After finishing the initial “smoke tests” on the 5.6-5.9 GHz receiver setup – in particular the KU LNC 5659 C PRO down converter, we decided to measure the sensitivity of the WBX receiver over the whole range. We define sensitivity in this respect as the weakest CW signal that:
Gives an SNR of at least 5dB on the spectrum scope in GNU Radio (before filtering)
Produces a clearly audible signal in the speaker
This corresponds roughly to the weakest Morse code signal we could decode using the receiver and while it is definitely not a universal measure for the performance of the receiver, it is a good figure to compare with other ham radio equipment. Noise figure and IP measurements are available from Ettus Research.
The USRP/WBX receiver was connected directly to a well calibrated signal generator using 50 cm of AIRCELL 5 coax cable. The computer was running the Simple CW Receiver V0.3 🙂
We found that a -130 dBm signal is clearly detectable over the whole range, which is quite good. The screenshots below were all taken using a -130 dBm signal, except the 70 MHz measurement, which was taken using -133 dBm. As you will see, we increased the RF gain at the higher frequencies to get a better SNR. Click on the images to get a full resolution screenshot.
I had some time to improve on the simple CW receiver by adding the frequency translating filter to allow easy tuning within the sampled spectrum. This feature was missing from the previous versions because I did not know how to specify the filter taps parameter of the gr_freq_xlating_fir_filter_xxx block in GRC. Of course, all I had to do was RTFM to find out. GRC is smart enough to allow entering complete python statements (including function calls) for parameter values, so all I had to do was to create a variable filter_taps and use firdes.band_pass(1, samp_rate, low, high, trans, firdes.WIN_HAMMING, 6.76) as value, then use filter_taps as parameter for the xlating filter.
I was playing with GNU Radio, the USRP and the WBX daughterboard tonight preparing for the tests of the 5.7/5.8 GHz receiver setup tomorrow. For some reason that I can not remember, I have decided to tune in to 432.471 MHz – the UHF frequency of the OZ7IGY beacon, which is located approximately 50 km (24 mi) from me.
I knew I could receive it even when I am inside using my FT-817 but I didn’t really expect to receive it with the USRP+WBX. Well, I was wrong. Already with the lousy multi-band whip I could hear it. It wasn’t strong but I could hear it. I decided to try with the Arrow antenna and voila, suddenly I could receive it with 40+ dB SNR!
The software was a slightly modified version of the simple CW receiver I posted yesterday, implemented in GNU Radio Companion. The modification consisted of adding a waterfall display after the band pass filter. I have included the GNU Radio flow graph at the end of the video, the one below is without the waterfall display:
So, what do you think, how does the CW sound in a so simple software defined radio?
I have been playing with GNU Radio and GRC (GNU Radio Companion) over the weekend and I ended up implementing a very simple CW receiver. This will be very handy on Tuesday when we will be testing the 5.8 GHz UNITEC-1 setup for the first time using the OZ7IGY beacon on 5.76093 GHz. Here is a quick video demo of the receiver where I use my Yaesu FT-817ND to transmit a test signal.
Next step will be to connect it to the USRP equipped with the WBX and TVRX daughterboards (need 400…700 MHz receiver) and use some basic GNU Radio receiver software to test it using the OZ7IGY beacon, which transmits on 5.7 GHz (5760.930 MHz to be precise) and is definitely within our range. We can probably also find a signal generator for 5.6/5.7/5.8/5.9 GHz somewhere for better tests.
If everything goes according to current plans, JAXA will launch their PLANET-C spacecraft towards Venus on May 18, 2010. To fill out the empty space and available payload mass on the H-IIA rocket, they will also bring four university built cubesats into orbit. One of these cubesats, UNITEC-1, is very special in that it will follow PLANET-C all the way to Venus, although without any propulsive capabilities to make trajectory corrections it might end up somewhere else – we’ll see about that. Another special thing about UNITEC-1 is that it will be using the 6 cm amateur radio band. Cubesats have a tendency to be stuck on VHF and UHF frequencies for understandable reasons, so a 5.8 GHz signal from deep space will be an interesting challenge!
