check for proper use of ac resistance

* (exec-spice "ngspice %s" t)

Vin 1 0  dc 5.0  ac 3.0

R1 1 2  1k ac=4k
C2 2 0  1n
R2 2 0  4k


.control

let success_count = 0

op

let H0 = 1 / (1 + 1k / 4k)
let gold = 5.0 * H0
let err = v(2)/gold - 1

echo "Note: err[op] =" $&err
if abs(err) > 1e-12
  echo "ERROR: op test failed"
else
  echo "INFO: [op], success"
  let success_count = success_count + 1
end

ac lin 3 100kHz 200kHz

let H0 = 1 / (1 + 4k / 4k)
let tau = 1 / (1/4k + 1/4k) * 1n
let s = 2 * pi * i * frequency
let gold = 3.0 * H0 / (1 + s * tau)

let err = abs(v(2)/gold - 1)

echo "Note: err[ac] =" $&err
if vecmax(err) > 1e-12
  echo "ERROR: ac test failed"
else
  echo "INFO: [ac], success"
  let success_count = success_count + 1
end

pz 1 0  2 0  vol pz

let tau = 1 / (1/4k + 1/4k) * 1n
let gold = -1/tau

let err = abs(pole(1)/gold - 1)

echo "Note: err[pz] =" $&err
if err > 1e-12
  echo "ERROR: ac test failed"
else
  echo "INFO: [pz], success"
  let success_count = success_count + 1
end

if success_count ne 3
  quit 1
else
  quit 0
end

.endc