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Banggood's $30 100kHz-1.7GHz build-it yourself SDR kit B ack in the November 2013 issue of Silicon Chip, in an article explaining how to use our SiDRADIO SDR project to receive DRM30 broadcasts, I had a sidebar on pages 70-71 discussing the "direct sampling" approach to adapting a DVB-T dongle for MF and HF radio reception. A number of readers had asked why we hadn't used this approach as an alternative to the up-converter approach we had used in the SiDRADIO. In the sidebar, I tried to explain not only how a DVB-T dongle could be modified for MF and HF reception using direct sampling, but also the shortcomings of this approach with regard to reception performance, compared with the use of a preselector and an up-converter. I concluded by suggesting that the direct sampling approach would be fine if you just wanted to use a spare DVB-T dongle for SDR reception of local AM radio signals. But for proper reception on the LF or shortwave bands, we believe that our LF-HF Up-Converter (June 2013) or the SiDRADIO (October/November 2013) would be preferable. 92 Silicon Chip Review by JIM ROWE This low-cost Software-defined Radio uses a standard DVT-B USB dongle to provide wide-range radio coverage from 100kHz to 1.7GHz. How do they manage it at such low cost? It turns out that they use a “direct sampling” approach which eliminates some of the circuitry which would otherwise be required. So how good is it? Read on. . . The new Banggood low-cost SDR kit reviewed in this article (siliconchip. com.au/l/aag8) uses (you guessed it) the direct sampling approach for reception below 30MHz. And they've worked out a way to do this in addition to the standard no-mods-to-thedongle VHF and UHF reception so that switching between the two bands is achieved entirely in software. In short, they've taken advantage of the direct sampling approach to come up with a very neat little DVB-T dongle based SDR solution, in kit form and at a surprisingly low price. It does have a few tricky aspects in terms of kit assembly and also some limitations in terms of performance. But it would still make a very good introduction to Software Defined Radio. The assembled kit is claimed to tune from 100kHz to 1.7GHz, in two overlapping ranges: 100kHz to 30MHz using the direct sampling input and 25MHz to 1.7GHz using the regular dongle antenna input. Celebrating 30 Years And it all runs from the 5V DC from a PC's USB port, which is also used for communications between the dongle and PC. In operation, it draws around 280mA. The complete, assembled SDR is housed in a neat little metal case measuring only 83 x 50 x 20mm, finished in matte black enamel. What you get in the kit As shown in the photo, the kit comes with pretty well everything you'll need to build it: a very compact DVB-T dongle, a PCB for the rest of the circuitry, the parts for the metal case (including the M2 assembly screws), two edgemounting SMA sockets for the RF input connectors, a mini USB socket and all of the minor passive components. This includes the SMD capacitors and resistors, three small electrolytic capacitors and two LEDs. There's also a tiny ferrite toroid (5mm outer diameter) for winding the coupling balun, plus a length of very fine (0.06mm siliconchip.com.au A quick look at the circuit The Banggood SDR kit comes with everything needed to build it. However, you may find an external and/or active antenna improves its performance. outer diameter) enamelled copper wire to wind it. There's also a length of 0.5mm outer diameter enamelled copper wire to wind the two small low-pass filter coils. You also get a 1m USB cable to hook up the finished SDR to your PC, plus a 250mm-long loaded whip antenna with a 2.9m cable fitted with an SMA plug for connecting it to either of the SDR input sockets. You can download a 7-page PDF from the Banggood website which includes the kit assembly instructions. The text is a bit sketchy in places and doesn't explain some things particularly well but there are quite a few photos which help clarify things. The kit comes with one spare component for each of the SMD capacitors and resistors, in case one of any of these tiny parts is lost. There's also one additional M2 screw along with the eight needed to assemble the SDR case. Very helpful! One thing you don't get, though, is the software needed to run the SDR on your PC. For this, you need to download an SDR application like SDR# (available from www.airspy.com). siliconchip.com.au If you haven't played with donglebased SDR before it's also a good idea to go to the RTL-SDR website (www. rtl-sdr.com) and download their Quick Start Guide file, which explains a lot about installing SDR# and the drivers it needs in order to communicate with a dongle-based SDR. We published an article which explained the process of setting up SDR# starting on page 12 of the May 2013 issue. We have also reproduced the series of steps required to install SDR# in a panel in this article and even if you're referring to the May 2013 article, you should read that as some things have changed slightly to suit newer versions of Windows. Just before we discuss assembling the kit, take a look at the circuit diagram, Fig.1. This is much clearer and easier to follow than the one included in the instructions from Banggood. The circuitry of the dongle itself is shown in simplified form inside the light green filled rectangular area in the centre. As