Design Ideas: January 20, 1994
The circuit in Fig 1 stretches the output pulses of an ECL comparator by operating the ECL line receivers with unusually large pulldown resistors to the -15V supply. The large load resistance maximizes the gains of the emitter-follower output stages and speeds up the rising edges. At the same time, the low pulldown current, together with the input and wiring capacitances, slows the falling edges. These actions stretch the positive pulses by approximately 2 nsec/stage.
ECL comparators usually have low gains, typically about 500. The receivers of IC1 increase this gain by a minimum of 60 to 100 times. IC2 is an unusual single shot, which stretches only the short pulses but leaves the long pulses unchanged. The circuit wire ORs the input of IC2A to its output, so when the input goes high, it instantly pulls up the output. After one gate delay, the output of IC2A becomes active and pulls the input up, latching it into high position. Now the output of IC2B turns off, and its output drops with the slope determined by C1 and the pulldown current. After crossing the switching threshold, IC2B turns off IC2A, which resets the latch. If the input pulse is longer than this delay, the latch has no effect.