Brian,
We've always done the calibration at 1% steps. Yes, it stinks. Although
for a full run, both polarities it'll take us only around 3.5 hrs to take
all the data. Why do you need to run both 3V/m and 10V/m? We do the
constant power method and just use a forward power high enough to capture
both in the same run.
On amplifier saturation, we sometimes get those slightly greater than 5.1
dB differences too. About 5.2, max. I chalk it up to the uncertainty of
the field probe (I measure the delta with both the wattmeter and the field
probe). As such I also put a little extra headroom into my 2dB compression
point (I put the cutoff at 3.5 dB instead of 3.1dB). The most likely cause
of 2dB compression on our amplifiers is the input level. If our amps are
running at 100% gain, the maximum level I can pipe to the amp from the
synth (and be just outside the 2dB point) is between -8 and -5dBm depending
on frequency.
Keep in mind that the harmonics requirement is still in place as well.
You'll need to check that your highest harmonic (third) is more than 6dB
lower than the fundamental in the field. You can measure it after your amp
(or directional coupler) but you have to take into account your antenna
factors and cable losses. On our equipment I found this to be an easy
requirement to meet as long as I'm outside the 2dB compression point
anyway.
The low pass filter the standard mentions is to allow you to meet the
harmonic distortion requirement even if your amp is saturating. If you can
roll off your third harmonic you can get a little more power out of your
amp. I used to use the cable to my antenna as a low pass filter when we
first started doing the test above 1GHz. Then we got a bigger amp and
replaced the cable with a low loss one, making things much easier.
About the only thing you can do to fix the compression around 80MHz is to
get a bigger amp (expensive) or change antennas. You could use a filter to
roll off the 240MHz (and higher) harmonics, but it may be more trouble than
just using a different antenna (and probably won't buy you a whole lot
unless you are just on the edge). Luckily I haven't had to do that yet.
Regards,
Rob
"Kunde, Brian"
<brian_kunde@LECO
TC.COM> To
Sent by: <[email protected]>
[email protected] cc
Subject
10/26/2007 12:00 Radiated Immunity Calibration Test
PM
I recently obtained the IEC 61000-4-3:2006 radiated immunity test standard
which I'm trying to get geared up for. In addition to testing above 1Ghz,
there are some major changes/improvements to the calibration methods.
Calibration Freq Steps:? In the older version of this standard,
calibration was done at 10% frequency steps, but now it is 1% steps. This
change is huge regarding the amount of time?calibration takes. Just from
80Mhz to1Ghz with 10% it took us about 2.5 hours. At 1% steps it will take
us 12 hours per power level. To do 3V/m and 10V/m will take 24 hours. To
test two power levels to 6Ghz will take us over 40 hours of testing. This
is considering a highly automated calibration test routine and NOT
counting the time for verifying the amp saturation.? I know the older
version of the standard called out 10% frequency steps?because 1%
steps?was impractical. Why did this change?
Amp Saturation:? In section 6.2.1, it calls out a procedure for verifying
your amp is not operating in saturation. It says the difference between
the two forward power levels should be within 3.1 and 5.1 db. Then is
says, "If the difference is less than 3.1 db, then the amplifier is
saturated and is not suitable for testing".? What if the difference is
greater than 5.1?? Is the amp still in saturation?
Fixing Saturation:? So you run the procedure and you find at a few
frequencies your amp is in saturation. What is the most likely cause and
what do you do to fix it?? Is the most likely cause the amp itself?? In
appendix D, the standard refers to an "adjustable/tracking/tunable
low-pass filter". What is that? There is no other practical advice that I
see in the standard.? It explains that saturation is caused by strong
harmonic signals, but what causes it if not the amp??? Can it be caused by
cabling, high?antenna factor, or poor SWR?
More specifically to my case, the power level difference for most
frequencies are recorded about 5.1db, which is a 1:1 change from the amps
input, which I think is a good thing. However, I have a couple frequencies
that record slightly higher than 5.1db, such as 5.15 or 5.2db. Is this ok?
If not, what is the cause.
I also have a few frequencies at the very low end, around 80Mhz, that has
a difference around 2db.? I know the antenna is not very efficient down
that low and the amp has to really put out a lot of power to?hit 10V/m.
The SWR is real bad (5:1)?at these low frequencies. I understand this is
common?with most?antennas.?If this could be the cause, what can be done to
fix it? (Attenuators?)
Thanks everyone.
The Other Brian.
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