Skills For the Coming Market - PCB Electrical Testing
Once a PCBA prototype is returned from assembly, it will need to go through some level of testing and evaluation to ensure the product design operates as intended. There are many tests that can be performed to evaluate a design before it is scaled to higher volume. One important set of tests for a PCB designer or test engineer is electrical measurements. These measurements could also be performed in addition to environmental measurements or while debugging firmware, so it is important to know how to perform PCB electrical measurements in multiple settings.
In this article, we’ll look at some of the essential PCB electrical measurements that will help designers test and evaluate their assemblies during the prototyping or debug phases. We’ll also show some equipment examples that are essential for any lab setup.
PCB Electrical Testing Skills and Equipment
2-Point and 4-Point Resistance Measurements
The standard DC resistance measurement is a 2-point measurement with a multimeter. The same measurement would be used for DC voltage and current. 4-point measurements use four probes and are important for eliminating the influence of electrical contacts in order to accurately determine low resistances. This is important in power systems that must pass power over rails, through current-sense resistors, high-precision resistors, and any other measurement where the resistance of an electrical contact can skew a measurement result.
Time-Domain Measurements With an Oscilloscope
The most basic measurement a designer may need to perform beyond using a multimeter is a time-domain measurement of signal or waveform behavior. The primary instrument used for this purpose is an oscilloscope. These instruments come in analog or digital versions, although the newer instruments available today are primarily digital devices.
Time-domain waveform measurement with an oscilloscope.
With an oscilloscope, you can perform several important measurements with components or a PCBA. Some examples include:
- RMS noise
- Serial bitstreams
- Signal rise/fall times and ringing
- Eye diagrams with more advanced oscilloscopes
The important specifications for performing these measurements are bandwidth (sampling rate) and input sensitivity (as a voltage). These will limit the voltage range and maximum frequency content you can measure with the scope before you start to see artifacts in the measurement.
Frequency-Domain Measurements With a Spectrum Analyzer
A frequency analyzer is used to determine the frequency content contained in a signal or waveform, essentially giving a power spectrum measurement. Like an oscilloscope, these devices have a bandwidth rating that defines the maximum frequency they can be used to accurately measure.
Frequency-domain power spectrum measurement with a spectrum analyzer.
Note that your oscilloscope measurement can be converted into the frequency domain using a Fourier transform. Once you capture a repeated waveform from the time domain data, or you simply apply windowing to a time-domain trace, you can then get a good measurement of the spectral content in the waveform.
S-parameter Measurements With a Vector Network Analyzer
Not all designers will need this, but it is important to know about this piece of equipment and what it can measure. A vector network analyzer (VNA) is used to measure S-parameters for a printed circuit, discrete circuit, semiconductor, or some other device under test (DUT). These systems can be very expensive, especially if you need to operate into the high GHz range. However, there are some low-cost VNA units available that can capture measurements at WiFi frequencies.
Within the scope of VNA measurements, there are several techniques that can be used to extract S-parameters for high-impedance, matched impedance, and low-impedance devices:
- Standard 2-port S-parameter measurements
- N-port networks reduced to 2-port measurements
- 2-port shunt measurements for low-impedance devices
This measurement is important for high-speed interconnect evaluation. It’s another case where not all designers will do this measurement regularly, but it is important to at least know what the measurement means and the information it provides.
This measurement involves sending a pulse through a cable or transmission line and measuring the reflected power. Based on the time at which the reflected signal is measured, the signal’s phase, and the signal’s power, a time-domain reflectometer can determine the impedance at each point along the interconnect. This can be used to identify impedance discontinuities and their magnitude. Essentially, it gives the input impedance at each point along the interconnect.
Example time-domain reflectometry result and corresponding S11 result in the frequency domain.
Arbitrary Waveform Responses
An arbitrary waveform response could be measured with one of the above instruments in the time domain or the frequency domain (or both). This involves using an arbitrary waveform to source a repeating waveform, which could be programmed piecewise linearly to have nearly any repeating shape. The test engineer then measures the response of the circuit or DUT. From this you can determine any number of important properties. For a linear circuit, one goal could be to determine the device or circuit transfer function with respect to that particular waveform. Commonly sourced waveforms would be square or triangle waves, or modulated waves as shown below.
This is a basic EMI measurement that can be used as a baseline for more advanced EMC measurements like line-impedance stabilization network (LISN) measurements. The simplest way to perform this measurement is with a near-field probe. This inductive probe is held close to an assembly and is used to measure the magnetic field component of any EMI being produced by the board.
These probes can be used with an oscilloscope or spectrum analyzer to visualize the characteristics of the radiation being generated by a device. By scanning over the device, it’s possible to locate sources of EMI and examine the magnitude of the emission at a certain distance as part of EMC pre-qualification.
The coming market demands designers with a broad skill set, and PCB electrical testing is only part of the set of competencies required in electronics design. Professional digital and analog engineers trust the complete set of system analysis tools from Cadence for evaluating and simulating their system functionality. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.
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