Abstract: A termination circuit for RGB signal lines provides a high frequency termination impedance that matches the impedance of a monitor cable and monitor termination, without adversely affecting the monitor-sense circuitry of an ASIC that generates the RGB video signals. The termination circuit includes a first resistor connected in parallel with a diode. The diode is reversed biased when a DC voltage is present on a VGA line. The parallel circuit is connected in series with a second resistor such that at DC the termination circuit has a resistance equal to the sum of the resistances of the two resistors. At high frequencies the termination circuit has a resistance equal to the second resistor. When used with a VGA ASIC designed to work with 150.OMEGA. pull-down sense resistors on the RGB lines, the circuit provides the proper 75.OMEGA. AC termination impedance required for VGA lines while producing the 150.OMEGA. DC termination required for proper monitor sense operation.

Abstract: Noise generated in a computer system on a modem adapter card is effectively suppressed by providing an extension to a voltage reference plane. The extension reference plane is coupled to the voltage reference plane of the adapter card by an impedance circuit. The extension reference plane causes the adapter card to have a composite reference plane which has an overall length greater than one-quarter of the wavelength of the signal to be suppressed such that the composite reference plane does not resonate at the frequency of the signal which generated the noise. The reference planes are preferably connected by a resonant circuit so that the two reference planes are effectively electrically connected at high frequencies and are electrically disconnected at low frequencies so that resonance of the boards is also suppressed at the lower frequencies.

Abstract: A method for defining traces on a multi-layer circuit board suppresses electro-magnetic emissions radiated from the traces. The defined traces carry a differential signal. Layers of signal and return trace pairs are formed. Each succeeding layer of traces is spaced above and follows the traces below, with the succeeding signal trace positioned above the preceding return trace and the succeeding return trace positioned above the preceding signal trace. All of the signal traces are conductively connected and all of the return traces are conductively connected. The symmetry of the arrangement causes the electro-magnetic fields generated by the conductors (traces) to cancel at any appreciable distance. Additionally, varying combinations of layers and/or varying parameters such as dimension, spacing and dielectric material, results in a desired impedance for matching to external cabling and the like.

Abstract: A probe comprising at least one unshielded sensor tip and at least one transformer shielded within a conductive housing for sensing the electric field strength of a precise near field when connected to a radio frequency meter. Each enclosed transformer includes an annular ferrous core having a primary wire and a secondary wire wound thereon, with the input end of the primary wire extended slightly outside the distal end of the housing providing each sensor tip, and with the reference end of the primary wire connected to an annular conductor of a coaxial connector. The output end of the secondary wire is connected to an annular conductor of the coaxial connector. The reference end of the secondary wire and the shielded housing are connected to the annular conductor of the coaxial connector to provide a ground reference for the circuit.

Abstract: A multi-layer printed circuit board is constructed to suppress radio frequency interference (RFI) generated by high frequency clock and data signals therein. Suppression is achieved by positioning clock lines carrying the clock signal on a first voltage reference layer proximate to a second voltage reference layer. The two layers shield the clock signal from the signal lines on other layers. Noise may be further reduced by forming bridges in the second voltage reference layer proximate to the clock lines so that the bridges span the width of the clock line. Capacitors are also preferably utilized to further suppress radiated noise. The capacitors provide AC coupling between the first and second voltage reference layers so that a low impedance path is provided for high frequency noise generated by the clock signal. The second voltage reference layer thus operates as an effectively continuous shield between the bridges.