Hello Gert,
It was Ken Javor who was asking, but I will note that ETS-Lindgren probe
documents specify an accuracy of +/- 2 dB transfer impedance, which for
*formal* calibration requires as you say 0.5 dB accuracy. It also takes
precision 50 Ohm test fixtures, better loads that I used, and a better
instrument, too. Call my process a sanity check, adequate only to get
an ad-hoc setup back to readings that more closely approximate what was
seen at outside test houses. That's what the customer wanted.
However, any deviation from a smooth response may be detected fairly
easily, and if I saw a "jog" in a current probe function I would
recommend replacement. I did recommend the customer get their analyzer
and probe calibrated.
Regards,
Cortland Richmond
KA5S
On 12/6/2011 1:03 AM, ce-test, qualified testing bv - Gert Gremmen wrote:
If at 100 MHz your sensor indicates during calibration 10 dB
and at 120 MHz 12 dB, how are you going to know the value at
110 MHz. It maight be 20 as well as 0 dB, you don't not know as you did
not measure/ calibrate.
This requires some knowledge about the behavior of the sensor,
and the 1/BW is and indication of that.
There is another aspect in EMI measurements.
As the measuring receiver interpolates
between the calibrated samples, the max interpolation error
(lin interpolation) is 50% of the vertical differences between
calibration points.
If the measuring receiver is ± 0.5 dB you should register a
calibration
each time the difference between previous and current sample is 0.5 dB.
Then the total error will be slightly more than 0.5 dB.
There are more sophisticated interpolation methods such as
cubic spline and polynomial interpolation, and the error item
is subject to higher mathematics.....
Play with it at:
http://terpconnect.umd.edu/~petersd/interp.html
or athttp://www.ibiblio.org/e-notes/Splines/Lagrange.htm
you can actually move the points on the latter page.
Gert Gremmen
-
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