High Performance Data Acquisition
Systems
Electro-Magnetic Interference (EMI) in Instrumentation Cables During Explosive Testing
Scott
Mr. W. Scott Walton, US
Abstract: Electro-Magnetic Interference (EMI) in
Instrumentation Cables During Explosive Testing
An experimental evaluation of Electro-Magnetic Interference (EMI) in various
instrumentation cables was conducted using Exploding Bridge Wire (EBW)
detonators and C-4 explosive. This paper provides field data to support
statements made during meetings at the 74th. Shock and Vibration Symposium in
"Anyone using standard RG-58 cable and BNC connectors should not buy high
performance (16 bit) data acquisition electronics, because the cable noise will
render the 16 bit resolution back to less than 12 bit resolution."
Comparison testing of triax, twinax,
coax, and shielded twisted pair cables in the EBW explosive environment was
needed to choose cabling that has performance necessary to take advantage of
the high resolution of the BTSX data acquisition
system that Aberdeen Test Center (ATC) is developing for ballistic
measurements. Prior to the test, a variety of conflicting factors both
supported and refuted the need to change from standard RG-58 cable:
Supporting Continued Use of Standard RG-58 Coax Cable:
• Industry standard. Has worked well in almost all applications for more than
40 years
• Economical. Universally accepted.
• Attempt to change to Twinax cable in 1980 was
unsuccessful.
• A single connector for triax costs $84 (more than
an entire reel of coax).
• Cost to change cable might exceed $5,000 per BTSX.
• No point in changing if significant improvement is not demonstrated in ATC
test environment.
Supporting Change to New Cable:
• New high SNR electronics will use low signal levels.
• Doug Firth of Precision Filters Inc. says signal conditioning spec’s are meaningless with existing cable.
• Fred Bloennigen of Bustec,
Inc.says don’t buy high
performance electronics if you are using old cable.
• Streuther Smith says any instrument with BNC
connectors is low performance, ‘Old School’ instrumentation.
• There has been a significant price reduction on triax
& twinax cable.
The experiment provided credible data to support the selection of a new cable
to replace RG-58 coax cable. The data show that the EBW is a mediocre EMI
‘point source’, but the ‘C’ cable used to fire the EBW is a dramatic,
distributed EMI source. It was noted that length and proximity of ‘C’
cable to RG-58 cable controls the amplitude and frequency of
the EMI ‘spike’. Measurement of the electric field from various
detonations was contaminated by RG-58 cable noise.
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