Under lTspice 17.0.36, I found an effective lowest limit in defining Tr and Tf, of >10ps.

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Very simple model, only one V source:

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;PULSE(0 1 4m {Trf} {Trf} 1m 2m)

.step param Trf list 1f 10f 11f 20f 50f 100f 1p

.meas T0 WHEN V(A)=0.01 rise=1

.meas T1 WHEN V(A)=0.99 rise=1

.meas Td param T1-T0

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Log:

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.step trf=1e-015

.step trf=1e-014

.step trf=1.1e-014

.step trf=2e-014

.step trf=5e-014

.step trf=1e-013

.step trf=1e-012

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Measurement: t0

? step???????????? v(a)=0.01

???? 1???????????? 0.00400001

???? 2???????????? 0.00400001

???? 3???????????? 0.004

???? 4???????????? 0.004

???? 5???????????? 0.004

???? 6???????????? 0.004

???? 7???????????? 0.004

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Measurement: t1

? step???????????? v(a)=0.99

???? 1???????????? 0.00400056

???? 2???????????? 0.00400056

???? 3???????????? 0.004

???? 4???????????? 0.004

???? 5???????????? 0.004

???? 6???????????? 0.004

???? 7???????????? 0.004

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Measurement: td

? step???????????? t1-t0

???? 1???????????? 5.537e-007

???? 2???????????? 5.537e-007

???? 3???????????? 1.07796e-014

???? 4???????????? 1.95989e-014

???? 5???????????? 4.90007e-014

???? 6???????????? 9.79997e-014

???? 7???????????? 9.8e-013

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Anything 10ps or shorter results in 554ns, but 11ps works within 3%

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Dave

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