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

Option 001 is a mumetal shield that snaps over the CRT deflection coils. This both reduces the external fields exiting the oscilloscope and, more important, reduces the susceptibility of the display to low-frequency external fields (such as power transformers). Unfortunately, even with the shield in place, we are unable to comply fully with the RE01 (magnetic radiated) or RS02 (magnetic susceptibility) tests of MIL-STD-461C because the fields from these tests can enter through the front of the CRT display and upset the displayed waveform by moving the electron beam. This is a disadvantage of all raster-scanned CRT displays.

Option 002 is a conductive filter shield that is attached in front of the display screen. This shield reduces the E fields at the horizontal scan frequency of 25 kHz exiting the front of the CRT display. This filter screen allows the oscilloscope to meet the RE02 emission requirements of MIL-STD461C. Additionally, it solves the problem of 25-kHz CRT emissions entering the oscilloscope probe tip.

Acknowledgments

I would like to acknowledge the entire HP HP 54600 design team for their assistance in preparing this article and in persevering throughout the EMC design effort. I would also like to thank Dean Chaney for all the EMC testing.

References

1. A more detailed description of the harmonic comb generator measurement techniques along with construction details can be found in K. Wyatt and D. Chaney, "RFI Measurements Using a Harmonic Comb Generator," RF Design Magazine, January 1991, pp. 53-58.

2. H. Ott, Noise Reduction Techniques in Electronic Systems, 2nd edition, Prentice-Hall, 1988.

3. R. German, H. Ott, and C. Paul, "Effect of an Image Plane on Printed Circuit Board Radiation," IEEE International Symposium on EMC, Washington, D.C., August 21-23,1990, pp. 284-291.