Many types of signal generators and oscillators are used for EMC testing. Of them, lightning simulators are, without doubt, the most exciting. The maximum peak current that designers need to consider is about 200 kA with actual lightning-strike statistics showing an average of 20 kA and 140 kA exceeded 2% of the time.
Lightning strikes are not simple events but instead consist of a variable number of pulses with a range of rise times and durations. High-fidelity lightning-strike simulation involves fast, high-energy discharges. In addition to being dangerous, such simulators are expensive and very large.
Lightning Simulation
A paper detailing the upgrading and recommissioning of the lightning simulator at Sandia National Laboratories goes into fascinating detail.1 For example, “When fired, both [capacitor] banks typically erect to approximately 1 MV. At 1 MV, the stored energy is 176 kJ in the 200-kA bank and 88 kJ in the 100-kA bank….”
The banks referred to are so-called Marx banks, an arrangement of capacitors and spark gaps that initially is charged in parallel but when triggered reconfigures (erects) itself automatically into a series configuration as successive spark-gaps arc. Each bank is housed in its own insulating tank together with 16,000 gallons of transformer oil.
The Sandia simulator originally was built to test nuclear weapon safety components and systems so the design was scaled to accommodate the most severe requirements where possible. Although lightning testing is done by direct simulation in some cases, it’s far more practical to replicate the results of a lightning strike; that is, create the much lower amplitude voltage and current test waveforms that would be induced on the DUT rather than try to duplicate the strike itself.
This is the philosophy behind a series of several waveforms now specified by RTCA DO-160F Environmental Conditions and Test Procedures for Airborne Equipment Section 22 for aircraft lightning tests. In general, the topic of aircraft EMC testing has become complicated by the large amount of carbon composite materials used in modern planes. The series of waveforms addresses the different types of scenarios that have developed as a result of this.
Louis Feudi, global channel manager for EMC at Thermo Fisher Scientific, described the situation, “Formerly, when a plane was struck by lightning, the metal airframe presented a highly conductive, low-impedance path for the lightning to travel over the exterior of the vehicle. This minimized voltage generation and acted as a Faraday cage, preventing coupling of voltage or current waveforms to the interior.
“With composite materials, the impedance of the exterior surface is greatly increased,” he continued. “As a result, voltage and current waveforms are generated that can couple onto aircraft wiring bundles routed along the airframe close to the interior surface and passing through the structural support ribs.”
The present complement of six test waveforms is applied to interior cable bundles and coupled via ground or direct injection to the internal electronic components. How these waveforms are applied and at what signal level vary according to cable location within the airframe, the physical air separation from current paths, and the cable’s mission-critical nature.
According to Mr. Feudi, a new seventh waveform is expected to be published in the next revision to DO-160, and MIL-STD-461 may be modified to adopt DO-160 Section 22.
Thermo Scientific, part of Thermo Fisher Scientific, makes a modular Lightning Test System. It is available with five waveforms for testing metal and composite aircraft to Level 3, 4, or 5 and with a sixth waveform for testing to Level 5.
Comb Generators
The comb generator is another type of specialized signal source with unique capabilities although much less dramatic than those of lightning simulators. As its name suggests, this device produces a succession of narrow spectral lines that resembles the teeth in a comb. The lines are spaced a distance apart equal to the repetition frequency of the pulses driving the generator.
Applications
A calibrated comb generator is at the heart of an emissions measurement technique explained by David Mawdsley, managing director of Laplace Instruments.2 His company makes the Emissions Reference Source (ERS), a small battery-operated comb generator with 2-MHz line spacing and output lines from 30 MHz to 1,000 MHz (Figure 1). Each of the 485 points is calibrated for vertical and horizontal polarization on an open air test site (OATS).
With the help of the ERS, a parking lot or even a large conference room can be used for precompliance testing. If the ERS is placed in the makeshift test environment, any difference between measured values and calibrated ERS levels must be caused by the test site. The differences are used to correct readings made with your EUT in place of the ERS.
Mr. Mawdsley described measurement uncertainty as great as 20 dB that was reduced to 6 dB or less when readings from these kinds of impromptu test sites had been corrected. Also key to the process is identification of ambient emissions accomplished by comparing site measurements made with the ERS on and with it off.
