Keysight Technologies Achieving Accurate RF and Microwave Power Measurements for Satellite Thermal Vacuum Test Application Note
Introduction Equipment used in space applications needs to go through stringent environmental tests in order to prove that it can survive the extreme conditions experienced during launch and while in space. Some of the key environmental tests include vibration, thermal vacuum, and electromagnetic compatibility. Vibration test is important to ensure that the equipment is able to sustain high impact during rocket launch. Thermal vacuum test is crucial to confirm the equipment is able to operate under the space vacuum condition. Electromagnetic compatibility test is required due to densely-packed electronic systems operating at various frequencies. RF signals radiated from high power equipment (transmitters) can affect the performance of low power equipment (receivers). This application note focuses on thermal vacuum (TVAC) test, which is typically the final test to be conducted, and the most complex and expensive test. Detailed planning of the TVAC test is required, and normally done six to eighteen months ahead of the test in order to ensure success. TVAC testing helps to uncover possible problems such as outgassing of equipment that could contaminate the environment, design flaws due to overheating of materials, and a corona (metal vapor arcing) effect that can cause problems at low pressure. This application note compares the commonly-implemented power measurement test setups for TVAC with a new test method that uses TVAC-qualified LAN power sensors. The paper explains how this new test method provides a simplified test setup, and more accurate and reliable microwave power measurements for TVAC testing of satellite equipment. Table of Contents Introduction... 2 Typical TVAC Test Setup... 3 New TVAC Test Setup... 4 Design and Qualification Process of the Keysight TVAC Power Sensor.... 5 Manufacturing Process Ensures the Highest Quality and Reliability.... 6 Key Specifications and Benefits... 6 Monitoring and Controlling the Power Sensor s Temperature in a TVAC Chamber... 7 Bulkhead Connectors... 8 Conclusion... 9 More Information.... 9
03 Keysight Achieving Accurate RF and Microwave Power Measurements for Satellite Thermal Vacuum Test Application Note Typical TVAC Test Setup Figure 1 shows the typical test setup for TVAC test. The whole satellite, or satellite components or modules are placed inside a TVAC chamber. All electrical connections to the system under test must include hermetically-sealed connector feedthroughs in order to maintain the vacuum. In general, all the measurement equipment, including the power meter and sensors, are located outside of the chamber and connect to the satellite or components using the sealed feedthroughs via switch matrices or a test interface subsystem. Long RF cable runs are often needed to connect the power sensor to the satellite inside the chamber. This can result in accuracy degradation due to poor SWR and insertion loss due to the length of the RF cable. Complex calibrations are required to calibrate the path loss as the RF cable performance changes with temperature. Typical TVAC test setup RF power meter and sensor are typically installed outside the chamber and being routed into the chamber via long RF cables. Degraded accuracy Complex test setup Signal generator Signal analysis Network analysis RF switch matrix Bulkhead connector interface Test interface subsystem Complicated calibration routine Thermal vacuum chamber Power meters Sensors Multiplier Satellite or satellite components DC power system Computer Typivcally 40 to 80 ft. Figure 1. Typical TVAC test setup with all the measurement equipment located outside the chamber and connected to the satellite or components via switch matrices or a test interface subsystem
04 Keysight Achieving Accurate RF and Microwave Power Measurements for Satellite Thermal Vacuum Test Application Note New TVAC Test Setup With the Keysight Technologies, Inc. U2049XA LAN power sensor equipped with option TVA, the TVAC-qualified sensor can be directly connected to the satellite inside the TVAC chamber (Figure 2). The benefits include: Simplified test setup with less complex calibration routines The sensor now can be connected directly to the output of the device under test within the TVAC environment and while testing is being conducted. This removes the use of RF cables and the need to perform the associated complex calibrations. The power sensor immediately provides accurate output power measurements, without path loss calibration. More reliable and