Comparing RF Demo Kits

10.09.2021 15:48

Earlier this year I was writing about the RF Demo Kit. It's a small circuit board with a sample of different radio frequency circuits that's sold as a learning tool. Among other things it includes a set of on-board short, open and load calibration standards. I was using these standards on the RF Demo Kit for VNA calibration when measuring antennas with U.FL connectors since I lack a better U.FL calibration kit. Having a better model of calibration standards leads to more accurate measurements and in the previous blog post I wrote about measuring some of the model parameters.

At the time I managed characterize the fringing capacitance C0 = 0.58 pF of the open standard on the RF Demo Kit. When I mentioned my measurements on the NanoVNA forum, it was pointed out to me that at least two versions of the RF Demo Kit exist and that the other version might have better characteristics due to the different PCB layout. Since I recently improved my home-made VNA, I decided to revisit this measurement and see if the second RF Demo Kit indeed shows different characteristics.

SOLT standards on the NWDZ RF Demo Kit.

Here is a close-up of the SOLT standards on the NWDZ Rev-01-10 RF Demo Kit. This is the board I measured in my earlier blog post. The load standard is a 0603 size SMT chip resistor. It measures 50.0 Ω at DC. Short and open standards are terminated directly at the U.FL connector, which means they have a different delay compared to the load standard. The substrate looks like a typical 1.6 mm thick FR-4.

SOLT standards on the Deepelec RF Demo Kit.

This is the other board mentioned in the NanoVNA forum, the Deepelec RF Demo Kit. Here the load standard is a smaller, 0402 size SMT chip resistor. It also measures 50.0 Ω at DC.

All three standards are terminated after an approximately 2 mm long trace. Compared to the NWDZ Demo Kit, this forms a more consistent calibration plane. The trace is covered in solder resist, which isn't ideal. The dimensions of the trace and gap to ground seem to be slightly different between the standards. The substrate looks identical to the other board, also most likely a 1.6 mm thick FR-4.

Compared to the NWDZ Demo Kit, U.FL connectors have thermal reliefs around their ground pads. Using thermal reliefs is usually discouraged in high frequency circuits.

My measurement setup for characterizing the RF Demo Kits.

I again used my home-made VNA for the measurements. I connected the RF Demo Kits to the VNA using a 20 cm SS405 coax and a reasonably expensive U.FL-to-SMA adapter. I measured at the frequency span from 500 to 3000 MHz. The VNA was calibrated at its port using a SMA calibration kit I wrote about previously. I applied port extension to null out the effects of the coax using the same method I described in the earlier blog post about RF Demo Kits.

In general, this setup yielded much more accurate results as before. The new version of my VNA has significantly better characteristics in terms of the dynamic range and phase noise. The SS405 coax with the U.FL-to-SMA adapter also performed much better and gives more consistent results than the 20 cm RG-316 pigtail I used previously. Compared to my earlier measurements the new ones look cleaner and with less random noise.

Smith chart for the NWDZ Demo Kit with port extension.

This is how the open, short and load standards measure on the NWDZ RF Demo Kit. Ideally, the standards should have 1, -1 and 0 reflectivity respectively and the plots should just be dots on the Smith chart. Due to various imperfections however the plots spread out from their ideal values as frequency increases.

Fitting a capacitance value to the measurement of the open standard gives a value of C0 = 0.57 pF. This is very close to the 0.58 pF figure I got in my previous measurement.

The load standard has more than 13.5 dB return loss up to 3 GHz. In my previous measurement I've seen return loss down to 12 dB.

Smith chart for the Deepelec Demo Kit with port extension.

The measurement of the Deepelec RF Demo Kit shows a similar picture, but differs in a few details. The calculated one-way time delay for the port extension was 30 ps longer compared to the NWDZ kit. 30 ps is equivalent to 9 mm in free space. This seems too large for the 2 mm line seen on the PCB, even when accounting for a reasonable velocity factor. There might be some other effects here or maybe it's just an error in my measurement.

Fitting a capacitor to the open standard yields C0 = 0.54 pF. The value is similar to the one I get for the NWDZ Demo Kit, however this fit is worse near 3 GHz. On the Smith chart, the trace for the NWDZ open increases in phase in a linear fashion with very little deviation from the ideal capacitor characteristic. On the other hand the trace for Deepelec open folds back at itself and makes a loop near 3 GHz.

Interestingly, the match of the load standard is slightly worse than with the NWDZ board. The worst return loss is 11.9 dB. I would expect the physically smaller 0402 resistor to behave better at higher frequencies.

Smith chart for the SMA calibration kit with port extension.

For comparison, this the same measurement done with the SMA calibration kit connected to the end of the same 20 cm SS405 cable (although with a SMA-to-SMA adapter instead of the U.FL-to-SMA). You can see that all 3 standards are much better defined compared to the two Demo Kits. The port extension comes out at exactly 1.00 ns, which is roughly consistent with the 20 cm length of the cable and the 0.7 velocity factor for SS405.

Setup for measuring the thru on Demo Kits.

I also measured these same three SMA standards by connecting them using a short U.FL pigtail and the thru on both Demo Kits. This way I wanted to get an estimate of the quality of the thrus on these boards since I can't measure them directly with my one-port VNA.

Measurements of the thru on the NWDZ Demo Kit.

Compare this measurement with the case above where the SMA calibration standards were connected directly to the 20 cm SS405 cable without the NWDZ thru in between. The three standards are much less well defined. This is the effect of the two U.FL connectors on the Demo Kit, the PCB trace between them and the U.FL pigtail. I can't say which one had a stronger effect.

Measurements of the thru on the Deepelec Demo Kit.

This is the same measurement, but with the thru on the Deepelec Demo Kit. Interestingly, it shows a much worse picture. I was so surprised that I repeated the measurement to make sure I was getting a consistent result. The thrus on both Demo Kits are effectively the same, with the only obvious difference being the dimensions of the PCB trace and the gaps to ground.

It seems unlikely this is an effect of the U.FL connectors. They seem identical on both boards and the connectors would also ruin the response of other standards. The most suspect is the 4 mm long trace between the connectors. Deepelec's trace does look a bit too narrow for 50 Ω on a typical two layer board. My guess is it's about 50 mil wide, with a 20 mil gap to ground. This makes the gap too small for a microstrip mode. A co-planar wave guide model with some typical PCB parameters gives a characteristic impedance of about 125 Ω, not accounting for the effect of the solder mask filling the gap.

For 3 GHz, the 1/10 wavelength rule is between 5 and 10 mm, depending on the velocity factor. The trace for the thru is about 4 mm long so it's surprising to me that it would have this much of an effect, even if the trace has a wrong characteristic impedance. However a quick simulation of the measurement did in fact show very similar results if I used a 4 mm, 125 Ω transmission line segment as a thru:

Simulated effect of a 4 mm, 125 ohm transmission line segment.

In summary, the SOLT calibration standards on the NWDZ RF Demo Kit seem to be better than on the Deepelec one. The return loss of the load standard is slightly better and the open standard is more accurately described with the fringing capacitance model at high frequencies. The thru on the NWDZ is significantly better compared to Deepelec, which seems to have a significant impedance mismatch.

I need to stress that these Demo Kits sell for about 20 €, which is an order of magnitude less than similar, semi-professional kits. A cal-kit with a brand name will have at least one more figure in its price on top of that. They can't really compare. The RF Demo Kits I've tested are mostly meant to be used with the NanoVNA that works best up to a few hundred MHz and are perfectly fine as a learning tool.

Posted by Tomaž | Categories: Analog

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