Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
1999-08-25
2002-03-19
Bost, Dwayne (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S296000, C379S056300, C348S189000
Reexamination Certificate
active
06360090
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed toward data communications and, more particularly, a method and apparatus having improved immunity from infrared noise from fluorescent lights and other sources.
BACKGROUND OF THE INVENTION
Infrared wireless data communication is a useful method for short range (in the approximate range of 0-10 meters) wireless transfer of data between electronic equipment; such as, cellular phones, computers, computer peripherals (printers, modems, keyboards, cursor control devices, etc.), electronic keys, electronic ID devices, and network equipment. Infrared wireless communication devices typically have the advantages of smaller size, lower cost, fewer regulatory requirements, and a well defined transmission coverage area as compared to radio frequency wireless technology (i.e. the zone of transmission is bounded by physical walls and therefore more useful in an office environment). In addition, infrared wireless communication has further advantages with regard to reliability, electro magnetic compatibility, multiplexing capability, easier mechanical design, and convenience to the user as compared to cable based communication technology. As a result, infrared data communication devices are useful for replacing 0-10 meter long data transfer cables between electronic devices, provided that their size and costs can be reduced to that of comparable cable technology.
Infrared data communications devices typically consist of transmitter and receiver components. The infrared data transmitter section consists of one or more infrared light emitting diodes (LEDs), an infrared lens, and an LED current driver. A conventional infrared data receiver typically consists of an infrared photodiode and a high gain receiver amplifier with various signal processing functions, such as automatic gain control (AGC), background current cancelling, filtering, and demodulation. For one-directional data transfer, only a transmitter at the originating end and a receiver at the answering end is required. For bi-directional communication, a receiver and transmitter at each end is required. A combined transmitter and receiver is called a transceiver.
In typical high volume applications, it is now standard practice to fabricate the receiver circuitry and transmitter driver in a single integrated circuit (IC) to produce a transceiver IC. In turn, a transceiver IC, infrared photodiode and LED along with lenses for the photodiode and LED are assembled together in a plastic molded package designed to be small in size and allow placement in the incorporating electronic device so as to have a wide angle of view (typically through an infrared window on its case). The transceiver IC is designed to digitally interface to some type of serial data communications device such as an Infrared Communication Controller (ICC), UART, USART, or a microprocessor performing the same function.
A representative example of a conventional infrared data transmitter and receiver pair is shown in FIG.
1
. Infrared transmitter
10
includes LED
16
which generates a modulated infrared pulse in response to transistor
14
being driven by the input data signal D
IN
. The modulated infrared signal is optically coupled to an infrared detector, such as photodiode
24
normally operated in current mode (versus voltage mode) producing an output current which is a linear analog of the optical infrared signal falling on it. The infrared pulses generated by LED
16
strike photodiode
24
causing it to conduct current responsive to the transmitted input data signal D
IN
thereby generating a received data signal at D
IR
.
Data can be modulated on the infrared transmitted signal by any of a number of well known methods. Two of the most popular methods are defined by the Infrared Data Association (IrDA) and Sharp corporation (Sharp ASK). IrDA Physical Layer Link Specification 1.1e specifics two main physical layer infrared modulation methods. One method is a low-speed (2 Kbp/s to 1.15 Mbp/s) on-off infrared carrier asynchronous modulation where the presence of a pulse indicates a 0 bit and the absence of a pulse indicates a 1 bit. The second method is a high speed (4 Mb/s) synchronous Four Pulse Position Modulation (4 PPM) method in which the time position of a 125 ns infrared pulse in a 500 ns frame encodes two bits of information. The Sharp ASK method is similar to the low speed IrDA method but also modulates the infrared carrier with a 500 Khz signal to facilitate differentiating between a valid signal and ambient infrared signals.
In receiver
20
, the received signal at D
IN
is transformed into a voltage signal and amplified by amplifier
26
. The signal output from amplifier
26
then feeds into comparator
42
which demodulates the received signal by comparing it to a detection threshold voltage V
DET
in order to produce a digital output data signal at D
OUT
.
The received signal waveform will have edges with slope and will often include a superimposed noise signal. As a result, V
DET
is ideally placed at the center of the received signal waveform so that the output data signal has a consistent waveform width despite the slope of the received signal edges. Also, placing V
DET
at the center of the received signal improves the noise immunity of receiver
20
because the voltage difference between V
DET
and both the high and low levels of the received signal is maximized such that noise peaks are less likely to result in spurious transitions in D
OUT
.
The received signal, however, can vary in amplitude by several orders of magnitude due primarily to variations in the distance between transmitter
10
and receiver
20
. The strength of the received signal decreases proportional to the square of the distance. Depending on the range and intensity of the infrared transmitter, the photodiode outputs signal current in the range of 5 na to 5 ma. plus DC and AC currents arising from ambient infrared sources of sunlight, incandescent and florescent lighting. As a consequence, the center of the received signal waveform will vary, whereas V
DET
must generally be maintained at a constant level. To address this problem, receivers typically include an automatic gain control mechanism to adjust the gain responsive to the received signal amplitude. The received signal is fed to AGC peak detector
36
which amplifies the signal and drives current through diode
32
into capacitor
28
when the signal exceeds the AGC threshold voltage V
AGC
in order to generate a gain control signal. The gain control signal increases in response to increasing signal strength and correspondingly reduces the gain of amplifier
26
so that the amplitude of the received signal at the output of amplifier
26
remains relatively constant despite variations in received signal strength.
At a minimum, infrared receiver
20
amplifies the photodetector signal current and then level detects or demodulates the signal when it rises above the detect threshold V
DET
thereby producing a digital output pulse at D
OUT
. For improved performance, the receiver may also perform the added functions of blocking or correcting DC and low frequency AC ambient (1-300 ua) signals and Automatic Gain Control (AGC) which improves both noise immunity and minimizes output pulse width variation with signal strength.
As noted above, infrared data receivers are vulnerable to infrared ambient noise in their environments. This noise produces spurious outputs and degrades performance by causing bit errors. The predominate sources of noise for infrared receivers in most common environments are (1) photocurrent shot noise from background ambient infrared light; (2) other infrared data transmitters; and (3) fluorescent lights.
Of these three sources, infrared noise from fluorescent lights is typically the most disruptive and most difficult and expensive to mediate. For wideband IrDA devices, receiver optical sensitivity is limited to a value that is as much as 10 times less than is practically possible so as to limit interference from fluorescent lights. Consequently,
Holcombe Wayne T.
North Brian B.
Bost Dwayne
Fraucissen Vernon W.
Integration Associates Inc.
West Lewis G.
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