No abstract

When two or more analog computers are to be connected to each other or to peripheral devices, it is necessary to do so without creating excessive ground or AC-line noise in any of the devices and without performing major modifications on any of them.It is assumed that each device can operate by itself without excessive ground noise and that each device has a ground node connected to its chassis in order to avoid pickup between ground and chassis. Each chassis may be connected to earth ground at this node.The ground noise which must be avoided is that resulting from coupling through the AC power line and earth grounds.In addition, each device must not create excessive radiofrequency interference (RFI) on the AC line since it could thence be coupled into the power supplies of the other devices. Also, each device should reject AC-line RFI. These decoupling properties may be tested for-say-minus 60 db peak RFI on the line and for 60 dB of line RFI rejection in the signal or DC power. (120 mV peak on the 120 VAC corresponds to -60 dB and will appear as no more than 12 [tV peak on a 10V DC output which is down -60dB, for a total of -120 dB.) This line noise isolation may be improved by an RFI line filter for any device.Ground noise is caused by the AC potential difference between any one chassis and other chassis in the same system. If each chassis is connected to earth ground at a separate location, the difference in ground potential difference may cause noise. If only one of the chassis is connected to earth ground, or if all chassis are connected to earth ground at a common point, electromagnetically induced potential difference may still exist between the device chassis. A heavy bus buried in the earth and grounded to each chassis has been found effective in reducing the ground potential difference between chassis.Since ground noise is coupled between devices by means of the AC line, it may be reduced by isolating the AC lines of the devices. A line-isolation transformer decouples the DC current component, while electrostatic transformer shielding and RFI filters decouple the AC components. The line-isolating transformer is also necessary to avoid the potential difference in the AC neutral line from being imposed between the ground nodes of the devices.Signal lines (both analog and digital) are usually coaxial or twisted-pair cables whose &dquo;outer&dquo; conductor is connected to ground at each end and is thus across the ground potential difference between chassis. Low-frequency components on an outer conductor between chassis are then shorted by the ground bus between chassis, while the high-frequency components are attenuated by the common-mode rejection of the cable.If the signals are digital, more &dquo;noise&dquo; can be tolerated, and the ground bus may not be necessary in a digital computer. The ground potential difference between devices may be measured; effective capacitance between the primary and the other windings of a transformer may be measured -with and without electrostatic shielding; the transfer...