Ken brings up a nasty dilemma; what do you do when the correction factor
doesn't follow the theoretical prediction.
In my example of the factor set of an H-field loop antenna, which I used
because the amplitude might go from 28 dB @ 300 kHz to 88 dB @ 20 Hz (data
>from my EMCO 7604 passive loop), I would expect the lower-frequency end of the
plot to be linear. Yet, when I lay a straight-edge to the actual plot, I find
a few slights humps and bumps.
Are these deviations attributable to my cal lab's equipment errors or setup
errors? Is my straight-edge warped? Or could these deviations be real? While
watching for the signs of measurement overloads or noise-floor limitations, I
also have to admit that maybe there is something in my probe which creates
tiny resonances or coupling effects that really does affect the theoretical
results.
I tend to give more credence to data in front of me than calculations on
paper. Calculations are not inherently wrong, indeed, they are "perfect." If
formulae had enough terms to account for the "real world", then we could
eliminate a lot of experimental calibration work.
Ken mentions standard gain horns as an example of physical reality accurately
predicting electrical performance. He's right, those horns are very
predictable, unless.... My favorite "unless" involves the waveguide-to-coax
adapter (sometimes external, but often built-in to the antenna). The coax
connector female center pin often supports the tiny probe in the waveguide
throat. Positioning of this pin is critical to horn performance. Ever wonder
what happens to the position of this little coupling element when you screw on
a coax cable with an improperly assembled or damaged male center pin? You now
have a very "unstandard" standard gain antenna!
So, when I'm faced with data that contradicts theoretical predictions, unless
I can catch something in my filters of skepticism, I generally go with the
measured data, wiggles and all.
Ed Price
[email protected] <blocked::mailto:[email protected]> WB6WSN
NARTE Certified EMC Engineer
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780
Military & Avionics EMC Is Our Specialty
________________________________
From: [email protected] [mailto:[email protected]] On Behalf Of Ken
Javor
Sent: Monday, June 08, 2009 1:53 PM
To: [email protected]
Subject: Re: [PSES] Measurement Accuracy and antenna factors
Ed’s use of the current probe / H-field antenna examples brings up
something I sort of implied earlier, but meant to discuss in more detail, but
forgot.
Both of Ed’s examples function like transformers at the low end of their
frequency range: the correction factor changes linearly with frequency. This
is inherent in how the devices work. If I were to get data from the cal lab
that showed departures from linear operation below, say 100 Hz, whereas from
100 Hz to 10 kHz I was seeing linear changes with frequency, I would not
blindly input the non-linear calibration. I would likely check to see if the
dynamic range of the measurement set-up had been exceeded, and I was getting a
plot of the measurement system noise floor.
That’s a crude example, but it gets directly to the more subtle problem
of
the OP and his subsequent post about including all local maxima and minima and
also looking for inflection points, etc. That’s all well and good assuming
that what you get from the cal lab is guaranteed the antenna performance
itself. But if the swept data point step sizes are a tiny fraction of what it
would take to get the real antenna factor to change a dB, and yet you are
seeing dB changes, then what you are seeing is artifacts of the measurement
system errors, not the antenna factor itself. We ought not be correcting our
data for errors or uncertainty in the calibration process, and a knowledge of
how the antenna works goes a long way towards “smoothing out” the raw
data.
Case in point, a standard gain horn. The antenna factor can be
calculated
>from the gain, and the gain is calculated from the physical dimensions. The
measurement and tolerances on the physical dimensions are much better (orders
of magnitude better) than the measurement errors on the antenna range, and it
makes no sense at all to prefer measured rf data over measured dimensional
data. The only reason for measuring the antenna gain is if you think there is
a problem, which you ought to be able to check by visual inspection.
Now a biconical is more complex than a SGH, but the same principal
applies
that there is some frequency interval over which the antenna factor will be
flat, or at least smooth. Apparent ripple in the AF at frequency intervals
shorter than what is theoretically possible is “noise” from the test setup.
I strongly suggest that more emphasis be placed on understanding our
test
antennas, and less on manipulating data with unknown sources of error.
Ken Javor
Phone: (256) 650-5261
________________________________
From: "Price, Edward" <[email protected]>
Date: Mon, 8 Jun 2009 13:09:06 -0700
To: <[email protected]>
Conversation: [PSES] Measurement Accuracy and antenna factors
Subject: RE: [PSES] Measurement Accuracy and antenna factors
I think that's the method I also use when the correction factor is
moving
over a large range, say, for the low frequency end of a current probe or
H-field antenna.
