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.
Hello again. Now we have returned from our fantasy trip we need to look at realistic methods of removing the elephant. That is with a practical budget and with a practical time for implementation. Previously we came up with a list of practical means of dislodging the elephant. Let’s look at each in turn.
a) The Installation of Low Pass Filters
A low pass filter will allow the wanted test signal through (e.g. a 1GHz signal from a 1-2GHz TWTA) and reduce the level of the unwanted harmonic (e.g. at 2GHz). The harmonic is a high power one (at around half the power of the wanted signal) so the filter must be absorptive, that is it must dissipate the harmonic power). A reflective type filter will bounce the harmonic power back at the amplifier, not a good thing as TWT amplifiers are prone to damage from high reflected power.
The low pass filter is switched in only when the test frequency is at the start section of the TWT amplifier’s band. Once the test frequency gets to the point where amplifier produces more reasonable harmonic levels the filter is switched out (bypassed).
A back of the envelope calculation using sensible assumptions says the achievable test field strength is around 275v/m:
That is:
Assumptions:
Amplifier power P at start of band is 250W
Antenna gain G at 1GHz is 10dB
The test distance d is 1m
Test field E = sqrt (30.P.G) / d
E = sqrt (30. 250.10) / 1
E ≈ 275 v/m
We need 200v/m so we do not have a lot of margin to play with. The back of the envelope estimate does not include current system losses never mind the additional losses due the filter / associated switches / cabling. It looks like we will need to source a very high gain 1-2GHz antenna or alternatively use a higher power amplifier if we are to salvage the filter solution.
To be continued…..
Please note, this is all straight from the top of my head so if the thinking contains fundamental errors, now would be a good time to point them out so we don’t get too far down the road before realizing the mistake.
Oh, and if anyone wants to know how the ‘magic factor’ 30 gets into the formula, just let me know and I will explain how it arises.
b) The Use of Band Overlap
This is where the low-harmonic tail-end of one amplifier band overlaps the start-of-band of the next amplifier in the suite. So for instance if part of the suite comprised a 1-2.5GHz TWT amplifier followed by a 2-4GHz TWT amplifier, the test would use the 1-2.5GHz amplifier to 2.5GHz and then switch in the 2-4GHz amplifier over 2.5-4.0GHz.
The approach taken here is to start with what we would like to see using ‘virtual’ amplifiers, followed by an attempt to match our wish list with amplifiers that actually exist in the marketplace.
What we would like to see (including the sub 1GHz solid-state amplifier used in 200-1000MHz testing), is the following overlapping suite:
200-1400MHz 1kW / 1-2.5GHz 250W / 2.0-4GHz 250W / 2.5-7.5GHz 250W / 7.5-18GHz 250W
Note we will be forced to resort to the using a low-pass filter for the start of the final amplifier band, however this is feasible as the antenna gain at 8GHz is superb creating a healthy power margin to overcome additional system losses.
Also, a sub-1GHz power amplifier going to 1400MHz is simply not available (ALL available amplifiers stop at 1GHz), so we must use a low-pass-filter here too, but with a 400W amplifier to cover the additional losses introduced by the filter arrangement.
So the practical ‘overlapping’ suite becomes 200-1000MHz 1kW / 1-2.5GHz 400W (filtered) / 2.0-4GHz 250W / 2.5-7.5GHz 250W / 7.5-18GHz 250W (filtered)
To be continued – comments welcome
-Tom Mullineaux
