HackRF

My HackRF SDR arrived on Friday and I had a little time to play with it yesterday. Here's a picture of what the commercial product looks like:

To see what the performance is like I attached my 4-element yagi to a small preamp (to overcome the 15m of cable loss) and fed the output to a MiniCircuits ZFSC-2-1splitter which fed both a HackRF and a RTLSDR dongle. The antenna was pointed in roughly a SW direction (and vertical polarisation). This allows me to pick up both the GB3VHF beacon and the GRAVES signal (via scattering off meteors etc as I've discussed previously) in the same 2MHz passband.

Setting up was easy enough - I'm using my own GNUradio scripts and the gr-osmocom driver (which I compiled from git). I just had to install the udev scripts (to make it accessible to non-root users) and I needed a frequency correction of -11ppm due to the offset of about 1600Hz I found when looking at GB3VHF.

Looking at the stability of the system by looking at the received frequency for GB3VHF shows:

HackRF

RTLSDR

As can be seen from the above plots the HackRF is much more stable than the RTLSDR (which could also double in function as a thermometer!). The SNR for both devices is about the same.

For these tests I'm using gains of RF=0, IF=24 & BB=24. If I used less then I start to see a reduction in the SNR. One thing I have noticed is if I use more gain than 0,24,24 I start to see more spurious signals appearing - i.e. signals which appear strongly in the HackRF spectrum, but not in the RTLSDR spectrum. Even when I turned the gain down to 0,24,24 I still saw the odd signal, for example:

HackRF

RTLSDR

You can see that there is signal breakthrough occurring at about position 2000Hz, 1900 (time 'pixels') - along with a number of meteor detecions later on. I'm not sure what frequency is causing this but presumably it can be reduced by an external filter (I'm assuming its not an 'in-band' signal but a result of the complex RF chain before the ADC). I shall be investigating this further, but at least it does not occur too often - less than once every few hours or so.

Over the next few weeks I'll be looking at other aspects of the HackRF, especially its ability to receive up to 6GHz.

My multichannel RTLSDR has stalled a little of late - I'm having difficulties with the PCB version of the frequency tripler - the veroboard version was causing wideband noise however the PCB version doesn't make enough drive to power 2 RTLSDR dongles. One thing I'll hopefully be trying later this year is using 2 HackRF's with their built in external clock input/output to see how well they perform as a coherent multichannel receiver.

Bistatic Radar Results

Below is an example of what I can now get out of my radar. The video show a bunch of frames, each about 1s in real time, of range (vertical, about 0 - 70km path length difference) against Doppler (which goes from around -350m/s to +350m/s). Visible are a bunch of aircraft which all go from positive Doppler to negative as expected.

There's lots more experimentation to do with regards to frame rates and pulse length (how long to do the autocorrelation over). Hopefully the data I recorded also had a pass of ISS in it, so it would be good to search for that.


Update: It looks like the video gets compressed to nothing when its uploaded, I'll sort out a better way to show it....

Update 2:  Lets see if this works better....

Hmmm.....not really....

Coherent Receiver

For the last couple of months I've been trying to put together a 2 channel coherent receiver. For this I'm using a 9.6MHz TXCO oscillator followed by a simple tripler (from the bottom of this page). This gives me the 28.8MHz clock signal that's required by the RTLSDR dongle. I've then taken 2 RTLSDR dongles, desoldered their crystals and replace with a coax feed from the tripler. I should then have 2 receive channels which should be always in phase with each other, and due to the use of a TXCO should not drift (too much!) with temperature.

It all seems to work well - by which I mean if I cross-correlate the signals from both receivers I get something which looks like:

which shows that the 2 channels are very correlated (the peak isn't at zero as the dongles aren't started at exactly the same time).

However, when I look in detail I see that I'm actually dropping samples - every so often the peak will shift by around 100-1000 samples. To figure out what is happening I now have each dongle in a different machine which has certainly reduced the occurrence of the drop-outs, but they still occur (the drop-out rate went from around once every minute or two, to once an hour). I need to see what I can do about this (better disk drive, update to bios, tweak USB settings somewhere) - but I'm not sure at the moment where to start (with spending lots of money!).

I'm doing this to make a simple passive bistatic radar using DAB radio as the illuminator - mainly to look at meteors and perhaps satellites (but it should also pick up planes). This is currently the sort of output I get - range is up and down and Doppler shift is left to right.

A lot of the things which are visible in this are just artefacts from structure that the format of the DAB signal. I'm in the process of verify some of the signals I think I can see!

At the moment the engineering  is a bit poor, so the whole system works as a bit of a local jammer! Fortunately this isn't a problem at the DAB frequencies, but is at ~144MHz which makes it tricky to receive either GRAVES or the GB3VHF beacon. So the next step is to properly engineer the whole tripler/oscilator/dongle system into a well shielded box. I also want to increase the number of receive channels to 4 to allow me some amount of interferometric or beam forming capability. I've also started to look into using a SI5351C as the frequency driver which could make up to 8 frequency outputs from a TXCO or a rubidium clock.