Sweep Frequency Response Analyzer (SFRA) User Guide
C-4
72A-2570-01 Rev. K 07/2011
It must be noted that when using the voltage relationship,
H(j
)
is not always
directly related to
Z(j
)
. For
Z(j
)
to be directly related to
H(j
),
a current must
be substituted for the output voltage and then ohm’s Law can be realized.
However, SFRA uses the voltage-ratio relationship to determine
H(j
)
. Since
SFRA uses a 50-ohm impedance- match measuring system, the 50-ohm
impedance must be incorporated into
H(j
)
. The next equation shows the
relationship of
Z(j
)
to
H(j
)
:
It is often useful to plot the magnitude and phase relationship of the transfer
function in logarithmic format. The units of magnitude and phase are in decibels
(dB) and degrees, respectively. Magnitude and phase are represented as follows:
This format takes advantage of the asymptotic symmetry by using a logarithmic
scale for frequency. Plotting the phase relationship with the magnitude data helps
determine whether the system is resistive, inductive, or capacitive. It is often
useful to compare resonance in the magnitude plots with the zero crossings in the
phase relationship.
Test Cable Lengths
The cables and connectors supplied with the M5000 instruments are made from
low- loss RG-58 RF coaxial cable, with the shields grounded to the instrument
chassis through a standard connector. The instrument requires a matched
impedance signal cable and performs a single-end measurement—that is, the
signal is measured with respect to the instrument ground. The shield of the signal
cable must be connected to the chassis using a 50-ohm impedance-matched RF
BCN connector. The test leads should not be modified in any manner.
Practical field experience indicates that the leads be 18 m / 60 ft. This is the
shortest length useful to test the largest transformers from a location on the
ground, adjacent to the unit. Nevertheless, lead length determines the maximum
effective frequency.
The standard cable shield grounds connect the cable shields to the transformer
ground at the base of the bushing. These ground connections are 3.6 m / 12 ft back
from the terminal connection, on the measurement ends of the cables. If this is not
long enough to reach from the bushing terminal to the base of the bushing, a
30 m / 100 ft cable is available, with ground connections 5.4 m / 18 ft back from
the terminal connection.
H j
50
Z j
(
)
50
+
--------------------------
=
A dB
20
10
H j
(
)
log
=
A
1
–
tan
H j
(
)
=
Summary of Contents for SFRA
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