Microwave Transition Analyzer

The microwave transition analyzer brings time-domain analysis to RF and microwave component engineers. A very wide-bandwidth, dual-channel front end, a precisely uniform sampling interval, and powerful digital signal processing provide unprecedented measurement flexibility, including the ability to measure magnitude and phase transitions as fast as 25 picoseconds.

As signal processing capabilities advance, modern microwave and radio frequency (RF) systems are becoming more and more sophisticated. Pulsed-RF signals, once used only for radar applications, are increasingly being used in communication systems as well. These signals routinely have complex modulation within the pulse, especially frequency and phase variations (see Fig. 1). Operating frequencies and bandwidths continue to increase, placing additional demands on the components of the systems.

Engineers responsible for the design and testing of such components and systems often need to measure them under the same dynamic conditions as those in which they are used. For example, it may be necessary to measure a device's response to phase coding or linear frequency chirp inside an RF pulse.

Measurements with traditional frequency-domain instrumentation are often insufficient to characterize and understand fully the operation of components in dynamic signal environments. Before the microwave transition analyzer introduced in this article, no single instrument could handle the diverse range of measurements required for dynamic testing at microwave frequencies. In addition to the new measurements it makes, this analyzer can perform many of the measurements previously requiring the use of network, spectrum, dynamic signal, and modulation analyzers, as well as oscilloscopes, counters, and power meters.

Importance of the Time Domain

A key benefit of the microwave transition analyzer is that it brings time-domain analysis to RF and microwave component engineers. In addition to its use in pulsed-RF testing, the time domain is essential to characterizing and understanding nonlinear devices because one can clearly and intuitively see the relationship between the input and output signals. As an example, both signals in Fig. 2 would appear identical if displayed on a spectrum analyzer. Even if the phase of the harmonics were known, the differences between the signals would not be immediately obvious. When viewed in the time domain, however, it is clear that signal 1 is clipped (the output of a limiter, say), while signal 2 has crossover distortion (what might be seen at the output of a Class-B amplifier, for example). Without the time domain, engineers have had to guess at the underlying causes of observed frequency-domain behavior. The ability to view microwave signals in the time domain has also proved to be extremely valuable to designers that are using CAE microwave design simulators, such as HP's MDS. Now simulations based on circuit models can be easily compared to actual measurements in both the time domain and the frequency domain.

Historically, most measurements on high-frequency nonlinear devices have been performed in the frequency domain. Often, this has been because of inadequacies in time-domain instrumentation. When frequency-domain information is of prime concern, spectrum analyzers are superb in their ability to display harmoniC, modulation, and spurious signals with a large dynamic range. However, without the phase of the frequency components, the time-domain signal cannot be reconstructed. Network analyzers are excellent for performing linear, small-signal, frequency-domain testing, but they are limited in their ability to characterize nonlinear devices. The addition of harmonic and offset sweep capability in network analyzers has helped, but the time-domain perspective is still missing.

For envelope analysis of pulsed-RF signals, spectrum analyzers offer some limited time-domain capability. Recently, network analyzers have been adapted for pulsed-RF timedomain testing as well. Because of the architecture of these instruments, the intermediate frequency (IF) bandwidth imposes an upper limit on the measurement bandwidth. The result is minimum measurable edge times of greater than 100 ns. The microwave transition analyzer's architecture does not have this restriction. Edge speed is limited only by the RF bandwidth. Consequently, magnitude and phase measurements on pulses with rise times as fast as 25 ps are possible. Fig. 3 shows an example of a microwave transition analyzer measurement.

The ability to measure narrow pulses in the time domain can also be used to determine the impulse response (and therefore magnitude, relative phase, and group delay) of frequency translation components such as mixers and receivers. By stimulating these devices with a narrow 'pulse of RF energy, -time-domain distortion can be directly observed. Often, it is the time-domain distortion that is of interest, even though it may be specified indirectly as magnitude and phase flatness versus frequency. By transforming the input and output pulses to the frequency domain with the built-in fast Fourier transform (FFT) and computing their ratio, the transfer function is obtained. From this, familiar results of magnitude and group delay versus frequency can be displayed. Network analyzers are only able to measure the phase and group delay of frequency translation components relative to a reference or "golden" device.