Saturday, 1 March 2014

Dangly Dongles

Because of work, ive not had much time for radio for a few days. But I have been playing with a bit of SDR.

For a bit of fun, I ordered a few of these DVB-T USB dongles from China

These are intended for use in receiving off air digital TV, but the antenna that comes with them, a piddly little magmount, is frankly dire. Of far greater interest is that these have very wide band software defined radio receivers. The R820T tuner chip in these can cover from around the mid 20 MHz, up to about a gig and a half, if used with suitable SDR software.

An inside view shows how these are made up. The little biddy chip to the left of the 28.8MHz clock crystal is the Rafael R820T tuner. The other square IC to the right is the Realtek RTL2832U DVB-T demodulator and ADC. These sticks are popular because of the wide tuning range of the R820T and the very low cost of the device. However, as with most things bought from the far east, quality control is rather variable. This particular model seems to be a good one, but if nothing else, always check that the little SMT dual schottky diode that can be seen just above the MCX connector is present. This is ESD protection and is crucial, as the chip is rather sensitive!

As I said, the stock antenna is abysmal. But I dont have an adaptor from MCX to anything useful, and I certainly wasnt going to buy one, as an adaptor is almost as expensive as the dongle! So I had a hunt about for something to use. As it happens, I intend building this one into a die cast box, with a USB-B socket and proper antenna connector, so it can be mounted as far as possible from the PC. In its present state, and plugged direct into a computer, it is very susceptible to RF noise from the PC and power supplies. On the casing of the old Marconi RC-690 ive been dismantling, there are three TNC connectors. OK, so not as easily used as BNC or N types, but far better than the MCX. As it happens, one of these is a bulkhead socket with a tail of RG-178, perfect for this task. So, a little cutout into the plastic case (which incidentally, is a silk matt black and feels so smooooth!) and I modified the dongle for an external antenna

I like to use RG-178 as its silver plated and makes for a very good and neat job. Be aware these are RoHS compliant and use lead free solder, so are a bit tricky to get melted. Ive left the MCX connector on, as it might come in handy, and its too big a pain to take off.

Im using this with SDR#. This software is fairly easy to use, but there is a specific version to use for these dongles, otherwise it doesnt give you the USB option! Setting up the software is a bit of a bother until you find your way about. With the dongle on its supplied antenna, you can barely get even the strongest of FM broadcasts, but on my discone, things are much different. It is prone to easy overload though, which results in a lot of spurious indications on the waterfall.

But for under £6 shipped, its a bit of fun, and the things it can be used for are almost endless! I have a 3m USB extension cable coming. Keep it away from RFI sources, shielded, and with a proper antenna, and it promises to be an interesting bit of kit.

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