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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