Ringing in transient simulation.

Dear all,

I encountered an unexpected ringing while running transient simulation for a high-speed receiver.

I am using ADE Assembler and the version is ICADV12.3-64b.

The detail is as following,

There are CTLE, source follower, interleaving switch and SAR ADC in series in my test bench. The differential input of CTLE is proved by an ideal voltage source through 50Ohm ideal resistor.  And there are also 8 "vpulse" voltage source realizing 8 phase 8GHz clock driving interleaving switches. There is some s-parameter file to model the induction inside CTLE. And there are a few tens of ideal voltage sources providing dc control signals. I saw unexpected ringing on top of the CTLE input signal in the transient waveform.

In order to debug this issue, I remove all circuitry after the source follower. So there are only input voltage source, CTLE, source follower and 8 clock voltage sources (which are open), and some dc control voltage source in the test bench. The ringing remained there.

If I removed those 8 clock voltage sources in the schematic, or did not save their waveform , the ringing went away.

Below is the my transient simulation setup parameters
start = 0 s
outputstart = 0 s
stop = 10 ns
step = 10 ps
maxstep = 200 ps
ic = all
useprevic = no
skipdc = no
reltol = 1e-03
abstol(V) = 1 uV
abstol(I) = 1 pA
abstol(U) = 1 u
temp = 27 C
tnom = 27 C
tempeffects = all
errpreset = moderate
method = traponly
lteratio = 3.5
relref = sigglobal
cmin = 0 F
gmin = 1 pS
rabsshort = 1 mOhm

Do you have any idea about this issue?

Thanks and regards,

Yutao