you can see, it uses two main chips: a Rafael Micro R820T VHF-UHF tuner chip and the Realtek RTL2832U digital demodulator chip with its inbuilt USB interface. The latter is really the heart of the dongle and also that of the overall SDR. Notice that by using a dongle with the R820T tuner chip ahead of the RTL2832U, Banggood's designers have made it easier to use the direct sampling approach. That's because the R820T has only one pair of differential outputs, rather than the two pairs used by other popular tuner chips like the Elonics E4000 or the Fitipower FC0013. Since the outputs from the R820T only use the I+ and I- inputs of the RTL2832U, this leaves its Q+ and Qinputs free for feeding in the LF-HF input signals for direct sampling. The components and wiring outside the green rectangle in Fig.1 is the additional circuitry used in the Banggood SDR kit, to extend its frequency range downwards to 100kHz and also to improve its performance and flexibility. The circuitry at lower left is used for direct sampling of lower frequencies, and as you can see is fairly straightforward. The signals first pass through a two-stage low pass filter comprising coils L1 and L2 plus their associated capacitors; then balun transformer T1 The first step to take when assembling this kit is to remove the case on the DVB-T dongle as shown. After the two connectors are removed (they aren’t reused), the board is then attached to the main SDR board. The two photos shown here are of the top (left) and bottom (right) of the dongle’s PCB. Celebrating 30 Years November 2017  93 Fig.1: the heart of this software-defined radio (SDR) is the DVB-T dongle shown in the centre, it uses a multi-band tuner chip and the Realtek RTL2832U COFDM digital demodulator chip which also provides the USB interface. is used to change them into differential form to feed into the Q+ and Q- inputs of the RTL2832U. Finally, note that the kit designers have also made provision for both of the SDR inputs to be provided with 5V DC "phantom" power, by using the A setting of link header CON2 (at upper right). This makes it easy to use active antennas with the SDR, or to use a preselector with gain in the case of the direct sampling LF-HF input. It's a nice feature which doesn't seem to be explained in the Assembly Instructions. To make things easier for myself, I first used a jeweller's saw to cut off both connectors level with the ends of the PCB, leaving only their inner portions to be desoldered and removed. Once the connectors have been removed and their holes in the PCB cleaned up, it is ready to be fitted inside the main SDR PCB, in the rectangular cut-out in the centre. But before you do so, it's a good idea to fit most of the other components to the main PCB. Assembling the kit The first step in building the kit is to prise open the DVB-T dongle's plastic case to reveal the tiny PCB assembly; the PCB itself measures only 28 x 17mm and is shown on the preceding page. The next steps are to remove the USB type A plug from one end of the PCB and the Belling-Lee RF socket from the other end. These steps turn out to be a little tricky because you have to do them with a fairly high-powered soldering iron while at the same time being careful not to damage the many tiny SMD components already fitted to both sides of the PCB. 94 Silicon Chip A close-up of the assembled PCB showing the various connections required between it and the dongle. The most important thing of note is the connection from toroidal transformer T1 to pins 4 & 5 of the RTL2832U micro. Celebrating 30 Years siliconchip.com.au The photos above show the top (left) and bottom (right) of the completed SDR board. There are a reasonable number of through-hole and SMD components that need to be soldered to the board along with securing the dongle PCB in the cutout. It’s best to solder many of the smaller components to the board before attaching the dongle PCB as there isn’t a lot of space to work with. I added the SMD capacitors and resistors first, followed by the 4.7µH SMD inductor and SMD LED2. By the way, these are all 0805 parts (2.0 x 1.2mm), so you need a soldering iron with a fine and well-tinned tip. Next, I fitted the two edge-mounted SMA sockets at the input end, followed by the SMD mini USB connector at the output end. Then I decided to solder the dongle PCB in place. This needs to be done carefully; it's attached to the main PCB using short lengths of 1mm diameter tinned copper wire or tiny pieces (3 x 4mm) of thin brass shim, soldered to each corner of the smaller PCB. I found the easiest way to do this was to first solder these pieces to the ground copper on each corner of the top of the dongle PCB. Then I could lower the assembly into the main board cutout, so the added pieces held it in place while I soldered the outer ends of each to the earth copper on the top of the main PCB. The kit designers have left these areas unmasked and pre-tinned. Next came the really tricky steps: first winding the tiny balun transformer T1, then fitting it to the main PCB in the location shown and finally soldering the ends of its outer secondary wires to pins 4 (Q+) and 5 (Q-) of the RTL2832U demodulator chip on the dongle PCB. Winding T1 isn't too hard but because it's wound as a trifilar (three wires at once) coil using very fine wire (0.063mm diameter) on a very tiny (5mm OD, 3mm ID) toroidal ferrite core, it ain't easy either. You first need to straighten the