Measuring Henrik's RF bridge

19.11.2020 17:03

I've recently made a copy of a bridge-based directional coupler that was published by Henrik Forstén on his blog. After some difficulties with soldering the components I now have one fully assembled device. The question is of course whether it also functions correctly. Along with his designs Henrik also published the results of some measurements of his couplers with a professional VNA. Unfortunately I don't have access to an instrument like that. Nonetheless I performed some similar measurements with the equipment I have at hand and tried to compare the performance of my copy with the original.

Henrik's RF bridge connected to a NanoVNA-H.

The first test I did was to measure the S-parameters. I replicated Henrik's measurements with a NanoVNA-H. I measured the coupler as a two port device. For each pair of ports I connected one port to CH0 and the other to CH1 while terminating the third port on the coupler with the calibration load that came with the NanoVNA.

Measured gain S-parameters of Henrik's RF bridge.

Ports are numbered as following: 1 - DUT, 2 - IN and 3 - DET.

S12 is the through path from signal generator to the device-under-test. Ideally it should be 0 dB (I think a more correct value to show here would be S21, but Henrik shows S12 - in any case, the difference is minimal). S13 is the coupled path for the reflected wave to the detector. The design of the bridge sets this at -16 dB. S23 is unwanted coupling of the forward wave to the detector. Ideally it should be at minus infinity dB. My measurements are the thicker lines that go up to 1500 MHz (the limit of my NanoVNA). Henrik's measurements with a commercial VNA are the thinner lines. They extend up to 10 GHz so only a small part is shown.

As you can see, the gain measurements match almost perfectly for the small frequency range that I could cover. Directivity (S13 - S23) is better than 25 dB.

Measured reflection S-parameters of Henrik's RF bridge.

The reflection part of S-parameters doesn't match that well. Again, the bold traces that go to 1500 MHz are my measurements while the faded out lines are taken from Henrik's graphs. I'm seeing around 10 to 20 dB worse return loss.

I don't know whether this due to a problem with my measurement or a problem with my bridge construction. On one hand, I suspect that NanoVNA can't compensate for imperfect matching of CH1 in two-port measurements. It implements only one-half of a full two-port VNA and lacks the CH1 reflection measurement it needs to compensate for that. On the other hand, later measurements also show that impedance matching seem to be a persistent problem in my setups.

Next test I did was the same scalar directivity measurement that I did with the "Transverters Store" bridge. I used ERASynth Micro as a signal source connected to IN and a rtl-sdr DVB-T dongle as a power detector connected to DET. I measured the power when the DUT port was terminated with a short and when it was terminated with the calibration load from the NanoVNA set. Estimated directivity was the ratio of these two measurements.

Comparison of the directivity of Henrik's and Transverters store bridges.

Comparing the two bridges it appears that the Transverters bridge has a higher directivity at low frequencies. However above 1 GHz Henrik's bridge works better. Based on the construction of the two devices that kind of makes sense. I'm not sure what's with the directivity spiking up like that at 1500 MHz. Anyway, I don't plan to use this bridge for scalar measurements since I now have a vector setup. I was just curious how it compares to the old bridge.

Assembled rtl-sdr vector measurement setup with Henrik's bridge.

Finally I performed a 1-port short-open-load calibration of my vector measurement setup using the new bridge and the NanoVNA calibration kit. The graph below shows the error network terms that I calculated from the calibration measurements using scikit-rf. Again, Henrik's results are shown as well for comparison.

Error network terms when using Henrik's bridge.

This isn't a comparison of bridge performance but rather of the whole measurement system, since these error terms include the effects of other components as well. Obviously the two systems are completely different in these two cases, but still it gives some idea of how well my setup performs compared to Henrik's home-made VNA.

Note that the directivity in my previous scalar measurement is defined slightly differently than here. It is roughly equivalent to reflection tracking divided by directivity from the error terms graph. Slide 48 in the Agilent Network Analyzer Basics presentation has a good illustration of the meaning of the error terms.

A difference in reflection tracking is expected since my and Henrik's VNA probably have different overall gains in the system. I'm happy that it's reasonably flat, meaning that the losses in the bridge don't increase much towards higher frequencies.

My directivity seems slightly better and this result also shows a peak at around 1500 MHz that I saw in the scalar measurement. Source match is roughly 20 dB worse. This agrees with the NanoVNA measurements that have shown quite a bad reflection loss on all bridge ports and also my previous results showing that overall I have quite bad matching in the whole system. I might experiment a bit with this in the future and try to correct it.

Error network terms when using Transverters bridge.

For another comparison, here are the error terms calculated using the same method, but using the "Transverters Store" bridge. Here the bridge starts attenuating the signal towards higher frequencies, and hence losing the effective directivity. The source match seems to be better at low frequencies though.

In conclusion, my copy of the bridge design seems to work just fine. Its effective directivity is around 25 dB in the range I was able to measure. The multiplex board doesn't seem to affect it much and the only problem seems to be relatively poor 50 Ω matching, but that might be because of my other equipment. For low frequency measurements below 1 GHz using the Transverters Store bridge should yield better results. A true test of the new bridge however would be at higher frequencies, beyond the 2 GHz that my current setup is able to handle. The only limiting factor is the rtl-sdr receiver however. I plan to get a better one though in the future and that should enable me to make measurements up to 6 GHz. Judging by Henrik's results the new bridge should handle that as well.

Posted by Tomaž | Categories: Analog

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