Choosing the correct test equipment and knowing how to use it can help improve the speed and accuracy of measurements made on high-speed pulsed signal waveforms.
Radar pulses must be characterized to determine the health of a new prototype design or an existing system. Measurements can be made with such RF/microwave test instruments as oscilloscopes and spectrum analyzers, although it helps to understand the capabilities of these instruments and how to specify enough performance for a given set of pulse characteristics. Some useful guidance for this can be found in a 28-page application note from Tektronix, "Advanced Radar Analysis: Tools for Measuring Modern Radars."
Measurement equipment for studying radar pulses emulates the functionality of the radar system, with a pulse generator acting as the radar transmitter and an oscilloscope acting as a radar receiver in the time domain or a spectrum analyzer as a radar receiver in the frequency domain. Matching the equipment to the requirements of a particular system involves sorting through a number of specifications that include frequency range, pulse bandwidth, modulation bandwidth, and dynamic range.
When selecting a pulse generator, for example, questions to consider are whether the generator can cover the carrier frequency range of the system to be tested or, if it is to be operated within the system's baseband frequency range, if suitable upconversion equipment is available to translate the baseband signals to the frequency range required for the system under test. Additional questions should address whether the generator provides suitable output amplitude to drive an amplifier or antenna feeding the system to be tested, and whether its time-domain characteristics, such as pulse width, rise time, and fall time are within the operating parameters of the system to be tested.
On the receive side, oscilloscopes have typically been used to display pulsed waveforms when evaluating radar system performance, often at baseband frequencies. As the bandwidths of oscilloscopes have increased, they can now be used to characterize the actual RF/ microwave output pulses from radar, rather than their baseband equivalents. Because modern radar systems may operate with extremely fast pulse rise and fall times, often in the subnanosecond range, adequate oscilloscope bandwidth of as much as 20 GHz may be needed to accurately display such pulses. As the application note points out, a digital sampling oscilloscope (DSO) is suitable for testing repetitive pulses. But when evaluating a radar system on a pulse-topulse or single-pulse basis, a real-time oscilloscope, or its frequency-domain counterpart, the real-time spectrum analyzer, is required.
Capturing a transient event such as a radar pulse requires sophisticated triggering capabilities, generally based on the characteristics of a pulse edge or the voltage amplitude of the pulse. For applications with simple pulse measurements, basic triggering capabilities may suffice. But when seeking to isolate pulse anomalies or random events amidst a series of uniform pulses, more complex triggering functionality may be required, to the level that multiple conditions, including a combination of edge triggering, amplitude level settings, or even a specified pulse width, can be programmed as trigger settings. Delay capability is also a part of an instrument's triggering functionality, and the delay adjustment range should provide the resolution required to provide fine-grain capture of even high-speed pulses.
The application note explains the difference between making measurements on simple pulses and on pulse-modulated signals, where it is necessary to capture the full signal envelope. Often, it is desirable to remove the carrier so that the pulse information alone can be examined. Swept-frequency (superheterodyne) spectrum analyzers can be operated in a zero-span mode in which the analyzer's intermediate-frequency (IF) bandwidth is essentially "parked" at a center frequency of interest (when the frequency of the pulse is known) and the pulsed signals are captured within that IF bandwidth. In contrast, real-time spectrum analyzers digitize a wide instantaneous IF bandwidth and store the information for digital processing. Real-time spectrum analyzers, with their ability to trigger on transient events, are also useful tools in capturing and displaying information about pulsemodulated carriers.
More on evaluating the characteristics of single radar pulses can be found in the application note, "Advanced Radar Analysis: Tools for Measuring Modern Radars." It includes information on other useful pulse test capabilities, such as measurements of occupied bandwidth, spurious signals, and unintended emitters, and explains the roles of both oscillsoscopes and spectrum analyzers in the measurements. Copies are available in PDF form for free download from the Tektronix web site at www.tek.com.