xvia

A PCB Via Model

LC Model


VRML Model

Plot Files

v1.xy Input voltage
v2.xy Transmitted voltage
i1.xy Input current
i2.xy Transmitted current
pulse_time.xy Time domain pulses
pulse_freq.xy Frequency domain pulses
sp.xy S-parameters of the via

Anatomy Of A Circuit Board

The circuit board to be studied has four layers.
  1. Top signal layer
  2. Ground layer
  3. Power layer
  4. Bottom signal layer
Layers 1 and 2 form a pair, as do 3 and 4. The layers of a pair are separated by a laminate dielectric material, and the assembled pairs are separated by a prepreg dielectric.

Only a small portion of the circuit board containing a single via will be analyzed, along with its associated pads and signal traces.

Through The Layers

Thin metallic traces are used to conduct signals horizontally within a circuit board within a signal layer. When a multi-layer circuit board is created, vertical interconnects must also be added. These interconnects are vias, and are usually round (because they are drilled). Pads are placed at the ends of the traces to provide a large spot to connect the via.

When a ground or power (solid metal) layer is created, a special techinque is used to propagate signals through the layer, rather than shorted to the layer. Relief (or isolation) pads are etched out of the metal for this purpose. The space left after removing the metal is eventually filled in with a prepreg material (sort of a circuit board glue).

Running The Simulation

Bring up the Run Simulation dialog.

Press Start, which will display a prompt dialog. Enter 3000 for the number of time steps, and press Ok.

Plotting The Probe Data

The raw data saved by the probes can be plotted with the Plot Probes dialog of the Analysis menu.

Time-domain plot of the voltage measured on the input trace. The incident pulse is the first (70 volt!) peak, and the reflection returns from the via around 25 picoseconds.

Time-domain plot of the voltage measured on the output trace. This is the transmitted pulse, reduced from 70 to 50 volts after negotiating the via.

Defining The Pulses

The raw (time-domain) probe data needs to be windowed in time before it can be converted into frequency domain data. This windowing identifies the pulses recorded by the probes. To calculate two-port S-parameters, three pulses need to be defined: incident, reflected, and transmitted. Each pulse can group four types of probes: voltage, current, charge,and magnetic flux. In this case, we only have voltage and current probes.

A start and end time step must be set for each pulse to define the time window. Note that time steps (integers) are used, rather than time in seconds. This is a hold-over from the good-old days, and may change in the future.

The pulses extracted from the probe data can be plotted with the Plot Pulses dialog of the Analysis menu.

Nearly all of the low frequency content of the incident pulse goes through the via, but much of the high frequency content is reflected.

Calculating The S-Parameters

Using the data from the three pulses, the S-parameters of the via can be calculated with the S-Parameters dialog of the Analysis menu.

Conveniently, the dialog assumes that the first pulse defined is the incident, the second is the reflected, and the third is the transmitted. Since we defined the pulses in that order earlier, then the defaults can be used. Also, since both the voltage and current data was saved by the probes, the power (V*I) can be used to calculate the S-parameters. If only the voltage or the current was available, V**2 or I**2 could be used.


See also the complete PCB via frequency domain analysis on the LC web site.