Fluke PM6690 - Page 60

The nominal trigger point is 0 V with an un
-
certainty of ± 10 mV.
Asinewaveexpressedas
Vt V ft
P
() sin( )=´ 2p
, has a slew rate
D
D
V
t
of
Vf
P
´ 2p
close to the zero-crosssing. That gives
us the systematic time error when crossing
10 mV, instead of crossing 0 mV.
10
2
mV
V FREQ
s
P
()
()
´´p
And the corresponding phase error in degrees
is:
10 360
2
mV FREQ
V FREQ
P
´°´
´´p
which can be reduced to:
06.
()
V
P
°
This error can occur on both inputs, so the
worst case systematic error is thus:
06 06.
()
.
()
()
VA VB
PP
+°
n
Methods of Compensation
The calculations above show the typical un
-
certainties in the constituents that make up the
total systematic error. For a given set of input
signals you can compensate for this error
more or less completely by making calibration
measurements. Depending on the acceptable
residual error, you can use one of the methods
described below. The first one is very simple
but does not take the inter-channel propaga
-
tion delay difference into account. The second
one includes all systematic errors, if it is car
-
ried out meticulously, but it is often not practi
-
cable.
Common settings for the two inputs are:
Slope:
Coupling:
Impedance:
Trigger:
Trigger Level:
Filter:
Pos or Neg
AC
1MW or 50 W depending
on source and frequency
Man
0V
Off
Method 1:
Connect the test signals to Input A and Input
B. Select the function Phase A rel A to find
the initial error. Use the MATH/LIM menu to
enter this value as the constant L in the for-
mula K
*
X+L by pressing X
0
and change sign.
Now the current measurement result (X
0
) will
be subtracted from the future phase measure
-
ments made by selecting Phase A rel B.A
considerable part of the systematic phase er
-
rors will thus be cancelled out. Note that this
calibration has to be repeated if the frequency
or the amplitude changes.
Method 2:
Connect one of the signals to be measured to
both Input A and Input B via a 50 W power
splitter or a BNC T-piece, depending on the
source impedance. Make sure the cable
lengths between power splitter / T-piece and
instrument inputs are equal. Select the func
-
tion Phase A rel B and read the result. Enter
this value as a correction factor in the same
way as described above for Method 1.
Measuring Functions
4-20 Possible Errors
Vpeak
(A)
Vpeak
(B)
Worst case
systematic error
150 mV 150 mV
4°+4° =8°
1.5 V 150 mV
0.4°+4° =4.4°
1.5 V 1.5 V
0.4°+0.4°=0.8°
Table 4-3 Systematic trigger level timing
error (examples).