PASSIVE PEAKING CIRCUIT COMPRISING A STEP-DOWN IMPEDANCE TRANSFORMER
A passive peaking circuit is formed in part from a passive step-down impedance transformer that interconnects the light source driver to the light source. The step-down impedance transformer has impedance that decreases in a continuous or discrete manner in the direction from the light source driver circuit to the light source. The passive peaking circuit peaks the electrical drive signal being delivered from the light source driver circuit to the light source, thereby widening the eye opening.
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The invention relates to a passive peaking circuit for performing laser peaking in an optical transmitter or transceiver.
BACKGROUND OF THE INVENTIONIn optical transmitters and transceivers, a laser diode driver circuit generates electrical drive signals that are used to drive a laser diode. The laser diode driver circuit and the laser diode are typically formed in separate integrated circuits (ICs). In particular, the laser diode driver circuit is typically formed in a transmitter (Tx) IC and the laser diode is typically formed in a laser diode IC. The ICs are typically mounted on a printed circuit board (PCB) or a flex circuit and electrically interconnected by electrically-conductive traces of the PCB or flex circuit.
It is known in optical Tx and transceiver technology to include active peaking circuitry in the Tx IC for performing peaking of the electrical drive signals generated by the laser diode driver circuitry. Active peaking circuitry in the Tx IC typically comprises a pre-distortion or pre-equalization circuit, such as a feed forward equalizer (FFE), for example, that shapes the electrical drive signal before it is input to the laser diode. Such pre-distortion or pre-equalization circuits emphasize or de-emphasize the amplitude of the electrical drive signal. The pre-distortion or pre-equalization process is often referred to as “laser peaking.” In general, laser peaking involves using a pulse to add or subtract current from the rising and/or falling edge of the electrical drive signal. The goal is to balance the rise/fall times of the electrical drive signal to improve the eye opening of the optical signal produced by the laser diode.
While active peaking circuitry currently used in many Tx ICs generally works well at achieving this goal, it increases the cost of the Tx IC due to the additional die area that the circuitry consumes and the increased complexity of the IC design. In addition, the active components (e.g., amplifiers) of active peaking circuitry consume a relatively large amount of power.
A need exists for a way to achieve peaking that is less costly in terms of die area, IC design complexity and power consumption.
SUMMARY OF THE INVENTIONThe invention provides a passive peaking circuit comprising a light source circuit, a light source driver circuit, and a step-down impedance transformer electrically interconnecting the light source driver circuit and the light source circuit. The light source circuit comprises at least a first light source. The light source driver circuit comprises at least a first light source driver that produces a first electrical drive signal for driving the first light source. The step-down impedance transformer passively peaks the first electrical drive signal.
In accordance with an illustrative embodiment, the passive peaking circuit comprises a laser diode circuit comprising at least a first laser diode, a laser diode driver circuit comprising at least a first laser diode driver that produces a first electrical drive signal for driving the first laser diode, and a step-down impedance transformer electrically interconnecting the laser diode driver circuit and the laser diode circuit. The step-down impedance transformer comprises at least a first electrically-conductive trace having a first end electrically connected to the laser diode driver circuit and a second end electrically connected to the laser diode circuit. At least a first portion of the first trace that includes the second end of the first trace has a width that increases in a direction from the first end of the trace to the second end of the trace. The increase in width result in a decrease in impedance along the first portion of the first trace in the direction from the laser diode driver circuit to the laser diode circuit. The step-down impedance transformer passively peaks the first electrical drive signal.
In accordance with another illustrative embodiment, the passive peaking circuit comprises a light-emitting diode (LED) circuit comprising at least a first LED, an LED driver circuit comprising at least a first LED driver that produces a first electrical drive signal for driving the first LED, and a step-down impedance transformer electrically interconnecting the LED driver circuit and the LED circuit. The step-down impedance transformer comprises at least a first electrically-conductive trace having a first end electrically connected to the LED driver circuit and a second end electrically connected to the LED circuit. At least a first portion of the first trace that includes the second end of the first trace has a width that increases in a direction from the first end of the trace to the second end of the trace. The increase in width results in a decrease in impedance along the first portion of the first trace in the direction from the LED driver circuit to the LED circuit. The step-down impedance transformer passively peaks the first electrical drive signal.
