Introduction
This article reviews MIL-STD-461 CS101 including the updates contained in revision “G” the current version. The test title “Conducted Susceptibility, Power Leads” pretty well defines the test, where interference is applied to the power input of the Equipment Under Test (EUT). During application of the interference the EUT is monitored to verify acceptable performance. Note that the use of prior revisions may remain in use because of maintaining compatibility of new components with existing systems.
Sources, coupling paths and victims are present in an Electromagnetic Compatibility (EMC) situation and CS101 is no different. Noise and even more prominent power frequency harmonics from any item on the power bus could place interference on the power line. That interference could conduct via the bus to other equipment sharing the bus. If the other equipment was sensitive to that frequency, susceptibility could be produced. Along the way the cables with the interference could be routed in parallel to a sensitive signal cable such as the audio of a radio receiver and inductive coupling supports the susceptibility. So, this risk has been around since the early days of radio communication with analog circuitry and remains a threat with today’s technology.
CS01 has been one of the test methods since the initial release of MIL-STD-462 back in 1967 and MIL-STD-461A-C applied this test in limited cases. The purpose was to determine if the test article was susceptible to low-frequency interference appearing on the power lines. The early days specified the frequency range of 30 Hz to 50 kHz with CS02 starting at 50 kHz. Testing within 10% of the power frequency was exempt. The test procedure called for transformer coupling to add the interference to the power line and that basic approach is still used today.
The early standard also established the dual limit for testing. The goal was to apply an interfering voltage measured across the power input terminals. But what happens if the characteristic impedance of the power input is very low. The voltage is not developed and efforts to create the voltage by increasing the drive amplitude with the associated current resulting in overheating from the high current. Note that testing is not necessary on the power return lead since the interfering voltage is across the input terminals, the interfering current is flowing through the entire power input circuit loop.
To support this dual limit the MIL-STD-461C stated that the EUT was considered compliant if not susceptible with the test voltage applied to the power input terminals. It was also considered compliant if the interfering signal source output was adjusted to dissipate 50W into a 0.5-ohm load while the interference was applied to the input. The goal was to pre-calibrate this 50W drive but since the procedure did not include an obligation to do so, many simply used a 50W source and measured the applied voltage when the source was driven at maximum.
In 1993 MIL-STD-461 and MIL-STD-462 Revisions “D” were issued with a few changes that would help standardize the test.
- Name changed to CS101 to align with other methods
- Two test voltage limits were introduced – one for power voltage greater than 28 volts and one for 28 volts or less as the EUT nominal power input.
- The maximum drive level was increased to 80W instead of the previous 50W and a procedure for pre-calibration of this 80W dissipation was implemented.
- The test start frequency for AC power was set at twice the power frequency.
- Applicability was assigned to all systems.
Release of MIL-STD-461E eliminated MIL-STD-462 by incorporating the procedure document into MIL-STD-461. CS101 was also affected with an increase of the test end frequency to 150 kHz.
MIL-STD-461F kept the CS101 test as present in the revision E standard except the applicability removed testing for devices that had no operating current demand greater than 100A.
MIL-STD-461G brought forth a few changes that have affected how the testing is accomplished and reported. In addition, an alternate test approach was included to simplify measurements of the interfering signal in the presence of high voltage AC power. Since MIL-STD-461G is the current version we will expand on the changes as part of the upcoming detailed discussion below. Revision G also limited applicability to devices with a current demand of less than 30A with some specific exceptions for sensitive operation in the CS101 test frequency range.
Procedure Details
Pre-test calibration is used to establish the drive levels to limit the applied current in case the EUT presents a very low impedance at selected interfering frequencies. The standard provides for 80W drive power dissipation by a 0.5 ohm resistive load up to 5 kHz and then decreases to 0.09W at 150 kHz.
Figure 1 shows the basic calibration configuration where the signal source that will be used for test is connected to the coupling transformer primary winding. The 0.5 ohm resistor is connected across the coupling transformer secondary winding. Voltmeters are connected to measure the signal – note that the upper frequency for test is 150 kHz so be mindful of the voltmeter frequency limitations.
Figure 1: CS101 Calibration Configuration
Once configured, adjust the signal source amplitude to produce the 80W or 6.32 Vrms measured across the resistor at the test start frequency. Record the coupling transformer voltmeter measurement as the drive level maximum required during the EUT testing. Repeat this process as the frequency is increased. During the process on increasing the frequency monitor the interfering signal to see that it remains sinusoidal – no amplifier overdrive compression or distortion. Since you need to monitor, it may be good to select an oscilloscope as one of the voltmeters. If the measurements vary outside of your selected tolerance, you will need to make drive signal adjustments to calibrate at the specified levels.
