Keithley 2520, User Manual, Page: 298 / 329

F-14 Measurement Considerations Model 2520 User’s Manual
Transmission line model
Figure F-12 shows the transmission line scheme that provides the least impedance mis-
match. The laser diode appears as a low-impedance path to a current pulse. By placing the
laser diode in series with the source and termination of the current pulse, and by maintain-
ing uniform impedance in the current path, the magnitude of reflections is minimized. This
method, however, is not practical. The electrical shielding must be compromised to pro-
vide access to the laser diode as shown in Figure F-13.
The compromised shield provides access to the laser diode and upsets the continuity of the
impedance of the transmission line. To minimize the effects of the discontinuity, the shield
of the two compromised cable sections must be connected and the length of the inner con-
ductor must be minimized.
Figure F-12
Minimal impedance mismatch
Figure F-13
Compromised shield provides laser diode access
Current
Output HI
Current
Output LO
Current
Output HI
Current
Output LO
Integrating
Sphere