These and other features and advantages of the invention will become apparent from the following description, drawings and claims.
In accordance with embodiments described herein, a passive peaking circuit is formed in part from an electrically-conductive trace that interconnects the laser diode driver IC to the laser diode IC. The trace comprises a group of interconnected trace segments that are configured as a step-down impedance transformer having impedance that steps down in a continuous or discrete manner from the laser diode driver IC to the laser diode IC. Illustrative, or exemplary, embodiments will now be described with reference to
A second end of the trace segment 2a is connected to a first end of the trace segment 2b. A second end of the trace segment 2b is connected to a first end of the trace segment 2c. A second end of the trace segment 2c is connected to a first end of the trace segment 2d.
The trace segments 2b-2d form the step-down impedance transformer. Segments 2b-2d are assumed for exemplary, or illustrative, purposes to be of equal length and to be made of the same material, e.g., copper. Consequently, each segment 2b-2d delays the electrical drive signal generated by the laser diode driver circuitry 7 by an equal amount of time, e.g., 17 picoseconds (ps). Segment 2a is assumed, for illustrative purposes, to have the impedance of 25 ohm and to provide a delay of 25 ps. Segment 2b is assumed, for illustrative purposes, to have the impedance of 21.87 ohm. Segment 2c is assumed, for illustrative purposes, to have the impedance of 18.64 ohm. Segment 2d is assumed, for illustrative purposes, to have the impedance of 16.29 ohm. Thus, the impedance of the trace formed by trace segments 2b-2d steps down, or decreases, in the direction from the Tx IC 5 toward the laser diode IC 6. The source and load impedances, ZSOURCE and ZLOAD, respectively, are 25 ohm and 2.5 ohm, respectively.
The group of trace segments 2b-2d interconnected as shown in
The simulation results shown in
The process of selecting the impedance values Z2-Z4 should take into consideration the frequency range of the electrical drive signal and the amount of insertion loss that is deemed acceptable. Given these parameters and the source and load impedance values, impedance values for the trace segments of the step-down impedance transformer are selected using the above-defined constraints. Through routine experimentation or simulation, a determination can be made as to which impedance values provide a generally constant insertion loss that is acceptable over a range of frequencies of interest.
As indicated above, the trace that interconnects the laser diode driver circuitry of the Tx IC with the laser diode IC has impedance that decreases along the trace in the direction from the laser diode driver circuitry to the laser diode. The impedance of the trace having the decreasing impedance can be made in a variety of ways. In accordance with illustrative embodiments, the decrease in impedance is provided by increasing the width of the trace in the direction from the laser diode driver circuit to the laser diode. A few of such examples will now be described with reference to
The discrete increases in the widths of the trace segments described above with reference to
The passive peaking circuit 1 shown in
It should also be noted that the electrical connections between the laser diode driver circuitry and the laser diode may be single ended connections or differential connections. If the connections are differential, then each of the two traces will have the step-down impedance transformer configuration shown in
It should be noted that the invention has been described with respect to illustrative embodiments for the purpose of describing the principles and concepts of the invention. The invention is not limited to these embodiments. For example, while the invention has been described with reference to varying the width of a trace in order to vary its impedance, variations in impedance can be accomplished in other ways, such as by varying the material comprising the trace along the length of the trace in order to vary the impedance of the trace along its length. Also, it is possible to vary respective lengths of individual segments instead of varying their widths. Also, it is possible to insert other passive components (e.g., resistors) in line with the trace to vary its impedance, although care should be taken so as to not increase insertion loss to unacceptable levels. Given the goals of the invention described herein, persons of skill in the art will be able to provide other designs that achieve the same or similar goals. As will be understood by those skilled in the art in view of the description being provided herein, these and many other modifications may be made to the illustrative embodiments described above to achieve the goals of the invention, and all such modifications are within the scope of the invention.
