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, with the rest of the test field created by an unintended frequency.
Further expansion on the culprit:
TWT amplifiers create high levels of harmonic noise at the start of their operational band (lower band edge). These high levels subside steadily with increasing fundamental signal frequency until around 20 percent into the band, by which point the level has dropped from an unacceptably high harmonic level at half the power of the fundamental (3dB down) to an acceptable twentieth of the power (13 dB down).
The significance of this can be seen when the 1-18GHz suite of amplifiers is described.
First, a little history. In days of old, the suite was comprised of a 1-2GHz 200W amplifier, a 2-4GHz 200W amplifier, a 4-8GHz 200W amplifier and an 8-18GHz amplifier. As explained above, high harmonic content compromised the test field at the start of each amplifier band. As you can imagine, this suite represented a significant financial outlay (of the order of $240k since the amplifiers are around $60k each).
Where there is a will there is a way, and around two decades ago some bright spark came up with a way to win business on price by reducing the amplifier count to three. This required ‘detuning’ three of the existing amplifiers. The suite now comprised a 1-2.5GHz 200W amplifier, a 2.5-7.5GHz 200W amplifier and a 7.5-18GHz amplifier.
On the face of it, this was an attractive move (system buyer pays less, system provider wins the bid), but was a bad move from a test-field fidelity viewpoint, as a trade-off in stretching the band beyond the original amplifier design remit was worse harmonic performance. In short, the elephant got bigger.
In the next post, we will look at how other EMC sectors deal with test-field fidelity, and we will begin to look at possible engineering approaches to the issue of MIL-STD test-field integrity.
-Tom Mullineaux
Anonymous says
Tom Mullineaux :- There were three comments posted about the first post on the MIL-STD Elephant. I will give brief answers on each for now and weave fuller answers into the next post
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Anonymous May 31, 2013 at 12:08 AM
I beg to differ. I agree that there are elephants in the EMC world but harmonics from a TWT should be removed by band pass (or low pass)filters. As in your example a 1-2GHz amplifier should be fitted with a 2 GHz low pass filter to reduce the level of the harmonics. We 1-6 GHz TWT that has high harmonics but a 2,4,6 GHz low pass filter to connect to the same amplifier. It is just a matter of doing it right
Tom's answer: Thank you for your comment. I note you say 'should' be fitted with … I have visited many many test houses and only saw this once. In actual fact filters appear in the list of possible solutions to this issue as you will see in upcoming posts
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Anonymous May 31, 2013 at 2:49 AM
Hi Tom, This is why some amp manufacturers are looking towards solid state, with the decreasing cost of solid state devices it's getting such that there is very little difference between the two technologies. There is actually a way round this problem. We have advised that it is always possible to conduct your sweep from HF down to LF, TWT's have very little radiation outside their operating bandwidth so the potential for misleading harmonic interference goes away.
You do always have the potential problem in 2 signal mixing products within the EUT but that's a different story!
Best Regards
Tim Hague
European Technical Manager
AR Europe.
Reply
Tom's answer: – Thanks Tim, good comments. Solid state with band overlap is one of the possible solutions I will be discussing in upcoming posts. I am familiar with the 'get around' where the test is run backwards to try and establish which frequency the equipment under test failed at (the intended or the harmonic, I will expand on this for others later), but see this as a fix for an embarrassing situation (for the provider of a very expensive system)
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Anonymous May 31, 2013 at 4:32 AM
Hmmm so if I understand correctly you would prefer an antenna system coupled to an , lets say Spectrum analyzer, and validate , calibrate the field this way iso of with a field probe. I have one comment only on the whole subject of immunity testing. The real world environtmental RF conditions that cause real problems with Objects be it equipment or otherwise can't be replicated or used for testing due to cost and time consuming reasons. So our testing is already compromised by the real world why bother with the harmonics (just joking)
Tom's answer: – Thank you for your comments. Actually you will find that RF immunity is starting to lean towards ascertaining the level of unwanted fields in exactly the manner you describe. I agree with you that the test fields do not represent real situations but I think there is a case for replicating the real threats faced. For instance, regarding the ubiquitous cell phone, this threat can be created fairly inexpensively. Modern comms waveform generator, amplifier, antenna. Thanks again, good points. Keep them coming!!