Test System Options
The block diagram for an improved multi-paction test system is shown in Figure 2; a photograph of an actual system is shown in Figure 3. This system relies on multiple-channel, high-speed, analog-to-digital converters (ADCs) to capture test signals entering and exiting the DUT. As with a traditional test system, the DUT remains in a vacuum chamber, with high-power coaxial transmission lines to route test signals to it and from it. The transmission lines are housed within a high-power cart, which provides mobility for ease of connection to different DUTs.
The test system uses phase-nulling techniques to study changes in the linearity of the input signal versus the output signal of a DUT. In a conventional system, incident and reflected test signals are summed and results displayed on a spectrum analyzer or peak power meter. But unlike a traditional system, in which the combination of a variable attenuator and a phase shifter and manual tuning are used to null the phase of the incident and reflected test signals, this type of system digitizes the incident and reflected signals by means of a dual-channel, high-speed, analog-to-digital converter (ADC) and uses digital signal processing (DSP) techniques to manipulate and analyze the data. By analyzing the digitized signals by means of dedicated software, any adjustments needed between the incident and reflected signals in order to detect multi-paction events can be done almost instantaneously while still controlling the time constant to simulate what were formerly manual adjustments of variable attenuators and phase shifters to account for changing conditions at the DUT.
This technology differs from traditional test systems in that it can acquire data continuously, with zero dead time in the measurements. The ADCs digitize and send all test signals to the system controller, which runs the specialized measurement and analysis software. The software operates according to predefined parameters, such as threshold voltages, phase nulls, amplitude changes, increases in third-harmonic levels, etc., to acquire test data of interest. The hardware, in the form of the computer-controlled digitizer, continuously monitors the test signals for any kind of multi-paction-related event, and the software triggers on various event thresholds to either store or disregard the captured signal data.
The combination of hardware and software ensures that measurement channels are phase matched and phase coherent, even accounting for manufacturing tolerances in passive components, such as couplers and attenuators. Each measurement channel is sampled and measured for any deviations in phase, and corrections are made automatically under software control.
This approach uses various techniques to identify possible multi-paction events. For example, it measures changes in system and DUT return loss under different signal conditions, since a progressive or dramatic change in return loss can indicate that a DUT is defective or likely to suffer multi-paction discharges at high power levels. The system is designed to evaluate a device under swept power levels, beginning with relative low voltage and current levels and ramping to levels as high as multiple kilowatts of power, monitoring DUT performance for changes during the increases in power levels.
An ADC-based system configured in this manner can test as many as four DUTs sequentially, with only about five seconds required to switch a test sequence from one DUT to another. The system can pro-
vide test results in a variety of formats. In one approach, the degradation of pulsed waveforms under changing signal conditions can help identify impending failure of a DUT due to multi-paction effects (Figure 4). Once a test routine is begun, it is completely under software and computer control, with no human intervention needed. It can run continuously, acquiring test results continuously according to preset conditions stored in the software prior to running the test.
A typical system could be configured to perform testing in bands from 1 to 18 GHz at power levels as high as 10 kW at 2 GHz. It provides extensive monitoring and display capabilities, allowing an operator close scrutiny of real-time events over a wide range of frequencies and power levels. The system is also calibrated to account for variations in performance as a function of temperature and power. For example, the insertion loss of the system test cables will increase as temperature rises due to high power levels. The initial calibration accounts for these temperature-dependent system performance variations in order to provide measurement results that are accurate in amplitude within ±0.1 dB and in phase within ±0.1 deg.
For those interested in predicting multi-paction effects on their own DUTs, the European Space Agency (ESA) offers a free online multi-paction calculator at http://multipactor.esa.int/. The calculator includes a variety of RF software tools, such as a mismatch calculator, skin depth calculator, waveguide matching transformer, rectangular and circular waveguide insertion loss calculator, venting and outgassing calculator, and shielding effectiveness (SE) calculator. It can be used for analyzing microstrip, stripline, coaxial connectors, and standard waveguide gaps.
This article was written by Howard Salvesen, Vice President, Engineering at In-Phase Technologies, Inc., Clarksburg, NJ. For more information, visit http://info.hotims.com/28056-121.
- S. J. Feltham and S. Johns, “The ESTEC Multipaction Support Facility,” Preparing for the Future, ESA Newsletter, European Space Agency, Vol. 7, No. 4, 1997, p. 2.
- Harlan S. Hurley and Dennis J. Kozakoff, “RF Multicoupler Design Techniques to Minimize Problems of Corona, Multipaction, and Stability,” NASA Contractor Report, NASA CR-61387, NASA-George C. Marshall Space Flight Center, AL, December 1971.