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LT1720/LT1721

APPLICATIONS INFORMATION

tPULSE (ns)

100

1000

10000

1720/21 F14

MEASURED

EQUATION 1

Figure 14. Log Pulse Stretcher Output Pulse vs Input Pulse

SPLITTER

CIRCUIT OF

FIGURE 12

n FOOT CABLE

1 FOOT CABLE

NANOSECOND

INPUT RANGE

MICROSECOND

OUTPUT RANGE

tOUT

(SEE TEXT)

1720/21 F15

Figure 15. RG-58 Cable with Velocity of Propogation = 66%;

Delay at Y = (n – 1) • 1.54ns

You don’t need expensive equipment to confirm the actual

overall performance of this circuit. All you need is a

respectable waveform generator (capable of >~100kHz), a

splitter, a variety of cable lengths and a 20MHz or 60MHz

oscilloscope. Split a single pulse source into different

cable lengths and then into the delay detector, feeding the

longer cable into the Y input (see Figure 15). A 6 foot cable

length difference will create a ~9.2ns delay (using 66%

propagation speed RG-58 cable), and should result in

easily measured 1.70

µs output pulses. A 12 foot cable

length difference will result in ~18.4ns delay and 2.07

µs

output pulses. The difference in the two output pulse

widths is the per-octave response of your circuit (see

equation (3)). Shorter cable length differences can be

used to get a plot of circuit performance down to 1.5ns (if

any), which can then later be used as a lookup reference

when you have moved from quantifying the circuit to using

the circuit. (Note there is a slight aberration in perfor-

mance below 10ns. See Figure 14.) As a final check, feed

the circuit with identical cable lengths and check that it is

not producing any output pulses.

10ns Triple Overlap Generator

The circuit of Figure 16 utilizes an LT1721 to generate

three overlapping outputs whose pulse edges are sepa-

rated by 10ns as shown. The time constant is set by the RC

network on the output of comparator A. Comparator B and

D trip at fixed percentages of the exponential voltage decay

across the capacitor. The 4.22k

Ω feed-forward to the C

comparator’s inverting input keeps the delay differences

the same in each direction despite the exponential nature

of the RC network’s voltage.

There is a 15ns delay to the first edge in both directions,

due to the 4.5ns delay of two LT1721 comparators, plus

6ns delay in the RC network. This starting delay is short-

ened somewhat if the pulse was shorter than 40ns be-

cause the RC network will not have fully settled; however,

the 10ns edge separations stay constant.

The values shown utilize only the lowest 75% of the supply

voltage span, which allows it to work down to 2.7V supply.

The delay differences grow a couple nanoseconds from 5V

to 2.7V supply due to the fixed VOL/VOH drops which grow

as a percentage at low supply voltage. To keep this effect

to a minimum, the 1k

Ω pull-up on comparator A provides

equal loading in either state.

Fast Waveform Sampler

Figure 17 uses a diode-bridge-type switch for clean, fast

waveform sampling. The diode bridge, because of its

inherent symmetry, provides lower AC errors than other

semiconductor-based switching technologies. This cir-

cuit features 20dB of gain, 10MHz full power bandwidth

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