April 25, 2012 8 Comments
Balint Seeber’s new software supports not only the USRP, but also a range of low-cost USB dongles designed for receiving DVB-T television signals. When I read about this, I felt that I can’t not try one, so I ordered an ezcap dongle for $20 from a Chinese seller on eBay. It arrived a couple of days ago, and my experience so far is very positive.
The dongle I ordered was listed as an ezcap dongle, which is one of the models supported by Balint’s software. The dongle that arrived is exactly the one that was shown in the picture on eBay; inside, it was labeled as EzTV666. It came with a 15cm whip antenna, so I plugged in the antenna, plugged in the dongle, configured the driver using Zadig, and started up HDSDR. Balint’s library for the USRP and the TV dongles was already installed, so all I had to do is tell the software that I wanted to use a TV dongle and it worked without a problem. I didn’t have to install any new software.
I was immediately able to receive some local narrow-band FM traffic. I ran the dongle with a bandwidth of 1.6 MHz, so you can see a nice piece of spectrum on the screen at once. It goes up to 3.2 MHz, but Balint warns that high bandwidths cause some signal degradation. In the screenshot below you can see shorts bursts from a hand-held trancseiver at the input frequency of a local repeater and the repeater’s output 600 kHz higher. The repeater output bursts are longer because the repeater controller adds a “tail” to each transmission. It’s pretty neat to see both the input and the output of a repeater at the same time. The dongle’s frequency is a bit off, so the displayed frequency is incorrect (about 10 kHz too high), but this is easy to correct in HDSDR (Options -> ExtIO Frequency Options).
I installed the software that came with the dongle on another computer. Reception of local broadcast FM stations was good, but I was not able to receive DVB-T television signals, even with the bundled whip antenna outside the window.
Clearly, I needed better antennas if I was to receive more interesting stuff. To use better antennas, I needed some way of connecting them to the dongle. From the picture on eBay it was not easy to determine the type of RF connector used on the dongle; it did not seem to be the IEC 169-2 connector that is used in some of the other SDR-capable DVB-T dongles, but I didn’t know what it was. When the dongle arrived, I realized that the connector is an MCX connector. I didn’t have any cables with MCX connectors or any MCX adapters, so my initial thought was to cut the cable from the rather useless whip and terminate it with a BNC, but before I did that a friend gave me an MCX-to-N patch cable that he bought at a surplus store. That was good enough to connect any of my existing antennas, so I continued the experiments.
I first tried a 145 MHz dipole (1m end to end). It improved the reception of local UHF and VHF narrow-band FM dramatically, and it also worked very well for DVB-T (which is at 514 MHz here); I was able to watch TV on the computer. It also worked reasonably well for air-band signals. The next stage was to try to receive satellite signals. I experimented previously with a borrowed FUNcube dongle and I new that the FUNcube was very capable of receiving satellite signals. The FUNcube dongle uses the same tuner chip as the ezcap (an Elonics E4000, but its analog-to-digital converter is higher resolution than the ezcap’s; the FUNcube also has a low-noise preamplifier ahead of the tuner, and I am not sure if the ezcap uses one (if somebody knows, let me know).
A satellite called VO-52 was supposed to fly over last night, so I turned my cheap Yagi towards it and listened. Initially, I attached a preamplifier between the antenna and the dongle, but I later discovered that it did not make a huge difference. VO-52 has both a Morse beacon and a linear transponder. The beacon came through very clearly. In the screenshot on the right you can see the beacon’s signal move down in the audio passband due to Doppler shift. It it well above the noise and easy to hear. Next, I checked whether I can receive SSB signals in the transponder’s passband; I could. Here is a short recording of one Greek station (the Doppler shift causes the speaker’s voice to become lower over time).
While I was listening to VO-52, the local FM repeater sprang into activity. It is only about 100kHz away from the satellite’s signal; when the repeater was transmitting, it was causing pretty severe distortion of everything else the dongle was receiving. The satellite still came though, but not as well. You can see the effect in the screenshot on the right. I discovered that this was caused by too much gain in the preamplifier+dongle chain. There are two ways to solve this: to not use a preamplifier, or to drastically lower the gain of the dongle (you can set the gain with a slider).
The main benefit of the preamplifier in my setup is that it includes a good bandpass filter. Without the preamplifier, strong-out-of band signals sometimes overwhelm the dongle. With it, reception is much more consistent. But the dongle does work reasonably well even without anything between it and the antenna.
Another interesting thing that you can see in the last screenshot is the strong CTCSS tone coming out of the repeater, at 91.5 Hz. It is clearly visible in both the audio spectrum display and in the audio waterfall.