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.