11.8. VECTOR NETWORK ANALYZER
One of the prominent laboratory instruments used for radio-frequency and microwave design
purposes is the vector network analyzer (VNA). A VNA is ideally suited for measuring the
response of a DUT (device under test) as a function of frequency. The primary output of a
VNA is reflected and transmitted power ratios or the square roots thereof. Through
mathematical manipulation of the power ratios, a large amount of valuable data can be
extracted. A few examples that can be measured with a VNA are skin effect losses, dielectric
constant variations, characteristic impedance, capacitive and inductive variations, and
coupling coefficients. Furthermore, since VNAs are typically designed with the microwave
engineer in mind, they can make reliable measurements to extremely high frequencies. The
primary disadvantage is that they provide data in the frequency domain. Subsequently, it is
necessary to translate the extracted data into a useful format that can be used in the time
domain.
In the past, most digital designs had no need for extracted electrical characteristics in the
hundreds of megahertz range, simply because their operating frequencies were low
compared to today's systems. Modern bus designs, however, are becoming so fast that such
measurements are a necessity. As bus speeds pass the 500-MHz region, rise and fall times
are required to be as fast as 100 ps. Subsequently, the frequency content of the digital
waveforms can easily exceed 3 GHz. As bus frequencies increase, the resolution required
for measuring capacitance, inductance, and resistance increases. To the design engineer,
proper characterization to a fraction of a picofarad, for example, might be necessary to
characterize the timings on a bus accurately. If used properly, the VNA is the most accurate
tool available to extract the electrical parameters necessary to simulate high-speed designs.
The primary focus of this discussion is on VNA operation to extract relevant electrical
parameters prominent in digital high-speed bus designs. This includes propagation delay,
crosstalk, frequency-dependent resistance, capacitance, and inductance parameters. Basic
procedures used to measure items such as connectors, sockets, vias, and PCB traces are
covered. It should be noted that these techniques may or may not apply to the reader.
However, for the novice VNA user, these procedures should help provide some helpful
concepts for utilizing the VNA to yield accurate results.
Because the focus of this chapter is on validating and developing models for high-speed
digital design, the measurement techniques discussed are limited to the common methods
most useful for the characterization and development of models. The discussion includes
measurement techniques, sources of error, and instrument calibration.
11.8.1. Introduction to S Parameters
The primary output of the VNA is the scattering matrix, also know as S parameters. The
scattering matrix provides a means of describing a network of N ports in terms of the incident
and reflected signals seen at each port. As frequencies approach the microwave range, the
ability to extract voltage and currents in a direct manner becomes very difficult. Under these
circumstances, the only directly measurable quantities are the voltages (or power) in terms
of incident, reflected, and transmitted waves, which are directly related to the reflection and
transmission coefficients at the measurement ports. Therefore, the scattering matrix is
defined in terms of the reflected signals from each port of a device and the transmitted
signals from one port to another, as shown in
Figure 11.25
.