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Diffuse Infrared Receiver

IP.com Disclosure Number: IPCOM000039126D
Original Publication Date: 1987-Apr-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 4 page(s) / 36K

Publishing Venue

IBM

Related People

Bustamante, C: AUTHOR [+2]

Abstract

This article describes an improved circuit arrangement for a diffuse infrared receiver. It provides solutions to problems in the areas of dynamic range, fluorescent bulb interference, and common-mode noise rejection. (Image Omitted) In the diffuse infrared (IR) media, there is a large amount of attenuation since there is no waveguide in the system. Signal strength drops off as 1/(radius)2. Therefore, an IR receiver must have a large dynamic range (70 dB). Because of this, existing IR receivers have saturation problems at the high end. High frequency fluorescent bulbs, which produce 70 kHz IR noise, will be marketed in the future. These bulbs will be popular because they are efficient. Today's fluorescent bulbs run at 120 Hz and use a large inductor to induce the voltages required for the fluorescent process.

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Diffuse Infrared Receiver

This article describes an improved circuit arrangement for a diffuse infrared receiver. It provides solutions to problems in the areas of dynamic range, fluorescent bulb interference, and common-mode noise rejection.

(Image Omitted)

In the diffuse infrared (IR) media, there is a large amount of attenuation since there is no waveguide in the system. Signal strength drops off as 1/(radius)2. Therefore, an IR receiver must have a large dynamic range (70 dB). Because of this, existing IR receivers have saturation problems at the high end. High frequency fluorescent bulbs, which produce 70 kHz IR noise, will be marketed in the future. These bulbs will be popular because they are efficient. Today's fluorescent bulbs run at 120 Hz and use a large inductor to induce the voltages required for the fluorescent process. The new bulbs use 70 kHz and, like switching power supplies, use small inductors to produce the same voltages. Existing IR systems do not have filtering that is sufficient against this noise source.

Referring to Fig. 1, the diffuse IR receiver disclosed herein is composed of six distinct parts: 1. Optical front end 2. First stage amplifier 3. Second stage amplifier with automatic gain control (AGC) 4. Filtering 5. Comparator decoder 6. Carrier Sense The optical front end comprises an IR singal source, dual bias

(Image Omitted)

networks and a preamplifier. For the first stage amplifier the reactance of capacitors C5 and C6 should be small at 200 kHz. The IC used is the Motorola MC1350 differential intermediate frequency (IF) amplifier. Two are used, one in the first and second stage. It is meant as a video IF amplifier and has the following advantages: 1. Differential and, therefore, a large common mode

rejection ratio. 2. Output is open collector. This allows defining the

impedance at the output node and, therefore, the

operating conditions. 3. Can act as a subtracter, differentiator, amplifier,

AGC, and has a self-biased input stage. R11 to ground holds the gain at maximum. Capacitor C16 rolls off any noise at U2 pin-5 including the thermal noise of R11. C21 and C22 are ceramic and electrolytic decoupling capacitors. R42 completes the decoupling filter. R9 and R10 are selected as large as possible such that the MC1350 does not saturate. We found this to be 560 ohms. Capacitors C7 and C8 are the dominant poles of the system. They are set to 120 pF to yield a 3 dB bandwidth of 1.89 MHz. Diodes D9 and D10 act as clamps to solve the saturation problems experienced with older

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receiver versions. This clamp also limits the maximum signal-to-noise ratio obtainable. If silicon diodes are used, the maximum signal-to- noise ratio is 40 dB. This will correspond to a bit error rate of less than 10e-11, which is excellent. The signal is not useable at this stage because it lets in large amounts of noise that will be produced by future high frequency fluorescent bulbs. The noise must be filtered out i...