accurate power measurements The new TVAC test setup TVAC qualified Po/LAN power sensors are connected directly to the input and output of the satellite inside the thermal vacuum chamber. Signal generator Signal analysis Network analysis Multiplier DC power system Computer By eliminating the long RF cable, the power measurement uncertainties due to insertion loss, poor SWR performance, and temperature effects of the cable are removed. The power sensor is fully calibrated and offers warranted accuracy specifications over a temperature range of 0 to 55 C. Its internal zero and calibration function also allow users to perform zero and calibrations-on-the-fly to improve overall measurement accuracy. Remote monitoring By connecting the LAN power sensor to a shared network via a network switch or power-over-ethernet (PoE) hub, the power sensor can be controlled remotely from any part of the world using a Web browser and the Keysight Technologies, Inc. BenchVue software. Table 1 summarizes the benefits of using the TVAC power sensor compared to a traditional bench power meter and sensor solution. RF switch matrix Bulkhead connector interface Test interface subsystem 40 to 80 ft. typical Reliable solution High accuracy Simplified test setup Remote monitoring Thermal vacuum chamber Power sensor Power sensor Satellite or satellite components Input Output Table 1. Comparison of traditional and TVAC test methods Item Power meter and power sensor TVAC power sensor Sensor s location Outside chamber, perhaps 60 ft away from payload Inside chamber, directly connected to payload input/ output or through some adapters Losses due to cable (based on a typical TVAC cable) 34 db at 18 GHz Minimum losses Input power to sensor 10 to 40 dbm +24 to 6 dbm Mismatch uncertainty 2.56% (between sensor and long cable, cable SWR 1.25) 1.30% (between sensor and source, source SWR 1.12) Measurement uncertainty at 18 GHz Measurement speed Total dynamic range and supported signal types 4.59% (E4413A-H33 at 25 dbm) 4.59% (U2049XA at +9 dbm) 3.72% (8485D-033 at 25 dbm) 1 400 readings per second (E4413A-H33) 20 readings per second (8485D-033) 90 db CW-only signal (E4413A-H33) 50 db CW/modulated signal (8485D-033) 1. E4413A-H33 and 8485D-033 are Keysight power sensors widely used in satellite testing, and enable power measurement up to 33 GHz. Figure 2. New TVAC test setup with sensors connected directly to the input or output of the satellite for improved accuracy and simplified test setup 50,000 readings per second 90 db CW/modulated signal
05 Keysight Achieving Accurate RF and Microwave Power Measurements for Satellite Thermal Vacuum Test Application Note Design and Qualification Process for the Keysight TVAC Power Sensor The Keysight U2049XA option TVA (TVAC power sensor) uses low-level materials with minimum outgassing properties. Under vacuum, different materials have different outgassing rates and have the capability to contaminate the vacuum chamber. Therefore, to ensure that all the materials used in U2049XA option TVA are vacuum compatible, they are matched against the NASA/ESA outgassing database. Items not listed in the database are subject to material testing in accordance with ECSS-Q-ST-70 02C 1 to ensure any outgassing is within prescribed limits. The full sensor assembly is also sent for TVAC qualification based on a typical customer TVAC profile, as shown in Figure 3. The test shown in Figure 3 was carried out over a duration of 10 days and 8 hours per temperature cycle. Performance tests were carried out throughout the different hot and cold temperatures to ensure that the sensor measured accurately over a wide range of temperatures under the vacuum environment, and that repeated temperature cycling did not degrade performance. In this example, the sensor was subjected to operation tests ranging from 10 to +60 C and non-operating storage tests from 40 to 100 C. TVAC qualification tests are performed in addition to Keysight s standard environmental qualification program, known as ETM (environmental test manual). The Keysight ETM is a collection of test methods and best practices that Keysight uses to ensure that its product meets applicable regulatory requirements, client expectations, and Keysight s published environmental specifications for storage, transportation, and use. Off On Off Unit on cycles 100 + 0/ 3 C 60 + 0/ 3 C 23 + 0/ 2 C 10 + 3/ 0 C 40 + 0/ 3 C Note: Dwell times are 40 mins Figure 3. Keysight TVAC qualification test profile for U2049XA option TVA 1. Thermal vacuum outgassing test for the screening of space materials by European Cooperation for Space Standardization.
