Don,
I agree completely with the additional checks that you perform.
In my opinion, performing the calibration at 18v/m instead of 10v/m is not a
good idea. I understand and agree with the intent, but in practice it can
cause problems.
In a chamber that does not perform well, you may be overdriving the amp at
some probe positions (nulls) just trying to level to 18v/m. This can cause
harmonics that can affect the probe readings and give erroneous field
uniformity information. I have seen harmonics affect the probe readings when
trying to calibrate below 80 MHz with bi-log type antennas, where the antenna
factor really stinks at the lower frequencies.
Even though the standard says what it says, I think it is better to calibrate
at a lower level so as to make the harmonics a non-issue, make the
calculations for the new drive levels (to include the 80% peak power) then
perform the checks you describe. You also need to peform the radiated
harmonics check as described in the standard. I would then feel much more
comfortable defending the results.
Bob Richards, NCT
--- On Tue, 8/5/08, don_borow...@selinc.com <don_borow...@selinc.com> wrote:
.... And the standard suggests doing
field calibrations at the peak level of the RF during AM modulation,
e.g.,
when calibrating for 10 V/m testing, calibrate the field at 18 V/m,
which
is the peak level of the RF with 80% AM modulation turned on (I have
always
favored this method).
I do two additional checks not discussed in the standard: 1. I use the
uniform drive level method of field calibration. After the calibration,
I
compare the drive level used during the calibration to that obtained
from
the 16-point calculation (for most points, the resultant drive from the
16-point calculation is lower than the uniform drive used for the field
calibration); 2. Using the drive table obtained from the 16-point
calculation, I make a power measurement at the output of the power
amplifier to make sure the output power is comfortably within the
maximum
capability of the amplifier.
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