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.
How other EMC sectors control test field fidelity
Commercial Sector
Test field integrity was an issue with commercial RF immunity testing due to amplifiers (note not TWT amplifiers) being run into hard saturation and thereby creating high harmonic power levels. As previously explained high harmonic power levels combined with antenna gain characteristics can make a significant unintended contribution to the measured test field.
The elephant has been removed from the room in that the commercial standard now restricts rogue test field components to one quarter of the total test field. This stipulation is checked at all test frequencies during system calibration.
Automotive Sector
To address what once was poor test field integrity, current automotive standards limit amplifier harmonics to 20dBc maximum (Ford and GM) or 13dBc maximum (Chrysler). 20dBc seems arbitrary and will be a topic in an upcoming ‘Automotive elephant in the test room’ thread.
Addressing MIL-STD Test-Field Integrity
So what are our options in trying to dislodge the elephant? We will take a ‘money no object’ stance later, but first let’s examine the pros and cons of the more commercially minded approaches. A non-exhaustive list is: –
• 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
In actual fact readers outguessed me by suggesting the first and third solutions too (see below).
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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
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)
To be continued …
-Tom Mullineaux