I want to take a moment here to provide an example of this to help clarify. With the signal source frequency set to 120 Hz increase the signal source amplitude to measure 6.5 Vrms across the resistor. The 6.5 Vrms provides for the interference to be above the minimum of 6.32 Vrms for 80W without excessive over testing. In our example, record the coupling transformer voltmeter measurement of 14.5V. Increase the frequency while observing both voltmeter measurements. As you increase the frequency you notice that the voltage across the resistor is decreasing but remains above the 6.32V minimum until you reach 650 Hz. At this point you will increase the signal source to 6.5V across the resistor. Record the frequency and coupling transformer voltmeter measurement of 14.9V. If the resistor voltmeter measurement had increased above our tolerance of 6.75V we would have decreased the amplitude. When we reach 5 kHz, the amplitude limit will start decreasing so we will need to work our way down the limit slope by increasing the frequency until reaching the over-test tolerance then reducing the amplitude to the minimum voltage. This process continues until the end frequency is reached. This establishes the calibration drive just above the minimum value of the limit. At the end of this process we have recorded the amplitude maximum required through the test frequency range to show compliance with CS101.
Next comes the test segment of the testing where the EUT is subjected to interference. The basic test configuration is shown in Figure 2 using the signal source and coupling transformer that were in the calibration configuration. Check you test configuration closely – there is a high potential for a wiring error that cause shock, burn or test measurement errors. For example:
- Failing to power the oscilloscope with isolation could make the oscilloscope ground terminals such as the BNC connector body to be live and a shock hazard or burn the oscilloscope probe – flame about 15 cm high.
- Connecting the coupling transformer secondary across the power terminals instead of in series with the phase lead results in damage unless the circuit breaker trips in time.
- Not realizing that the capacitor body is negative and the posts are both positive I a feedthrough capacitor construction.
- Connecting the oscilloscope to the power source side of the coupling transformer measures a voltage that is not across the EUT terminals.
Figure 2: CS101 Test Configuration
Your test configuration checks out so you’re ready to start testing. You have confirmed what constitutes acceptance – pass/fail criteria can be monitored. Applying power to the EUT results in failure to start and you finally find that getting rid of the coupling transformer allows start-up. The open circuit in the transformer primary reflects a high impedance and causes a voltage drop in the secondary because of the inrush current. Placing a small resistor across the primary temporarily, allows the EUT to start and then the resistor is removed for testing.
We get everything running and the EUT performing properly for test. The oscilloscope shows the 120 Vac at 60 Hz – the power input. Now we apply the interfering signal at a low amplitude at the start frequency. We increase the amplitude monitoring the voltage at the EUT power terminals but find that the low-level interfering signal is nearly invisible in the presence of the power input voltage. Expanding the vertical display of the oscilloscope, that is setting a smaller volts/division causes the 120 V to display only part of the vertical scale and that allows the smaller interfering signal to be more visible. Since you are at the start frequency of 120 Hz the power and interference could be synchronized, so going to 121 Hz could allow better visibility. Now that we can see the interfering signal, we increase the amplitude to the test voltage limit observing that we stop increasing if we reach the pre-calibrated drive for that frequency. This process continues until the end frequency is reached.
The goal of applying the test voltage is met with obstacles in the test configuration. LISN presence inserts two 50 µH inductors in series with the circuit loop. As the interference signal frequency increases, the LISN inductive reactance causes a voltage drop. This drop forces an increased drive to overcome the losses, so we become limited by the pre-calibration. The capacitor serves to bypass the LISN inductance and prevent the voltage drop because as the inductive reactance is increasing, the capacitive reactance is decreasing.
MIL-STD-461G added instructions to determine where the losses occurred if the test voltage was not attained. Losses could be across the LISN but the capacitor should compensate. Testing in a shielded enclosure could see losses across the power source power filters in the room power are using inductance in the filtering. You may need to locate the testing outside the room to eliminate the filter loss. Changing the bypass capacitor may help reduce the losses. Most of the information about this loss investigation appears in the appendix so contractual obligation to take these actions could be in question.
In the interest of automated testing, some labs have adopted a strategy of doing the pre-calibration with the computer recording the settings and then applying the pre-calibration levels. Obtaining measurements of the interfering signal in the presence of AC power makes a feedback of the actual test voltage difficult. This approach could apply over-test conditions and damage the EUT. This approach was used during test of one of my products and the charred remains of my circuit board confirmed that the test was not power limited.
MIL-STD-461G also supported testing with a receiver/transducer configuration. The transducer provides isolation to the receiver and reduces the amplitude to a safe level. The standard does not provide information about the transducer except that it cannot be a phase shift network. If you plan to use this type of test device, it will need to be documented in the test procedure to get approval for use.
Summary
CS101 testing applies to all systems so it is included in nearly every MILSTD-461 qualification test program. It is very difficult to automate while maintaining the dual condition limits of a test voltage or pre-calibrated drive level. The test technician I very busy during test with the need to simultaneously monitor:
- Test voltage applied
- Pre-calibrated drive level
- EUT performance for assessing susceptibility
- Keeping away from shock hazards
- Potential investigation for voltages losses
Although difficult to measure the interfering signal in the presence of AC power, using peak to peak excursions and converting to the rms equivalent can be easily accomplished if you allow an over-test.
CS101 testing is somewhat complex and requires attention to the details with test configuration checks. During the test, the technician must remain focused to keep up with all of the details.
