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Using Cheap Asian Electronic Modules – by Jim Rowe New w.i.d.e.b.a.n.d RTL-SDR modules In the November 2017 issue we reviewed a low-cost RTL-SDR kit from Chinese firm Banggood Technology. Since then, fully assembled RTL-SDRs have become available from Banggood and other Chinese suppliers. So we decided to put them through their paces. W e described how softwaredefined radios (SDRs) work in our May 2013 issue (siliconchip.com.au/Article/3778), and gave details on using the popular SDR# (“SDR-sharp”) software. Then we followed that up with an up-converter project for low-frequency reception in the June 2013 issue (siliconchip.com.au/Article/3810). That design was then expanded into the SiDRADIO integrated SDR, which was described in the October-December 2013 issues (siliconchip.com.au/ Series/130). And, as mentioned in the intro, we reviewed the $30 Banggood SDR kit in November 2017 (siliconchip.com. au/Article/10879). So we won’t go back over all the details of how SDRs operate. If you want the full treatment, read the May 2013 ANTENNA +3.3V 3.3V REGULATOR 1 SMA SOCKET DIGITALLY PROGRAMMABLE MULTI-BAND VHF & UHF TUNER CHIP (RAFAEL MICRO R820T2 ) OPTIONAL RECEIVER FOR IR REMOTE SC 2020  I+ I– Q+ 5 Q– 2 4 REALTEK RTL2832U COFDM DIGITAL DEMODULATOR CHIP WITH USB 2.0 I/F USB TYPE A PLUG EEPROM INSIDE A BASIC 25MHz – 1.7GHz VHF–UHF SDR DONGLE Fig.1: the configuration of a basic RTL-SDR dongle. The R820T2 provides preselection and RF gain, while the RTL2832 converts the RF signals to digital data, to feed to the PC via its USB port. 62 Silicon Chip Australia’s electronics magazine article. But for those who just need a quick refresher, let’s go over the basic details. A software-defined radio is essentially a device capable of converting a PC into a radio receiver, tuned and controlled by software running on the PC. “RTL-SDR” refers to an SDR based on a Realtek RTL2832U digital demodulator chip, usually in conjunction with a multi-band VHF/UHF tuner chip like the Rafael Micro R820T2. The first products using devices like the RTL2832U and the R820T were low-cost DVB-T dongles, released around 2009 to provide a cheap way to receive digital TV with a PC. It was only a little later that people realised that the same dongles could be used to receive AM, FM, CW and SSB radio signals. That was the birth of low-cost SDRs. The Banggood SDR kit we reviewed in 2017 was claimed to provide wide range reception from 100kHz to 1.7GHz. It turned out to be rather tricky to assemble, but gave quite respectable performance even on the LF-HF siliconchip.com.au +3.3V 3.3V REGULATOR 1 VHF-UHF INPUT SOCKET I+ I– Q+ 5 Q– 2 DIGITALLY PROGRAMMABLE MULTI-BAND VHF & UHF TUNER CHIP (RAFAEL MICRO R820T2 ) 4 REALTEK RTL2832U COFDM DIGITAL DEMODULATOR CHIP WITH USB 2.0 I/F USB TYPE A PLUG RTL-SDR dongle is limited to VHF and UHF reception. While there are many signals on these bands, there are also plenty on the LF and HF bands below 25MHz. Some additions are needed for RTLSDR reception on these lower bands. Direct sampling OPTIONAL RECEIVER FOR IR REMOTE LF-HF INPUT SOCKET SC 2020  EEPROM T1 LF-HF BANDPASS FILTER INSIDE A WIDE RANGE SDR USING HF DIRECT SAMPLING Fig.2: an SDR dongle like that shown in Fig.1, but modified to provide LF-HF reception using direct sampling. The lower frequency signals are fed to transformer T1, which couples them to the RTL2832’s Q+ and Q- pins for sampling. bands, despite using the cheaper ‘direct sampling’ approach rather than an upconverter. Since then, Banggood and various other Chinese suppliers have come up with several new fully-assembled RTL-SDR units, and they are what we are investigating in this article. A basic RTL-SDR dongle Fig.1 shows the block diagram of a basic RTL-SDR dongle. The main components are a Realtek RTL2832U COFDM digital demodulator chip and a Rafael Micro R820T2 digitally programmable multi-band VHF and UHF tuner chip. The RTL2832U chip includes a USB 2.0 interface which receives commands from the PC software and also feeds the demodulated signal samples back to the PC. It also includes the core of an 8051 CPU and a hardware FIFO to handle the bulk USB transfers. Fig.1 also shows an infrared receiver. This is basically a carry-over from the