Read other posts in the “Elephant in the Test Room” series here.
I apologize for the prolonged absence, my attention was drawn elsewhere for a while – back in the saddle now for the foreseeable future at a rate of at least one blog post per month.
For this new raft of posts, I decided to highlight open EMC controversies that everyone seems to know about, yet few talk about. I shall christen this new series of controversies ‘Elephants in the Test Room’.
Elephant in the Test Room #1 – Compromised MIL-STD Test Fields
Aside: Before launching into this – let me state 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
So let’s begin the scrutiny : –
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.
Let’s expand on the culprit first – The culprit is the characteristically high level of harmonic power created by TWT amplifiers at the start of the amplifier band. Look at any TWT amplifier data sheet and you will see worst-case harmonic levels are just 3dB below the intended frequency power. This means that a 1-2GHz 200W TWT amplifier creating an intended 1GHz signal at 200W, is simultaneously creating a 2GHz signal at 100W.
The 200W at 1GHz and the 100W at 2GHz are presented to the antenna. The antenna dutifully creates two fields, generally favouring the higher frequency as antenna gain tends to improve with frequency.
A field probe is used to measure the field strength one meter away from the antenna. The field probe is unable to differentiate between the two fields and simply sums the contribution of both fields. This is the 200v/m presented to the equipment under test.
To be continued…
– Tom Mullineaux
