S-parameter utilities spreadsheet

Updated February 3, 2008

Click here to go to our main page on S-parameters

Click here to go to our page on mismatch loss, loss factor etc.

Click here to go to our page on VSWR

Click here to learn some basic network theory

Using manufacturers' S-parameters, you can make all manner of useful plots, which will help you evaluate candidate parts, as well as the give you some insight into how accurate the data really is. The file is called "S-Parameter_Utilites_101.xls", and its latest version is located in our download area. Go get it, behold its sheer awesomeness, and use this page as your guide to working its magic!

Update for June 2007! We updated our S-parameter file example using new data downloaded from Hittite's web site for the HMC467LP3 switchable attenuator in its 0 dB state (thanks Mike V. for pointing out the new data!) If you visited this Microwaves101 page before, you'll remember that the original data on this switchable attenuator had two flaws, first, that the S12 measurement was flakey, and second, the part did not work down to DC as advertised (our guess is that they measured it in a fixture that had blocking caps). Looks like they corrected both problems, so we redid the analysis. You'd think by now they might open the Hittite wallet and sponsor this page!

Before we go further on this topic, let's give kudos to both Hittite and TriQuint for offering downloadable S-parameters of their MMICs on their respective web sites. We'd like to think we shamed TriQuint into this (on this very page) but Texans never admit it when they latch on to other people's ideas, nor do they pick up the cue even as the rest of the world rolls their eyes...

Eleven plots are available in the latest version of the spreadsheet. If you have any suggestions to make the analysis more versatile, send us an email!

S-parameter magnitude

S-parameter magnitude in dB

Input/output impedance

Normalized input/output impedance

VSWR

Mismatch loss

Loss factor

Efficiency factor

Group delay (with and without smoothing!)

Unwrapped phase (with and without reference plane extensions!)

K-factor and maximum available gain

Smith chart plots

Sniff tests for "good" S-parameter data

Below we give you some things to consider when you use our S-parameter utility spreadsheet to evaluate manufacturer's S-parameters. For more info, see our pages on basic network theory and K-factor.

1. For passive reciprocal parts, S12 must equal S21. Example of passive non-reciprocal parts are circulators and isolators.
2. For passive parts, S21 and S12 can't be greater than 0 dB.
3. In the intended band of operation, mismatch loss should be just a few tenths of a dB for a passive part, maybe a little higher (0.5 dB) for an amplifier.
4. For a passive part, K-factor cannot be less than 1.0, by definition.
5. For an amplifier, K-factor should be greater than 1 at all frequencies, by design, even outside of the band, or you'll have stability problems using the part.

Example plots: Hittite HMC467LP3, 0 dB state

We downloaded the S-parameters for this passive reciprocal device from the Hittite web site, which is very friendly, and provides S-parameters in "S2P" format. This is a nine column text file with some specific formatting in the header which you can ignore for now. The nine columns that you need are:

Frequency
Magnitude S11
Angle S11
Magnitude S21
Angle S21
Magnitude S12
Angle S12
Magnitude S22
Angle S22

Every once in a while you'll find data that is not in this order, it's your job to correct it!

To use our spreadsheet, you first need to read the S-parameter data into Excel in a separate file (choose "all files (*.*)" as the file type, using the "delimited" option, with "spaces" as the delimiter). Then copy nine columns of data from the Hittite file into our spreadsheet that you just downloaded, on the "Enter_data" sheet, starting in the first frequency box. That was easy! Save it with a file name that indicates the data that you entered. You'll need to do three more things before you even look at the plots. There are two pull down boxes, one says whether you are in dB or magnitude or real/imaginary format, and one tells the spreadsheet what the frequency units are, make sure you have these correct. You also need to specify the system characteristic impedance (almost always 50 ohms, but once in a while it will be 75 ohms). You should copy some of the header info onto the "Enter_data" sheet on the blank rows near the top of the sheet so you will remember what data you were looking at, then save it under a new filename. Now all you need to do is go to the plots, and adjust the axes and/or make other formatting changes to pretty them up, and add as much detail to the titles as you want. Voila!

The example we chose is a digital attenuator, Hittite HMC467LP3, measured in its low-loss state. This is a passive circuit, with bandwidth advertised from DC to 6 GHz. Let's look at some of the plots:

S-parameter magnitude

This is what the S-parameter magnitude looks like if it is not converted to decibels. Boring! Because this is a passive reciprocal circuit, the magnitude of S21 and S12 should be exactly the same. Here's the original data on the HMC467LP, which we pointed out was problematic. The S12 data has some ugly ripples, and the magnitudes of S12 and S21 go to zero at 0 GHz, indicating that the part doesn't work at DC as advertised. If you ever measure a passive 2-port device where S12 and S21 aren't equal in magnitude, you've messed up the measurement and need to start again.