Claims
1. A passive peaking circuit comprising:
- a light source circuit comprising at least a first light source disposed on one of a printed circuit board or a flex circuit;
- a light source driver circuit comprising at least a first light source driver that produces a first electrical drive signal for driving the first light source disposed on a remaining one of the printed circuit board or the flex circuit; and
- a step-down impedance transformer having a first portion and a second portion and electrically interconnecting the light source driver circuit and the light source circuit, the first portion disposed with the light source driver circuit on the remaining one of the printed circuit board or the flex circuit, the step-down impedance transformer passively peaking the first electrical drive signal.
2. The passive peaking circuit of claim 1, wherein the step-down impedance transformer comprises at least a first electrically-conductive trace having a first end electrically connected to the light source driver circuit and a second end electrically connected to the light source circuit, wherein at least a first portion of the first trace that includes the second end of the first trace has a width that increases in a direction from the first end of the trace to the second end of the trace, and wherein the increase in width results in a decrease in impedance along said first portion of the first trace in the direction from the light source driver circuit to the light source circuit.
3. The passive peaking circuit of claim 2, wherein the first electrically-conductive trace comprises N electrically-conductive trace segments, where N is an integer having a value that is equal to or greater than two, wherein the trace segments are connected end to end to form the first electrically-conductive trace, and wherein the trace segments comprising said first portion of the first trace increase in width in the direction from the light source driver circuit to the light source circuit.
4. The passive peaking circuit of claim 3, wherein each trace segment has a constant width, and wherein the widths of the trace segments comprising said first portion of the first trace increase in a discrete manner in the direction from the light source driver circuit to the light source circuit such that the width of said first portion of the first trace increases in discrete steps in the direction from the light source driver circuit to the light source circuit.
5. The passive peaking circuit of claim 4, wherein the increases in width are equal.
6. The passive peaking circuit of claim 4, wherein at least some of the increases in width are unequal.
7. The passive peaking circuit of claim 3, wherein each trace segment of said portion of the first trace has a variable width.
8. The passive peaking circuit of claim 7, wherein the width of each trace segment of said first portion of the first trace increases in a linear manner such that the width of said first portion of the first trace increases in a nonlinear manner in the direction from the light source driver circuit to the light source circuit.
9. The passive peaking circuit of claim 8, wherein the width of each trace segment of said first portion of the first trace increases in a nonlinear manner such that the width of said first portion of the first trace increases in a nonlinear manner in the direction from the light source driver circuit to the light source circuit.
10. The passive peaking circuit of claim 2, wherein said first portion of the first trace is disposed on the printed circuit board.
11. The passive peaking circuit of claim 2, wherein said first portion of the first trace is disposed on the flex circuit.
12. The passive peaking circuit of claim 11, wherein the second portion of the first trace is disposed on the printed circuit board.
13. The passive peaking circuit of claim 12, wherein the light source driver circuit is disposed on the printed circuit board and the light source circuit is disposed on the flex circuit, and wherein said second portion of the first trace is electrically connected to the light source driver circuit.
14. The passive peaking circuit of claim 1, wherein the step-down impedance transformer passively peaks the first electrical drive signal by increasing a maximum magnitude of the electrical drive signal along a rising edge of the electrical drive signal.
15. The passive peaking circuit of claim 14, wherein the step-down impedance transformer passively peaks the first electrical drive signal by decreasing a minimum magnitude of the electrical drive signal along a falling edge of the electrical drive signal.
16. A passive peaking circuit comprising:
- a laser diode circuit comprising at least a first laser diode on one of a printed circuit board or a flex circuit;
- a laser diode driver circuit comprising at least a first laser diode driver that produces a first electrical drive signal for driving the first laser diode on a remaining one of the printed circuit board or the flex circuit; and
- a step-down impedance transformer having a first portion and a second portion and directly electrically interconnecting the laser diode driver circuit and the laser diode circuit, the first portion proximal to the laser diode driver circuit on the remaining one of the printed circuit board or the flex circuit, the second portion adjacent to the laser diode circuit, the step-down impedance transformer passively peaking the first electrical drive signal, wherein the step-down impedance transformer comprises at least a first electrically-conductive trace having a first end electrically connected to the laser diode driver circuit and a second end electrically connected to the laser diode circuit, wherein at least the first portion of the first trace that includes the second end of the first trace has a width that increases in a direction from the first end of the trace to the second end of the trace, and wherein the increase in width results in a decrease in impedance along said first portion of the first trace in the direction from the laser diode driver circuit to the laser diode circuit.
