[SI-LIST] Re: local and global ground
- From: Ray Anderson <Raymond.Anderson@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx, gedlund@xxxxxxxxxx
- Date: Mon, 4 Feb 2002 11:34:28 -0800 (PST)
Greg-
I haven't run your spice deck, but by just visually
inspecting your connections I notice that
you have 2 different grounds (gnd, gnd3) defined
in your model while in the real world those
ground locations are actually at 4 physical locations
(input to the first tline, input to the connector,
output from the connector, output of the second tline).
It seems to me that you are forcing some rather strange
conditions by forcing multiple locations to 'gnd' while
referencing the output of the connector to 'gnd3'.
Your 'gnd' grounds are in essence spice node 0. Noise measurements
are made relative to a "local" ground very near to the point of noise
measurement. Measuring things relative to a global ground can
lead to interesting but not real measurements.
I think that you probe statements by default, measure values
relative to node 0 or global ground. You need to arrange things
to measure the noise voltage differentially relative to a local
ground.
-Ray Anderson
Sun Microsystems
>* connector di/dt simulation
>* two pins from a straight-through header
>* pins are 0.5 x 0.5 mm each at 2.0 mm pitch
>* pin length = 20 mm
>
>
>* vsrc is a ramp function with rounded corners
>
>vsrc src gnd pwl
>+ 0.00000000e-09 0.00000000e+00,
>+ 1.00000000e-09 0.00000000e+00,
>+ 1.00728536e-09 1.04749925e-03,
>+ 1.01230955e-09 2.98950612e-03,
>+ 1.01666498e-09 5.47700003e-03,
>+ 1.02537584e-09 1.26837911e-02,
>+ 1.04279745e-09 3.59374061e-02,
>+ 1.07717645e-09 1.15284845e-01,
>+ 1.11771131e-09 2.61263251e-01,
>+ 1.15862179e-09 4.56881702e-01,
>+ 1.20053422e-09 6.94177032e-01,
>+ 1.24460769e-09 9.66125727e-01,
>+ 1.29132187e-09 1.25672615e+00,
>+ 1.32953060e-09 1.47916722e+00,
>+ 1.34953058e-09 1.58539677e+00,
>+ 1.37969661e-09 1.72766232e+00,
>+ 1.44002855e-09 1.92984247e+00,
>+ 1.48411775e-09 1.99502504e+00,
>+ 1.50000000e-09 2.00000000e+00,
>+ 2.00000000e-08 2.00000000e+00
>
>
>rsrc src sig1 50
>
>tline1 sig1 gnd sig2 gnd td=2ns z0=50
>
>xconn sig2 gnd sig3 gnd3 conn_2pin
>
>tline2 sig3 gnd3 sig4 gnd td=2ns z0=50
>
>rterm sig4 gnd 50
>
>
>
>*****************
>* SUBCIRCUITS *
>*****************
>
>.subckt conn_2pin pin1_in pin2_in pin1_out pin2_out
>
>c12a pin1_in pin2_in 0.1pF
>
>l1 pin1_in pin1_x 15.7nH
>r1 pin1_x pin1_out 0.01
>
>l2 pin2_in pin2_x 15.7nH
>r2 pin2_x pin2_out 0.01
>
>c12b pin1_out pin2_out 0.1pF
>
>k12 l1 l2 0.53
>
>.ends
>
>
>******************
>* RUN CONTROLS *
>******************
>
>.option post probe csdf
>
>.tran 20ps 10ns
>
>.probe v(src)
>.probe v(sig1) v(sig2) v(sig3) v(sig4)
>.probe v(gnd3)
>
>.end
>
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