Use a switching-regulator controller to generate fast pulses

A source of pulses with fast-rising edges that approximate the step function can help you perform many useful laboratory measurements, including characterization of coaxial cables’ rise times and location of cable faults using time-domainref lectometry methods. For example, evaluating the rise time of a 10- to 20-ft-long RG-58/U cable requires edge-transition times of 1 to 2nsec. Agilent’s (www.agilent.com) HP8012B, a workhorse pulse generator that finds use in many electronics labs, can deliver pulses with rise times of 5nsec that are adequate for many applications but not for cable characterization.

As an alternative, switching regulator-controller ICs can deliver gate-drive pulses with rise and fall times of less than 2nsec, making them ideal candidates for laboratory pulse-generation service. A simple implementation uses Linear Technology’s (www.linear.com) LTC3803 constant frequency flyback controller, IC1 (Figure 1). The controller selfclocks at 200kHz, and applying a sample of its output to its Sense pin causes the controller to operate at its minimum duty cycle and produce a 300nsec-wide output pulse.



The LTC3803’s output can deliver more than 180mA into a 50Ω load, so use a low-series inductance bypass capacitor that connects as directly as possible between IC1’s power and ground (pins 5 and 2). The decoupling components, C1, a 10μF ceramic capacitor, and R1, a 200Ω resistor, minimize pulse-top aberrations without introducing amplitude droop. The circuit’s output directly drives a 50Ω termination at amplitudes as high as 9V. For applications that require maximum pulse fidelity, use a back termination resistor, RBACKTERM, to suppress triple-transit echos and absorb reflections from the cable and any mismatch in the cable’s far end termination impedance. Back termination also helps when driving passive filters, which expect to see a specific generator impedance. The LTC3803’s output impedance is approximately 1.5Ω, which affects the value of the back termination resistor. The back termination technique works well with load impedances of at least 2kΩ. At impedances higher than that value, parasitic impedances associated with the terminating resistor and IC1 degrade bandwidth and pulse fidelity.

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In a back-terminated, 50Ω system, the circuit delivers a 4.5V output pulse with symmetric rise and fall times of 1.5nsec, pulse-to amplitude aberrations of less than 10%, and amplitude droop of less than 5%. Directly driving a 50Ω load doesn’t degrade the output’s rise and fall times. For best pulse fidelity, use stripline techniques to route IC1’s output directly to the termination resistor and output connector J1. Using a 100mil-wide trace on a 1/16-in., double-sided, glass-epoxy pc board approximates a 50Ω surge impedance.