original use of these dongles for DVB-T reception (to receive signals from a remote control), and isn’t needed for SDR operation. The R820T2 chip is only able to receive signals between about 25MHz and 1.7GHz (1700MHz), so the basic NOTE: S1 MAY BE ELECTRONIC RATHER THAN MECHANICAL The cheapest way of adding LF and HF reception capability is shown in Fig.2. Here, the LF-HF signals are fed into the SDR via a second input, then passed through a bandpass filter to reduce interference from signals outside this range. Then they go through a small RF transformer (T1) and into the Q+ and Q- inputs of the RTL2832U demodulator chip. These pins are not used for VHF/UHF reception. So, these signals can be received by the PC software directing the RTL2832U to perform direct sampling from the Q+ and Q- pins, rather than from the I+ and I- pins. So changing between VHF-UHF reception and LFHF reception can be done by software command. With this approach, the LF-HF signals receive no input gain or preselection. As a result, the sensitivity and selectivity of this type of ‘wide range’ RTL-SDR on the LF-HF bands is not marvellous – although it can be acceptable for some applications. Some of the newer RTL-SDRs using the direct sampling approach have a +3.3V 3.3V REGULATOR +5V MINI USB SOCKET 1 VHF–UHF INPUT SOCKET VHF–UHF S1 LF–HF DIGITALLY PROGRAMMABLE MULTI-BAND VHF & UHF TUNER CHIP (RAFAEL MICRO R820T2 ) OPTIONAL RECEIVER FOR IR REMOTE LF–HF INPUT SOCKET SC 2020 4 REALTEK RTL2832U COFDM DIGITAL DEMODULATOR CHIP WITH USB 2.0 I/F  EEPROM VHF–UHF MIXER LOW-PASS FILTER I+ I– Q+ 5 Q– 2 HIGH-PASS FILTER LOCAL OSCILLATOR S2 +5V LF–HF (100MHz OR 125MHz) INSIDE A WIDE RANGE SDR WITH A BUILT-IN LF-HF UPCONVERTER Fig.3: adding an upconverter provides better LF-HF performance than the direct sampling approach shown in Fig.2. The LF-HF signals are mixed with a much higher frequency local oscillator signal, and the resulting sum-product (a higher frequency again) is fed to the SDR’s UHF input via a high-pass filter that rejects the unwanted signal components from the mixer. siliconchip.com.au Australia’s electronics magazine May 2020  63 Internal front and back views of the “V3” RTL-SDR, showing board construction and the SMA and USB sockets on each end (the SMA is the input and USB the output). The upper board (left) is identical to the old DVB-T dongle. The upconverter option The upconverter approach provides improved reception below 25MHz. This is shown in Fig.3. The LF-HF signals again come in via a separate input socket, but they then go through a low-pass filter to attenuate any signals above 25MHz which could cause interference. Then they are fed into a mixer, along with a local oscillator (LO) signal, typically either 100MHz or 125MHz. The mixer output incorporates the sum and difference frequencies. It goes through a high-pass filter, with its corner frequency set to be a little above the local oscillator frequency. This removes the original, local oscillator and difference signals, leaving only the sum signal. So the output from the high-pass filter is effectively the incoming LF-HF signals shifted up by the local oscillator frequency. With a 100MHz LO, an incoming signal of say 200kHz becomes a signal of 100MHz + 200kHz or 100.200MHz, while an incoming signal of 8.35MHz is shifted up to become a signal of 108.35MHz, and so on. Switch S1 selects either the VHFUHF signals from the upper input socket, or the upshifted LF-HF signals from the mixer and high-pass filter. This can be either a mechanical or an electronic switch. Switch S2 at lower right is used to control the operation of the local oscillator, switching it on for reception of LF-HF signals, or off for reception of VHF-UHF signals. This upconverter approach is more complicated and expensive than the direct sampling approach, but it does deliver somewhat better reception for LF and HF signals. That’s mostly because the upshifted signals go through the same R820T2 digitally-programmed multi-band tuner as the VHF and UHF signals. The fact that the SDR is receiving LF-HF signals at a higher frequency than they are broadcast is taken care of by the reception software (eg, SDR#). These packages have an option to allow the effective (and displayed) tuning frequency to be shifted up or down by any desired figure. So if your upconverter has a local oscillator frequency of 100MHz, all you have to do is instruct the application to subtract 100MHz from the upshifted frequency, and it will be shown at the correct frequency. The main shortcoming of the upconverter is that the added local oscillator degrades the tuning stability, unless it has exceptional frequency stability. In other words, the LF-HF tuning tends to ‘drift’ or ‘wander’ with temperature variations. 