A few months ago we brought out a poll asking readers of this blog what they might like to see in a revised RTL-SDR dongle. We’ve now taken some of those suggestions and implemented them into a brand new dongle. For now the price of the new dongle will remain the same as before at $24.95 USD for the dongle + antenna kit and $19.95 USD for the dongle only, but we may need to increase the price by $1 – $2 within the next few weeks due to our slightly increased manufacturing costs. Worldwide shipping remains free from the Chinese international warehouse, and US customers can order either from the Chinese international warehouse or from Amazon who will give you free shipping if you are a Prime member, or spend over $49. The Chinese warehouse is currently stocked and ready to ship, and Amazon is now stocked and should be ready to ship by the end of this week.
Please go to our store page at rtl-sdr.com/store for information on purchasing.
Here is the short version of the biggest changes:
1) HF support via direct sampling. Connect an HF antenna directly to the SMA connector and tune from 500 kHz – 24 MHz with the direct sampling mod. (No hardware modding or soldering required)
2) Lower internal noise. Less spurs, lower noise floor etc.
3) Software switchable bias tee. No need to do any soldering to enable the bias tee. Can be turned on and off in software.
We call this version three of our RTL-SDR Blog dongles. The first was version zero and was simply the standard MCX dongles with better antennas. Next came version 1 with the bias tee and SMA connector, and version two introduced the metal case.
Here is the long list of improvements and changes, and why they were made:
1) Improved ESD protection on the radio front end. The BAV99 diode which is used on most dongles is not a true ESD rated diode. We have added a real ESD rated diode for better protection. The BAV99 remains in the circuit as a strong signal clipper, to prevent damage to the R820T2 from overly strong signals. Please remember that not even this will save your radio from a lightning strike, and any permanently outdoor mounted antenna system must have its own lightning protection.
2) Longer SMA connector. One or two customers had problems with the shorter SMA plugs which could not fit some of their antenna connectors. The longer shaft fixes this and also allows us to add a nut to fasten it to the aluminum body which provides a better low impedance connection (although this is not strictly needed as the PCB side ground tracks already provide a good connection).
3) Improved front end circuit. The standard matching circuit on the RTL-SDR was designed for DVB-T use, and tends to attenuate signals above ~1 GHz. The new matching circuit has less attenuation above 1 GHz and similar performance below. We used very high quality, high SRF, high Q inductors in this circuit.
4) Added a software switchable 4.5v bias tee. In previous versions of our units the 4.5v bias tee needed to be activated manually, by soldering a bridge between two pads on the PCB. However we found that many customers who want to use the bias tee do not have the skills or tools to be able to perform this mod. The new unit makes use of a low noise LDO and one of the GPIO pins on the RTL2832U to activate the bias tee in software. This of course requires a modification to the drivers, but we will shortly upload a program called rtl_biast and batch files to turn the bias tee on and off in Windows and Linux.
This bias tee is great for powering a remote LNA (like Adams PSA5043+ based LNA4ALL) or something like the SpyVerter upconverter. We’ve tested it with both and found them to be running just fine.
Warning: The bias tee LDO can be damaged if you short circuit it. Before turning on the bias tee, ensure the circuit to be powered is not shorted, or that the RTL-SDR is not connected to a DC shorted antenna!
5) Added several access pads on the PCB. Access pads for the unused GPIO pins, CLK in/out, 3.3V, GND and I2C pins have been added. The CLK input/output is disconnected by default (see change 6). Access pads for the I branch have also been added as some users and industrial customers are using these in special projects. These pads are only for advanced users who need them for special projects. Take care as these pins are not ESD protected.
6) Added a clock selector jumper. By soldering in a 4 pin 1.27mm pitch jumper header and removing the default 0 Ohm resistor, one can now easily select between the onboard clock, an external clock, or having the on board clock be the output for another dongle. This is for advanced users only who want to experiment with things like passive radar, and coherent receivers.
7) Reduced noise with a modified PCB design. This significantly reduces spurs and noise pickup due much lower impedance grounding and blocking of interference. Also added a USB common mode choke to reduce USB noise, several ferrite chokes on the PCB, and a lower noise LDO. A larger ground plane also improves on heat dissipation.
8) Added an experimental HF direct sampling circuit, which is diplexed out from the SMA connector. This has little to no effect on VHF/UHF operation, but allows us to make use of the Q branch on the RTL2832U chip for direct sampling, which allows us to receive from about 500 kHz to about 24 MHz. (Below 500 kHz is unavailable due to attenuation from the bias tee circuit). We used a ~10dB 50 Ohm preamp as a buffer and to overcome losses in the transformer and filter. We also added a strong 24 MHz low pass filter, and added an impedance matching transformer coil to ensure good direct sampling performance.
Of course direct sampling can never be as good as using an upconverter. It can overload easily if you have strong signals since there is no gain control. But this should at least give the majority of users a decent taste of what’s on HF. If you then find HF interesting, then you can consider upgrading to an upconverter like the SpyVerter (and the SpyVerter is of course compatible with our bias tee for easy operation).
We’re still classing this mode as experimental (and will be interested to hear any feedback on results), but we have had good results in our testing of this mode when receiving signals that are not too strong, getting sensitivity as good as an upconverter. We found that very good reception was obtainable with a long wire antenna and 9:1 unun combination.
9) Antenna bases now come with a stronger magnet and a conductive copper sticker on the bottom. The stronger magnet adds very good stability when using our large 1.5m antenna and the copper sticker ensures that good electrical contact can be made between the base and whatever piece of metal you use underneath as the ground plane. This significantly improves the antenna’s performance as a quarter wave ground plane.
10) Added corner mounting holes for those who want to stack PCBs. Some customers have been building devices that require multiple RTL-SDR dongles, and these standoff holes should aid in stacking.
As from the previous innovations the units still come with:
1) SMA connector – The most common connector in the radio world. Easy to adapt to other connectors and low loss over a wide range of frequencies.
2) Thermal pad – A thin thermal pad allows heat to transfer from the PCB to the metal case easily. The metal case then cools off to the surrounding air. This helps to solve L-band insensitivity problems.
3) Metal case – Helps block out interference and provides cooling.
We now have a V3 users guide available which explains how to use the new features such as the bias tee, HF mode and CLK jumpers.
What’s coming next?
We think that our unit is now pretty much at the peak of how good a cheap R820T2 RTL-SDR can be, so apart from minor tweaks this is likely to be our last major revision of this model of the RTL-SDR. In a 1-2 months we hope to bring out a FM bandstop filter with metal enclosure and SMA plugs with a target cost of $14.95 shipped. Further into the future we also hope to bring out supporting products like a wideband bias tee powered LNA and wideband antennas. These supporting products will of course be compatible with other SDR’s like the Airspy or SDRplay, or other RTL-SDR dongles.