York EMC makes the CGE01 Comb Generator Emitter that can be used in much the same way as the ERS. Interestingly, the product datasheet states that the CGE01 originally was developed as a tool to evaluate shielding effectiveness of small enclosures. Like the ERS, it’s battery powered, and the datasheet highlights the plated metal cylindrical enclosure that contributes to the uniform field.
The output from the CGE01 extends from 50 MHz to 18 GHz in steps of either 80 MHz or 100 MHz. It is available with a 50-? SMA output connector or an integral or detachable monocone antenna.
Technologies
Several technologies are used today in comb generators although most designs continue to be based on step recovery diodes (SRD). This type of diode dates back to the 1970s and 1980s, with output edge speeds as fast as 12 ps reported in a 1991 HP research paper.3
SRDs are P-type, intrinsic, N-type (PIN) structures designed to store charge in the central, intrinsic region while forward biased. When the bias voltage is reversed, the stored minority carriers must be swept out, initially through diffusion. Until the charge is removed, the diode remains in a low-impedance state. As the last of the charge is removed, the diode no longer can sustain significant current flow and quickly switches to a high-impedance, low-capacitance state.
The 12-ps HP device showed improved output edge speed partly as the result of a doping profile linearly graded from one side of the intrinsic region to the other. The effect of this refinement is to constrain the charge storage regions to lie totally within the intrinsic layer rather than extending into the P and N regions to either side. Because the diffusion transit time depends on the effective width of the intrinsic layer, the reverse recovery process is faster in devices with graded doping.
Both the edge speed of the final transition to a high-impedance state and the overall time taken by the reverse recovery process are important. The faster the edge speed, the higher the frequency at which useable harmonics are generated. On the other hand, if the generator is driven at a 100-MHz rate as many are, the diode must have recovered within 10 ns.
Depending on your application, the close harmonic spacing can be a problem when using a comb generator. For EMC testing, generally you want as many closely spaced lines as possible, but they also must be accurately calibrated. In contrast, if you are using a 100-MHz drive frequency and want to isolate the harmonic at 2 GHz, it’s not trivial to design a filter with significant rejection at 1.9 GHz and 2.1 GHz.
One way to minimize filter design problems is to use a comb generator that includes a phase-locked frequency multiplier. MITEQ, a microwave transmission equipment company, makes a range of comb generator multipliers that increases the drive pulse repetition rate by a factor of 10 from an input of 100 MHz to 1 GHz. This means that the comb generated at the output only has lines at 1, 2, 3…up to 18 GHz.
In addition to simplifying the filter design, this change to the comb-generator architecture reduces the SRD multiplication order by a factor of 10. Instead of the 18-GHz harmonic representing 180 x the 100-MHz input, it now is only 18 x the 1-GHz PLL-multiplied drive frequency. Of course, the SRD has only 1 ns in which to complete the reverse recovery process, so device selection becomes more critical.
Picosecond Pulse Labs (PPL) has taken a different approach to comb generation basing a series of devices on the company’s nonlinear transmission line (NLTL) technology. One motivation for seeking an alternative design is the phase noise associated with SRD-based comb generators.
An SRD’s reverse recovery period precedes the fast transition to a high-impedance state. During the time that the stored minority carriers are being swept out, recombination noise and shot noise are being generated, and these processes add timing jitter to the output pulse. In the frequency domain, this translates to additional phase noise.
NLTLs use majority carrier devices and therefore are more closely related to Schottky varactor-based frequency multipliers than SRD circuits. This means that recombination and shot noise are much less important, and NLTL-based generators have better phase noise as a result. A PPL technical note describes residual phase noise measurement tests that confirm the NLTL-based comb generator phase noise to be well below the noise floor of the instrument setup.4
The PPL NLTL consists of a transmission line periodically loaded by several varactors. Because the varactor capacitance is much larger at low reverse-bias levels than at higher levels, a positive-going pulse entering the line exhibits a voltage-dependent propagation velocity at successive points in time along the rising and falling edges. On a rising edge, the lower voltages are delayed more than those near the top. This means that the overall transition is shortened, making the rising edge sharper.
