Saturday, 19 February 2011

Sweep Testing vs. PIM Testing

Sweep Testing vs. PIM Testing
PIM testing and sweep (VSWR) testing are not the same. Each test has a specific function. VSWR testing is measuring the frequency response of the Device Under Test (DUT) in terms of return loss and insertion loss. PIM testing is measuring the level of inter-modulation [interference] created within the DUT. A device or antenna system can pass VSWR/Sweep testing and fail PIM testing, and vice-versa.Static vs. Dynamic Testing
Static testing is, well, static. In a static test, the test equipment is connected to the DUT and the tech begins the testing cycle. Everyone stands very still while the test results are recorded. This is an easier test to pass and it is also invalid. PIM testing is a dynamic test.

PIM testing is dynamic! Really. Dynamic testing means every component, connector, and connections in the DUT must be tapped and/or wiggled. If you are not tapping, you are not testing.The PIM Test Process

Typically, the technician will have the tower hand start tapping at the antenna and work his way back to the main feed line connector. Then the same process is completed from the bottom main feed line connector back to the carrier’s radio. During all the tapping and moving, the PIM tech monitors the PIM levels. If PIM levels remain under the specified threshold, results are recorded and the DUT passes. If PIM levels exceed acceptable levels, the tech will work with the crew to isolate the bad component(s).PIM Testing is a Team Sport

PIM testing is a team sport. Preferably, there are 3 or more people on the team. The team consists of the PIM tech, at least one tower hand, and one ground person/installer. Each member of the team has responsibilities during the testing process.On the Tower

The person at the top of the tower has three main responsibilities: safety, communication, and perfecting the art of the tap. First and foremost, the tower hand needs to be safe at all times. During PIM testing, especially during troubleshooting, there is the added safety consideration of protecting against an RF burn. Most approved PIM test sets have a transmit power of 20 watts or more. Although an RF burn is unlikely, it is extremely important that the tower hand and the tester are in constant communication. The person on the tower must let the tester know when the load is attached and confirm that the RF has been turned off prior to breaking any connections or removing the load. During the testing process the tower hand needs to communicate clearly where he is tapping/wiggling to the tester. The tester has no way of knowing where the tower hand is tapping without open communication between the two. Each tester will have a preferred method/technique for dynamic testing at the top of the tower. The tester and tower hand should discuss expectations and answer any questions prior to anyone climbing the tower. The person on the tower must keep safety in mind and communicate constantly with the tester to keep the test process moving forward.What to Expect

For the person on the tower, PIM Testing can seem like a dog chasing his tail. As long as the tech behind the test equipment has been trained and has the appropriate experience, trust that there is a method to the madness. During the testing process, if the system is passing, you will be moving between the antenna and main feed line connector, tapping and/or wiggling every component and connector as go.

During troubleshooting the tech will ask you place the load at a given juncture to help isolate any PIM issues. The tech may have you move to one point, say the end of the main feed line, run a test, and then ask you to move to the end of the top jumper. After another test, you may be asked to move back to the main feed line connector. Anytime you are more than six feet off the ground, this moving back and forth may seem like the tester is chasing his shadow. If you begin to get frustrated, ask the tech for a little insight into what is going on during the process. Remember: breathe, be patient, and resist the urge to make any strategic “accidental” tool drops. [Should be “patient,” not patience]

When testing is going well, if you are the ground support, PIM testing is fast, efficient, and just another day in field. If the system is passing, the person on the ground will be tapping from the main feed line connector back to the test jumper during each test. It’s not difficult, but you can expect to be up and down the ladder repeatedly. When PIM issues arise and troubleshooting and repair begins, the person on the ground can expect to ride the same emotional roller coaster as the person on the tower. The tech will have you place the load strategically to isolate any PIM issues in the line. The tech may ask you to rework a connector (sometimes repeatedly) to resolve a PIM issue. Even after reworking a problem, a PIM issue may still exist in the same location. PIM testing is highly sensitive and can detect workmanship and manufacturing issues that are not visible to the human eye. During the testing, remember: breathe, be patient, and resist the urge to throw that wrench, even though you have good aim and a clear shot at the tech.The Tech

Despite appearance sometimes, the PIM tech/tester has a lot going on during the testing process. In addition to collecting, interpreting, and recording the data, the tester is responsible for coordinating the team. In addition to the technical aspects of testing, the tech has three additional obligations to the testing team. Like the tower hand and the installer, the tech has to communicate constantly with the team to make sure he never turns the RF on before the team is ready. He should let everyone know prior to turning RF on and off. There is beauty in clarity. The tester should make a conscious effort to explain the process and explain what he doing especially during troubleshooting. There are limitations to the PIM test set. Let the crew know what they are and how they affect the troubleshooting process. Finally, troubleshooting and PIM testing in general can be frustrating for all involved. The tech needs to be mindful of the demands on the people on the tower and on the ground and do all that is reasonable to ease the pain of testing while insuring the validity of the test.
What to Expect
The PIM tech/tester is usually either a member of the installation crew or at least works for the same company or a third party tester. Either way, when the troubleshooting gets thick and the day long, the tech becomes the center of attention. The tech can expect to be put under a good deal of pressure to relax the standards or allow a static test to slide for the sake of goodwill. You cannot. As the technician, you are responsible for the validity of the test. That means sticking to the test protocol and customer specification. And, once a problem is identified, sticking to your guns until the problem is resolved. While in the middle of troubleshooting, concentration focused, it’s easy to forget to communicate with the rest of the team. When you feel the frustration level begin to rise, take a minute to let everyone know exactly what you’re doing. Constant communication helps keep everyone engaged and frustration levels low. When you are the PIM tech/tester, you may feel like the bad guy, but remember integrity is everything.

After hundreds of PIM tests, we have seen the value of PIM testing, especially as an integral part of a quality assurance program. However, PIM testing can be tedious and frustrating. Testing often takes longer than the allotted time. And a lack of understanding by the crew or a lack of training for the PIM tech only compounds matters. Yet, when the crew is informed, the tech trained, expectations are communicated, and the team understands their roles: PIM testing becomes one more way for a tower crew to prove just how good their workmanship really is.Recommendations for Improving PIM Test Results

PIM testing helps to ensure the installation is of the highest quality. Here are a few tips to improve the PIM test results and testing experience. On the Ground

The responsibilities of the installer on the ground with the PIM tester are the same as those of the tower hand’s, with one addition. Like the tower hand, the installer should always keep safety in mind. The installer should not break any connection without confirming the RF power has been turned off. The installer must also communicate with the tester and follow any instructions regarding the dynamic testing (the tapping). Finally, we generally look to the installer to help keep the testing momentum going.What to Expect

  • Use connector tools.
  • Over or under torque on connectors affect PIM levels: Use a torque wrench.
  • Make sure connectors are clean. Use alcohol swabs and canned air (like those used to clean computer keyboards) to remove dirt, metal shavings, and other contaminates prior to sealing the connector.
  • Make sure connectors are clean. Not a typo, it is that important. Clean connectors can dramatically improve PIM levels.
  • Pay attention to detail. In the field, there appears to be a direct correlation between time and attention to detail during the installation and the amount of time, energy, and money spent PIM testing and troubleshooting. A fast and furious installation usually results in slow, tedious, PIM testing.
  • Experience and training matters: whether through on-the-job training or manufacturers certification courses. Generally, the more experience and training crews have, the better the PIM testing goes. The quality of the installation is better and the lower PIM levels prove it.

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