Atec Solar-6220-1A - Manual (Page 6)

impedance or the correction curve, whichever is
desired. Actually, there is no need to plot the
answer as transfer impedance, since the desired
end product is the correction factor to be applied
to the meter reading to obtain decibels above
one microampere. The correction must be
obtained for each configuration. In other words,
if you want to use the method for maximum
sensitivity, the calibration is performed with just
a 50 ohm load on the primary winding simulating
the EMI meter. If the flattening networks will be
used, then they must be connected to the primary
winding during the calibration and must be
further loaded with 50 ohms to simulate the EMI
meter input.
At each test frequency, the output of the audio
signal generator is adjusted for a level which
delivers the same current to the secondary (S) of
the transformer. This is accomplished by setting a
constant voltage across the 10 ohm resistor. A
convenient level is 0.1 volt across 10 ohms which
is 10,000 microamperes (80 dB/uA).
Adjust the gain of the EMI meter to assure a one
microvolt meter reading for a one microvolt R.F.
input from a standard signal generator. Then
connect the 50 ohm input circuit of the EMI meter
to the primary of the 6220-1A. If the EMC-10E is
used, insert a 10 dB pad in series with the input. If
the calibration is for maximum sensitivity, no
additional loading is necessary. If the calibration
is for the flattened versions discussed above, the
appropriate resistance must be connected across
the primary of the transformer.
At the frequency of the test, set the output of the
signal source to obtain 1.0 volt across the 10 ohm
resistor. Carefully tune the EMI meter to the test
frequency and note the meter reading on the dB
scale. The difference between the meter reading
in dB and 80 dB represents the correction neces-
sary to convert the meter reading to dB above
one microampere for narrowband measurements.
In most cases, the correction will have a negative
sign. For example, at 100 Hz the EMI meter
may read 88 dB above one microvolt. Since the
reference is 80 dB above one microampere, the
correction is -8 dB to added algebraically to
the meter reading to obtain the correct reading
in dB above one microampere.
If the 10 dB pad has been used, this loss must be
accounted for in deriving the correction. If the
pad will be used in the actual test setups, its loss
becomes part of the correction factor. In this case,
the meter reading obtained in the foregoing
example would be 78 dB above one microvolt
and the correction factor would be +2 dB for
narrowband measurements.
Repeating this procedure at a number of test
frequencies will produce enough data to plot
a smooth curve for use when actual tests are
being conducted.
DERIVING THE BROADBAND
CORRECTION FACTOR
When making broadband measurements as
required by MIL-STD-461A in terms of “dB above
one microampere per megahertz,” use the
average of the narrowband factors over the range
30 Hz to 14 KHz and add a bandwidth correction
factor of 37 dB.
In the case of Method CE01 of MIL-STD-461A, use
the 20 KHz wideband mode of the EMI meter,
determine the average of the narrowband factors
over the range 30 Hz to 20 KHz and use this figure
as the bandwidth correction factor.
When using high pass filters at the input to the
EMI meter to eliminate the first few harmonics
of the power line frequency as allowed by
MIL-STD-461A, the range covered will depend
upon the cutoff frequency of the filter. For
example, on 60 Hz power lines and using Solar
Type 7205-0.35 High Pass Filter between
the 6220-1A Transformer and the EMI meter,
obtain the average narrowband correction
between 350 Hz and 14 KHz and add the band-
width correction factor of 37 dB. On 400 Hz lines
when using the Solar Type 7205-2.4 High Pass
Filter between the transformer and the EMI
meter, determine the average of the narrowband
factors in the range of 2.4 KHz and 14 KHz and
add the bandwidth correction factor of 38.5 dB.
SUMMARY
Some of the material given in this Application
Note is terse and given without much explana-
tion. If your are confused by this simplification,
just call us. Incidentally, the Signal Corps liked this
method so well that they included it in Notice #3
to MIL-STD-462 date 9 Feb 71.
AN622001 (continued)
FIGURE 4 – TEST SETUP
FOR DETERMINING CORRECTION FACTOR
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