Atec Solar-6220-1A - Manual (Page 5)

above 5 KHz when using an EMI meter capable of
measuring 1.0 microvolt into 50 ohms. For EMI
meters such as the NM-7A and the EMC-10E,
the meter sensitivity is a decade better and
it is possible to measure EMI currents of
.005 microamperes at 5 KHz and above.
FLATTENING THE RESPONSE
At a sacrifice of sensitivity, the upper portion of
the frequency vs. correction factor curve can be
flattened to provide a constant correction factor
from about 1 KHz up to 150 KHz. This is depicted
in curve #2 of Figure 3, where a -20 dB correction
is suitable over this part of the frequency range.
The flattening is obtained by loading the primary
with a suitable value or resistance. The resistance
value used in this example is 10 ohms. The
flattening still allows the measurement of a
0.1 microampere signal when using an EMI meter
with 0.1 microvolt sensitivity. An advantage
of this response curve is the sloping correction
at frequencies below 1KHz which acts like a high
pass filter to remove some of the power line
harmonics from wideband measurements.
If you are only interested in frequencies above
150 Hz, a 2 ohm resistor is all that is needed. See
curve #3.
STILL MORE FLATTENING
Like the girdle ads say, you can firmer and flatter,
with a loss in sensitivity , by further reducing the
value of the shunt resistor. This is illustrated
in curve #4 of Figure 3 where a 0.5 ohm shunt
resistor (Solar Type 6920-0.5) is connected
across the transformer primary winding used as
an output winding to the EMI meter. The overall
flatness is achieved at the sacrifice of considerable
sensitivity, but the sensitivity is well under the
requirements of existing specifications and the
correction network utilizes no active elements.
LIMITATIONS OF THE METHOD
When measuring EMI current on d.c. lines, there
are no problems, but on a.c. lines there are limita-
tions. The a.c. voltage drop across the winding (S)
due to power current flowing to the test sample
is the principal problem. This voltage induces
twice as much voltage in the output winding (P)
at the power frequency. Since we prefer to limit
the power dissipation in the 50 ohm input to the
EMI meter so that it will not exceed 0.5 watts, the
induced voltage must be kept below a safe limit.
For 400 Hz lines, the power frequency current
must not exceed 16 amperes to avoid too much
400 Hz power dissipation in the input to the EMI
meter. Also, the resistance ‘R’ used across the
output winding (P) must be at least a 50 watt
rating on 400 Hz lines. This resistor should be
noninductive to avoid errors due to inductive
reactance.
THINGS TO BE WARY OF
The 10 ȖF feed-thru required by present
day specs has appreciable reactance at 30 Hz
(
LJ54 ohms) and acts to reduce the actual EMI
current flowing in the circuit. This means less
trouble in meeting the spec, but when calibrating
the test method described herein, it is wise to
short circuit the capacitor.
In the case where the input circuit to the
EMI meter is reactive, such as the EMC-10E, it is
necessary to use a minimum loss ‘T’ pad at
the input to the meter. The Eaton NM-7A and
NM-12/27A units do not require this pad and
its loss.
DETERMINING THE NARROWBAND
CORRECTION FACTOR
The test setup of Figure 4 describes the simple
method of determining either the transfer
AN622001 (continued)
dB TO BE ADDED TO EMI
METER READING (IN dB) TO
OBTAIN dB / ȖA.
+20
+10
0
–10
10 100 1K 10K
FREQUENCY HERTZ
100K 1M
–20
–30
CURVE #4
CURVE #3
R LJ0.5 OHM
R LJ2 OHMS
CURVE #1
CURVE #2
R=
∞
R LJ10 OHMS
FIGURE 3 – TYPICAL CORRECTION DATA VS. FREQUENCY
61