The mission for us radio amateurs is much more than just trying to listen for UNITEC-1. As I see it, radio amateurs can:
Receive telemetry from an interplanetary spacecraft. UNITEC-1 will transmit telemetry on 5.84 GHz using very simple ON/OFF keying at 1 bit per second. This is quite exciting – I believe it will be the first time that we can receive unencrypted telemetry from an interplanetary spacecraft.
Support the UNITEC-1 operators by tracking their craft. By providing them with accurate measurements of antenna pointing and Doppler shift, the operators can estimate the actual trajectory of the craft (remember, UNITEC-1 does not have any coherent tracking transponder on-board). More tracking data from around the world will improve their statistics significantly.
There is a formal call for support on the UNITEC-1 website requesting the global amateur radio community to help with tracking. You can also read about the actual mission UNITEC-1 will carry out during its journey to Venus. Be sure to check out the PDF file that gives many details relevant to tracking.
We are going to need a C-band downconverter. From Kuhne we have several choices. One kind uses 1.4…1.7 GHz as IF, the other kind uses 400…700 MHz as IF. Since we already have about 40 meters of H1000-class cable between the antenna and the control room, we chose the UHF version to reduce the cable losses.
The LNC has been ordered and it should arrive next week. We have everything else on stock but will need to write some software. There is plenty of time though until 17 May. In any case, stay tuned for updates during the coming weeks.
One of the reasons I have been very excited to get the WBX transceiver daughterboard for the USRP is that with one single RF board I can have a full duplex transceiver covering both the 2 m and 70 cm amateur radio bands. This is where most of the amateur radio satellite traffic takes place, including FM, SSB, CW and packet, though modulation is really not an issue when we have a software defined radio transceiver implemented in the GNU Radio framework.
In order to use the WBX transceiver for this purpose, I need to add low noise preamplifiers (LNA) in front of the receiver and power amplifiers (PA) to boost the output of the transmitter. I need them for both bands, since some satellites are in V/U mode, others in U/V mode. Using the usual single-band devices is not optimal, because it would require 4 external devices and a bunch of wiring and switching. That could become too expensive. Therefore, I was looking for devices that cover both bands.
I have often seen wide band LNAs and PAs that would cover both bands and much more; however, I was slightly surprised to find one box that contains both LNA and PA for 2 m and 70 cm – and can even work in full duplex! This is the Microset VUR-30 which is available from Wimo.
Dual band (144-148 and 430-440 MHz)
NF 1 dB on vhf, 1.2 dB on UHF
30W RF out
Requires 1W input. The WBX can give up to 100mW. Hope it will work at reduced output without needing a buffer amplifier between the WBX and the VUR-30.
For some reason I can not find much details about the VUR-30, not even a photo of the rear end to see how it connects to the radio and antennas (one or two connectors, etc.). But from the brief specs and the front photo it looks promising and the price is low enough so that I can just go ahead and buy one.
This video shows my first on-the-air tests with the WBX transceiver using the USRP (Universal Software Radio Peripheral) and GNU Radio.
The receiver was tested using wide band FM broadcast, APT signal from NOAA 17 satellite and Copenhagen VOLMET. I have also performed some tests using DVB-T signal and wireless sensor signals but I wanted to keep the video short so these were not included. I can post them in separate videos if there is interest.
I have been quite busy during the last weeks doing overtime at work, nonetheless, I have managed to carry out some small on the air tests of my newly acquired WBX transceiver boards for the USRP and GNU Radio. Tonight, I tuned in to Copenhagen VOLMET that transmits AM on 127.000 MHz.
I had a few hours to spare tonight and I decided to do a quick check of the WBX receiver. Didn’t have time for much so I just compared it to the TVRX tuner using a strong FM broadcast station. The software was a simple WFM receiver constructed in the GNU Radio Companion graphical editor.
As you can see, both receivers have roughly 40 dB SNR, though the WBX seems to be slightly better. I also observed that the WBX has a “flatter” spectrum profile than the TVRX in particular at wider bandwidths.