S-Parameter Viewer

A Touchstone file at a glance - what it states, its reflections on a Smith chart, its transmissions on a polar chart, and every term in dB and phase over the file's own frequency range.

Opening the viewer

Tools › S-Parameter Viewer opens an empty viewer, with or without a workspace, and its Open Touchstone file… button reads a .s1p, .s2p, … .sNp. It is a window of its own, like the Match Designer and the Signal Designer, and it opens centred on the screen that holds the window you opened it from. Several may be open; asking for a file that is already on show brings that window forward.

You can also drag a Touchstone file from your file manager onto the viewer. It opens the file in that window, just as Open Touchstone file… does.

A Touchstone file in the Workspace opens the same way. Open in S-Parameter Viewer is on its right-click menu, and double-clicking the file does it too — for a file you imported and for a loose .sNp sitting in the workspace folder.

A placed SnP component — the library's S1P, S2P, S3P, S4P and SnP are all this one component — has three buttons on its File row. Browse… chooses the file, the folder button shows it in your file manager, and the eye button opens the viewer on it. The eye is available once the file is chosen and is there.

In a viewer opened from the eye button, a Touchstone file dropped on the window also becomes the component's File, and its port count is set from the file. A file that cannot be read changes nothing. The change is an ordinary schematic edit: Edit › Undo in the schematic gives the component its previous file back, even after the viewer is closed.

A Data Display's dataset list offers the same View on any row whose file is a Touchstone file.

File information

The left pane reads the file back to you, which is most of the point of opening it. The path under the file name opens that file in the system file manager.

The plots follow the file. Nothing is resampled and nothing is interpolated: each curve joins the file's own points.

Choosing the plots

Four buttons sit above the plots: the Smith chart, the polar chart, magnitude and phase. The window opens with the Smith chart and magnitude on, and polar and phase off. Each button turns its plot on or off, and with none of them on the pane is empty.

The polar chart draws the transmissions only — S21, S12 and the other off-diagonal terms — never a reflection.

The names of the traces sit beside each plot. Nothing is written above the curves.

The four plots

Reflection is a Smith chart of the reflection at each port — S11, S22 and so on.

Transmission is a polar chart of the off-diagonal terms only — S21, S12 and the rest.

Magnitude is every ticked term in dB against frequency, and Phase is the same terms in degrees. The frequency axis runs from exactly the file's lowest frequency to exactly its highest, rather than out to the nearest round number, and a logarithmically swept file is drawn on a logarithmic axis. The phase axis is fixed at ±180°, which is the whole of what a wrapped phase can be.

The axis unit follows the span. A file written in Hz that runs from 1 to 5 GHz reads in GHz, because an axis of 1E+09 to 5E+09 says the same thing badly.

Hover the magnitude or phase plot and a vertical line crosses every trace at that frequency, with each value in the box beside the pointer. On the Smith chart and the polar chart the marker sits on the nearest sample. A Smith reflection reads S(1,1) @ 2.4 GHz, then Γ as a real and imaginary pair and as a magnitude and angle — Γ=0.5+j0.1 (0.51 ∠ 11.31°) — then the impedance at the file's reference as Z=Z0*(1+j0) Ω. A double-click still drops a marker you can drag and keep, and Delete removes the selected ones. Arrow keys step a kept marker one sample at a time.

A file of Y- or Z-parameters is drawn as S-parameters at the file's own reference, because a Smith chart of an admittance would not be a reflection. The file pane still says Y or Z, which is what the file is.

Choosing which terms to draw

Every term is a tick box, and a reflection's name is the heavier one. The four buttons above them:

Up to four ports, Default is every term. Above that it is the reflections plus each forward neighbour, S21, S32, S43 and so on, over the first eight ports — an .s16p has 256 terms and a file can have hundreds of ports, so drawing all of them is a choice rather than a greeting. Past six terms the list beside a plot is left off, because it would take the width the curves need; each tick box carries its term's colour.

A file with many ports lists its terms one matrix row at a time, in a list that scrolls on its own.

Findings

Three things about a file are defects, and each is stated as a sentence naming where:

• the file's name claims a port count the data does not hold, so its matrices may have been assembled the wrong shape;
• the frequencies are not strictly increasing, and the plots draw them in file order anyway so the defect is visible;
• the reference impedance has a resistance that is not positive, so the data cannot be renormalised.

The plots are drawn either way. A file that cannot be read at all shows the reader's own reason and no plots.

Passivity, reciprocity and causality are not reported here. An amplifier has gain and a circulator is not reciprocal, and neither is a defect in a file; circuitrf check measures all three and leaves the judgement to you.

Saving it as a Data Display

The viewer keeps nothing of its own. Save as Data Display in the header writes the plots as they stand to a .cdd and opens it, so markers, axes and terms can be edited afterwards in the Data Display. The file itself is referenced, not copied. The Smith and polar charts sit across the top and magnitude and phase across the bottom. Each round chart names its terms beside the chart, or the first few and how many there are when it has more than six, with one frequency row under it.

Reload reads the file again, which is what you want after a measurement or a run has rewritten it. The terms you had ticked stay ticked.

Running it headless

circuitrf snp prints this pane's lines, and -o draws the four plots or writes the same Data Display the button does.

$ circuitrf snp amp.s2p
$ circuitrf snp coupler.s4p --params reflections,S21 -o coupler.svg

What it will not do

It does not check passivity, reciprocity or causality.

It reads the Touchstone files the rest of circuitRF reads. A Touchstone 2 file written with keyword blocks ([Version] 2.0, [Number of Ports], [Network Data]) is refused with the reader's own reason, the same as everywhere else.

It edits nothing. To change a file, change the file; Reload picks the change up.