Fiber-optically isolated instrumentation for pulsed power system diagnostics

Advances in high sample rate/wide bandwidth analog-to-digital converters have enabled the development of increasingly sophisticated instrumentation suited for the highly stressing electromagnetic environment associated with many pulsed power systems. This application shows a high bandwidth digitizer packaged in a compact configuration where the sampled data is transmitted digitally using TCP/IP network protocol over fiber-optic cabling, providing completely isolated measurements.

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 is important to maintain measurement quality and to ensure personnel safety.

Several approaches have been used to build specialized instrumentation to meet specific requirements. Traditional methods of instrumentation involve placement of an oscilloscope inside a shielded container in close proximity to the measurement point, or the use of 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 the wide analog bandwidth, and be as small as possible to minimize field perturbations. In response to these requirements the Compact Remote Digitizer or CRDAQ was developed and tested in comparison to the current pulsed power measurement systems.

Compact Remote Digitizer Analog fiber-optic telemetry systems require regular calibration and careful treatment of fiber-optic cable and terminations. These restrictions can be significantly reduced by digitizing the analog signal at the source and transmitting digital data via fiber. The compact remote digitizer (CRDAQ) seeks to lift these restrictions. This system is a fiber-optically coupled, battery-operated device contained in an EM-hardened case with dimensions of 8 3⁄8” x 4 3⁄4” x 5 1⁄4”.

The instrument is based on the 3U CompactPCI standard using commercial-off-the-shelf (COTS) computers and digitizers. For this application the computer was chosen for low power consumption since the need for computing power is minimal. Two digitizers, a 250 MHz and 1.5 GHz model, have been tested in the system. The 250 MHz digitizer or DC110 is a current product from Acqiris while the 1.5 GHz (DC152) is new product developed through a cooperative effort between NSWC and Acqiris USA.

Table 1 presents a summary the CRDAQ specifications and Figure 1 shows the system block diagram.

Table 1

CRDAQ consist of the electronic subassembly containing all of the system components in a single package and an enclosure into which the subassembly is inserted for EM shielding. The electronic subassembly is designed to be removed from the enclosure and remain fully functional while allowing access to the system components for maintenance or repair. Figure 2 shows how the CompactPCI computer, digitizer, battery pack, and controller board (power conversion, communications, power control, and system health) make up a self contained assembly.

The hardware to contain the electronic subassembly consists of a custom designed part produced by Stereolithography (SLA) that attaches to the CompactPCI backplane to make the self contained assembly. This complete assembly is then inserted into the shielded enclosure which has ST connections for fiber Ethernet, a SMA connector for the input signal, EMI air filter for cooling, and a separate cover for easy removal of the battery (Figure 3).

In order to keep the CRDAQ as efficient and compact as possible the controller board is a simple two layer printed circuit board populated with COTS components. Using COTS components reduces the dependence on extremely specialized circuits, thus making maintenance and repair for the end user easier (Figure 4). The same statement is true for the battery pack since it is comprised of 4 Sony Lithium-Ion camcorder batteries connected in series to produce the 28.8 V 5.5 A/hr pack.

Software Control of the CRDAQ is done using any Windows XP/2000 based computer with Ethernet by two programs; one to control/monitor the CRDAQ system via a microcontroller and one to control and obtain the data from the Acqiris digitizer. The microcontroller program monitors battery voltage, internal temperature, controls the attenuator setting, and provides for remote startup and shutdown to conserver battery life (Figure 5).

The second program, AcqirisMAQS or 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 at a later date (Figure 6).

Digitizer performance Testing of both the 250 MHz and 1.5 GHz CRDAQ models has been performed with exceptional correlation with commercial measurement equipment. The DC110 or 250 MHz model was used in a recent field test of a pulsed power source at the NSWC Dahlgren MOATS test facility. In this test a Eaton 91550-2 150 MHz current clamp was used to measure the coupled current on a power cable with the DC110 CRDAQ and an EG&G ODT-E6 200 MHz analog fiber transmitter. Figure 7 shows how well the CRDAQ signal is correlated with the EG&G signal. One significant observation is that the noise floor of the CRDAQ is less than that of the EG&G.

The DC152 or 1.5 GHz CRDAQ verification measurement was made in the laboratory where it was compared with a Nanofast OP300 1 GHz analog fiber link and a Lecroy Wavemaster 8300A using a Bournlea pulse generator. The signal from the Bournlea was sent through a 3-way power splitter to all three instruments where a single pulse was sampled and used for comparison. In Figure 8 it can be seen that the CRDAQ measurement is in agreement with the Lecroy oscilloscope, used to directly digitize the signal, as well as exhibiting less noise than the Nanofast measurement system.

The authors would like to thank the Test Resource Management Center (TRMC) Test & Evaluation/Science & Technology (T&E/S&T) Program for their support. This work was funded through the U.S. Army Program Executive Office for Simulation, Training & Instrumentation (PEO STRI) and the Directed Energy Technology Office at NSWC Dahlgren.

Ben Grady has been with the NSWC Dahlgren Directed Energy Technology Office since 2000, investigating the susceptibility of critical infrastructure systems to intentional EMI and developing pulsed power/HMP measurement instrumentation. His previous experience includes RF and microwave work at Ericsson Telecom and NASA Langley Research Center. Ben received an AASET in 1987 and a BSEET in 1993 from Old Dominion University, and a MSEE from Virginia Tech in 2000.

Naval Surface Warfare Center, Dahlgren Division 17320 Dahlgren Rd., Code B20 Dahlgren, VA 22448 USA

For more information on the Agilent-Acqiris technology mentioned, please visit www.acqiris.com