Mitchell Lee
EDN
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-domain-reflectometry 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 2 nsec. Agilent's HP8012B, a workhorse pulse generator that finds use in many electronics labs, can deliver pulses with rise times of 5 nsec 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 2 nsec, making them ideal candidates for laboratory pulse-generation service. A simple implementation uses Linear Technology's LTC3803 constant-frequency flyback controller, IC1 (Figure 1). The controller self-clocks at 200 kHz, and applying a sample of its output to its SENSE pin causes the controller to operate at its minimum duty cycle and produce a 300-nsec-wide output pulse.
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| Figure 1. | Switching-regulator-controller IC1 delivers pulses with 1.5-nsec rise and fall times into a 50 Ω load. |
The LTC3803's output can deliver more than 180 mA into a 50 Ω 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 10-µF ceramic capacitor, and R1, a 200 Ω resistor, minimize pulse-top aberrations without introducing amplitude droop. The circuit's output directly drives a 50 Ω termination at amplitudes as high as 9 V. 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 1.5 Ω, which affects the value of the back-termination resistor. The back-termination technique works well with load impedances of at least 2 kΩ. At impedances higher than that value, parasitic impedances associated with the terminating resistor and IC1 degrade bandwidth and pulse fidelity.
In a back-terminated, 50 Ω system, the circuit delivers a 4.5 V output pulse with symmetric rise and fall times of 1.5 nsec, pulse-top-amplitude aberrations of less than 10%, and amplitude droop of less than 5%. Directly driving a 50 Ω 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 100-mil-wide trace on a 1/16-in., double-sided, glass-epoxy pc board approximates a 50 Ω surge impedance.