And one more thing; all my cal data is swept data (or at least, stepped
in
really, really tiny increments). I get a continuous plot, plus periodic
tabulated points along the curve. As Ken says, you can see the undulation of
the curve, and scale intermediate points such as every 1 dB. (Most times, I
can visually resolve a half dB, so picking the 1 dB transition points feels
pretty accurate to me.
Ed Price
[email protected] <blocked::mailto:[email protected]> WB6WSN
NARTE Certified EMC Engineer
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780
Military & Avionics EMC Is Our Specialty
________________________________
From: [email protected] [mailto:[email protected]] On Behalf
Of Ken Javor
Sent: Monday, June 08, 2009 12:43 PM
To: [email protected]
Subject: Re: [PSES] Measurement Accuracy and antenna factors
That is only true if the calibration is done like in the old
days, like my
old biconicals tuned at 20, 30, 40, 50, ... MHz. The query of the original
post is, given you have received swept data from the cal lab, how many of
those points do you import into your AF data correction file, and at what
point do you start using interpolation. In that case, what I was referring
to was that you could simply eyeball the data and enter data points that are,
say, 1 dB apart. Which is what I do.
That is a specific answer in terms of the OP and your
question. But even
granting your premise, that it is somehow an unknown antenna, and I am the
antenna calibrator working without the original antenna factor data for the
antenna, there is still no need to record volumes of data. All I need do is
set up the two antenna test, run a sweep, and observe where the isolation is
relatively constant, and where it changes rapidly, and once again I can
concentrate my efforts so that across the entire antenna range, I take data at
points roughly 1 dB apart.
Ken Javor
Phone: (256) 650-5261
________________________________
From: "Grasso, Charles" <[email protected]>
Date: Mon, 8 Jun 2009 12:29:43 -0600
To: Brent G DeWitt <[email protected]>, Ken Javor
<[email protected]>, <[email protected]>
Conversation: [PSES] Measurement Accuracy and antenna factors
Subject: RE: [PSES] Measurement Accuracy and antenna factors
But Ken – You don’t know a priori where the AF slope changes.
________________________________
From: [email protected] [mailto:[email protected]] On Behalf
Of Brent G
DeWitt
Sent: Sunday, June 07, 2009 8:35 PM
To: 'Ken Javor'; [email protected]
Subject: RE: [PSES] Measurement Accuracy and antenna factors
Well said Ken. I have also dealt with the absurd number of
points taken by
automated systems. Many years ago I wrote an QuickBasic program (yeah, that
many years ago), that decimated the data based on exactly the same thought.
It did a simple piecewise first derivative as well as looking for total
changes around .5 to 1 dB, depending entirely on how skeptical I was. It
resulted in a huge reduction in frivolous data.
Brent DeWitt
Westborough, MA
From: Ken Javor [mailto:[email protected]]
Sent: Sunday, June 07, 2009 1:43 PM
To: [email protected]
Subject: Re: [PSES] Measurement Accuracy and antenna factors
I’m not a fan of all this tenth of a dB concern with
uncertainty. I also
disagree that antenna factors are selected randomly to be entered into a data
file.
It seems obvious to me that intelligent data entry would use an
analog
simulation of what the questioner is after: Lots of data points (high
density) where the factors change rapidly with frequency, fewer points where
the factor is relatively constant.
I once had a commercial facility calibrate an antenna, and
they did so at
hundreds of frequencies, with the values bouncing around hundredths or tenths
of a dB from data point to data point. I’m sorry, but that seems a moronic
waste of time and money. All they were plotting was the error bounds of
their measurement system, not the actual performance of my antenna.
It’s time a for a little common sense to be displayed on this
topic.
Apologies in advance if I have hurt anyone’s feelings!
Ken Javor
Phone: (256) 650-5261
________________________________
From: "ce-test, qualified testing bv - Gert Gremmen"
<[email protected]>
Date: Sun, 7 Jun 2009 18:47:36 +0200
To: <[email protected]>
Conversation: Measurement Accuracy and antenna factors
Subject: Measurement Accuracy and antenna factors
A lot of effort has been put into specification of
measurement accuracies in radiated emissions.
CISPR 16-4-2 has a number of uncertainty budgets listed.
One factor that I have not seen in any budget is the
error introduced by interpolation between
antenna factor calibration points by the measuring receiver.
