Electrical Measurement Techniques in Time and Frequency Domains

Cadence System Analysis

Key Takeaways

There are several different types of measurements used to examine electrical behavior in the time and frequency domains.
These measurements require some fundamental pieces of equipment that are adaptable to multiple analyses.
Measurements can be compared with simulations to validate models and ensure measurements were gathered correctly.

Test fixtures used for measurement techniques in the time and frequency domains

These test fixtures are used for RF measurement techniques in time and frequency domains

Anytime you need to test a new electronic product, there will be a range of measurements involved and multiple performance benchmarks to check performance. Test requirements in signaling standards, product-specific performance metrics, and component interface requirements all influence how a product behaves in the field and how signals propagate in the system. Then, there are aspects like regulatory requirements on EMI/EMC compliance, safety standards, reliability standards, and industry-specific performance standards.

With the complex nature of modern electronics, designers often have a role to play in developing performance benchmarks and test procedures for their boards. Although not all designers are test engineers, they still need to consider how their boards can be tested during prototyping. Manufacturers can help with some basic measurements or impedance controlled measurements and validation, but designers should consider how they will evaluate their prototypes before scaling to production. In this article, we will explore the measurement techniques in the time and frequency domains that designers should consider.

Measurement Techniques in Time and Frequency Domains

Measurement techniques in electronics are not necessarily focused around specific procedures, but rather around the capabilities of different instruments. There are some fundamental instruments that are used to assess signal and power behavior in the time and frequency domains. Understanding these instruments and what they can measure gives designers a foundation for developing test requirements for their systems.

Here are some of the common instruments and basic measurement techniques test engineers use to evaluate their designs in the time and frequency domains:

Source-Measure Unit

These instruments are programmable multimeters with an integrated power supply. They operate by sourcing some power at the desired signal level, and the device measures the voltage and/or current passing through a circuit or interconnect. These devices are commonly used for DC sweep measurements with semiconductor devices and can be programmed over USB, GPIB, serial, or parallel protocols.

Oscilloscopes

Oscilloscopes are the fundamental meter for time-domain measurements. They are receiver instruments in that they only measure inputs and do not source any signal. However, an external source can be connected to an input on an oscilloscope and compared with the received signal to extract phase, attenuation, and dispersion. Modern oscilloscopes include many features that automate time-domain measurements and calculations. Some example measurements that can be gathered in the time domain include:

Transient response in interconnects, components, and circuits
Harmonic signals at a single frequency to visualize distortion
Jitter, intersymbol interference, and bit error rates from eye diagrams

Spectrum Analyzer

A spectrum analyzer is used for signal measurements in the frequency domain and they are the conjugate instrument to oscilloscopes. In other words, the signal viewed on a spectrum analyzer is the Fourier transform of the signal viewed on an oscilloscope. These instruments are also pure receivers; they do not source any signal, although some units can be used to compare input and output signals in the frequency domain. The test engineer can then take this data and determine the magnitude of the transfer function for a device under test (DUT).

Spectrum analyzer measurement techniques in time and frequency domains

A spectrum analyzer is used to measure electrical behavior in the frequency domain

Network Analyzer

A network analyzer essentially combines a spectrum analyzer and a signal generator into a 2-port instrument. This device can be used to probe 2-port networks at frequencies reaching into the high GHz range. Network analyzers come in the scalar (magnitude) and vector (magnitude + phase) varieties. All network analyzers source a signal at one port and measure the received signal at the second port. The two signals can then be compared to determine a transfer function or network parameters for a 2-port network. These devices provide broadband measurements reaching hundreds of GHz, making them a mainstay instrument for RF test engineers.

Newer Techniques for High Speed Measurements: IEEE P370

Oscilloscopes, VNAs, spectrum analyzers, and SMUs are not going anywhere, but newer techniques are supplanting or complementing the use of these instruments. The newest measurement technique is IEEE P370, an impulse response measurement that is meant to assess S-parameter quality in the frequency domain and determine accurate S-parameters within a desired bandwidth.

This measurement technique is used to examine S-parameters of a DUT through a de-embedding procedure. In the IEEE P370 standards, a 2x-thru test structure is used to measure S-parameters using a standard test structure (usually the Beatty standard) with SMA connectors. The S-parameters of the test structure and the 2x-thru structure are known and can be de-embedded, leaving behind the S-parameters of the channel or DUT.

Validate Your Designs Against Simulations

The measurement techniques listed above are important for validating designs before production, but they are also important for validating simulation models. Today’s complex electronics often require some level of simulation to ensure the design can meet basic performance benchmarks, and an entire industry has been developed to produce simulation tools for pre-layout and post-layout modeling of electronics. Newer CFD and multiphysics packages even let you evaluate heat transfer in your board to ensure reliability and design a cooling strategy.

Simulations give you progressively higher levels of evaluation that can be compared with results from any of the above measurement techniques in the time and frequency domains. The table below shows which simulation tools and analyses can be used to evaluate some of the above measurements.

Measurement	Simulation
Impulse response	Impulse response calculation using the simulated or measured transfer function.
Eye diagram	Uses a pseudorandom bit sequence with randomly generated jitter on the driver. Convolution can be used to calculate the signal shape and eye diagram at the receiver.
Bit error rate	The bit error rate incurred in a channel can be calculated using simulated or measured eye diagrams. This is a statistical measurement that requires comparing the fraction of signal levels with the receiver’s threshold.
Near and far field radiation	Radiated EMI from the system can be simulated using a full-wave 3D field solver.
Time-domain responses	Oscilloscope measurements can be validated using transient analysis in circuit models. Advanced 3D field solvers may include a transient response simulator (using FDTD methods) that will show how the system converges to its steady state.
Transfer function	A transfer function can be calculated directly from circuit models using SPICE packages or it can be calculated from field solver results, if necessary.
Network parameters	This is an extension of transfer function methods. Various network parameters can be calculated with SPICE packages, but the most accurate results that account for parasitics are calculated using integrated 2D or 3D field solvers.
Impedance	Just like Z-parameters (one set of network parameters), the impedance of a circuit or channel can be calculated using a SPICE package or field solver (FDFD methods).

When you need to validate your measurement techniques in the time and frequency domains, use the best set of PCB design and analysis tools with integrated field solver features. Cadence offers a range of applications that automate many important tasks in systems analysis, including signal and power integrity analysis through a set of integrated field solvers.

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