Experiments with the "Transverters Store" RF bridge

07.06.2020 17:54

The "Transverters Store" RF bridge, for a lack of a better name, is a low-cost bridge circuit that can be used to measure reflection loss or voltage standing wave ratio (VSWR) at radio frequencies. It claims to be usable from 0.1 MHz to 3 GHz. Basic design and operating principle of a similar device is described in "A Simple Wideband Return Loss Bridge Revisited", an article by Paul McMahon from 2005. In it he also gives measurements of its performance up to 500 MHz. The exact device I have seems to be very much related to Paul McMahon's design. It came from a web page called Transverters-Store and shipped from Ukraine. Very similar looking products with more or less exact copies of the PCB layout are available from various other sources. Since there's no product number or a clear name associated with it, most often people refer to these as simply "that cheap bridge from eBay".

The "Transverters Store" RF bridge.

In short, the bridge operates similarly to the typical resistor bridge networks, like the Wheatstone bridge. The network is composed of two 50 Ω resistors on the PCB itself (each made out of two parallel 100 Ω resistors), the reference load on the REF port and the device under test on the DUT port. The biggest difference is the addition of a balun circuit. This makes the detector output referenced to ground, instead of floating between the two bridge branches like in a low-frequency bridge. The balun is implemented here as a high-frequency transformer made out of a row of black ferrite beads and two lengths of coax.

The bridge can in principle be thought of as a directional coupler. The signal power on the OUT port only corresponds to the reflected power coming back in to the DUT port, but (ideally) not the forward power going out of that port to the device under test. Compared to a true directional coupler however the bridge can't be operated in the reverse. You can't measure forward power by connecting a signal source on the OUT port and a detector on the IN port.

Ideal bridge output versus return loss.

This is how the bridge would behave if everything was ideal. The vertical axis shows the power on the OUT port relative to the power on the IN port. The horizontal axis shows return loss of the device under test. Using directional coupler terminology, the bridge has a coupling factor of 16 dB if used with a 50 Ω detector. It's also interesting to see that if using such a detector on the OUT port, the output of the bridge is slightly non-linear in respect to return loss. The difference is small - an open circuit will measure around 1.5 dB too high and a short will measure around 1.5 dB too low. Considering other inaccuracies, this detail probably isn't significant in practice.

Below you can see the setup I used for the experiments. Signal source on the top left is an ERASynth Micro. The detector on top right is an Ezcap DVB-T dongle (using Elonics E4000 tuner) and rtl-power-fftw. Both the source and the detector are controlled from a PC through a USB connection. Above the bridge you can see the terminations I used in the experiments: A borrowed professional Narda Micro-Pad 30 dB attenuator (DC - 18 GHz) which I used as a terminator, a couple of home-made 50 Ω SMA terminators (using two parallel 100 Ω 0603 resistors in a SMA connector), a home-made short and a no-name terminator that has a through-hole metal-film 51 Ω resistor inside.

The setup for experiments with the RF bridge.

Using this setup I tried to measure the directivity of the bridge. Directivity is the measure of how well the bridge selects between the forward and reflected power. The higher directivity, the lower return loss and VSWR you can reliably measure with it. Anritsu has a good application note that describes how directivity affects measurement error. I measured directivity by measuring OUT port power twice: once with a short on the DUT port and once with my home-made terminator. Dividing these two values gave an estimate of bridge directivity. I performed two measurements: once with the Narda attenuator on the REF port and once with my home-made terminator using two 0603 SMT resistors.

There is an approximately 200 MHz wide gap in my measurements at around 1.1 GHz because the DVB-T receiver cannot tune on those frequencies.

Measured directivity of the RF bridge.

You can see that my two measurements differ somewhat. In both, the bridge shows good directivity up to around 1 GHz. Above that, it's below 20 dB which introduces a large error in measured VSWR as you'll see below. The DVB-T receiver I used also shows a decrease in sensitivity above 1 GHz, however I've repeated this measurement using different input power levels (from -10 dBm to -30 dBm on ERASynth micro) and they all show similar results, hence I believe the measured decrease in directivity is not due to the limited dynamic range of my measurement setup.

In general, I think the biggest source of errors during these experiments are the terminators I've used. The directivity measurement assumes that the bridge can be measured with perfect termination on both the DUT and REF ports. By testing various combinations of terminators I had at hand I've seen significant differences in output port power, which suggest they are all slightly imperfect in different ways.

My measurements of directivity compared to seller's.

This is how my measurements compare with the measurements published on the Transverters Store website. The blue and orange plots are same results as above, only rescaled. Red plot is the Transverters' result. Again, my results differ from theirs. Below 500 MHz mine show a bit better directivity. Above 500 MHz mine are worse. Both show a slow decrease and then a sharp fall at around 1 GHz. I'm not claiming their measurements are wrong. My setup is very much improvised and can't compare to professional equipment. It's also very likely that different devices differ due to manufacturing tolerances.

Measured VSWR of the cheap SMA terminator.

Finally, here's an example of a VSWR measurement using this setup. I've measured the bad attenuator I've mentioned in my previous blog post that's made using a standard through-hole resistor. Again, the blue and orange plots show measurements using the two different references on the REF port of the bridge. The shaded areas show the error interval of the VSWR measurement due to bridge directivity I measured earlier. The VSWR of the device under test can be anywhere inside the area.

Interestingly, the terminator itself doesn't seem that bad based on this measurement. Both of my measurements show that the upper bound of the VSWR is below 1.5 up to 1 GHz. Of course, it all depends on the application whether that is good enough. You can also see that above 1 GHz the error intervals increase dramatically due to low bridge directivity. The lower bounds do carry some information (e.g. the terminator can't have VSWR below 1.5 at 2 GHz), but the results aren't really useful for anything else that a rough qualitative estimate.

In the end, this seems to be useful method of measuring return loss and VSWR below 1 GHz. Using it at higher frequencies however doesn't look too promising. 3 GHz upper limit seems to me like a stretch at this point. The largest practical problem is finding a good 50 Ω load to use on the REF port, a problem which was also identified by others (see for example a comment by James Eagleson below his video here). Such precision loads are expensive to buy and seem hard to build at home.

I was also surprised how well using the DVB-T tuner as a power meter turned out in this case. I was first planning to use a real power meter with this setup, but the device I ordered seemingly got lost in the mail. I didn't see any indication that the dynamic range of the tuner was limiting the the accuracy of the measurement. Since all measurements here use only ratios of power, absolute calibration of the detector isn't necessary. With the rtl-sdr device you only need to make sure the automatic gain control is turned off.

Posted by Tomaž | Categories: Analog

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