Vector measurements with the HackRF

27.03.2021 20:09

Over the last year I slowly built up a small, one-port vector network analyzer. The instrument consists of a rtl-sdr USB receiver dongle, the ERASynth Micro frequency synthesizer, an RF bridge, a custom time multiplex board I designed and, of course, a whole lot of software that glues everything together and does final signal processing in the digital domain. In the past months I've written pretty extensively about its development here on this blog and I've also used the system in practice, mainly to measure VSWR and matching networks of various multiband LTE antennas.

The original instrument could perform S11 measurements up to around 2 GHz. However the 2 GHz limit was only due to the E4000 tuner in the rtl-sdr. Since ERASynth Micro can generate signals up to 6.4 GHz and I designed the multiplexer with signals up to 8 GHz in mind, getting measurements at higher frequencies was only a matter of upgrading the receiver. I was tempted to do that since the popular 2.4 GHz ISM band was just out of reach of my measurements. I did a lot of searching around for a suitable SDR that would cover that frequency range. The top of my list was an USRP B200, but amid the lockdowns and the general chaos in international shipping last year I couldn't find a supplier. In the end I settled for a HackRF One.

My small, home-made vector network analyzer, upgraded with a HackRF.

On one hand, moving from the rtl-sdr to HackRF was pretty simple. After I got hackrf_tcp up and running most of my old code just worked. Getting things to work reasonably well took longer though. I lost a lot of time figuring out why the gain in the system varied a lot from one measurement to the other. I would optimize signal levels for best dynamic range, only to try it again next day and find that they have changed considerably. In the end, I found out that the USB hub that I was using could not supply the additional current required by the HackRF. Interestingly, nothing obvious broke with the USB bus voltage wildly out of spec, only the analog performance became erratic.

I wish HackRF came with some hardware settings (jumpers or something) that would allow me to semi-permanently disable some of its more dangerous features. As it is right now I need to use a DC block to protect my multiplex board from a DC bias in case the antenna port power gets enabled by a software bug. I also had to put in a 20 dB attenuator to protect the HackRF in case its pre-amplifier gets turned on, since that would get damaged by the signal levels I'm commonly using.

Error network terms when using Henrik's bridge with HackRF.

Here are the error network terms with the upgraded system. The faded lines are the measurements by Henrik Forstén from whom I copied the RF bridge design. Up to 2 GHz the error terms match pretty well with those I measured with the rtl-sdr. As I noted in my previous blog post, the error terms are a measure of the whole measurement system, not only the bridge. Hence my results differ from Henrik's quite significantly since apart from the bridge his system is quite different.

Unfortunately, my new system still isn't very well behaved beyond 2 GHz. I suspect this has to do with the construction that has a lot of connectors everywhere and RG-316 cables of questionable quality. Every contact introduces some impedance mismatch and in the end it's hard to say what is causing what. I also know that I made an error in calculating the dimensions of the coplanar waveguides on my multiplex board. I'm sure that is not helping things.

Estimated dynamic range of the system.

This is an estimated dynamic range of the vector measurement, using the method described in this post. Let's say it's better than 50 dB below 1.5 GHz and better than 30 dB below 3.5 GHz. It quickly gets worse after that. At around 5.5 GHz I suspect there's some kind of a resonance in the 10 dB attenuator I have on the multiplex board that's made out of 0603 resistors. Beyond that point the signal that passes through the attenuator is stronger than the un-attenuated signal and I can't measure anything anymore.

I really would like to improve the dynamic range in the future. Basically all the practical measurements I did with this system was with the device-under-test being behind a U.FL connector. The problem with that is that the connector introduces a significant mismatch before the device-under-test. You can calibrate this out, but this comes at a cost of the effective dynamic range. Hence it may be true that 30 dB of dynamic range is enough for practical measurements. However as soon as you start moving the measurement plane away from the port of the instrument, you really want to start with as much dynamic range as possible.

I believe most of the total noise in the system now actually comes from the phase noise. Currently the signal source and the receiver use independent clocks and each of those clocks has its own drift and various artifacts introduced by individual phase locked loops. I suspect things would improve significantly if I could run both ERASynth Micro and HackRF from a common clock source, ideally from the ERASynth Micro's low phase noise TCXO. Unfortunately they use incompatible interfaces for reference clock in/out. I need to make an adapter circuit before I can try this out.

In the end, this is all kind of an endless rabbit's hole. Every measurement I take seems like it could be done better and there appears to be a lot of room for improvement. I've already accumulated a wish list of changes for the multiplex circuit. At one point I will likely make a new versions of the bridge and the multiplexer PCBs using the experience from the past year of experimenting.

Posted by Tomaž | Categories: Analog


Physicists SM5BSZ wrote me once on SDR subject: if you do frequency conversion, use single low noise oscillator.

My birthday regards from Jagodina, Serbia.

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