06 Keysight Achieving Accurate RF and Microwave Power Measurements for Satellite Thermal Vacuum Test Application Note Manufacturing Process to Ensure Highest Quality and Reliability Every TVAC power sensor manufactured by Keysight is subjected to thermal vacuum bake out in Keysight s in-house TVAC test facility located in the United States. Each batch of power sensors is placed inside a TVAC chamber and baked at the pressure of < 5 x 10-6 Torr and a temperature of 85 C for a duration of at least 8.5 hours. Immediately after the completion of TVAC bake out, the sensors are packed in vacuum sealed bags to minimize re-absorption of moisture. This process ensures that the sensors are contaminant-free and ready to use in a TVAC chamber. Key Specifications and Benefits Table 2 summarizes the key features and specifications of the TVAC power sensor that make it ideal for satellite thermal vacuum tests. Table 2. TVAC power sensor use benefits Key features and specifications 1 Benefits Wide frequency coverage of 10 MHz to 33 GHz A single sensor that can cover a wide range of satellite bands Wide power range of 70 to +20 dbm Ability to measure both output signals and small reflected signals Power over Ethernet (PoE) Enables remote monitoring and control of power measurements over Ethernet with the use of a standard Web browser and the Keysight BenchVue power meter software. Additional DC power wiring is unnecessary Internal zero and calibration Performs zero and calibration-on-the-fly at any point of the TVAC test, without having access to the sensor inside the TVAC chamber TVAC-qualified Improves overall accuracy with sensor connected to the DUT output directly inside the TVAC chamber Provides greater confidence in power measurement results 1. For more detailed specifications, please refer to U2040 X-Series data sheet, literature number 5992-0040EN, on www.keysight.com.
07 Keysight Achieving Accurate RF and Microwave Power Measurements for Satellite Thermal Vacuum Test Application Note Monitoring and Controlling the Power Sensor s Temperature in a TVAC Chamber In general, the temperature range for thermal vacuum test can be anywhere from 196 to +140 C. It can easily be beyond the operational temperature range of the TVAC power sensor. Therefore, it is a good practice to monitor the temperature of the sensor inside the chamber to ensure that it is operating within its supported temperature range. In common with any powered equipment within a TVAC chamber, the power sensor will generate heat that can only be removed by radiation or conduction. Each Keysight TVAC power sensor is shipped together with thermal interface material (TIM) and two C-brackets. Keysight recommends attaching the sensor to a metal plate or similar conductive surface, and sandwiching the TIM between the sensor and the metal plate. The sensor then should be secured tightly onto the metal plate with the C-brackets (Figure 4). During operation, monitor the temperature of the sensor s exterior surface with a thermocouple to ensure it is within its operating temperature of 0 to +55 C. When operating the sensor in a cold environment, below 0 C, wrap the sensor with heat reflective material. This allows its self-generated heat to keep it in the operating temperature range. When operating in a hot environment, beyond +55 C, the sensor needs to be cooled. Alternatively, turn off the power sensor when operating beyond its operating temperature range. The non-operating temperature range of the sensor is 40 to +100 C. C-bracket TIM Figure 4. A thermal interface material (TIM) and two sets of C-brackets are supplied with every TVAC power sensor (U2049XA option TVA only) for thermal control purposes. (Refer to U2040 X-Series User s Guide for mounting dimensions and instructions)
08 Keysight Achieving Accurate RF and Microwave Power Measurements for Satellite Thermal Vacuum Test Application Note Bulkhead Connectors A custom-made bulkhead connector is needed to connect the TVAC power sensor (with PoE interface) inside the chamber to the external computer or PoE switch. Figure 5 shown an example of a sample bulkhead cable adapter (D-sub 50 to Cat5 cable adapter) that Keysight uses in TVAC qualification tests. A few manufacturers provide custom-made cable adapter service such as: Airbus Defense and Space Test Laboratories. http://airbusdefenceandspace.com/ Pave Technologies Co. www.pavetechnologyco.com Douglas Electrical Components. www.douglaselectrical.com D-sub 50 pins connector TVAC safe LAN cable CAT5 connectors (connect to sensor inside chamber or connect to computer/poe switch outside of chamber Figure 5. Example of a six-sensor bulkhead cable adapter connected to the sensor inside a TVAC chamber to a computer or PoE switch located outside the chamber. Connects through a standard hermetically-sealed, 50-way D-sub connector on the bulkhead
09 Keysight Achieving Accurate RF and Microwave Power Measurements for Satellite Thermal Vacuum Test Application Note Conclusion The Keysight Technologies U2049XA Option TVA TVAC power sensor is the ideal solution for satellite thermal vacuum testing inside a TVAC chamber. The TVAC power sensor comes with numerous benefits. It enables accurate and reliable power measurements inside a TVAC chamber, simplifies measurement setup, and is suitable for any signal format. It also offers 100 times faster measurement speed and comes with a wide 90 db dynamic range. Careful considerations have been given to the thermal control of the sensor operating inside a chamber. Follow the sensor s mounting instruction for the most effective heat dissipation in a vacuum environment. More Information Keysight U2040 X-Series Wide Dynamic Range Power Sensors data sheet, literature number 5992-0040EN Keysight U2040 X-Series Wide Dynamic Range Power Sensors User s Guide, document number U2041-90002.
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