Avian’s Blog

The RF Demo Kit is a small printed circuit board with several simple RF circuits on it. There are several variants out there from various sellers. It's cheap and commonly sold together with NanoVNA as a learning tool since it is nicely labeled. Among random circuits like filters and attenuators, it also contains a set of short, open, load and thru (SOLT) standards that can be used for VNA calibration. These are all accessible over U.FL surface mount coaxial connectors.

I've been using the SOLT standards on the Demo Kit for calibrating my home-made vector network analyzer. I've been measuring some circuits with an U.FL connection and I lack a better U.FL calibration kit at the moment.

Of course, for this price it's unrealistic to expect the Demo Kit to come with any detailed characterization of the standards. Initially I just assumed the standards were ideal in my calculations. However I suspected this wasn't very accurate. The most telling sign was that I would often get an S11 measurement of a passive circuit that had the absolute value larger than 1. This means that the circuit reflected more power than it received and that wasn't possible. Hence the calibration must have given my instrument a wrong idea of what a perfect signal reflection looks like.

I suspected that my measurements would be more accurate if I could estimate some stray components of the standards on the Demo Kit and take them into account in my calibration. I did the estimation using a method that is roughly described in the Calibrating Standards for In-Fixture Device Characterization white paper from Agilent.

I did my measurements in the frequency range from 600 MHz to 3 GHz. First I used my SMA cal-kit to calibrate the VNA with the measurement plane on its SMA port. I then measured the S11 of the Demo Kit short standard, using a SMA-to-U.FL coax cable:

I used this measurement to calculate the port extension parameters to move the measurement plane from the VNA port to the position of the short on the Demo Kit PCB. I verified that the calculated port extension made sense. Calculated time delay was approximately 0.95 ns. With a velocity factor of 0.7 for the RG-316 cable, this gives a length of 19.9 cm which matches the length of the 20 cm cable I used almost exactly.

Using the port extension "unwinds" the Smith chart for the short standard. Ideally the whole graph should be in one spot at Z = 0 (red dot). In reality, noise of the VNA and various other imperfections make it a fuzzy blob around that point.

I then applied the same port extension to the measurement of the open standard. Ideally, this graph should be in one point at Z = infinity (again, marked with a red dot). This graph is still very noisy, like the previous one. However one important difference is that it clearly shows a systematic, frequency dependent deviation, not just random noise around the ideal point. Some calculation shows that this deviation is equivalent to a stray capacitance of about 0.58 pF. The simulated response of an open with 0.58 pF stray capacitance is shown in orange:

The Agilent white paper goes further and also estimates the stray inductance of the load standard. This is how my measurement of the Demo Kit load standard looks like with the same port extension applied as before. Again, the ideal value is marked with the red dot:

It looks pretty messy, with maximum reflection loss of about -12 dB at frequencies up to 3 GHz. Note that the U.FL connectors themselves are only specified up to about -18 dB reflection loss, so a lot of this is probably due to connector contacts. The white paper describes using time gating to isolate the effect of the load from the effect of contacts, but the mathematics of doing that based on my measurements escape me for the moment. I also suspect that my setup lacks the necessary bandwidth.

I stopped here. I suspect estimating the stray load inductance wouldn't make much difference. Keep in mind that the graph is drawn with port extension in place, which removes the effect of the cable. Hence the circling of the graph must be due to phase delays in the load resistor and the U.FL connector. The phase delay also cannot be due to the short length of microstrip between the U.FL and the 0603 resistor on the RF Demo Kit PCB. That shouldn't account for more than about 15° of phase shift at these frequencies.

At the very least I'm somewhat satisfied that the Figure 7(b) in the white paper shows a somewhat similarly messy measurement for their load standard. I'm sure they were using a much higher quality standard, but they were also measuring up to 20 GHz:

Image by Agilent Technologies, Inc.

In the end, here is the practical effect of taking into account the estimated stray capacitance of the open standard when measuring S11 of a device:

When the VNA was calibrated with the assumption of a ideal open standard, the log magnitude of the measured S11 went above 0 dB at around 1400 MHz. This messed up all sorts of things when I wanted to use the measurement in some circuit modeling. However, when I took the stray capacitance of the open standard into account, the measured S11 correctly stayed below 0 dB over the entire frequency range. Since I don't have a known-good measurement I can't be sure in general whether one or the other is more accurate. However the fact that the limits seem correct now suggests that taking the estimated stray capacitance into account is an improvement.