1. Understanding Sources of NoiseThere are many sources of noise that can affect a measurement system. In general, three elements are present in a system susceptible to noise. A source of electromagnetic emissions must exist, a receiver that is susceptible to electromagnetic emissions must be present, and some path must exist between the two.
A common source of noise is radiated electromagnetic energy that travels through the air. Any nonshielded or insufficiently shielded conductor in a measurement system can act as an antenna picking up noise that can distort the signals that travel over the conductor. Cables, essentially antennas that pick-up noise, are the link between the ever-present electromagnetic energy and your measurement system. Sources of noise include high and low frequency radiation generated from sources such as rotating mechanical machinery, power lines, fluorescent light fixtures, RF signal sources, and poorly shielded electrical equipment.
The development of the IEEE 488 standard included extensive testing to understand the requirements for neutralizing the affects of electromagnetic interference (EMI). Based on these characterizations, today it is generally accepted that the cable assembly (that is, the interconnecting cable and the associated connectors) and the connector/cable interface immediately inside the instrument will, to a large extent, determine the electromagnetic compliance (EMC) performance of the overall instrumentation system. Anticipating the need to minimize the effects of noise, the IEEE 488 standard specifies a connector and cable assembly that provides acceptable EMC performance under a broad range of circumstances. However, in many applications, it is advisable to further reduce the effects of EMI.
2. Reducing the Effects of Radiated Interference
Further reduction of EMI does not require the test engineer to do extensive research or EMI measurements to characterize the noise immunity of the system, although in certain circumstances this may be advisable. The IEEE 488 standard does document additional methods for reducing the effects of radiated interference.
The following paragraphs briefly discuss the recommended methods for reducing the effects of radiated interference.
The IEEE 488 standard suggests a minimum coverage of 85 percent against radiated emissions. However, it also recommends that 90 percent coverage would provide further improvement. A combination of both a copper braid shield and a metallized Mylar or foil shield within the cable provides significant improvement in screening cables and the signals they carry from radiated emissions.
The IEEE 488 standard also recommends metallic connector housing to reduce the effects of radiated emissions
Radiation screening can be further improved by placing a metallic cover over the ends of the piggyback GPIB connectors. Connector covers also provide a barrier against dust and other particles that may invade the connector area.
3. Identifying Noise-Resistant GPIB Cables
National Instruments offers a wide assortment of solutions that meet the requirements outlined above. To further protect your system from radiated emissions, choose shielded, ruggedly constructed GPIB cables and connector covers that can satisfy requirements for basic and high-performance applications.
Double-Shielded GPIB Cables
The National Instruments X2, X4, and X5 cables are double-shielded for superior noise immunity when compared to standard single-shielded GPIB cables. They have shielded plug/receptacles and an overlapping connector shell seam for superior shielding and grounding. The connector shell is constructed with corrosion resistant, nickel-plated, cast aluminum for ruggedness and superior noise immunity. The combination of the characteristics delivers a minimum of 90 percent coverage and very low capacitance characteristics, going beyond the suggested shield requirements of the IEEE 488 standard.
Looking at figure 1, it can be seen that the double-shielded cables contain three distinct shielding elements. It is less susceptible to noise because it includes an outer copper braid and two inner aluminum/Mylar shields for a total of three shields. This following benefits result:
- Protecting signal lines from external noise
- Cutting radiated emissions from the cable itself
- Minimizing the effects of cross-talk between signal lines, thanks to the inner shields Table 1 shows the double-shielded cable specifications.
Figure 1. Double-Shielded Cable Assembly Diagram
|Conductors||4 twisted-pair conductors 26 awg (7/34),
16 twisted-pair conductors 26 awg (7/34),
2 drain wires 24 awg
|Shield||Braided shield 36 awg tinned copper,
2 aluminum/Mylar inner shields (90% minimum coverage)
|Impedance||< [0.25 + 0.153 (L)] W/conductor; L = length in meters|
|Capacitance||< 140 pF/m at 25 °C|
To complete your system, National Instruments also offers male and female connector covers to prevent dust or other contaminants from corroding or damaging the exposed GPIB cable contacts. The covers also shield the cable contacts from radiated emissions from surrounding equipment.
There are many ways to reduce the effects of noise in a GPIB system. Engineers often engage in detailed testing to characterize the noise immunity of a system. A basic yet often overlooked method of minimizing the effects of noise is simply investing in shielded GPIB cables and connector covers to help you increase the noise immunity of your system with minimal cost and minimal effort.
ANSI/IEEE Standard 488.1-1987, IEEE Standard Digital Interface for Programmable Instrumentation, Institute of Electrical and Electronics Engineers, 345 East 47th Street, New York, NY, 10017