Data Fig1

Fiber-Optically Isolated Instrumentation Application

When traditional instruments became too cumbersome, a compact high-bandwidth digitizer provided the solution.Instrumentation of high-voltage pulsed power experiments can be problematic because of the strong electromagnetic (EM) fields present around the measurement location and the need to maintain isolation through the walls of shielded enclosures. Isolation in these applications is important to maintain measurement quality and ensure personnel safety.

A typical experiment consists of a high power source designed to radiate a tens of nanoseconds rise time damped sine pulse intent on damaging the electronics present in the radiated field. EM field levels in excess of 10 kV/m are typical.

In comparison, the EM field level that consumer electronics are tested to as part of EMC standard EN 50082-2:1995 is 10 V/m. The test environment of a radiated pulsed power source can cover large areas depending on the radiation pattern of the source, requiring multiple channels of data acquisition to cover the area of interest.

At the Naval Surface Warfare Center Dahlgren Division (NSWCDD), we typically use more than 16 channels and always have a need for more. A representative pulsed power test scenario is shown in Figure 1.

Figure 1. A Typical Radiated Pulsed Power Experiment

Several approaches have been used to build instrumentation to meet the specific requirements for pulsed power testing. Traditional methods of instrumentation involve placing an oscilloscope inside a shielded container in close proximity to the measurement point or using analog fiber-optic telemetry systems to route the measurement signal to an external oscilloscope.

In general, the instrumentation needs to measure the signal in the time domain, digitize the signal close to the measurement point, offer wide analog bandwidth, and be as small as possible to minimize field perturbations. Large, bulky instruments such as oscilloscopes are cumbersome in this type of measurement, and analog fiber-optic telemetry systems require regular calibration and careful treatment of fiber-optic cables and terminations.

Recent advances in high sample rate and wide bandwidth analog-to-digital converters have enabled the development of increasingly sophisticated instrumentation suited for the EM environment associated with many pulsed power systems. One capability recently developed is a system based on the use of a high-bandwidth digitizer packaged in a compact configuration where the sampled data is transmitted digitally using TCP/IP network protocol.

Battery power and heavy shielding of the system allow measurements in regions of extremely high field strengths. Connection with external instrumentation and control systems is accomplished by using only fiber-optic cabling to provide completely isolated measurements.

Compact Remote Digitizer
Tested recently at the NSWCDD in a radiated pulsed power application, the compact remote digitizer (CRDAQ) met all of the requirements successfully. CRDAQ is a fiber-optically coupled, battery-operated remote digitizer contained in an EM-hardened case with dimensions of 8-3?8″ � 4-3?4″ � 5-1?4″. It is based on a 3U cPCI standard using commercial-off-the-shelf (COTS) computers and digitizers.

The CRDAQ consists of a four-slot cPCI backplane, a battery pack, digitizer, a cPCI computer, and associated components mounted in an EMI-shielded enclosure (Figure 2). The enclosure has ST connections for fiber Ethernet, an SMA connector for the input signal, an EMI air filter for cooling, and a separate cover for removing the battery. When not inserted into the shielded enclosure, the system is fully functional, allowing access to the system components for maintenance or repair.

Figure 2. The CRDAQ System

Consideration for repair and maintenance was designed into the system by the extensive use of COTS. The controller board, which features a simple, two-layer PCB, is populated with COTS components to reduce the dependence on extremely specialized circuits. In addition, the battery pack is composed of four lithium-ion camcorder batteries connected in series to produce the 28.8 V, 5.5 A-hour pack, further reducing maintenance requirements.

COTS cPCI components were chosen to meet the requirements for small size, fast digitizing speed, and low power consumption. Small size is required to minimize the effect that the enclosure has on the radiated EM field measurement.

The fast rise time of the pulse power measurements was the driving factor for a high sample rate digitizer to accurately capture the signal. Two commercial digitizers, operating at 1 GS/s and 4 GS/s, respectively, were used for testing the CRDAQ system (Table 1). Although digitizers were used for the pulsed power application, these units could easily be replaced with other cPCI instruments depending on the test application.

Table 1. Summary of Digitizer Specifications

Control of the CRDAQ is handled by two programs using any Windows XP/2000-based computer with Ethernet. One monitors the CRDAQ via a microcontroller, and one obtains the data from the digitizer.

