In the past, the PCB industry seemed to thrive on specialization of labor. So many of the jobs and the software tools of the time did not offer the level of productivity we enjoy today, and so much of a designer’s time was spent on tool configuration. Today, the situation is much different, and today’s design software allows designers to take much more active roles in all other areas of engineering a new electronic product.
Test engineering and precision measurements are one area where designers need to be knowledgeable in order to create the most value in product development. In the coming market, PCB designers need to take more of a multifunctional role in electronics development, so they will need to have an understanding of some precision measurements needed to evaluate system functionality.
This is about more than just knowing how test equipment works. The central idea here is to ensure a designer knows which measurements will be needed to evaluate a particular aspect of a circuit or PCB layout. This can then require understanding how to implement test structures, test connector options, or test procedures for particular designs.
Which Measurements Are Most Important?
The exact list of measurements designers should need to know is difficult to generalize as there are many tests available for different types of boards and applications. In general, electrical tests are performed in the time domain or in the frequency domain. The table below outlines some of the precision measurements that most commonly need to be performed by a designer and in what domain they will be taken.
This is just a cross section of the possible set of measurements a designer might need to perform. These can most often be performed with a small set of reasonable-cost instruments and standard probes provided by instrument vendors, as long as the signals involved do not start to reach into the GHz range.
Some of the more advanced measurements that might need to be performed can be found in this article.
What Equipment is Needed?
There is a short list of equipment that is needed to perform precision measurements in the above areas:
- Oscilloscope with built-in FFT and mathematics functions
- Function generator for providing test signals
- Low-noise isolated DMM
- Precision multimeter (down to uV or uA measurement range)
- Source-measure unit
Probes for these systems are also needed to ensure highly accurate measurements. Make sure that you understand probe bandwidth and probe parasitics when selecting these instruments, as well as their susceptibility to noise.
How to Build Your Capabilities
Although it’s important for designers to understand the process and methodology for gathering precision measurements, they don’t need to run out and spend millions of dollars on test equipment. It’s okay to start small and build your capabilities over time.
- Find some starter brands like Rigol, Siglent, and Hantek for basic equipment
- Look at eCommerce sites for deals on new and used equipment, including eBay
- Used test equipment vendors can be found online
- Get a basic lab setup started, including a power supply, scope, and function generator
Following this path can help an engineer accumulate a fair amount of viable equipment on a small budget. Aim for measurement capabilities above the 100 MHz range with reasonably high precision, and you’ll be able to confront most testing and debug tasks that require some precision measurements. Even some older equipment is still superior in terms of its precision and measurement capabilities.
This older oscilloscope may not provide the cleanest digital interface, but it can certainly compete in terms of precision.
Test Board Design
Many of today’s devices are not built on the first iteration. In some cases, a specialized test fixture is needed just to verify a proof of concept, specialized circuit, or embedded application. Test boards are often built specifically for these purposes whenever a reference design or evaluation board will not provide precision measurements or other required functions.
What should you put into a test board so you can get the most accurate measurements and a complete dataset during operation?
- Test points, including clip-on test points for probes
- Connectors for test sources (function generator)
- Indicators or displays that flash device states during tests
- Buttons for accessing certain test functions
- Experimental printed elements in the board (antennas, transmission lines, etc.)
Test boards allow designers to isolate specific structures or circuits, and these can be comprehensively evaluated before the test devices are compiled into a production board. When working on advanced RF systems and high-speed digital designs, it’s common to build dedicated boards with test circuits specifically for evaluating component functions.
Go Big With Custom Probes
In some cases, the equipment required to take a precision measurement simply does not exist. One case where this can occur is in probes that are needed to take a measurement. The test equipment industry has taken many steps to produce precision probes that will work with their instruments, but they cannot cover every application. Thus there is sometimes a need to build a custom probe to collect specialty electrical measurements.
There are very few generalizable guidelines for designing high-precision probes. Generally, probes are designed with a focus on eliminating parasitics that limit the frequency response or sensitivity of a traditional probe. The exact approach to be taken is different in each case. Some of the design goals in developing a custom probe might include:
- Very high or very low impedance response
- Very narrow or very broad frequency response
- Noise immunity, often with differential measurements
This test probe is designed to provide very high sample rate for low-level signals, essentially being an oscilloscope-on-board.
Field solvers and circuit solvers are often used to validate custom probe designs. Circuit solvers are needed simply to evaluate the measurement circuit, while electromagnetic field solver must be used to determine how parasitics in the physical design create any deviation from the ideal circuit behavior. With simulations and some experience, it’s possible to optimize a probe design for maximum precision.
Some precision measurements need to be done by hand due to the sensitive nature of the circuit being measured. However, there are some precision measurement methodologies where data capture can be automated. Developers who build test and measurement systems can use several pieces of equipment and standards to capture measurements from test rigs and fixtures, and the data can be viewed over time in a custom application.
Capturing automated measurements requires a few important applications and peripherals with a computer:
- A datalogger card, which usually connects over a serial port or USB
- USB ports on newer test equipment can stream data to an application
- GPIB cables (IEEE 488.2, see below) can capture data from older instruments
This IEEE 488.2 (GPIB) controller card can be used to interface with multiple devices that use an older bus hookup.
There are test and measurement applications, such as LabVIEW, that include libraries for interfacing with USB devices and with GPIB-based devices. If your computer is newer and does not have space for a GPIB expansion card in a PCIe slot, you can also find USB adapters for GPIB-connected devices.
Dataloggers can also be connected via USB for most low-speed serial measurements, such as the serial output from an ADC. Some dataloggers have an analog interface built into the front end of the component, so they can directly collect analog signals. Some of these have sample rates reaching into the MSps range with very high resolution (24 bits), so they can capture very precise changes in a time-domain signal. It’s common to see datalogger test setups that use multiple types of connections to acquire signals and stream data.
Example datalogger unit from Dataq. [Source]
Embedded Test Applications
There is another methodology with automated precision measurements, where the measurements are captured and evaluated on-device. Embedded test applications can be deployed on embedded systems, such as on single-board computers and IoT devices, which allow direct measurement, testing and evaluation of a target DUT.
Embedded test applications have to be custom-written to run on the embedded system and interface with the DUT. The DUT can be located on or off the board for the embedded device. The goal in this methodology is to allow a device to gather and process its own measurements, and then make a determination as to whether the target DUT is functioning as intended.
Embedded testing system topology.
The data captured by an embedded system that interfaces with a DUT is typically processed on-board, but it could be streamed back to a web platform or an edge server. These systems might use a high-precision ADC, for example, to test on-board devices or components that appear in the PCB. The results could then be used by the device, in a cloud application, or both.
Whether you need to verify measurements against simulation or you need to design exotic probes for precision electronics measurements, use the complete set of system analysis tools from Cadence to evaluate systems functionality. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity. Cadence PCB design products also integrate with a multiphysics field solver for thermal analysis, including verification of heat sink designs.