wire, then fold it into three, twist together and then thread the twisted wire trio through and around the miniature toroid eight times. siliconchip.com.au Then you need to cut them apart at each end and use a multimeter or DMM to carefully identify the start and finish of each wire. One of the wires becomes the transformer's primary, with its ends cut short and soldered to the pads between T1 and the board edge after you have attached T1 to the main PCB using a 5 x 6mm piece of double-sided adhesive tape. The start of one of the remaining wires is then twisted together with the finish of the other wire and after cutting them short, they are then soldered to the centre pad between T1 and the dongle PCB. The really tricky step is soldering the two remaining wire ends to pins 4 and 5 of the RTL2832U chip. This is because the wire is extremely fine; the pins of the chip are spaced less than 0.4mm apart and there are tiny SMD components mounted on the top of the dongle PCB only about 1mm away from the body of the RTL2832, very close to pins 4 and 5. See the close-up photo of the finished job directly left. Frankly, I found soldering these wires to the chip pins very difficult, even using a binocular microscope and soldering iron with a very slim tip. I ended up having to use a drop of epoxy cement (Araldite) to hold the ends of the wires in position over pins 4 and 5. Then when the cement had cured, I was finally able to solder them to the pins without any solder bridges. Once these steps had been done (whew!), assembling the rest of the kit was fairly straightforward. Completing the board assembly was mainly a matter of fitting the three small RB electrolytic caps plus the blue power LED Celebrating 30 Years and six additional wires making the connections between the two PCBs. Five of these wires go on the top, with one of them being a short length of 0.8mm diameter tinned copper wire connecting the input of the dongle PCB to the track on the main PCB coming from the VHF-UHF input connector (labelled "UV", for some reason). Three of the others are 7mm lengths of insulated hookup wire making the power and USB connections at the other end of the dongle PCB. The remaining wire is another 7mm length of insulated wire, used to connect one of the 22µF electrolytics (near the 1000µF electro) to the output pin of 3.3V regulator U2, at the end nearest the RTL2832U. The final wire goes under the PCB assembly, being a 14mm length of insulated wire used to connect the other 22µF capacitor on the main board in parallel with the SMD capacitor C52 on the dongle PCB (see photo at upper right). The very last components to fit on the main PCB are the two hand-wound low pass filter coils L1 and L2. These are each wound from the 0.5mm enamelled copper wire, with eight turns wound on a 5mm diameter former like the shank of a 5mm drill bit. Then the wire ends are bent out radially and tinned, to allow them to be soldered to the pads provided to the left of T1 (see photo at left above). Once these final components and wires have been fitted, the SDR board assembly is essentially complete and ready to be fitted into the lower part of the case. This is done by sliding it into one of the channels in the sides until the SMA input sockets are protruding out at the far end. Then you attach that end November 2017  95 Installing SDR# and the required drivers on your PC If you are using our instructions for installing SDR# from the May 2013 issue, please note that we published a follow-up on page 82 of the November 2013 issue. This points out that you may need to install the latest Microsoft .NET framework before you can install SDR# (SDR# since 2015 has required .NET 4.6 minimum to run). Having said that, most modern Windows machines (ie, Windows 7/8/10) should already have the .NET framework installed. Also, the latest versions of SDR# will not run on Windows XP. XP is no longer supported and its users should upgrade to a newer version to avoid security problems. Similarly, while it will likely run on Vista, the operating system is no longer supported. The other package that you may need on your system is the Visual C++ Runtime. The download locations for both packages are listed in the steps below. The steps to install SDR# on a Windows PC are as follows (based on the RTL-SDR quick start guide): 1) Install the Microsoft .NET 4.6 redistributable, available from www.microsoft.com/en-us/download/details. aspx?id=48130 This is not required if it’s already on your PC, which should typically be the case for Windows 10 users. 2) Install the Microsoft Visual C++ Runtime redistributable, available from www.microsoft.com/en-us/download/details. aspx?id=8328 Again, this is not necessary if you already have it; the installer should tell you if you are not sure. 3) Click on the downloads button at the top of www.airspy.com and download the x86 version of sdrsharp.zip, next to the heading titled “SDR Software Package”. 4) Unzip the contents of sdrsharp.zip but don’t run anything yet. 5) Double click install-rtlsdr.bat within the extracted files. This should download the files “rtlsdr.dll” and “zadig.exe” into the same directory (you may need to run this batch file as an administrator). 6) Plug in the dongle and wait for Windows to attempt to install the drivers (it will likely fail). 7) Right-click zadig.exe and select “Run as administrator”. 