Variable decision threshold apparatus

• US 20050062530A1
• Filed: 08/03/2004
• Published: 03/24/2005
• Est. Priority Date: 08/19/2003
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Agilent Technologies, Inc.
• Inventors: Wilkinson, Stuart, Bardsley, Richard, Ashmore, Sonia, Claybourn, Ian
• Granted Patent: US 7,091,793 B2
• Time in Patent Office: Days
• US Class Current: 330/136
• CPC Class Codes: H03K 5/082 with an adaptive threshold
• CPC Class Codes: H04B 10/695 Arrangements for optimizing the decision element in the receiver, e.g. by using automatic threshold control
• CPC Class Codes: H04L 25/062 Setting decision thresholds using feedforward techniques only

Abstract

A variable decision threshold apparatus includes a comparator for receiving the electrical signal and for comparing it to a threshold level. A signal strength monitor monitors the strength of the electrical signal and a scaler scales the decision threshold level to the measured amplitude of the electrical signal so that the percentage set as the threshold level is maintained.

Claims

1. A variable decision threshold apparatus comprising:

a comparator having at least one first input for receiving an input signal, a second input for receiving a decision threshold level control signal and at least one output for providing a signal based on a comparison of the input signal and a decision threshold level;

a signal strength monitor for determining the strength of the input signal and having an output for providing a signal depending on the strength of the input signal; and

a threshold level controller having a first input coupled to the output of the signal strength monitor and an output, coupled to the second input of the comparator, for providing the decision threshold level control signal depending on the strength of the input signal.

2. An apparatus as claimed in claim 1, wherein the threshold level controller is adjustable in response to a change in amplitude of the input signal so as to substantially maintain the correspondence of the decision threshold level to a predetermined percentage of the amplitude of the input signal.

3. An apparatus as claimed in claim 1, further comprising a microprocessor for receiving a signal indicating the predetermined percentage of the amplitude of the input signal that is required, the microprocessor having a plurality of first outputs for providing a plurality of discrete output levels to a digital to analogue converter having an output coupled to a second input of the threshold level controller.

4. An apparatus as claimed in claim 1, wherein the threshold level controller comprises a scaler having a first input coupled to the output of the signal strength monitor and an output coupled to an input of a decision threshold level control signal generator, wherein the scaler receives a signal indicative of the predetermined percentage of the amplitude of the input signal that is required, and controls the decision threshold level control signal generator so that an appropriate decision threshold level control signal is generated to the comparator.

5. An apparatus as claimed claim 1, further comprising a transimpedance amplifier having an input for receiving an input current and at least one output, coupled to the at least one input of the comparator, for generating the input signal from the input current.

6. An apparatus as claimed in claim 1, wherein the signal strength monitor has at least one input coupled to the at least one input of the comparator for directly monitoring the signal strength of the input signal to the comparator.

7. An apparatus as claimed in claims 1, wherein the signal strength monitor comprises a current mirror for generating a mirrored current corresponding to an input current, and a microprocessor coupled to the current mirror, wherein the microprocessor is programmable to determine the strength of the input signal from the mirrored current.

8. An apparatus as claimed in claim 7, wherein the scaler comprises an amplifier circuit having a first input and an output and a variable feedback resistor coupled between the first input and the output of the amplifier circuit, wherein the resistance of the variable feedback resistor is adjustable by the microprocessor in response to the determined strength of the input signal.

9. An apparatus as claimed in claim 8, wherein the variable feedback resistor is a digital potentiometer.

10. An apparatus as claimed in claim 4, wherein the decision threshold level control signal generator comprises a current injector for adjusting the decision threshold level of the comparator.

11. An apparatus as claimed in claim 4, wherein the comparator comprises a limiting amplifier.

12. An optoelectronic system comprising a photodiode for receiving an optical signal and for generating a photocurrent in response thereto, and an apparatus according to any preceding claim, wherein the input signal is generated from the photocurrent.

13. A method of controlling a variable decision threshold level comprising the steps of determining the strength of an input signal;

comparing the input signal to a decision threshold level corresponding to a predetermined percentage of the strength of the input signal; and

scaling the decision threshold level to the determined strength of the input signal.

14. A method of controlling a variable decision threshold level as claimed in claim 13 wherein the decision threshold level is adjusted in response to a change in amplitude of the input signal so as to substantially maintain the correspondence of the decision threshold level to the predetermined percentage of the amplitude of the input signal.

15. A method of controlling a variable decision threshold level as claimed in claim 14, further comprising the steps of generating a photocurrent;

and amplifying the photocurrent so as to generate the input signal.