| There are those who disagree with the r.f. conducted susceptibility test setup of Method CS02 of MIL-STD-462. Homemade rigs have sprouted to comply with the coupling capacitor requirement, each with it own disadvantage. Our little Type 7415-3 R.F. Coupler is the answer. A neat little box with BNC connectors and a pair of binding posts, it is rated at 270 V.A.C. at the LINE terminals and 20 volts PMS into the GEN port. Looks good and does a fine job.
The test setup diagram of the specification will result in power frequency voltages at the voltmeter terminals. If an untuned voltmeter is used, it is difficult to measure a one volt r.f. signal in the presence of the a.c. line voltage. It is not practical to use an EMI meter for this, unless the Type 7415-3 is placed in series with it.Otherwise, the power frequency voltage can damage the input
circuit of the EMI meter.
The Type 7415-3 contains a high pass filter in series with the detector circuit to eliminate power frequency voltages and allow r.f. signals from 50 KHz to 400 MHz to pass to the EMI meter as required. The high pass filter consists of three stages of an R-C network using series capacitors and shunt resistors. Using resistors instead of inductors enables the unit to cover a wide frequency range, with a 40 dB insertion loss in the pass band.This makes it necessary to multiply the detected voltage by a factor of 100
for the measurement of the injected voltage.
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The series capacitor in the Type 7415-3 consists of several styles of capacitors in parallel. Mica, ceramic and wrapped capacitors exhibit different characteristics versus frequency and the combination eliminates the need to change thevalue of the capacitor as frequency changes from
50 KHz to 400 MHz.
A tuned voltmeter such as an EMI meter is recommended as the detector for further isolation of unwanted frequencies, such as harmonics of the signal generator. If the generator waveform is “clean.” An untuned meter can be used if it is terminated in 50 ohms and preserves a 50 ohm coaxial circuit throughout the frequency range.
The reactance of the built-in series capacitor from the generator to the power line terminal presents very little loss at 50 KHz.At 400 Hz its reactance is about 362 ohms.Therefore, it represents a path by which 400 Hz power voltages can be fed back to damage the output circuit of the signal generator.
To avoid this, one suggestion is to use an isolating transformer at the output of a low impedance signal source. Figure 1 shows the use of the Model 6552-1A Audio Amplifier and the Type 7033-1 Impedance Matching Transformer. This arrangement can be used for injection levels up to 20 volts r.m.s. from 50 KHz to 500 KHz with satisfactory results.
At frequencies above 500 KHz, we recommend that a 50 ohm signal generator be used as indicated in Figure 2 except that a small capacitor must be connected in series between the generator and the
Type 7415-3 R.F. Coupler. |
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Solar Type 7525-1 is 0.1 microfarad, fitted with BNC connectors.
The reactance of this series capacitor reduces the power frequency voltage to a safe limit so that the output circuit of the generator will not be damaged. At 500 KHz, the r.f. signal is not greatly
attenuated.
At frequencies above 20 or 30 MHz, the connections from the banana
jacks to the test sample will create discontinuities that cannot be removed
from the setup. It is recommended that the wires from the banana jacks be less than one inch long to minimize VSWR anomalies.
DIMENSIONS
2.0" (51 mm) X 2.5" (63.5 mm) plus mounting flanges x 1.25" (31.25 mm) high.
TYPE 9132-1 R.F. COUPLER AND HIGH PASS FILTER
This unit is a high voltage version
of the Type 7415-3. It is electrically similar, but in a larger case and with
a rating of 500 VAC at the LINE terminals and 20 volts rms into
the GEN. Port.
Dimensions: 2.88" (73 mm) x 3.06"
(78 mm) plus mounting flanges x 2.0" (51 mm) high.
TYPE 9407-1 A THREE PHASE R.F. COUPLING NETWORK
MIL-STD-462 notice 5, method CS02 requires simultaneous coupling of the r.f. susceptibility |
