Performing Accurate Multi-Paction Measurements
- Sunday, 01 August 2010
Identifying Multi-Paction Events
Multi-paction effects can be detected by a number of different approaches, which make measurements globally (on the system) or locally (on a component). Global methods determine whether multi-paction is present somewhere in a satcom or other system, but don’t isolate which component is the source of the multi-paction effects. Local approaches are more useful during the system development stage to identify “weak links” in the system at the component level. Local evaluation methods can also be used to monitor part of a system under continuous operation in order to better understand long-term effects. Multi-paction effects can often be elusive to spot, and it can take years for a multi-paction discharge to occur. Over time, persistent multi-paction events can cause erosion of component surfaces, which degrade electrical performance and eventually lead to component and/or system failure.
Common global methods that analyze system performance under conditions conducive to multi-paction effects include detection of increased close-to-the-carrier noise, increased second and third harmonic levels, and changes in output power. Unfortunately, since the first two approaches rely on the generation of noise, they are prone to errors from noise caused by non-multi-paction causes. Multi-paction measurement systems typically suffer from a number of different sources of noise, including from microwave signal generators. Excessive noise from other sources can lead to establishing a too-low value for the multi-paction threshold of a system. Similarly, failing to detect a transient multi-paction event can lead to establishing an excessively high multi-paction threshold for a system.
Global techniques have also been developed based on the use of amplitude modulation (AM) detection, using high-speed sampling techniques to generate a Fast Fourier Transform (FFT) display of the test signal. This approach relies on the signal difference between the input test signal and the multi-paction threshold. When the signal carrier passes the multi-paction threshold, it produces a distinct modulated output signal. An FFT will reveal any periodicity in the measured signal.
Both global and local measurement methods have been developed for single-carrier and multiple-carrier test cases in order to closely emulate operating conditions of satcom and other high-power communications systems that employ complex modulation formats. In multicarrier systems, for example, the peak power levels of the system can vary widely, depending upon the relative signal phases of the carriers.
Traditional test systems have relied on a high-power RF/microwave source usually consisting of a frequency synthesizer and external pulse modulator boosted by means of a high-power traveling-wave-tube (TWT) amplifier (TWTA). Some means of power control is needed, usually in the form of a series of couplers and power dividers and precision variable attenuators. Phase shifters are typically used to control the phase of the test carrier signals, and often impedance tuners will be enlisted to ensure precise control of the impedance match to the device under test (DUT). The DUT itself, which may be a component, subsystem, or full system, is isolated in a vacuum chamber to emulate deep-space atmospheric conditions. Traditional monitoring equipment includes peak power meters, spectrum analyzers, and digital storage oscilloscopes (DSOs). Such a system (Figure 1) is designed to precisely compare signals entering and exiting the DUT for evidence of nonlinear behavior that might signal the onset of a multi-paction event. One of the goals of any multi-paction test system is not to instigate multi-paction events that lead to the destruction of a DUT, but to identify the threshold voltage and power levels for multi-paction events, below which the DUT can operate safely.
One of the problems with the traditional multi-paction test setup is its reliance on spectrum analyzers to identify spectral phenomena, such as increased levels of third-harmonic signals, that may be extremely short lived and difficult to capture, even by a high-speed DSO. Analog spectrum analyzers operate with swept analog resolution-bandwidth and video filters. Resolution increases as sweep speed decreases, and often an analog spectrum analyzer’s sweep speeds are incapable of capturing the short-term increases in third-harmonic distortion that may be the signals for a multi-paction event. Newer, real-time spectrum analyzers, which rely on digital capture of a fairly wide instantaneous input bandwidth, can improve the chances of capturing the transient events related to multi-paction effects, but these instruments still resolve signal information by means of swept filters, albeit using digital signal processing (DSP) for those filters.