In general the characteristics of a calibrated antenna
are entered into the measuring receiver as a number of
F/AF pairs, more or less randomly selected from
the calibration graph. Then the AF values for frequencies
in between those pairs a quadratic spline function is used
to interpolate. The function requires 4 calibration pairs to
operate
correctly
of which 2 must be lower and 2 must be higher then the
interpolated frequency. Especially near 30 MHz, where modern
antennas have steep AF graphs, a calibration point
below 30 MHz is not always available and I assume
the software duplicates the 30 MHz pair to
say 25 MHz to complete the function’s requirements.
This must introduce interpolation errors near 30 MHz.
I do now know the error that might be introduced by this
Type of function. I know that Taylor series have alternating
sign
In their expansion, and that the values diminish each term,
so the error of approximation remain smaller as the last term
used to interpolate. But Taylor does not suit itself
for approximation of non computable data (such as AF).
My questions for the group are:
What requirements are to be met for the F/AF pairs to
minimize errors?
What are the errors introduced by interpolation?
How do YOU handle this additional uncertainty…?
Gert Gremmen
Ce-test qualified testing bv
Van: [email protected] [mailto:[email protected]]
<mailto:[email protected]%5d> <mailto:[email protected]%5d> Namens Bill
Owsley
Verzonden: zondag 7 juni 2009 4:36
Aan: [email protected]; [email protected]; GheryPettit
Onderwerp: RE: CISPR 22-2005: testing on interconnecting DC
cables?
I routinely measure the same, but I have not been able to
establish that
there is any requirement for a direct measurement. In general, if the EMI
>from the DC cables causes a problem it will show in the usual required
tests. A test on the DC cables just focuses on the problem area and helps
with debug efforts, but I have not been able to claim that it is required by
CISPR 22 (or related standards) ps. Some of the DC cables are much longer
than any standard one normally used and so come fall under some of the
immunity tests, so by quantum leaps in logic, we apply the emissions test to
them. But when it comes time to ship, no problem...
- Bill
Indecision may or may not be the problem.
--- On Fri, 6/5/09, Pettit, Ghery <[email protected]>
wrote:
From: Pettit, Ghery <[email protected]>
Subject: RE: CISPR 22-2005: testing on interconnecting DC
cables?
To: "[email protected]" <[email protected]>,
"[email protected]"
<[email protected]>
Date: Friday, June 5, 2009, 2:25 PM Pat,
Annex C deals exclusively with telecommunication ports. This
is clear in
the first sentence of the annex. If a port isn't used for telecommunications
(see article 3.6 in CISPR 22:2008 for the definition) then Annex C doesn't
apply. And while the term "mains" isn't defined in the standard, it commonly
is taken to mean the low voltage distribution network in a building that is
supplied from the public power supply. Thus, the mains port is the port that
plugs into the wall socket. I don't see how the DC output port on your power
supply is either a telecommunications port or a mains port, so this test by
your customer doesn't make sense to me, at least not as a 'requirement' in
CISPR 22.
I hope this helps.
Ghery S. Pettit
-----Original Message-----
From: [email protected] </mc/[email protected]>
[mailto:[email protected] </mc/[email protected]> ] On Behalf Of
[email protected] </mc/[email protected]>
Sent: Friday, June 05, 2009 10:48 AM
To: [email protected] </mc/[email protected]>
Subject: CISPR 22-2005: testing on interconnecting DC cables?
Good Friday morning all,
We have a customer who is measuring conducted emissions on
the DC output
of our external switching power supply (laptop-style power
supply),
claiming it is required by CISPR 22. As I read through CISPR
22-2005 for
rebuttal material, the phrase telecom port was defined and
the measurement
details looked clear. Until I got to Annex C.
Clause C.1.5 is titled 'Flowchart for selecting test method',
and says the
flowchart in Figure C.6 is applied to different ports. The
flowchart has
a decision block at the top based on whether the port is a
telecom port.
If not, no testing is necessary.
If the port is a telecom port, you choose between 4 methods:
- Unscreened pairs
- Screened or coaxial
- Mains
- Other
Certainly, Mains ports need testing regardless of whether the
EUT has
telecom ports, so the flowchart has logic errors.
But does the port choice 'Other' mean you must test any port
not already
covered? Can a single statement in a flowchart define
testing
requirements not detailed elsewhere? BTW, the flowchart says
'Other'
ports must meet the telecom test limits.
Pat Lawler
EMC Engineer
SL Power Electronics Corp.
-
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