This is the second post in a multi-part blog series where we take a look at a few practical applications of spectrum analyzers.
Traditional spectrum analysis produces swept plots of power over frequency, revealing the strength of signals caught within the swept device’s analog resolution bandwidth filter. Though valuable for accurately describing continuous wave signals at a specific frequency, swept spectrum analysis may miss information of interest at frequencies not swept at the right moment. Fortunately, FFT-based spectrum analyzers with overlapping FFTs have a probability of intercepting signals in a frequency of interest with extremely short durations, and without missing any critical signal details. As the time-domain characteristics of signals and the time-domain response of RF and microwave devices—including radar, advanced modulation, power amplifiers, and etc—having the ability to measure the frequency- and time-domain response simultaneously can provide significant insight into the behavior of signals and operation of devices.
The Real-Time Advantage With Zero Span Measurements
For many decades, RF engineers have been leveraging a spectrum analyzer (SA) function, known as zero-span measurements. This measurement functions by setting the resolution bandwidth filter (RBW) on a single frequency, capturing signal energy at time-slices, and displaying this over time. This functionality could be used for simple amplitude modulation (AM). But, in the face of more complex modulation and ultra-wideband radar and telecommunication signals, real-time spectrum analyzers (RTSAs) bring a new features and uses for zero span measurements. Namely, an RTSAs wide instantaneous bandwidth and digital RBW filter can be used to capture tens of megahertz of frequency span, continuously, over time and analyze the frequency, time, and modulation domains of a signal simultaneously.
With radar measurements, acquiring this type of detailed multi-domain analysis would require several instruments that would be synced, with some error, in a large test system assembly. The setup and programming of this system would require substantial knowledge and experience. Similarly, studying advanced modulation techniques would require nearly as much rigor. However, an RTSA is capable of producing real-time triggering that can trigger and analyze signals instantly, or even trigger the recording of the behavior of signals for deeper analysis and playback in the future.
Additionally, having this ability performed by a single instrument eliminates the need to time-correlate data, as the display domains are all derived from the same time sampled signal. For radar and communications applications with complex modulation schemes, zero span measurements with an RTSA are extremely valuable in measuring the time-domain power behavior, particularly important with signal chain elements exposed to high power signals. For example, peak power, pulse width, pulse repetition frequency, demodulation, and mean signal power can all be measured with a simple setup of markers, triggers, bandwidth, and wait time settings.
Measuring Peak Power and Pulse Width With Zero Span Mode
Peak power and pulse width measurements are extremely important to understand when planning, designing, and troubleshooting signal chain components. Changes in modulation, pulsing, and other effects can create power levels that are potentially damaging or accelerate wear and tear with components for transmit and receive circuitry. An RTSA with zero span mode is able to provide valuable measurements that can be used to determine peak power and pulse width highly accurately.
Firstly, the RBW and VBW, limited by the instantaneous bandwidth of the RTSA, must be set wide enough for the pulse to settle within the window. A wider RBW will result in providing more accurate signal response detail and aid with calculating rise time and fall time, as a larger capture bandwidth will result in more of the sidelobe power and frequency components being included in the analysis. A general rule of thumb, is for the RBW to be 3 to 5 times the 3-dB bandwidth of the pulse for accurate characteristic measurements. Though if only power of the pulse is of interest, a smaller RBW could be used to reasonably estimate the pulsed power.
For repetitive signals, a video trigger can be used in zero span mode to provide a stable display of the pulse shape, which would allow for markers and delta-markers to provide pulse characteristic measurements. Also, if the sweep time is set long enough to capture multiple pulses, the pulse repetition frequency of the signal can be captured. With both the peak power measurement and the pulse repetition frequency, the mean power of the signal can be calculated:
Pmean = Ppeak + 10log(Pulse Width * Pulse Repetition Frequency).
For pulses that are faster than the settling time of the RBW, the narrow the pulse, the greater the inaccuracy of the peak power measurement will become.