20 15 Input Signal Level (dBm) -70 Banggood SDR kit “V3" RTL-SDR Blog V3 RTL-SDR.com -80 -90 10 SNR (dB) single RF input socket, with a ‘diplexer’ filter used to separate the incoming LF-HF signals (<24MHz) from the VHF-UHF signals (>24MHz). At least one also provides a 10dB RF preamp in the LF-HF branch, to compensate for losses in the bandpass filter and T1. We’ll look at this unit shortly. -100 -110 -120 -130 -140 0.1 1 10 100 Signal Frequency (MHz) 1000 1750 Fig.4: the sensitivity and signal-to-noise ratio figures for the reception of a range of frequencies from all three directsampling SDR units mentioned in this article. Note that the SNR (signal to noise ratio) figures are all very similar. 64 Silicon Chip Australia’s electronics magazine siliconchip.com.au SDR# grab: A screen grab from SDR# showing the performance of the RTL-SDR Blog V3 dongle when receiving a 1.600GHz CW signal at -127dBm (100nV). The received signal-to-noise ratio is 16.5dB – pretty impressive! That’s why upconverter type RTLSDRs generally claim to contain a high stability TCXO (temperaturecontrolled crystal oscillator), with a stability of say ±0.5ppm (parts per million). With a 100MHz local oscillator, that corresponds to a drift of ±50Hz. The other shortcoming of the upconverter approach is that because it doesn’t provide the incoming LF-HF signals with any preselection, strong signals near the signals you’re interested in can cause overload in the upconverting mixer, resulting in interference. Luckily, this can be remedied by using an external RF preselector ahead of the LF-HF input of the SDR. In the remainder of this article, we’ll look at RTL-SDRs that use the direct sampling approach. Next month, we’ll describe other units that use an upconverter. gles, with the metal case offering better electromagnetic shielding than the old plastic cases, and the SMA input socket offering better matching at UHF than the old Belling-Lee (PALtype) sockets. A typical example is shown in the photos below. This one came from Banggood, and cost A$30.16 delivered, including insurance and GST. It came with a short USB cable to connect it to the PC, and it carries the RTL.SDR label, together with a small “V.3” legend at the input end. It seems to be a clone of another similar looking unit sold online by RTL-SDR.com (www.rtl-sdr.com/ store) and various agents. The latter unit carries the label RTL-SDR.COM, and we’ll discuss that one shortly. If you open up the first unit, you’ll discover that it’s built on two small PCBs which are stacked, one on top of the other (see photos opposite). The upper PCB appears to be one of the original DVB-T dongle boards, A compact “V.3” RTL.SDR Currently, you’ll find quite a few low-cost RTL-SDRs available on the web. Many of them come in a compact aluminium case measuring 74 x 25 x 15mm, with a USB type-A plug at one end and an SMA input socket at the other. Basically, these are an improved version of the original DVB-T donsiliconchip.com.au The RTL-SDR dongles are supplied in metal cases, which assists in shielding from interference. This is the cheaper of the two units reviewed here – compare this to the higher-performing unit shown above right. Australia’s electronics magazine May 2020  65 (Above and left): The slightly more expensive (but much better performing) RTL-SDR Blog V3. Unlike the other dongle, this has a re-designed PCB incorporating the direct sampling components. complete with IR remote control receiver, indicator LED and holes for mounting an RF input connector. The lower PCB provides the additional components and circuitry for a direct-sampling (Q-branch) LF-HF input range, sharing the new SMA input socket. I found this unit to work fairly well. Its performance compares favourably with that of the Banggood kit SDR I reviewed