Read other posts in the “Elephant in the Test Room” series here.
Recap on this thread
The room: MIL-STD RF immunity testing – 1-18GHz 200v/m
The elephant: Everyone in the room is aware that a significant fraction of the ‘calibrated’ 200v/m test field is actually created at the wrong test frequency.
The culprit: Notoriously high ‘start of band’ harmonics produced by all octave band TWT amplifiers
The consequence: Equipments purportedly demonstrated to be resilient to 200v/m have in many instances been exposed to only around half this field strength, the rest of the test field created by an unintended frequency.
Things are getting a little messy so to maintain some semblance of order I will tackle the open topics as follows:
1. A restatement of my general stance regarding this particular elephant
2. The money no object approach
3. A reminder on the list of practical solutions
4. Replies to the latest readers’ comments
In the next post I will:
5. Discuss the ins and outs of each practical solution and
6. Keep my promise to provide a description of the ‘get around’ test houses are currently forced to adopt.
So here we go.
1. Restatement of my general stance
To ensure this is not lost in the course of the discussion, let me repeat that:
From my standpoint MIL-STD testing works as evidenced for instance by no planes dropping out of sky. However, in my view, this sector of the EMC industry should start to address this under-test situation
2. The “money and time no object” approach
This is where we have fun by letting our imaginations run wild.
We have an infinite amount of money and all the time in the world so we simply:
• Re-commission one of the space shuttles with a payload of 200 v/m remote controlled test equipment and the EUT (equipment under test). We won’t need the test chamber where we are going.
• Head for the quiet zone halfway to Alpha Centauri. At 4.37 light years away, Alpha Centauri is the closest system to us.
• Come to a halt halfway (Einstein would argue ‘at a halt’ relative to which coordinate system?), but we will go with our understanding of ‘come to a halt.’
• Unload the equipment to be tested along with a self powered radiator providing a perfect 200v/m over 1-18GHz at a distance of 1000m.
• By some miracle of science (we have all the time we need) arrange for the volume immediately surrounding the EUT to be filled with air at 25C.
• Employ the straightforward substitution method (as opposed to the questionable net-power method used in some automotive tests).
• Take the space shuttle a distance of one light year back towards Earth so that it does not disturb the test and so we are closer to home.
• Conduct the tests remotely
• Return to find everyone that waved us off died thousands of generations ago and no one cares about us, our tests, or the test results anymore.
Note we went to all this time and trouble to ensure no nearby bodies disturb the test field. We declined to use the ground plane table as we suspect this is there to try and give a measure of repeatability to a test that has notoriously high uncertainties. Talking of uncertainties we can now state categorically that if the EUT passed the test then it is immune to illumination by a field strength of 200v/m over 1-18GHz. Here on Earth imperfections in the chamber, the proximity of the antenna, and the presence of the ground planes (table and floor) give grounds (pun intended) for doubt in such a claim.
3. List of practical solutions
Getting back to Earth, repeated here is the list of solutions we proposed earlier (some of which readers guessed before the list was published)
• Switch in low-pass filters at the output of the amplifier
• Use frequency band overlap (tail-end of one amplifier overlaps start-of-band of the next amplifier in the suite)
• Replace TWT amplifiers with solid-state amplifiers
• A mix of solid state amplifiers and band overlap
These will be tackled one by one in the next post.
-Tom Mullineaux
4. Check out Tom’s responses to comments on his last post, Elephants in the Test Room #1 Continued – How Other EMC Sectors Dealt with the Elephant
