EMC design of the HP 54600 Series oscilloscopes: by a combination of electronic circuit design and mechanical shielding techniques, the design meets German FTZ standards and, with optional shielding, most U.S. military standards for electromagnetic compatibility - Technical

Hewlett-Packard Journal, Feb, 1992 by Kenneth D. Wyatt

Power Supply Filtering

Design constraints on the power supply filter circuit made for another major challenge. The oscilloscope must be able to use any power source worldwide, so it has to operate from 90 to 270 volts and from 44 to 440 Hz, automatically, without user-settable switches. We also had to meet safety leakage current limits (3.5 mA, maximum) at any voltage/frequency combination, and we had to meet both FTZ (Vfg) 1046 Class B (based upon the German VDE 0871 standard) and MIL-STD-461C, CE-03 (U.S. military) conducted emission limits. Finally, because of cost constraints, we could not use shielded power entry modules (PEMs). The entire filter circuit had to be designed with discrete components mounted on the power supply board. To say that this was a challenge would be an understatement.

One major difference between FTZ 1046 and MIL-STD-461 conducted emissions tests is that for FTZ 1046, noise voltages are measured, while for MIL-STD-461, noise currents are measured. Thus, the filter design for FTZ 1046 needs to look like a low impedance (shunt capacitor to limit the voltage), while the design for MIL-STD-461 needs to look like a high impedance (series inductance to limit the current). Thus, the filter topologies chosen for the two standards tend to be diametrically opposed.

Our filter vendor ultimately proposed a filter that resembles a typical FTZ 1046 topology with series inductors to reduce the MIL-STD-461 noise currents. The circuit is shown in Fig. 7.

System Design

Since the circuit board assembly contains connectors at each end, the usual nuts that attach the oscilloscope probe BNC connectors to the front panel were eliminated to avoid stresses on the enclosure. The BNC connectors are part of a cast shield assembly that attaches directly to the main circuit board and then to the steel deck with a screw. These connectors are simply pushed through matching holes in the molded plastic front panel as the circuit board is installed. This was another EMC design challenge in that we were originally counting on a fairly long path to ensure an adequate ground connection (or cable shield termination) between the BNC ground and the shielded enclosure of the oscilloscope. The result was a higher level of probe cable radiation then we could tolerate.

The solution to this is a thin, nickel-coated, phosphor-bronze shim placed between the cast BNC shield assembly and the front panel. This shim includes four connection points (90 degrees apart) which press around the perimeter of each BNC connector. Other tabs press between the deck and front panel. The shim provides a good connection between the circuit board and the enclosure, thereby reducing probe cable radiation. The other system problem stems from the use of a raster-scanned CRT display. Large magnetic fields 60-Hz vertical and 25-kHz horizontal) are used to drive the electron beam, and these fields radiate from the oscilloscope display. The 60-Hz field can interfere with other nearby CRT displays, and vice versa. Also, other low-frequency fields (from power transformers, typically) can interfere with the oscilloscope display, causing the display to swim or wiggle. Finally, the 25-kHz field radiates strongly out the front of the CRT and can be picked up by the oscilloscope probe tip as far away as one to two feet. If the user measures sensitive circuits directly in front of the CRT, they will tend to pick up this horizontal scanning signal. Passage of the radiated emission and susceptibility portions of MIL-STD-461C requires both of these fields to be considerably reduced. The optional shields previously mentioned were designed to help with both of these problems. While the product meets commercial EMC standards worldwide, some customers may need the additional shielding provided by these options.