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

[NOTE: SOME FORMULA'S HAVE BEEN OMITTED]

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.

The EMC design of the HP 54600 Series digitizing oscilloscopes consisted of a combination of circuit board suppression and mechanical design techniques. Since the entire product (including enclosure) was a completely new design, we had an opportunity to design in RFI suppression techniques from the very start of the development. This article describes the design and test methods employed to ensure that the products met international and military EMC standards.

RFI standards for HP products typically include the German FTZ 1046 Class B limit similar to the U.S.A. FCC Class B limit).* Additionally, we decided to attempt to meet the U.S. MIL-STD-461C RFI requirements as specified in the environmental standard MIL-T-28800D. Meeting these military standards imposed much more stringent design goals than normal. Since one of the primary drivers of the HP 54600 design was cost, early EMC/RFI integration was necessary. By considering EMC early in the system design, we hoped to minimize the cost of achieving the multiple goals of both international and military standards. In the end we were forced to offer two of the more costly suppression methods for some of the military requirements as options. We felt that the additional cost involved should not be imposed on the majority of customers, who would not require the additional shielding.

Enclosure Design

We knew that older enclosure designs included too many seams for easy RFI suppression. Therefore, we decided to start with fresh ideas. Several concepts were suggested and we finally settled on a two-part molded design, coated on the interior with a 2.5-[micro-m]-thick layer of vapor deposited aluminum.** One of the parts is the main cabinet and the other is the front panel. This two-part enclosure concept reduces the number of seams to the minimum required for access to the electronics. The aluminum is applied to all sides of the seam area so that good contact is made between the cabinet and the front panel.

The seam design is a continuous, overlapping "knife edge", which fits in a slightly zigzag manner between a series of raised bumps and stiffening ribs. These stiffening ribs are spaced about 3 to 6 cm apart all the way around the seam, providing a distributed pressure to ensure a good electrical connection. Molded bumps between the contact fingers act as additional contact points. Fig. I shows the main seam details. This design provides uniform contact while allowing easy disassembly without the need for special tools. The maximum spacing of about 3.5 cm between contact points yields a theoretical shielding effectiveness of 20 dB at 500 MHz. We performed several tests on both the seam itself and the enclosure as a whole to confirm the shielding performance.

Enclosure Testing

Several evaluation techniques were used to assess the RFI performance of the enclosure before building a finished electronic prototype. A number of proof-of-concept tests were performed on a glued-together prototype enclosure. These tests included near-field seam leakage tests using an HP 11940A close-field probe and far-field tests with a harmonic comb generator placed within the enclosure and measured as if it were a finished product.[1] The first enclosure testing consisted of a measurement of the basic shielding properties of various conductive plating choices. For this test, a pair of HP 11940A close-field probes were used tip to tip, one being the transmitting source and the other the receiver (Fig. 2). These probes were connected to an HP 8753B network analyzer and the transfer characteristic of the measurement system (without shielding material) was normalized to zero dB. The shielding samples were then placed between the probe tips and the near-field magnetic shielding effectiveness plot was obtained directly from the screen. This method allowed a quick comparison of several different shielding materials. Since the close-field probes were held by posts on the fixture, it was possible to measure various points of the actual molded parts to verify shielding performance and consistency.

Once molded prototypes were available, the comb generator was mounted within the enclosure. The comb generator simulated a very noisy product by emitting strong harmonics every 5 MHz from 30 to 1000 MHz. With the generator in place, seam emissions were measured with the 11940A close-field probe. A composite emissions characteristic was recorded using a spectrum analyzer and the signal leaks were noted. This emission recording was repeated for each mold trial to track the general progress of the design and to allow improvements to be incorporated into the mold before, the electronic prototype was available.

Finally, far-field testing was performed, again using the comb generator as a simulated product. By measuring the difference in signal strength between the generator in the enclosure and the generator without the enclosure, a rough idea of the system shielding effectiveness including all apertures and seams) was obtained. The video display module, keyboard, and steel deck were mounted within the enclosure to fill the larger apertures and more closely resemble the finished product. The prototype enclosure was tested at a 3-meter distance in an anechoic chamber. First, the harmonic levels of the generator were measured. Then, the generator was placed inside the enclosure and the harmonics were remeasured. For each measurement, the product was rotated in azimuth and the highest harmonic levels were recorded. The difference in readings then indicated the worst-case shielding effectiveness. Mg. 3 shows a typical plot of shielding effectiveness versus frequency. This technique exposed weak areas of the total system design before installing the electronics and allowed the mechanical and electronic design efforts to proceed in parallel.