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A Tunable Loss of Signal Threshold Scheme for the Digital Mode of Operation of Multi-Channel Rx PMD Chips

IP.com Disclosure Number: IPCOM000077161D
Publication Date: 2005-Feb-25
Document File: 4 page(s) / 516K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method that uses a multi-channel PMD receiver with a loss of signal (LOS) detect and adjust for each individual channel, as well as a global LOS. Benefits include conserving chip space and power, as well as improving mismatch and offset.

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A Tunable Loss of Signal Threshold Scheme for the Digital Mode of Operation of Multi-Channel Rx PMD Chips

Disclosed is a method that uses a multi-channel PMD receiver with a loss of signal (LOS) detect and adjust for each individual channel, as well as a global LOS. Benefits include conserving chip space and power, as well as improving mismatch and offset.

Background

In a single channel limiting amplifier (LIA) or trans-impedance amplifier which converts the electrical current to voltage (LIA/TIA) design, the LOS block is required to detect the optical signal level against an adjustable threshold level. This feature usually requires two or three pads, which is physically impossible for a multi-channel design. By operating the chip in a digital mode, the I2C bus is used to digitally adjust and monitor each channel’s LOS functions. Figure 1 shows the block diagram for a general multi-channel design that uses the I2C block. There are three major inputs: I2C control pins, a mode select pin (“0” for analog and “1” for digital), and the Rext pin (external adjust resistor); there is also one LOS output pin (“1” for no signal and “0” for signal presence).

In analog mode, all channels share the same LOS threshold. However, each channel has a different input level, so the LOS output is different. Since there is no way to observe each individual LOS output, all the LOS outputs are OR-ed into one, creating the global LOS out.

In digital mode, the I2C bus allows the user to set different LOS threshold a level for each individual channel. In addition, since the I2C bus allows for serial read/write, it also allows the user to read the different LOS out for each individual channel. As a result, the I2C controller sends the threshold value as a digital input to the DAC (DIGITAL TO ANALOG CONVERTER), adjusting the threshold level accordingly.

General Description

The disclosed method switches between parallel (global) mode and serial (digital) mode, and adjusts the LOS threshold against process/resistor variations. It accomplishes this by using a mux of digital switches to shift from one mode of operation to the other. The switches are controlled usi...