Techniques for Time Domain Measurements

Introduction

Components and subsystems of communication or radar systems need to achieve a certain level of specified electrical performance across the operating frequency range to pass testing and qualification. These specifications include, but are not limited to, voltage standing wave ratio (VSWR), return loss (RL), and insertion loss (IL). These specifications make it clear when the device under test (DUT) passes or fails its performance requirements as a function of frequency.

While a frequency measurement provides useful information, the sweeping frequency may not provide enough information to determine the root cause of a problem. When a system fails to meet specifications, troubleshooting is challenging due to component swapping until performance meets the specified requirements. Fortunately, there is another measurement technique that provides details into the location and magnitude of any such challenges.

This technique relies on measurements in the time domain — FieldFox can easily display the time domain characteristics of one and two-port components and systems.

Table of Content

• Time Domain Measurement Basics
• Gating and Frequency Response
• Masking in Coaxial Lines
• Relationship Between Frequency Span and Pulse Width
• Relationship Between Frequency Span and Time Span
• Configuring Bandpass and Lowpass Time Domain Options

Time Domain Measurement Basics

A time-domain analysis is very useful to observe the effects of mismatch along with a transmission line system. When an RF or microwave signal propagates along a transmission line, a portion of the signal reflects back from any discontinuities encountered along the path. Using time-domain analysis, the location of each discontinuity displays as a function of time along the x-axis — the amplitude of the reflected signal, or S11 plots along the y-axis.

Knowing the propagation velocity along the transmission line allows the FieldFox to scale the time measurement to physical distance. It is also possible to examine the time domain response of a transmitted signal or S21. However, this measurement requires a two-port connection. Due to distance constraints, field engineers cannot usually access two ports of a system. Because of that limitation, this application note primarily focuses on the time domain response of reflection measurements using one port.

The FieldFox can represent the measurement from any one-port or two-port device in the time domain and the frequency domain. If a measurement occurs in one domain, then the FieldFox calculates the other domain using a well-known mathematical technique called the Fourier Transform (FT). This transform provides a universal problem-solving method that allows you to examine a particular measurement from an entirely different viewpoint. If you record a measurement using a time-domain method, then an FT calculation results in a frequency domain representation of the data.

Alternatively, if you initially capture frequency domain data, then an Inverse FT (IFT) results in a time-domain representation of the data. Simultaneously displaying the same data in time and frequency creates a powerful analysis and problem-solving tool. Fortunately, modern test instrumentation, such as an NA, includes this mathematical transformation as part of the firmware. This feature allows users to display either time-domain data or frequency domain data or both.

Instruments with time-domain capabilities

There are two basic instruments capable of displaying the time domain response of individual discontinuities along with a transmission system — the time domain reflectometer (TDR) and the NA. A TDR uses a traditional method of launching an impulse or a step waveform into the test device and directly measures the response as a function of time. A fast edge launches into the transmission line when using a step generator and broadband oscilloscope. The broadband oscilloscope monitors the incident and reflected voltage waves and displays the position of each discontinuity as a function of time.

An NA, such as the FieldFox NA mode, is primarily a frequency domain instrument with the capability of measuring the reflected and transmission characteristics of one- and two-port devices. Using error-corrected data measured in the frequency domain, the FieldFox calculates the response of a network to an impulse or step function using the IFT. It then displays the response as a function of time. As the NA uses narrowband measurement receivers, the dynamic range is higher than oscilloscope-based TDR systems. Also, an NA includes time-domain capabilities for measuring band-limited devices, called bandpass mode, which we will discuss later in this application note. Lastly, there is a configuration of the FieldFox, known as cable and antenna (CAT) mode, which performs the same frequency-to-time domain transformation but scales the time measurements to an equivalent physical distance. This measurement aids the user in quickly locating faults in RF and microwave transmission lines in the field.

Measurement example using a horn antenna

Figure 3 shows a test configuration for an examination of the frequency domain and time domain responses of an over-the-air measurement. It uses an X-band waveguide horn antenna and a separate metal plate placed near the antenna. The high-gain horn antenna connects to a short length of WR-90 waveguide transmission line, and the transmission line attaches to a wave- guide-to-coaxial adapter for connection to the NA. Calibrate the NA for S11 at the plane where the adapter connects to the instrument port.