The microcontroller program monitors the battery voltage and internal temperature, controls the attenuator setting, and provides remote startup and shutdown to conserve battery life. The second program, commercial multichannel acquisition software, provides a virtual window into the digitizer for configuring the acquisition parameters, arming the unit, and displaying the digitized data, which can be saved to a file for further analysis.

Operation of the system can be explained using the block diagram in Figure 2 as a reference. The CRDAQ is placed where a measurement is desired with either an EM field probe or current probe connected to the SMA input with a coaxial cable. A standard Ethernet fiber cable also is connected between the remote digitizer and a monitoring computer for system and digitizer control.

With the unit in standby mode, the Ethernet media converter and microcontroller are powered up. In this state, they monitor internal temperature and battery voltage and will shut down the system in the event the battery voltage goes too low so the battery pack will not over discharge.

Approximately two minutes before the test event, the rest of the electronics are powered up using the microcontroller program. Once the system is fully operational, the attenuator is adjusted, and the digitizer is set based on the expected signal level from the probe to maximize the input range of the digitizer.

After the test event occurs, the measured signal will be displayed by the acquisition software and saved to a file. In pulsed power testing, the time between individual tests can range from several minutes to hours. To conserve battery life during those long delays, the CRDAQ can be put back into low-power standby mode.

A recent test at the NSWCDD Maginot Open Air Test Site (MOATS) facility compared the 1-GS/s (250-MHz) CRDAQ with the commercially available 200-MHz analog fiber telemetry system. For this test, the source radiated a high power damped sine signal, and the current coupled onto the power cables of the computers was measured.

Two current clamps were placed close together on the power cable, one connected to the commercial system and one to the CRDAQ. The result was two measurements that matched in time and amplitude with the commercial link exhibiting around 25 mV of noise due to the analog fiber link while the CRDAG noise was less than 5 mV.

The 4-GS/s configuration was verified in the laboratory using a pulse generator capable of producing a negative going pulse with an amplitude of 5 kV and a rise time of 10 ns. For this test, the pulse signal was sent through a three-way power splitter to the CRDAQ, a second commercially available analog fiber link connected to a 3-GHz oscilloscope, and directly into the 3-GHz oscilloscope (Figure 3).

Figure 3. Performance Comparison of the 1.5-GHz CRDAQ With Commercial Measurement Counterparts

The CRDAQ and oscilloscope measurements were highly correlated in time, amplitude, and the amount of noise present on the measured signal. As with the first commercial analog fiber link, the second commercial system exhibited more noise due to the transmission of the analog signal over the 50-meter fiber cable but still produced a measurement comparable to the CRDAQ and direct oscilloscope in time and amplitude.

Traditional methods of placing oscilloscopes in shielded enclosures and the use of analog fiber links to perform measurements during pulse power testing have been used for more than 20 years. Advancements in analog-to-digital converter speed and compactness have changed the way we are able to deploy remote measurement equipment.

The CRDAQ has shown by using the latest digitizer technology and a COTS form factor a remote measurement system can be developed that performs with exceptional correlation to commercial measurement counterparts.

In addition, the system has a lower noise floor than analog fiber links, which is inherent to the transmission of analog signals over long distances via fiber optic cable. It also eliminates the need for regular calibration and careful treatment of the fiber-optic cable and terminations.

Acknowledgements
This work was supported by the Office of the Secretary of Defense Test and Evaluation/Science and Technology Program and Directed Energy Technology Office at NSWCDD.

About the Author
Benjamin Grady has been with the NSWC Dahlgren Directed Energy Technology Office investigating the susceptibility of infrastructure systems to intentional EMI and developing pulsed power/high power microwave measurement instrumentation since 2000. Previously, he worked as an avionics technician, at NASA Langley Research Center as a microwave technologist, and as a cellular base-station product engineer at Ericsson Telecom. Mr. Grady received an A.A.S.E.T. in 1987, a B.S.E.E.T. in 1993, and an M.S.E.E. from Virginia Tech in 2000. Naval Surface Warfare Center Dahlgren Division, Code Q22, 17320 Dahlgren Rd., Dahlgren,VA 22448

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November 2006

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