Browse Prior Art Database

Methods to Minimize the Effects of LCD and LCD Backlight Noise on an Attached Sensor Screen

IP.com Disclosure Number: IPCOM000122606D
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 2 page(s) / 79K

Publishing Venue

IBM

Related People

Stein, F: AUTHOR

Abstract

Touch and stylus screens, referred to here as Sensor Screens, are very susceptible to the electromagnetic radiation generated by LCDs. Both the LCD and the fluorescent tubes of the LCD backlight generate this electrical noise. One sensor screen arrangment uses two arrays of parallel transparent conductors or wires, one array disposed in a vertical direction, the other in a horizontal direction. One typical approach to electromagnetic compatibility engineering is to place shielding material between the radiating element and the interfered element. This approach cannot be solely relied on for touch screens as significant shielding would also significantly reduce the level of light transmitted from the LCD to the user. Several other approaches have been developed and are described here.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Methods to Minimize the Effects of LCD and LCD Backlight Noise on
an Attached Sensor Screen

      Touch and stylus screens, referred to here as Sensor
Screens, are very susceptible to the electromagnetic radiation
generated by LCDs.  Both the LCD and the fluorescent tubes of the LCD
backlight generate this electrical noise.  One sensor screen
arrangment uses two arrays of parallel transparent conductors or
wires, one array disposed in a vertical direction, the other in a
horizontal direction. One typical approach to electromagnetic
compatibility engineering is to place shielding material between the
radiating element and the interfered element.  This approach cannot
be solely relied on for touch screens as significant shielding would
also significantly reduce the level of light transmitted from the LCD
to the user.  Several other approaches have been developed and are
described here.

      The stylus locating algorithm requires an accurate measurement
of the stylus signal strength.  Extraneous noise from the LCD and
backlight reduces the signal-to-noise ratio of the stylus signal
measurement.  One method to improve this signal/noise ratio is to
perform the measurement only when the stylus signal is expected.
Many circuits can be designed to do this; the one implemented uses a
JFET transistor across the feedback capacitor of a front end op amp.
When this transistor is cut off, the circuit amplifies the incoming
signal.  When the transistor is saturated, the op amp acts like a
voltage follower and the feedback capacitor is discharged resetting
the circuit.

      As a second line of defense, a synchronous detector is used.
Incoming noise is affected by the synchronous detector according to
the relationship between the noise frequency and the synchronous
detector's clock frequency. This detector is synchronized with the 40
KHz signal being transmitted to the se...