Yes, and in the real world that is what is done. But it has to meet the
accuracy requirement of the test. For example, see MIL-STD461E 4.3.1
(slightly snipped):
4.3.1 Measurement tolerances.
Unless otherwise stated for a particular measurement, the tolerance
shall be as follows:
c. Amplitude, measurement receiver: ±2 dB
d. Amplitude, measurement system (includes measurement receivers,
transducers, cables, and so forth): ±3 dB
If the receiver is already ±2 dB we don't have much to work with.
Calibration with X4 TUR might require an accuracy of 0.25 dB. Might
settle for less, no?
Sooner or later, someone will say their project passes by 0.5 dB; SHIP IT.
Same old story.
Cortland
KA5S
On 12/7/2011 6:50 AM, Ken Javor wrote:
Now we get to the heart of the matter that prompted the original post.
That spike in the 137 cm tip-to-tip biconical used with a T1 dipole
balun (28 -- 200 MHz) behavior might have been objectionable, but you
wouldn't miss it an analog sweep with even 1% frequency steps. You
would only miss it if you calibrated every ten MHz as was done with
the biconical antenna design in MIL-STD-461A (1968 -- when swept
performance was difficult to do). There is no need to use frequency
steps commensurate with the sorts of BWs used during EMI testing. A 1
- 5 MHz BW would more than suffice to sweep an antenna used above 30
MHz to ensure not missing anything.
The following quote is the rationale I have seen a few places with
which I take issue:
"Though the >sweep< may be continuous, if you can read amplitude only
by marking pixels, you need to insure .5 BW or less per pixel. That's
straight sampling theory, right?"
That is straight sampling theory applied to an unknown signal
characteristic. If a continuous sweep using 1/2 BW steps as per
sampling theory has been performed, then discrete points for an
antenna factor file may be chosen by inspection so that in any
particular frequency range either of the two following conditions are met:
1. If the data is noisy, with no obvious trend, then frequency points
should be listed so that the antenna factor between any two listed
points doesn't change by more than 1 dB (or whatever delta is
deemed necessary for adequate accuracy).
2. If a linear or log-linear approximation can be made over some
frequency interval, then only the two endpoints of that interval
need be recorded, with the understanding that between those two
points the appropriate interpolation will yield the desired
accuracy. The interpolation has to be one which the automated
software can be instructed to perform on its recorded antenna
factor files.
One of these two approaches should work when deriving a set of
discrete points from a continuous sweep of any test transducer
factor's performance.
The question is whether this is "cast in stone" anywhere or left up to
the person doing the calibration, or the test engineer abstracting a
subset of data points for inclusion in his files.
Ken Javor
Phone: (256) 650-5261
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