8) Make sure “List All Devices” is checked in the Options menu. 9) Makes sure either “Bulk-In, Interface (Interface)”, “RTL2832UHIDIR” or “RTL2832U” is selected in the drop-down list. 10) Ensure that WinUSB is selected in the box to the right of the green arrow. 11) Click the Replace Driver button. You may get a warning that the publisher cannot be verified; if so, select “Install this driver software anyway”. Note that you may need to run zadig.exe again if you move the dongle to another USB port. 12) Open SDRSharp.exe. Note that the first time you do this, you may get a message indicating that Windows has protected your PC. This is a false alarm, so click on “more info” and then “run anyway”. 13) Set the drop-down box in the “Source” tab at upper left to “RTL-SDR (USB)”. 14) Press the Play button. 15) Press the Configure button (looks like a gear) up the top, next to the Play button. By default, the RF gain is set at zero, so adjust it upwards until you start seeing the expected RF signals being picked up in the SDR display. That’s it, your SDR# software is ready to go. plate, with the sockets passing through the matching holes. After this, the front plate can be secured to the lower half of the case using two of the M2 screws. The rear plate is fitted in the same way, after bending the leads of power LED1 so its body lines up with the matching 3mm hole. The mini USB socket will also protrude slightly 224µV 20MHz AM test waveform in SDR#. 96 Silicon Chip through its matching hole. All that remains is to attach the top half of the case, which simply involves lowering it into place (with the correct orientation, since the two halves 5µV 1GHz NFM signal in SDR#. Celebrating 30 Years siliconchip.com.au have complementary ridges and slots) and then fitting a pair of M2 screws at each end. Trying it out I installed SDR# and its drivers (using Zadig, which comes with the SDR# package) on a couple of different Intel machines running Windows 7, 64-bit. I did strike a bit of trouble initially because I had downloaded and installed the 64-bit version of SDR# and it didn't seem to be able to find the SDR device and its driver on either machine. Happily, this problem was solved by downloading and installing the 32-bit version. Once I had SDR# up and running, I ran some tests on the 25MHz-1.7GHz tuning range, to verify the performance of the dongle. The results were quite promising. For example, a 5µV NFM (narrowband FM) signal could be received clearly at various frequencies from 30.1MHz to 1.5GHz, with peak carrier amplitudes and SNR (signal to noise ratio) figures as shown in Table 1. Then I ran some similar tests on the direct sampling 100kHz-30MHz tuning range, this time using a 224µV AM signal with 30% modulation. The results are shown in Table 2. These are still quite respectable, although the sensitivity on this range is understandably rather lower than that on the 25MHz-1.7GHz range because of the lack of front-end gain. There siliconchip.com.au were rather more spurious "birdies" too, because of the lack of any frontend tuning or preselection. Note that I went beyond the nominal upper-frequency limit of 30MHz, just to see what the effect of the SDR's input low-pass filter might be. As you can see, the performance is still quite respectable up to 36MHz, so the filter doesn't seem to be too savage. Since the performance with a signal level of 224µV was so promising, I decided to do a couple more tests at 15.02MHz (roughly in the centre of the tuning range), one with a signal level of 22.4µV, and the other with a signal level of 12.6µV. The results were still quite respectable, as shown in Table 3. The bottom line So here are the good points about Banggood's dongle-based SDR kit: • its very low price • quite respectable performance over most of the claimed tuning range, from about 100kHz to over 1.5GHz • quality and completeness of the kit, right down to those extra SMD components and the additional M2 case assembly screws On the other hand, here are the notso-good points: • there are some aspects of kit assembly that present quite a challenge, like winding the balun transformer T1 and then soldering the fine wires from its secondary to pins Celebrating 30 Years 4 and 5 of the dongle's RTL2832U chip • the sensitivity and selectivity of the finished SDR does leave a bit to be desired, especially on the LF/HF direct sampling range. For serious listening, you'd be advised to use either a very long external antenna with a good earth and/or (preferably) an active antenna to provide both some gain and some preselection. Actually, I can verify that the kit's performance does benefit from the use of an active indoor loop antenna because I tried it out with the low-cost SinoRadios TG34 antenna I reviewed back in the June 2013 issue of Silicon Chip (pages 32-33). It worked quite well, and when I looked on eBay to see if it was still available, I found it at: www.ebay.com. au/itm/130392486862 Banggood also had a very similar unit called the Degen DE31MS, which you'll find at: siliconchip.com.au/l/ aag5 One final suggestion: although the extruded metal case of the Banggood kit does provide some shielding for the SDR circuitry, this could be improved by adding some short wires between the PCB earth copper and solder lugs attached firmly to the inside of the upper and lower parts of the case. This ensures that the case is reliably connected to PCB earth, and so is able to provide full shielding, resulting in significantly lower interference. SC November 2017  97