Some improvements have been made in an instrument designed expressly for recording lightning-induced transient voltages on power and signal cables. The instrument as it existed prior to the improvements was described in "Fast Transient-Voltage Recorder" (KSC-11991), NASA Tech Briefs, Vol. 23, No. 10 (October 1999), page 6a.
The prior version of the instrument could sample transient voltages in four channels at a rate of 20 megasamples per second (MS/s). In the improved version, the rate can be easily increased to 100 MS/s. The prior version of the instrument could handle a peak input potential of 50 V, or more if an attenuator was used. The improved version accommodates typical input ranges of 10, 50, and 100 V. The input termination can be single-ended or differential, with input resistance selectable among 50 Ω , 120 Ω, or 10 kΩ.
A trigger circuit continuously monitors the signals on all four channels, comparing the signal level on each channel with a predetermined threshold level. The threshold for each channel in the original version could be set at any level from 5 to 95 percent of full scale, independently of the threshold levels for the other channels; in the improved version, threshold can be set at any level between 1 and 99 percent. When the signal level in any channel exceeds its threshold level, a trigger signal is generated, causing full recording of data to begin simultaneously on all four channels.
Even when data are not being recorded fully, 12-bit analog-to-digital (A/D) converters in the four channels operate continuously, temporarily storing their output data in first-in/first-out (FIFO) registers that are always kept half full. When a trigger signal is received, the remaining halves of the FIFO registers are filled up with data. Inasmuch as the full capacity of each FIFO register corresponds to an observation interval of 200 µs, this arrangement provides a 100-µs pretrigger recording capability.
In the previous version, once a transient had been thus recorded, and during intervals of inactivity, the data were transferred to a nonvolatile memory. In the improved version, the data are transferred to both the nonvolatile memory and a second set of FIFO registers. The second FIFO set can be made as deep as needed to store as many waveforms as required. The design of the improved version makes it easy to replace either or both sets of FIFO registers to change record lengths and waveform-storage capacities. The nonvolatile memory retains the data even if power is lost.
The previous version of the instrument was equipped with a clock, and the stored data were time-coded to establish the times of transients and to facilitate correlation with data on the same transients measured by other instruments. The improved version retains this internal clock, but is also equipped with a Global Positioning System (GPS) receiver and an Inter Range Instrumentation Group B (IRIG-B) decoder for accurate time stamping of any recorded waveform. If the IRIG-B signal is lost, the waveform is stamped with the time from the internal clock.
Like the previous version of the instrument, the present version is normally powered through an ordinary AC power line and includes backup batteries. During normal operation, the batteries are automatically charged. In the event of failure of AC power, the batteries can sustain operation for as long as 8 hours.
The design of the improved version is amenable to the addition of a cellular-telephone data link or other radio transceiver to enable remote interrogation of the status of the transient recorder or to retrieve data either on request or at scheduled intervals. For applications in which real-time retrieval of data is not feasible, one could use such other data-storage devices as rewriteable compact-disk read-only memories (CD-ROMs). The introduction of CD-ROMs would increase the data-acquisition and -storage capacity from the current level of 16 waveforms to several tens of thousands of waveforms.
This work was done by Pedro J. Medelius of Dynacs Engineering Co., Inc., for Kennedy Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers. KSC-12174