Now then, here's the new data. The data is looking good, and passes our passive reciprocal sniff test (S21=S12)!

From here down we discarded the old data.

One thing we'd like to pass on, if you ever try to put Excel plots onto a web site, you'll need to create jpg or other images of them. One thing you DON'T want to do is to scale an image of an Excel plot (we know this from experience) because the text and lines will soon become unreadable. Here's what we recommend in order to obtain a crisp five-inch wide image.

1. Change the plot type from portrait to landscape
2. Change the right side margin to 2.5 inches
3. Change the lower margin to 6 inches
4. Select all of the text and change it to 12 pitch.
5. Create an image using Paintshop Pro or other software

Now go back and compare the two S-parameter magnitude plots (above). The upper one was scaled, the lower one was done as suggested. Which one is easier to read? Yet another helpful hint for the unwashed masses provided as a service of Microwaves101...

S-parameter magnitude in dB

Here are the S-parameters, in decibels. In our spreadsheet we put S21 on the primary Y-axis and S12, S11 and S22 on the secondary Y-axis. This is helpful for an amplifier, but in the case of this passive device, we suggest that you move S12 onto the same axis as S21 as we did here.

Input/output impedances

In order to convert S-parameters to impedances, you must specify Z0. Usually it's 50 ohms, sometimes 75 ohms.

The calculation to get from S-parameters to impedances is more complicated than, for example, VSWR. This is one of the reasons the Smith Chart was invented, you could enter coordinates either way and the graph would solve the equations for you. Here's one form of the equations, sent by an alert engineer named Steve:

REAL=(Z0*(1-(MAG*MAG)))/(1+(MAG*MAG)-
(2*MAG*COS((ANG/360)*2*PI())))

IMAGINARY=(2*MAG*SIN((ANG/360)*2*PI())*50)/
(1+(MAG*MAG)-(2*MAG*COS((ANG/360)*2*PI())))

He sure like brackets! Here's the input and output impedance, with real and imaginary parts plotted separately. Ideally the real part is 50 ohms, and the imaginary is zero.

Normalized input/output impedances

This is just the above plot, with the impedances normalized to Z₀.

VSWR

Here we've converted S11 and S22 to VSWR. The part is well matched, under 1.5:1 over the entire DC to 6 GHz band. Again, input and output VSWR should be equal for a passive reciprocal part, here they're slightly different, but no measurement is perfect, and measurement errors are magnified when you look at them in terms of voltage standing waves. Here Click here to learn more about VSWR.

Mismatch loss

Hey, this part is pretty-well matched! You lose less than 0.2 dB due to reflection.

Loss factor

The concept of loss factor is discussed on this page. This is a measurement of how much energy is either radiated or goes up in heat, due to resistive or dielectric losses. In this case, no more than 15percent of the input power is "wasted". Loss factor should be equal in forward and reverse directions for a passive reciprocal part such as this.

Efficiency factor

Efficiency factor is also discussed further here. This measurement answers the question, "if I could perfectly match this part, how much loss would it have?"

Group delay

Group delay is a measurement of how long it takes electrical energy to go through the part. The bumpiness of the raw data (blue trace) is in our opinion because the data points are too close together for the angle measurement to be accurately resolved between frequency points. They might have improved this by applying more averaging to the measurement to reduce noise. At least the bumpiness tells us that they didn't smooth the S-parameter data.

You'd get a more realistic look at group delay if you smoothed the group delay. Which is something you can actually do using our spreadsheet! In the purple trace below we've added an "aperture" of five frequency points along the curve to smooth the data. From this trace we approximate the group delay at 0.045 nanoseconds, at least at the band center.

Using our 1 foot equals one nanosecond rule of thumb, let's calculate the 0.045 foot length of this part in smaller units:

0.045 foot x 12 inches/foot x 1000 mils/inch=540 mils

long or about 1.4 centimeters.

Unwrapped phase and reference plane extensions

This is described in more detail on a separate page. Here's the unwrapped phase of S21:

Now let's move the reference planes 0.7 cm into device on both sides (1.4 cm total, see group delay calculation above). The phase is much flatter over frequency.

K-factor and maximum available gain

Because this part is purely passive, its K-factor should be greater than 1 by definition (unconditionally stable).

Smith Chart

Conclusion

Does the Hittite HMC467 data pass the sniff tests? Indeed it does for the remeasured data!