17. The passive peaking circuit of claim 16, wherein the first electrically-conductive trace comprises N electrically-conductive trace segments, where N is an integer having a value that is equal to or greater than two, wherein the trace segments are connected end to end to form the first electrically-conductive trace, and wherein the trace segments comprising said first portion of the first trace increase in width in the direction from the laser diode driver circuit to the laser diode circuit.
18. The passive peaking circuit of claim 17, wherein each trace segment has a constant width, and wherein the widths of the trace segments comprising said first portion of the first trace increase in a discrete manner in the direction from the laser diode driver circuit to the laser diode circuit such that the width of said first portion of the first trace increases in discrete steps in the direction from the laser diode driver circuit to the laser diode circuit.
19. The passive peaking circuit of claim 17, wherein each trace segment of said portion of the first trace has a variable width, wherein the width of each trace segment of said first portion of the first trace increases in a linear manner such that the width of said first portion of the first trace increases in a nonlinear manner in the direction from the laser diode driver circuit to the laser diode circuit.
20. The passive peaking circuit of claim 17, wherein each trace segment of said portion of the first trace has a variable width, wherein the width of each trace segment of said first portion of the first trace increases in a nonlinear manner such that the width of said first portion of the first trace increases in a nonlinear manner in the direction from the laser diode driver circuit to the laser diode circuit.
21. A passive peaking circuit comprising:
- a light-emitting diode circuit on one of a printed circuit board or a flex circuit;
- a driver circuit on a remaining one of the printed circuit board or the flex circuit that produces a first electrical drive signal for driving the light-emitting diode circuit; and
- a step-down impedance transformer having a first portion and a second portion and directly electrically interconnecting the driver circuit and the light-emitting diode circuit, the first portion proximal to the driver circuit on the remaining one of the printed circuit board or the flex circuit, the step-down impedance transformer passively peaking the first electrical drive signal, wherein the step-down impedance transformer comprises at least a first electrically-conductive trace having a first end electrically connected to the driver circuit and a second end electrically connected to the light-emitting diode circuit, wherein at least a first portion of the first trace that includes the second end of the first trace has a width that increases in a direction from the first end of the trace to the second end of the trace, and wherein the increase in width results in a decrease in impedance along said first portion of the first trace in the direction from the driver circuit to the light-emitting diode circuit.
22. The passive peaking circuit of claim 21, wherein the first electrically-conductive trace comprises N electrically-conductive trace segments, where N is an integer having a value that is equal to or greater than two, wherein the trace segments are connected end to end to form the first electrically-conductive trace, and wherein the trace segments comprising said first portion of the first trace increase in width in the direction from the driver circuit to the light-emitting diode circuit.
23. The passive peaking circuit of claim 22, wherein each trace segment has a constant width, and wherein the widths of the trace segments comprising said first portion of the first trace increase in a discrete manner in the direction from the driver circuit to the light-emitting diode circuit such that the width of said first portion of the first trace increases in discrete steps in the direction from the driver circuit to the light-emitting diode circuit.
24. The passive peaking circuit of claim 22, wherein each trace segment of said portion of the first trace has a variable width, wherein the width of each trace segment of said first portion of the first trace increases in a linear manner such that the width of said first portion of the first trace increases in a nonlinear manner in the direction from the driver circuit to the light-emitting diode circuit.
25. The passive peaking circuit of claim 22, wherein each trace segment of said portion of the first trace has a variable width, wherein the width of each trace segment of said first portion of the first trace increases in a nonlinear manner such that the width of said first portion of the first trace increases in a nonlinear manner in the direction from the driver circuit to the light-emitting diode circuit.
Type: Application
Filed: Dec 2, 2013
Publication Date: Jun 4, 2015
Applicant: Avago Technologies General IP (Singapore) Pte. Ltd (Singapore)
Inventor: Vadim Heyfitch (Los Gatos, CA)
Application Number: 14/094,559