In addition to lower phase noise, an NLTL-based design has a much wider drive frequency range and greater power output at high frequencies and can operate at higher drive frequencies. As an example, PPL’s Model 7124 has 15-dB to 20-dB lower measured phase noise than a comparable SRD-based generator, provides -10-dBm output power at 50 GHz, and can be driven at any rate from 1.2 GHz to 2.3 GHz. SRD-based circuits have networks before and after the device to improve impedance matching and efficiency, but they also restrict the drive frequency range.
When RF/microwave devices are operated at relatively high power levels, their characteristics no longer are accurately represented by linear S parameters. The devices exhibit nonlinear behavior that requires measurement at several harmonics as well as the fundamental frequency.
Because the Agilent PNA-X VNA is a mixer-based instrument, DUT performance is measured one frequency at a time. However, each harmonic measurement must be made relative to the phase of the fundamental, and that information is no longer available within the VNA once the source frequency has changed.
The comb generator fills this role by providing a fixed set of reference frequencies and phases. According to an Agilent technical note, “The component characterization software for the N5242A-510 NVNA uses the phase data from the U9391C/F to calculate the nonlinear error terms for the PNA-X network analyzers.”5
Summary
Even if you don’t need EMC test equipment as specialized as a lightning simulator or comb generator, you still may want to include a bit of future-proofing when selecting a new instrument. Malcolm Levy, vice president of sales and marketing at Giga-tronics, explained, “The trend from RF to microwave frequencies is continuing, and as the applications move up in frequency, so have the regulatory testing requirements.
“Giga-tronics is responding to this trend with the introduction of a 50-GHz microwave signal generator (Figure 3) and 50-GHz microwave power amplifier,” he continued. “While most EMC testing has not yet extended to 50 GHz, the defense community is working at these frequencies. Also, we are seeing applications in automotive radar, satellite, and point-to-point radio.”
Vic Hudson, EMC product manager at Rohde & Schwarz, discussed the new pulse-train feature offered in the company’s signal generators. Basically, the option provides a separate Arb within the SMA100A 9-kHz to 3-GHz or 6-GHz and SMF100A 100-kHz to 43-GHz generators.
“These pulse trains often are required in radar, avionics, and automotive EMC applications. In contrast to single and double pulses, the term pulse train refers to combinations of different types of pulses, which can be either periodic or nonperiodic. Pulse width as well as pulse delay can be set independently and separately for each pulse,” he explained.
These kinds of EMC signal-source improvements are part of a broader trend toward comprehensive test instruments. A more flexible source may reduce test time and uncertainty because a single instrument or a single range can provide the needed measurements. At the other end of the spectrum, equipment that performs a very narrow, tightly specified function such as lightning simulation or comb generation addresses test requirements that general-purpose instruments can’t.
References
- Caldwell, M., and Martinez, L., “The Sandia Lightning Simulator; Recommissioning and Upgrades,” IEEE International Symposium on Electromagnetic Compatibility, 2005, Vol. 2, pp. 368-371.
- Mawdsley, D., “Test Site Recommendations for Radiated Emissions Testing,” Laplace Instruments.
- Tan et al., “A 12-ps GaAs Double Heterostructure Step Recovery Diode,” Hewlett-Packard Instruments and Photonics Laboratory, 1991.
- “Residual Phase Noise Measurements; Low Phase Noise Comb Generators,” Picosecond Pulse Labs, 2005.
- “U9391C/F Comb Generators Technical Overview,” Agilent Technologies, 2009.
| FOR MORE INFORMATION | Click below | |
| Agilent Technologies | U9391C/F Comb Generator | Click here |
| Giga-tronics | 2550B Microwave Signal Generator | Click here |
| Laplace Instruments | ERS Emissions Reference Source | Click here |
| Picosecond Pulse Labs | Model 7124 Comb Generator | Click here |
| Rohde & Schwarz | SMA100A/SMF100A Pulse Train Option | Click here |
| Thermo Scientific | LTS Lightning Test System | Click here |
| York EMC | CGE01 Comb Generator Emitter | Click here |
November 2009