passive components. It is used to measure the network parameters of electrical networks.
In today’s world, network analysers commonly measure s–parameters because at high frequencies, the reflectionand transmission of electrical networks are easy to measure. Network analysers are often used with two-port networks to characterise their parameters, but they can be used on networks with an arbitrary number of ports as well.

A vector network analyser (VNA) is a sub-type of a network analyser alongside the scalar network analyser (SNA). A VNA differs from an SNA as an SNA measures only the amplitude properties, whereas a VNA measures both amplitude and phase properties. A VNA may also be called a gainphase meter or an automatic network analyser.

“Once a passive or active component has been designed using the total measurement capability of a VNA, an SNA may be a more cost-effective measurement tool for the production line to reveal out-of-specificationcomponents. While SNAs require an external or internal sweeping signal source and signal separation hardware, they only need simple amplitude-only detectors, rather than complex (and more expensive) phase-coherent detectors,”—explains application note 1291-1B by Agilent Technologies.

Spectrum analyser. A spectrum analyser measures the magnitude of an input signal versus the frequency within the full frequency range of the instrument. The primary function of the analyser is to measure the spectral power of known and unknown signals. By analysing the spectra of the inputted electrical signals, dominant frequency, distortion, power, bandwidth, harmonics and other spectral components of a signal can be observed, which are not easily detectable in waveforms based on time-domain responses. These parameters are useful in the characterisation of electronic devices, passive and otherwise.

Spectrum analysers are among the most essential of RF/microwave instruments, showing in the frequency domain what a high-speed oscilloscope might show in the time domain, hence having the capability to reveal a great deal about the performance of a system and its components.

By adjusting the settings of a spectrum or signal analyser, such as the RBW filter, the measurement accuracy can be optimised. A narrow RBW filte can make it possible to display low-level signals at the expense of sweep speed. However, if RBW filter are too narrow, sideband information for signals with wideband modulation may be lost. Additional passive components in a test setup, such as coaxial cables and external filters, also serve t infuence the measurement accuracy of a spectrum analyser.

Newer spectrum analysers allow users to store the amplitude response of different passive components and measurement setups so that calibration is not necessary every time the test setup is used.

Challenges to navigate
Use of passive devices in low-voltage applications is increasingly becoming common. Laptops and cell phones require batteries with ultra-low internal resistance. So manufacturers and users of passive components are now starting to pay greater attention to the materials and processes used to build them, as well as the new rules about using them in low-voltage applications.

Thermoelectric EMFs are very common in passive components constructed of dissimilar materials, such as resistors. Design and manufacturing activities, such as testing batteries with ionic resistance, demand greater focus regarding the impact of noise and thermoelectric EMFs on measurement accuracy. This can be particularly challenging because very low test currents are required to measure these single milli-ohm resistances.

Understanding the challenges of using passive components in low-voltage circuits, the sources of error, and proper means for characterising them is extremely crucial to the proper design of low-voltage devices.

Commenting on the challenges faced by T&M engineers, T. Anand says, “One of the major challenges we face these days is ‘testing capability for quantity’. Most customers would not prefer to buy too many T&M equipment as their test quantum increases. They usually expect T&M solution to be capable of handling a large number of component tests that can be done faster. For instance, one of our existing customers recently changed their test sample size, which is critical from their quality acceptance perspective. However, this increase in test quantity is leading to demand of shortening test duration without any impact on test functionality and parameters. These types of pressures make the work challenging. T&M solutions must be ready to cater to ever-changing testquantities in future.”

Anjaneya Vara Prasad Gundu adds, “Testing is in high demand and some of the challenges that are hampering the growth include testing being compromised by clients due to overrun in the design and development phases by respective partners, lack of skilled resources with a knowledge of applying their knowledge and the non-integrity of the various measurement tools used during the life cycle.”

What to expect in the coming years
The development of new instrumentation technologies is driven by the needs of the users—the measurements and analysis required to support new technologies and applications. Amongst today’s needs are measurement speed, broadband signal measurements, modulation and demodulation capabilities within instruments, and greater integration with computer simulation and analysis. There are also increasing needs for high-performance portable instruments, flexibleand fast production test systems and a continuing push for the highest performance in laboratory instruments.

— Madhukar Tripathy, manager, telecom sector and indirect channel, Anritsu

The single biggest recent advancement in test equipment is inclusion of digitisation of signals and computer analysis capabilities. Many instruments now have internal PC platforms that operate the instrument and perform the necessary calculations to process the measurement data and deliver detailed displays and reports to the user.

Madhukar Tripathy, manager, telecom sector and indirect channel, An-ritsu, has a different take on the trends: “Technology is changing very fast, creating challenges for T&M manufacturers. Higher data speed, multifunctional test, future technology upgradability in a single box and minimum cost/less budget are major trends for passive T&M. Today’s customers are looking for short-term purchase. Getting T&M on rent is yet another big trend seen in the last couple of years.”

“In the last couple of years, many firmshave adopted microcontroller/computer-based design solutions. There is a major shift from old, bulky passive device boxes to classy, sleek and active units for testing. Counterfeit electronic components are an increasing threat to the electronics industry and many T&M solutions are now coming up with features that can identify such components. Some counterfeit parts are like counterfeit money; unauthorised copies. Other cases include mislabelled parts where parts meant for one purpose are relabelled for another by changing the part number. A similar device packaged and marked to appear as the correct device may even pass certain functional and parametric testing. A comprehensive set of tests is required to be carried out in order to spot fake part from the real one. This has been on agenda for many T&M solution providers including Knewron,” adds T. Anand.

“For fibre-opticcomponents various techniques are popular because they are quick and give easy access to all the key setup parameters. Present solutions focus on measuring insertion loss, return loss, polarisation-dependent loss, along with visual/cleaning inspection of connectors, across key wavelengths. Various tools deliver the reliable measurement results needed to meet end user requirements,” says J.K. Baldua, director, technical, Scientech Technologies.

“The proper way to inspect the fibre is to use vision-guided motion control to perform gross alignment of the fbre and lens. To automate the process completely, vision pattern matching tools are employed to locate the core and then automatically position the region of interest. With newer set-ups, the user gets quick and easy access to all the key setup parameters in a simple, easy-to-use, intuitive graphical user interface (GUI). This is very popular in qualificationtest where users want maximum control in a rapidly changing environment,” he adds.

When asked about radical changes affecting the T&M methodology for passive components, Mahesh Subramanyam, senior technical consultant, Agilent Technologies, says, “Synthetic instruments are a reality now. To achieve multiple functionality, individual sub-systems can be put together on a common platform. For example, on the digital processing side for a scope, spectrum analyser, network analyser and digital multimeter, the common block would be a digitiser. This can be built as a separate module. Other modules such as the source and the local oscillator can be built separately and integrated to form a network analyser or a spectrum analyser. The user interface and the software need to be accordingly modified.USB-based instrumentation is also getting popular.”

This article was published in Electronics For You, South Asia’s most popular electronics magazine