* One parameterized RC low-pass, three corner frequencies from one .subckt.
* Instance overrides the c param by name; Xdef takes the .subckt default.
V1 in 0 AC 1
Xdef in def rclp                 $ default c=1n  -> ~159 kHz
Xmid in mid rclp c=10n           $ override      -> ~15.9 kHz
Xlo  in lo  rclp c=100n          $ override      -> ~1.59 kHz
.subckt rclp a b r=1k c=1n
  R1 a b {r}
  C1 b 0 {c}
.ends
.ac dec 100 100 1meg
.control
  run
  meas ac fcdef WHEN vdb(def)=-3.0103
  meas ac fcmid WHEN vdb(mid)=-3.0103
  meas ac fclo  WHEN vdb(lo)=-3.0103
.endc
.end