in the November 2017 issue. The measured performance of both can be seen in Figure 4. This compares the performance of the kit SDR we previously reviewed, to both the new “V.3” RTL.SDR and the Blog V3 described below. This shows that the performance of the new unit is very close to that of the kit on the LF-HF direct sampling range, while its sensitivity on the VHF-UHF range is significantly worse, especially at the top end. Like all of the RTL-SDRs we’re discussing in these articles, the “V.3” unit is fully compatible with SDR PC applications like SDR#. It doesn’t come with this software, but you can download it for free from the Airspy website (www. airspy.com). You can also download a “Quick Start Guide” PDF from www.rtl-sdr. com, which explains a lot about installing SDR# and the drivers it needs to communicate with a dongle-based SDR. The RTL-SDR Blog V3 I also purchased one of the original units that was cloned: the RTL-SDR Blog V3 from rtl-sdr.com You can buy this from RTL-SDR (either directly or through Amazon) for US$21.95 plus postage, or from their Australian representatives, South Eastern Communications (www. secomms.com.au) for A$35.00 plus $11.60 postage. I ordered mine from South Eastern 66 Silicon Chip Communications. It comes in a neat little extruded aluminium case like the Banggood “V.3” unit, and it’s almost identical in size. But inside, all of the circuitry is on a single, completely redesigned PCB, as shown above. It has various additions and improvements, including a choke in the USB power line to reduce USB noise and a thermal pad under the PCB to keep the circuitry cooler by conducting heat to the metal case. There’s also a 10dB RF preamplifier in the LF-HF line between the diplexer and the bandpass filter, to improve the sensitivity. Other features include an additional shunt diode at the input to provide improved ESD protection, and a USBpowered ‘bias tee’ at the input to allow it to provide phantom power to RF amplifiers and active antennas. The bias tee is controlled by software, but SDR# and many of the other SDR applications don’t allow this to be done directly; it needs to be done using separate batch files. Before doing any serious testing of this unit, I downloaded and read both its data sheet and User Guide (from the rtl-sdr.com website). I was glad that I did, because I discovered that its ‘bias tee’ circuit is enabled by default, and can be damaged by connecting the RF input to a low-resistance antenna or signal generator – unless you disable it. I also discovered that the bias tee circuit can be disabled permanently by removing SMD inductor L13 (near the SMA input socket). This also improves the performance on the LF-HF range. So I fired up my soldering iron and carefully removed L13, before reassembling the RTLSDR Blog V3 and starting my tests. It soon became apparent that the performance of this unit is significantly better than that of either the RTL.SDR “V.3” or the original RTLAustralia’s electronics magazine SDR kit. The test results are summarised in figure 4, and if you compare them against the other curves, you’ll see that the Blog V3 is well ahead on both ranges. To summarise, the RTL-SDR Blog V3 is the best performer of the lot. It does cost a few dollars more (especially if you buy it via the local agents), but that’s worthwhile for the performance improvement. COMING NEXT MONTH: In the second part of this feature, we’ll test some of the larger RTL-SDR units with built-in upconverters, which should provide improved LF-HF reception. Stay tuned! SC Useful Links • www.secomms.com.au (Australian supplier of the RTL-SDR Blog V3) • www.airspy.com (best current source of the SDR# application) • https://rtl1090.com (ADS-B application; ADSB# is no longer available) • www.hdsdr.de (source of the HDSDR application) • https://zadig.akeo.ie/ (source of Zadig, the Windows generic USB driver installer needed by most SDR software) • www.rtl-sdr.com (an excellent source of information on RTL-SDR) • www.rtl-sdr.com/adsb-aircraftradar-with-rtl-sdr/ • www.rtl-sdr.com/big-list-rtl-sdrsupported-software/ • www.rtl-sdr.com/rtl-sdr-blog-v3-dongles-user-guide/ • www.rtl-sdr.com/rtl-sdr-quickstart-guide/ • www.rtl-sdr.com/sdrsharp-plugins/ • www.sdr-radio.com/download siliconchip.com.au